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Linux Advanced Routing & Traffic Control HOWTOBert HubertNetherlabs BV Gregory MaxwellRemco van Mook Martijn van Oosterhout Paul B Schroeder Jasper Spaans
Chapter 1. DedicationThis document is dedicated to lots of people, and is my attempt to do something back. To list but a few:
Chapter 2. IntroductionWelcome, gentle reader. This document hopes to enlighten you on how to do more with Linux 2.2/2.4 routing. Unbeknownst to most users, you already run tools which allow you to do spectacular things. Commands like route and ifconfig are actually very thin wrappers for the very powerful iproute2 infrastructure. I hope that this HOWTO will become as readable as the ones by Rusty Russell of (amongst other things) netfilter fame. You can always reach us by writing to the HOWTO team. However, please consider posting to the mailing list (see the relevant section) if you have questions which are not directly related to this HOWTO. We are no free helpdesk, but we often will answer questions asked on the list. Before losing your way in this HOWTO, if all you want to do is simple traffic shaping, skip everything and head to the Other possibilities chapter, and read about CBQ.init. 2.1. Disclaimer & LicenseThis document is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. In short, if your STM-64 backbone breaks down and distributes pornography to your most esteemed customers - it's never our fault. Sorry. Copyright (c) 2002 by bert hubert, Gregory Maxwell, Martijn van Oosterhout, Remco van Mook, Paul B. Schroeder and others. This material may be distributed only subject to the terms and conditions set forth in the Open Publication License, v1.0 or later (the latest version is presently available at http://www.opencontent.org/openpub/). Please freely copy and distribute (sell or give away) this document in any format. It's requested that corrections and/or comments be forwarded to the document maintainer. It is also requested that if you publish this HOWTO in hardcopy that you send the authors some samples for "review purposes" :-) 2.2. Prior knowledgeAs the title implies, this is the "Advanced" HOWTO. While by no means rocket science, some prior knowledge is assumed. Here are some other references which might help teach you more:
2.3. What Linux can do for youA small list of things that are possible:
Currently, not many people are using these advanced features. This is for several reasons. While the provided documentation is verbose, it is not very hands-on. Traffic control is almost undocumented. 2.4. Housekeeping notesThere are several things which should be noted about this document. While I wrote most of it, I really don't want it to stay that way. I am a strong believer in Open Source, so I encourage you to send feedback, updates, patches etcetera. Do not hesitate to inform me of typos or plain old errors. If my English sounds somewhat wooden, please realize that I'm not a native speaker. Feel free to send suggestions. If you feel to you are better qualified to maintain a section, or think that you can author and maintain new sections, you are welcome to do so. The SGML of this HOWTO is available via CVS, I very much envision more people working on it. In aid of this, you will find lots of FIXME notices. Patches are always welcome! Wherever you find a FIXME, you should know that you are treading in unknown territory. This is not to say that there are no errors elsewhere, but be extra careful. If you have validated something, please let us know so we can remove the FIXME notice. About this HOWTO, I will take some liberties along the road. For example, I postulate a 10Mbit Internet connection, while I know full well that those are not very common. 2.5. Access, CVS & submitting updatesThe canonical location for the HOWTO is here. We now have anonymous CVS access available to the world at large. This is good in a number of ways. You can easily upgrade to newer versions of this HOWTO and submitting patches is no work at all. Furthermore, it allows the authors to work on the source independently, which is good too.
If you spot an error, or want to add something, just fix it locally, and run cvs diff -u, and send the result off to us. A Makefile is supplied which should help you create postscript, dvi, pdf, html and plain text. You may need to install docbook, docbook-utils, ghostscript and tetex to get all formats. 2.6. Mailing listThe authors receive an increasing amount of mail about this HOWTO. Because of the clear interest of the community, it has been decided to start a mailinglist where people can talk to each other about Advanced Routing and Traffic Control. You can subscribe to the list here. It should be pointed out that the authors are very hesitant of answering questions not asked on the list. We would like the archive of the list to become some kind of knowledge base. If you have a question, please search the archive, and then post to the mailinglist. 2.7. Layout of this documentWe will be doing interesting stuff almost immediately, which also means that there will initially be parts that are explained incompletely or are not perfect. Please gloss over these parts and assume that all will become clear. Routing and filtering are two distinct things. Filtering is documented very well by Rusty's HOWTOs, available here: We will be focusing mostly on what is possible by combining netfilter and iproute2. Chapter 3. Introduction to iproute23.1. Why iproute2?Most Linux distributions, and most UNIX's, currently use the venerable arp, ifconfig and route commands. While these tools work, they show some unexpected behaviour under Linux 2.2 and up. For example, GRE tunnels are an integral part of routing these days, but require completely different tools. With iproute2, tunnels are an integral part of the tool set. The 2.2 and above Linux kernels include a completely redesigned network subsystem. This new networking code brings Linux performance and a feature set with little competition in the general OS arena. In fact, the new routing, filtering, and classifying code is more featureful than the one provided by many dedicated routers and firewalls and traffic shaping products. As new networking concepts have been invented, people have found ways to plaster them on top of the existing framework in existing OSes. This constant layering of cruft has lead to networking code that is filled with strange behaviour, much like most human languages. In the past, Linux emulated SunOS's handling of many of these things, which was not ideal. This new framework makes it possible to clearly express features previously beyond Linux's reach. 3.2. iproute2 tourLinux has a sophisticated system for bandwidth provisioning called Traffic Control. This system supports various method for classifying, prioritizing, sharing, and limiting both inbound and outbound traffic. We'll start off with a tiny tour of iproute2 possibilities. 3.3. PrerequisitesYou should make sure that you have the userland tools installed. This package is called 'iproute' on both RedHat and Debian, and may otherwise be found at ftp://ftp.inr.ac.ru/ip-routing/iproute2-2.2.4-now-ss??????.tar.gz". You can also try here for the latest version. Some parts of iproute require you to have certain kernel options enabled. It should also be noted that all releases of RedHat up to and including 6.2 come without most of the traffic control features in the default kernel. RedHat 7.2 has everything in by default. Also make sure that you have netlink support, should you choose to roll your own kernel. Iproute2 needs it. 3.4. Exploring your current configurationThis may come as a surprise, but iproute2 is already configured! The current commands ifconfig and route are already using the advanced syscalls, but mostly with very default (ie. boring) settings. The ip tool is central, and we'll ask it to display our interfaces for us. 3.4.1. ip shows us our links
Your mileage may vary, but this is what it shows on my NAT router at home. I'll only explain part of the output as not everything is directly relevant. We first see the loopback interface. While your computer may function somewhat without one, I'd advise against it. The MTU size (Maximum Transfer Unit) is 3924 octets, and it is not supposed to queue. Which makes sense because the loopback interface is a figment of your kernel's imagination. I'll skip the dummy interface for now, and it may not be present on your computer. Then there are my two physical network interfaces, one at the side of my cable modem, the other one serves my home ethernet segment. Furthermore, we see a ppp0 interface. Note the absence of IP addresses. iproute disconnects the concept of 'links' and 'IP addresses'. With IP aliasing, the concept of 'the' IP address had become quite irrelevant anyhow. It does show us the MAC addresses though, the hardware identifier of our ethernet interfaces. 3.4.2. ip shows us our IP addresses
This contains more information. It shows all our addresses, and to which cards they belong. 'inet' stands for Internet (IPv4). There are lots of other address families, but these don't concern us right now. Let's examine eth0 somewhat closer. It says that it is related to the inet address '10.0.0.1/8'. What does this mean? The /8 stands for the number of bits that are in the Network Address. There are 32 bits, so we have 24 bits left that are part of our network. The first 8 bits of 10.0.0.1 correspond to 10.0.0.0, our Network Address, and our netmask is 255.0.0.0. The other bits are connected to this interface, so 10.250.3.13 is directly available on eth0, as is 10.0.0.1 for example. With ppp0, the same concept goes, though the numbers are different. Its address is 212.64.94.251, without a subnet mask. This means that we have a point-to-point connection and that every address, with the exception of 212.64.94.251, is remote. There is more information, however. It tells us that on the other side of the link there is, yet again, only one address, 212.64.94.1. The /32 tells us that there are no 'network bits'. It is absolutely vital that you grasp these concepts. Refer to the documentation mentioned at the beginning of this HOWTO if you have trouble. You may also note 'qdisc', which stands for Queueing Discipline. This will become vital later on. 3.4.3. ip shows us our routesWell, we now know how to find 10.x.y.z addresses, and we are able to reach 212.64.94.1. This is not enough however, so we need instructions on how to reach the world. The Internet is available via our ppp connection, and it appears that 212.64.94.1 is willing to spread our packets around the world, and deliver results back to us.
This is pretty much self explanatory. The first 4 lines of output explicitly state what was already implied by ip address show, the last line tells us that the rest of the world can be found via 212.64.94.1, our default gateway. We can see that it is a gateway because of the word via, which tells us that we need to send packets to 212.64.94.1, and that it will take care of things. For reference, this is what the old route utility shows us:
3.5. ARPARP is the Address Resolution Protocol as described in RFC 826. ARP is used by a networked machine to resolve the hardware location/address of another machine on the same local network. Machines on the Internet are generally known by their names which resolve to IP addresses. This is how a machine on the foo.com network is able to communicate with another machine which is on the bar.net network. An IP address, though, cannot tell you the physical location of a machine. This is where ARP comes into the picture. Let's take a very simple example. Suppose I have a network composed of several machines. Two of the machines which are currently on my network are foo with an IP address of 10.0.0.1 and bar with an IP address of 10.0.0.2. Now foo wants to ping bar to see that he is alive, but alas, foo has no idea where bar is. So when foo decides to ping bar he will need to send out an ARP request. This ARP request is akin to foo shouting out on the network "Bar (10.0.0.2)! Where are you?" As a result of this every machine on the network will hear foo shouting, but only bar (10.0.0.2) will respond. Bar will then send an ARP reply directly back to foo which is akin bar saying, "Foo (10.0.0.1) I am here at 00:60:94:E9:08:12." After this simple transaction that's used to locate his friend on the network, foo is able to communicate with bar until he (his arp cache) forgets where bar is (typically after 15 minutes on Unix). Now let's see how this works. You can view your machines current arp/neighbor cache/table like so:
As you can see my machine espa041 (9.3.76.41) knows where to find espa042 (9.3.76.42) and espagate (9.3.76.1). Now let's add another machine to the arp cache.
As a result of espa041 trying to contact espa043, espa043's hardware address/location has now been added to the arp/neighbor cache. So until the entry for espa043 times out (as a result of no communication between the two) espa041 knows where to find espa043 and has no need to send an ARP request. Now let's delete espa043 from our arp cache:
Now espa041 has again forgotten where to find espa043 and will need to send another ARP request the next time he needs to communicate with espa043. You can also see from the above output that espagate (9.3.76.1) has been changed to the "stale" state. This means that the location shown is still valid, but it will have to be confirmed at the first transaction to that machine. Chapter 4. Rules - routing policy databaseIf you have a large router, you may well cater for the needs of different people, who should be served differently. The routing policy database allows you to do this by having multiple sets of routing tables. If you want to use this feature, make sure that your kernel is compiled with the "IP: advanced router" and "IP: policy routing" features. When the kernel needs to make a routing decision, it finds out which table needs to be consulted. By default, there are three tables. The old 'route' tool modifies the main and local tables, as does the ip tool (by default). The default rules:
This lists the priority of all rules. We see that all rules apply to all packets ('from all'). We've seen the 'main' table before, it is output by ip route ls, but the 'local' and 'default' table are new. If we want to do fancy things, we generate rules which point to different tables which allow us to override system wide routing rules. For the exact semantics on what the kernel does when there are more matching rules, see Alexey's ip-cref documentation. 4.1. Simple source policy routingLet's take a real example once again, I have 2 (actually 3, about time I returned them) cable modems, connected to a Linux NAT ('masquerading') router. People living here pay me to use the Internet. Suppose one of my house mates only visits hotmail and wants to pay less. This is fine with me, but they'll end up using the low-end cable modem. The 'fast' cable modem is known as 212.64.94.251 and is a PPP link to 212.64.94.1. The 'slow' cable modem is known by various ip addresses, 212.64.78.148 in this example and is a link to 195.96.98.253. The local table:
Lots of obvious things, but things that need to be specified somewhere. Well, here they are. The default table is empty. Let's view the 'main' table:
We now generate a new rule which we call 'John', for our hypothetical house mate. Although we can work with pure numbers, it's far easier if we add our tables to /etc/iproute2/rt_tables.
Now all that is left is to generate John's table, and flush the route cache:
And we are done. It is left as an exercise for the reader to implement this in ip-up. 4.2. Routing for multiple uplinks/providersA common configuration is the following, in which there are two providers that connect a local network (or even a single machine) to the big Internet.
4.2.1. Split accessThe first is how to route answers to packets coming in over a particular provider, say Provider 1, back out again over that same provider. Let us first set some symbolical names. Let $IF1 be the name of the first interface (if1 in the picture above) and $IF2 the name of the second interface. Then let $IP1 be the IP address associated with $IF1 and $IP2 the IP address associated with $IF2. Next, let $P1 be the IP address of the gateway at Provider 1, and $P2 the IP address of the gateway at provider 2. Finally, let $P1_NET be the IP network $P1 is in, and $P2_NET the IP network $P2 is in. One creates two additional routing tables, say T1 and T2. These are added in /etc/iproute2/rt_tables. Then you set up routing in these tables as follows:
Next you set up the main routing table. It is a good idea to route things to the direct neighbour through the interface connected to that neighbour. Note the `src' arguments, they make sure the right outgoing IP address is chosen.
Now, this is just the very basic setup. It will work for all processes running on the router itself, and for the local network, if it is masqueraded. If it is not, then you either have IP space from both providers or you are going to want to masquerade to one of the two providers. In both cases you will want to add rules selecting which provider to route out from based on the IP address of the machine in the local network. 4.2.2. Load balancingThe second question is how to balance traffic going out over the two providers. This is actually not hard if you already have set up split access as above. Instead of choosing one of the two providers as your default route, you now set up the default route to be a multipath route. In the default kernel this will balance routes over the two providers. It is done as follows (once more building on the example in the section on split-access):
Note that balancing will not be perfect, as it is route based, and routes are cached. This means that routes to often-used sites will always be over the same provider. Furthermore, if you really want to do this, you probably also want to look at Julian Anastasov's patches at http://www.linuxvirtualserver.org/~julian/#routes , Julian's route patch page. They will make things nicer to work with. Chapter 5. GRE and other tunnelsThere are 3 kinds of tunnels in Linux. There's IP in IP tunneling, GRE tunneling and tunnels that live outside the kernel (like, for example PPTP). 5.1. A few general remarks about tunnels:Tunnels can be used to do some very unusual and very cool stuff. They can also make things go horribly wrong when you don't configure them right. Don't point your default route to a tunnel device unless you know EXACTLY what you are doing :-). Furthermore, tunneling increases overhead, because it needs an extra set of IP headers. Typically this is 20 bytes per packet, so if the normal packet size (MTU) on a network is 1500 bytes, a packet that is sent through a tunnel can only be 1480 bytes big. This is not necessarily a problem, but be sure to read up on IP packet fragmentation/reassembly when you plan to connect large networks with tunnels. Oh, and of course, the fastest way to dig a tunnel is to dig at both sides. 5.2. IP in IP tunnelingThis kind of tunneling has been available in Linux for a long time. It requires 2 kernel modules, ipip.o and new_tunnel.o. Let's say you have 3 networks: Internal networks A and B, and intermediate network C (or let's say, Internet). So we have network A:
The router has address 172.16.17.18 on network C. and network B:
The router has address 172.19.20.21 on network C. As far as network C is concerned, we assume that it will pass any packet sent from A to B and vice versa. You might even use the Internet for this. Here's what you do: First, make sure the modules are installed:
Then, on the router of network A, you do the following:
And on the router of network B:
And if you're finished with your tunnel:
Presto, you're done. You can't forward broadcast or IPv6 traffic through an IP-in-IP tunnel, though. You just connect 2 IPv4 networks that normally wouldn't be able to talk to each other, that's all. As far as compatibility goes, this code has been around a long time, so it's compatible all the way back to 1.3 kernels. Linux IP-in-IP tunneling doesn't work with other Operating Systems or routers, as far as I know. It's simple, it works. Use it if you have to, otherwise use GRE. 5.3. GRE tunnelingGRE is a tunneling protocol that was originally developed by Cisco, and it can do a few more things than IP-in-IP tunneling. For example, you can also transport multicast traffic and IPv6 through a GRE tunnel. In Linux, you'll need the ip_gre.o module. 5.3.1. IPv4 TunnelingLet's do IPv4 tunneling first: Let's say you have 3 networks: Internal networks A and B, and intermediate network C (or let's say, Internet). So we have network A:
and network B:
As far as network C is concerned, we assume that it will pass any packet sent from A to B and vice versa. How and why, we do not care. On the router of network A, you do the following:
Let's discuss this for a bit. In line 1, we added a tunnel device, and called it netb (which is kind of obvious because that's where we want it to go). Furthermore we told it to use the GRE protocol (mode gre), that the remote address is 172.19.20.21 (the router at the other end), that our tunneling packets should originate from 172.16.17.18 (which allows your router to have several IP addresses on network C and let you decide which one to use for tunneling) and that the TTL field of the packet should be set to 255 (ttl 255). The second line enables the device. In the third line we gave the newly born interface netb the address 10.0.1.1. This is OK for smaller networks, but when you're starting up a mining expedition (LOTS of tunnels), you might want to consider using another IP range for tunneling interfaces (in this example, you could use 10.0.3.0). In the fourth line we set the route for network B. Note the different notation for the netmask. If you're not familiar with this notation, here's how it works: you write out the netmask in binary form, and you count all the ones. If you don't know how to do that, just remember that 255.0.0.0 is /8, 255.255.0.0 is /16 and 255.255.255.0 is /24. Oh, and 255.255.254.0 is /23, in case you were wondering. But enough about this, let's go on with the router of network B.
5.3.2. IPv6 TunnelingSee Section 6 for a short bit about IPv6 Addresses. On with the tunnels. Let's assume that you have the following IPv6 network, and you want to connect it to 6bone, or a friend.
Let's discuss this. In the first line, we created a tunnel device called sixbone. We gave it mode sit (which is IPv6 in IPv4 tunneling) and told it where to go to (remote) and where to come from (local). TTL is set to maximum, 255. Next, we made the device active (up). After that, we added our own network address, and set a route for 3ffe::/15 (which is currently all of 6bone) through the tunnel. GRE tunnels are currently the preferred type of tunneling. It's a standard that is also widely adopted outside the Linux community and therefore a Good Thing. Chapter 6. IPv6 tunneling with Cisco and/or 6boneBy Marco Davids <marco@sara.nl> NOTE to maintainer: As far as I am concerned, this IPv6-IPv4 tunneling is not per definition GRE tunneling. You could tunnel IPv6 over IPv4 by means of GRE tunnel devices (GRE tunnels ANY to IPv4), but the device used here ("sit") only tunnels IPv6 over IPv4 and is therefore something different. 6.1. IPv6 TunnelingThis is another application of the tunneling capabilities of Linux. It is popular among the IPv6 early adopters, or pioneers if you like. The 'hands-on' example described below is certainly not the only way to do IPv6 tunneling. However, it is the method that is often used to tunnel between Linux and a Cisco IPv6 capable router and experience tells us that this is just the thing many people are after. Ten to one this applies to you too ;-) A short bit about IPv6 addresses: IPv6 addresses are, compared to IPv4 addresses, really big: 128 bits against 32 bits. And this provides us just with the thing we need: many, many IP-addresses: 340,282,266,920,938,463,463,374,607,431,768,211,465 to be precise. Apart from this, IPv6 (or IPng, for IP Next Generation) is supposed to provide for smaller routing tables on the Internet's backbone routers, simpler configuration of equipment, better security at the IP level and better support for QoS. An example: 2002:836b:9820:0000:0000:0000:836b:9886 Writing down IPv6 addresses can be quite a burden. Therefore, to make life easier there are some rules:
The address 2002:836b:9820:0000:0000:0000:836b:9886 can be written down as 2002:836b:9820::836b:9886, which is somewhat friendlier. Another example, the address 3ffe:0000:0000:0000:0000:0020:34A1:F32C can be written down as 3ffe::20:34A1:F32C, which is a lot shorter. IPv6 is intended to be the successor of the current IPv4. Because it is relatively new technology, there is no worldwide native IPv6 network yet. To be able to move forward swiftly, the 6bone was introduced. Native IPv6 networks are connected to each other by encapsulating the IPv6 protocol in IPv4 packets and sending them over the existing IPv4 infrastructure from one IPv6 site to another. That is precisely where the tunnel steps in. To be able to use IPv6, we should have a kernel that supports it. There are many good documents on how to achieve this. But it all comes down to a few steps:
In other words, compile IPv6 as 'built-in' in your kernel. You can then save your config like usual and go ahead with compiling the kernel. HINT: Before doing so, consider editing the Makefile: EXTRAVERSION = -x ; --> ; EXTRAVERSION = -x-IPv6 There is a lot of good documentation about compiling and installing a kernel, however this document is about something else. If you run into problems at this stage, go and look for documentation about compiling a Linux kernel according to your own specifications. The file /usr/src/linux/README might be a good start. After you accomplished all this, and rebooted with your brand new kernel, you might want to issue an '/sbin/ifconfig -a' and notice the brand new 'sit0-device'. SIT stands for Simple Internet Transition. You may give yourself a compliment; you are now one major step closer to IP, the Next Generation ;-) Now on to the next step. You want to connect your host, or maybe even your entire LAN to another IPv6 capable network. This might be the "6bone" that is setup especially for this particular purpose. Let's assume that you have the following IPv6 network: 3ffe:604:6:8::/64 and you want to connect it to 6bone, or a friend. Please note that the /64 subnet notation works just like with regular IP addresses. Your IPv4 address is 145.100.24.181 and the 6bone router has IPv4 address 145.100.1.5
Let's discuss this. In the first line, we created a tunnel device called sixbone. We gave it mode sit (which is IPv6 in IPv4 tunneling) and told it where to go to (remote) and where to come from (local). TTL is set to maximum, 255. Next, we made the device active (up). After that, we added our own network address, and set a route for 3ffe::/15 (which is currently all of 6bone) through the tunnel. If the particular machine you run this on is your IPv6 gateway, then consider adding the following lines:
The latter, radvd is -like zebra- a router advertisement daemon, to support IPv6's autoconfiguration features. Search for it with your favourite search-engine if you like. You can check things like this:
If you happen to have radvd running on your IPv6 gateway and boot your IPv6 capable Linux on a machine on your local LAN, you would be able to enjoy the benefits of IPv6 autoconfiguration:
You could go ahead and configure your bind for IPv6 addresses. The A type has an equivalent for IPv6: AAAA. The in-addr.arpa's equivalent is: ip6.int. There's a lot of information available on this topic. There is an increasing number of IPv6-aware applications available, including secure shell, telnet, inetd, Mozilla the browser, Apache the webserver and a lot of others. But this is all outside the scope of this Routing document ;-) On the Cisco side the configuration would be something like this:
Chapter 7. IPsec: secure IP over the InternetFIXME: editor vacancy. In the meantime, see: The FreeS/WAN project. Another IPSec implementation for Linux is Cerberus, by NIST. However, their web pages have not been updated in over a year, and their version tended to trail well behind the current Linux kernel. USAGI, an alternative IPv6 implementation for Linux, also includes an IPSec implementation, but that might only be for IPv6. Chapter 8. Multicast routingFIXME: Editor Vacancy! The Multicast-HOWTO is ancient (relatively-speaking) and may be inaccurate or misleading in places, for that reason. Before you can do any multicast routing, you need to configure the Linux kernel to support the type of multicast routing you want to do. This, in turn, requires you to decide what type of multicast routing you expect to be using. There are essentially four "common" types - DVMRP (the Multicast version of the RIP unicast protocol), MOSPF (the same, but for OSPF), PIM-SM ("Protocol Independent Multicasting - Sparse Mode", which assumes that users of any multicast group are spread out, rather than clumped) and PIM-DM (the same, but "Dense Mode", which assumes that there will be significant clumps of users of the same multicast group). In the Linux kernel, you will notice that these options don't appear. This is because the protocol itself is handled by a routing application, such as Zebra, mrouted, or pimd. However, you still have to have a good idea of which you're going to use, to select the right options in the kernel. For all multicast routing, you will definitely need to enable "multicasting" and "multicast routing". For DVMRP and MOSPF, this is sufficient. If you are going to use PIM, you must also enable PIMv1 or PIMv2, depending on whether the network you are connecting to uses version 1 or 2 of the PIM protocol. Once you have all that sorted out, and your new Linux kernel compiled, you will see that the IP protocols listed, at boot time, now include IGMP. This is a protocol for managing multicast groups. At the time of writing, Linux supports IGMP versions 1 and 2 only, although version 3 does exist and has been documented. This doesn't really affect us that much, as IGMPv3 is still new enough that the extra capabilities of IGMPv3 aren't going to be that much use. Because IGMP deals with groups, only the features present in the simplest version of IGMP over the entire group are going to be used. For the most part, that will be IGMPv2, although IGMPv1 is sill going to be encountered. So far, so good. We've enabled multicasting. Now, we have to tell the Linux kernel to actually do something with it, so we can start routing. This means adding the Multicast virtual network to the router table: ip route add 224.0.0.0/4 dev eth0 (Assuming, of course, that you're multicasting over eth0! Substitute the device of your choice, for this.) Now, tell Linux to forward packets... echo 1 > /proc/sys/net/ipv4/ip_forward At this point, you may be wondering if this is ever going to do anything. So, to test our connection, we ping the default group, 224.0.0.1, to see if anyone is alive. All machines on your LAN with multicasting enabled should respond, but nothing else. You'll notice that none of the machines that respond have an IP address of 224.0.0.1. What a surprise! :) This is a group address (a "broadcast" to subscribers), and all members of the group will respond with their own address, not the group address. ping -c 2 224.0.0.1 At this point, you're ready to do actual multicast routing. Well, assuming that you have two networks to route between. (To Be Continued!) Chapter 9. Queueing Disciplines for Bandwidth ManagementNow, when I discovered this, it really blew me away. Linux 2.2/2.4 comes with everything to manage bandwidth in ways comparable to high-end dedicated bandwidth management systems. Linux even goes far beyond what Frame and ATM provide. Just to prevent confusion, tc uses the following rules for bandwith specification:
But when tc prints the rate, it uses following :
9.1. Queues and Queueing Disciplines explainedWith queueing we determine the way in which data is SENT. It is important to realise that we can only shape data that we transmit. With the way the Internet works, we have no direct control of what people send us. It's a bit like your (physical!) mailbox at home. There is no way you can influence the world to modify the amount of mail they send you, short of contacting everybody. However, the Internet is mostly based on TCP/IP which has a few features that help us. TCP/IP has no way of knowing the capacity of the network between two hosts, so it just starts sending data faster and faster ('slow start') and when packets start getting lost, because there is no room to send them, it will slow down. In fact it is a bit smarter than this, but more about that later. This is the equivalent of not reading half of your mail, and hoping that people will stop sending it to you. With the difference that it works for the Internet :-) If you have a router and wish to prevent certain hosts within your network from downloading too fast, you need to do your shaping on the *inner* interface of your router, the one that sends data to your own computers. You also have to be sure you are controlling the bottleneck of the link. If you have a 100Mbit NIC and you have a router that has a 256kbit link, you have to make sure you are not sending more data than your router can handle. Otherwise, it will be the router who is controlling the link and shaping the available bandwith. We need to 'own the queue' so to speak, and be the slowest link in the chain. Luckily this is easily possible. 9.2. Simple, classless Queueing DisciplinesAs said, with queueing disciplines, we change the way data is sent. Classless queueing disciplines are those that, by and large accept data and only reschedule, delay or drop it. These can be used to shape traffic for an entire interface, without any subdivisions. It is vital that you understand this part of queueing before we go on the the classful qdisc-containing-qdiscs! By far the most widely used discipline is the pfifo_fast qdisc - this is the default. This also explains why these advanced features are so robust. They are nothing more than 'just another queue'. Each of these queues has specific strengths and weaknesses. Not all of them may be as well tested. 9.2.1. pfifo_fastThis queue is, as the name says, First In, First Out, which means that no packet receives special treatment. At least, not quite. This queue has 3 so called 'bands'. Within each band, FIFO rules apply. However, as long as there are packets waiting in band 0, band 1 won't be processed. Same goes for band 1 and band 2. The kernel honors the so called Type of Service flag of packets, and takes care to insert 'minimum delay' packets in band 0. Do not confuse this classless simple qdisc with the classful PRIO one! Although they behave similarly, pfifo_fast is classless and you cannot add other qdiscs to it with the tc command. 9.2.1.1. Parameters & usageYou can't configure the pfifo_fast qdisc as it is the hardwired default. This is how it is configured by default:
9.2.2. Token Bucket FilterThe Token Bucket Filter (TBF) is a simple qdisc that only passes packets arriving at a rate which is not exceeding some administratively set rate, but with the possibility to allow short bursts in excess of this rate. TBF is very precise, network- and processor friendly. It should be your first choice if you simply want to slow an interface down! The TBF implementation consists of a buffer (bucket), constantly filled by some virtual pieces of information called tokens, at a specific rate (token rate). The most important parameter of the bucket is its size, that is the number of tokens it can store. Each arriving token collects one incoming data packet from the data queue and is then deleted from the bucket. Associating this algorithm with the two flows -- token and data, gives us three possible scenarios:
The last scenario is very important, because it allows to administratively shape the bandwidth available to data that's passing the filter. The accumulation of tokens allows a short burst of overlimit data to be still passed without loss, but any lasting overload will cause packets to be constantly delayed, and then dropped. Please note that in the actual implementation, tokens correspond to bytes, not packets. 9.2.2.1. Parameters & usageEven though you will probably not need to change them, tbf has some knobs available. First the parameters that are always available:
If the bucket contains tokens and is allowed to empty, by default it does so at infinite speed. If this is unacceptable, use the following parameters:
9.2.2.2. Sample configurationA simple but *very* useful configuration is this:
Ok, why is this useful? If you have a networking device with a large queue, like a DSL modem or a cable modem, and you talk to it over a fast device, like over an ethernet interface, you will find that uploading absolutely destroys interactivity. This is because uploading will fill the queue in the modem, which is probably *huge* because this helps actually achieving good data throughput uploading. But this is not what you want, you want to have the queue not too big so interactivity remains and you can still do other stuff while sending data. The line above slows down sending to a rate that does not lead to a queue in the modem - the queue will be in Linux, where we can control it to a limited size. Change 220kbit to your uplink's *actual* speed, minus a few percent. If you have a really fast modem, raise 'burst' a bit. 9.2.3. Stochastic Fairness QueueingStochastic Fairness Queueing (SFQ) is a simple implementation of the fair queueing algorithms family. It's less accurate than others, but it also requires less calculations while being almost perfectly fair. The key word in SFQ is conversation (or flow), which mostly corresponds to a TCP session or a UDP stream. Traffic is divided into a pretty large number of FIFO queues, one for each conversation. Traffic is then sent in a round robin fashion, giving each session the chance to send data in turn. This leads to very fair behaviour and disallows any single conversation from drowning out the rest. SFQ is called 'Stochastic' because it doesn't really allocate a queue for each session, it has an algorithm which divides traffic over a limited number of queues using a hashing algorithm. Because of the hash, multiple sessions might end up in the same bucket, which would halve each session's chance of sending a packet, thus halving the effective speed available. To prevent this situation from becoming noticeable, SFQ changes its hashing algorithm quite often so that any two colliding sessions will only do so for a small number of seconds. It is important to note that SFQ is only useful in case your actual outgoing interface is really full! If it isn't then there will be no queue on your linux machine and hence no effect. Later on we will describe how to combine SFQ with other qdiscs to get a best-of-both worlds situation. Specifically, setting SFQ on the ethernet interface heading to your cable modem or DSL router is pointless without further shaping! 9.2.3.1. Parameters & usageThe SFQ is pretty much self tuning:
9.2.3.2. Sample configurationIf you have a device which has identical link speed and actual available rate, like a phone modem, this configuration will help promote fairness:
The number 800c: is the automatically assigned handle number, limit means that 128 packets can wait in this queue. There are 1024 hashbuckets available for accounting, of which 128 can be active at a time (no more packets fit in the queue!) Once every 10 seconds, the hashes are reconfigured. 9.3. Advice for when to use which queueSummarizing, these are the simple queues that actually manage traffic by reordering, slowing or dropping packets. The following tips may help in choosing which queue to use. It mentions some qdiscs described in the Chapter 14 chapter.
9.4. TerminologyTo properly understand more complicated configurations it is necessary to explain a few concepts first. Because of the complexity and he relative youth of the subject, a lot of different words are used when people in fact mean the same thing. The following is loosely based on draft-ietf-diffserv-model-06.txt, An Informal Management Model for Diffserv Routers. It can currently be found at http://www.ietf.org/internet-drafts/draft-ietf-diffserv-model-06.txt. Read it for the strict definitions of the terms used.
Now that we have our terminology straight, let's see where all these things are.
The big block represents the kernel. The leftmost arrow represents traffic entering your machine from the network. It is then fed to the Ingress Qdisc which may apply Filters to a packet, and decide to drop it. This is called 'Policing'. This happens at a very early stage, before it has seen a lot of the kernel. It is therefore a very good place to drop traffic very early, without consuming a lot of CPU power. If the packet is allowed to continue, it may be destined for a local application, in which case it enters the IP stack in order to be processed, and handed over to a userspace program. The packet may also be forwarded without entering an application, in which case it is destined for egress. Userspace programs may also deliver data, which is then examined and forwarded to the Egress Classifier. There it is investigated and enqueued to any of a number of qdiscs. In the unconfigured default case, there is only one egress qdisc installed, the pfifo_fast, which always receives the packet. This is called 'enqueueing'. The packet now sits in the qdisc, waiting for the kernel to ask for it for transmission over the network interface. This is called 'dequeueing'. This picture also holds in case there is only one network adaptor - the arrows entering and leaving the kernel should not be taken too literally. Each network adaptor has both ingress and egress hooks. 9.5. Classful Queueing DisciplinesClassful qdiscs are very useful if you have different kinds of traffic which should have differing treatment. One of the classful qdiscs is called 'CBQ' , 'Class Based Queueing' and it is so widely mentioned that people identify queueing with classes solely with CBQ, but this is not the case. CBQ is merely the oldest kid on the block - and also the most complex one. It may not always do what you want. This may come as something of a shock to many who fell for the 'sendmail effect', which teaches us that any complex technology which doesn't come with documentation must be the best available. More about CBQ and its alternatives shortly. 9.5.1. Flow within classful qdiscs & classesWhen traffic enters a classful qdisc, it needs to be sent to any of the classes within - it needs to be 'classified'. To determine what to do with a packet, the so called 'filters' are consulted. It is important to know that the filters are called from within a qdisc, and not the other way around! The filters attached to that qdisc then return with a decision, and the qdisc uses this to enqueue the packet into one of the classes. Each subclass may try other filters to see if further instructions apply. If not, the class enqueues the packet to the qdisc it contains. Besides containing other qdiscs, most classful qdiscs also perform shaping. This is useful to perform both packet scheduling (with SFQ, for example) and rate control. You need this in cases where you have a high speed interface (for example, ethernet) to a slower device (a cable modem). If you were only to run SFQ, nothing would happen, as packets enter & leave your router without delay: the output interface is far faster than your actual link speed. There is no queue to schedule then. 9.5.2. The qdisc family: roots, handles, siblings and parentsEach interface has one egress 'root qdisc', by default the earlier mentioned classless pfifo_fast queueing discipline. Each qdisc can be assigned a handle, which can be used by later configuration statements to refer to that qdisc. Besides an egress qdisc, an interface may also have an ingress, which polices traffic coming in. The handles of these qdiscs consist of two parts, a major number and a minor number. It is habitual to name the root qdisc '1:', which is equal to '1:0'. The minor number of a qdisc is always 0. Classes need to have the same major number as their parent. 9.5.2.1. How filters are used to classify trafficRecapping, a typical hierarchy might look like this:
But don't let this tree fool you! You should *not* imagine the kernel to be at the apex of the tree and the network below, that is just not the case. Packets get enqueued and dequeued at the root qdisc, which is the only thing the kernel talks to. A packet might get classified in a chain like this: 1: -> 1:1 -> 12: -> 12:2 The packet now resides in a queue in a qdisc attached to class 12:2. In this example, a filter was attached to each 'node' in the tree, each choosing a branch to take next. This can make sense. However, this is also possible: 1: -> 12:2 In this case, a filter attached to the root decided to send the packet directly to 12:2. 9.5.2.2. How packets are dequeued to the hardwareWhen the kernel decides that it needs to extract packets to send to the interface, the root qdisc 1: gets a dequeue request, which is passed to 1:1, which is in turn passed to 10:, 11: and 12:, which each query their siblings, and try to dequeue() from them. In this case, the kernel needs to walk the entire tree, because only 12:2 contains a packet. In short, nested classes ONLY talk to their parent qdiscs, never to an interface. Only the root qdisc gets dequeued by the kernel! The upshot of this is that classes never get dequeued faster than their parents allow. And this is exactly what we want: this way we can have SFQ in an inner class, which doesn't do any shaping, only scheduling, and have a shaping outer qdisc, which does the shaping. 9.5.3. The PRIO qdiscThe PRIO qdisc doesn't actually shape, it only subdivides traffic based on how you configured your filters. You can consider the PRIO qdisc a kind of pfifo_fast on steroids, whereby each band is a separate class instead of a simple FIFO. When a packet is enqueued to the PRIO qdisc, a class is chosen based on the filter commands you gave. By default, three classes are created. These classes by default contain pure FIFO qdiscs with no internal structure, but you can replace these by any qdisc you have available. Whenever a packet needs to be dequeued, class :1 is tried first. Higher classes are only used if lower bands all did not give up a packet. This qdisc is very useful in case you want to prioritize certain kinds of traffic without using only TOS-flags but using all the power of the tc filters. It can also contain more all qdiscs, whereas pfifo_fast is limited to simple fifo qdiscs. Because it doesn't actually shape, the same warning as for SFQ holds: either use it only if your physical link is really full or wrap it inside a classful qdisc that does shape. The last holds for almost all cable modems and DSL devices. In formal words, the PRIO qdisc is a Work-Conserving scheduler. 9.5.3.1. PRIO parameters & usageThe following parameters are recognized by tc:
Reiterating, band 0 goes to minor number 1! Band 1 to minor number 2, etc. 9.5.3.2. Sample configurationWe will create this tree:
Bulk traffic will go to 30:, interactive traffic to 20: or 10:. Command lines:
Now let's see what we created:
We now do some bulk data transfer with a tool that properly sets TOS flags, and take another look:
It worked - all additional traffic has gone to 10:, which is our highest priority qdisc. No traffic was sent to the lowest priority, which previously received our entire scp. 9.5.4. The famous CBQ qdiscAs said before, CBQ is the most complex qdisc available, the most hyped, the least understood, and probably the trickiest one to get right. This is not because the authors are evil or incompetent, far from it, it's just that the CBQ algorithm isn't all that precise and doesn't really match the way Linux works. Besides being classful, CBQ is also a shaper and it is in that aspect that it really doesn't work very well. It should work like this. If you try to shape a 10mbit/s connection to 1mbit/s, the link should be idle 90% of the time. If it isn't, we need to throttle so that it IS idle 90% of the time. This is pretty hard to measure, so CBQ instead derives the idle time from the number of microseconds that elapse between requests from the hardware layer for more data. Combined, this can be used to approximate how full or empty the link is. This is rather circumspect and doesn't always arrive at proper results. For example, what if the actual link speed of an interface that is not really able to transmit the full 100mbit/s of data, perhaps because of a badly implemented driver? A PCMCIA network card will also never achieve 100mbit/s because of the way the bus is designed - again, how do we calculate the idle time? It gets even worse if we consider not-quite-real network devices like PPP over Ethernet or PPTP over TCP/IP. The effective bandwidth in that case is probably determined by the efficiency of pipes to userspace - which is huge. People who have done measurements discover that CBQ is not always very accurate and sometimes completely misses the mark. In many circumstances however it works well. With the documentation provided here, you should be able to configure it to work well in most cases. 9.5.4.1. CBQ shaping in detailAs said before, CBQ works by making sure that the link is idle just long enough to bring down the real bandwidth to the configured rate. To do so, it calculates the time that should pass between average packets. During operations, the effective idletime is measured using an exponential weighted moving average (EWMA), which considers recent packets to be exponentially more important than past ones. The UNIX loadaverage is calculated in the same way. The calculated idle time is subtracted from the EWMA measured one, the resulting number is called 'avgidle'. A perfectly loaded link has an avgidle of zero: packets arrive exactly once every calculated interval. An overloaded link has a negative avgidle and if it gets too negative, CBQ shuts down for a while and is then 'overlimit'. Conversely, an idle link might amass a huge avgidle, which would then allow infinite bandwidths after a few hours of silence. To prevent this, avgidle is capped at maxidle. If overlimit, in theory, the CBQ could throttle itself for exactly the amount of time that was calculated to pass between packets, and then pass one packet, and throttle again. But see the 'minburst' parameter below. These are parameters you can specify in order to configure shaping:
Internally, CBQ has a lot of fine tuning. For example, classes which are known not to have data enqueued to them aren't queried. Overlimit classes are penalized by lowering their effective priority. All very smart & complicated. 9.5.4.2. CBQ classful behaviourBesides shaping, using the aforementioned idletime approximations, CBQ also acts like the PRIO queue in the sense that classes can have differing priorities and that lower priority numbers will be polled before the higher priority ones. Each time a packet is requested by the hardware layer to be sent out to the network, a weighted round robin process ('WRR') starts, beginning with the lower priority classes. These are then grouped and queried if they have data available. If so, it is returned. After a class has been allowed to dequeue a number of bytes, the next class within that priority is tried. The following parameters control the WRR process:
Please note that all classes within an CBQ hierarchy need to share the same major number! 9.5.4.3. CBQ parameters that determine link sharing & borrowingBesides purely limiting certain kinds of traffic, it is also possible to specify which classes can borrow capacity from other classes or, conversely, lend out bandwidth.
Within such an agency class, there might be other classes which are allowed to swap bandwidth. 9.5.4.4. Sample configurationThis configuration limits webserver traffic to 5mbit and SMTP traffic to 3 mbit. Together, they may not get more than 6mbit. We have a 100mbit NIC and the classes may borrow bandwidth from each other.
As said before, CBQ requires a *lot* of knobs. All parameters are explained above, however. The corresponding HTB configuration is lots simpler.
These are our two classes. Note how we scale the weight with the configured rate. Both classes are not bounded, but they are connected to class 1:1 which is bounded. So the sum of bandwith of the 2 classes will never be more than 6mbit. The classids need to be within the same major number as the parent CBQ, by the way!
Both classes have a FIFO qdisc by default. But we replaced these with an SFQ queue so each flow of data is treated equally.
These commands, attached directly to the root, send traffic to the right qdiscs. Note that we use 'tc class add' to CREATE classes within a qdisc, but that we use 'tc qdisc add' to actually add qdiscs to these classes. You may wonder what happens to traffic that is not classified by any of the two rules. It appears that in this case, data will then be processed within 1:0, and be unlimited. If SMTP+web together try to exceed the set limit of 6mbit/s, bandwidth will be divided according to the weight parameter, giving 5/8 of traffic to the webserver and 3/8 to the mail server. With this configuration you can also say that webserver traffic will always get at minimum 5/8 * 6 mbit = 3.75 mbit. 9.5.4.5. Other CBQ parameters: split & defmapAs said before, a classful qdisc needs to call filters to determine which class a packet will be enqueued to. Besides calling the filter, CBQ offers other options, defmap & split. This is pretty complicated to understand, and it is not vital. But as this is the only known place where defmap & split are properly explained, I'm doing my best. As you will often want to filter on the Type of Service field only, a special syntax is provided. Whenever the CBQ needs to figure out where a packet needs to be enqueued, it checks if this node is a 'split node'. If so, one of the sub-qdiscs has indicated that it wishes to receive all packets with a certain configured priority, as might be derived from the TOS field, or socket options set by applications. The packets' priority bits are or-ed with the defmap field to see if a match exists. In other words, this is a short-hand way of creating a very fast filter, which only matches certain priorities. A defmap of ff (hex) will match everything, a map of 0 nothing. A sample configuration may help make things clearer:
Defmap refers to TC_PRIO bits, which are defined as follows:
The TC_PRIO.. number corresponds to bits, counted from the right. See the pfifo_fast section for more details how TOS bits are converted to priorities. Now the interactive and the bulk classes:
The 'split qdisc' is 1:0, which is where the choice will be made. C0 is binary for 11000000, 3F for 00111111, so these two together will match everything. The first class matches bits 7 & 6, and thus corresponds to 'interactive' and 'control' traffic. The second class matches the rest. Node 1:0 now has a table like this:
For additional fun, you can also pass a 'change mask', which indicates exactly which priorities you wish to change. You only need to use this if you are running 'tc class change'. For example, to add best effort traffic to 1:2, we could run this:
The priority map over at 1:0 now looks like this:
FIXME: did not test 'tc class change', only looked at the source. 9.5.5. Hierarchical Token BucketMartin Devera (<devik>) rightly realised that CBQ is complex and does not seem optimized for many typical situations. His Hierarchical approach is well suited for setups where you have a fixed amount of bandwidth which you want to divide for different purposes, giving each purpose a guaranteed bandwidth, with the possibility of specifying how much bandwidth can be borrowed. HTB works just like CBQ but does not resort to idle time calculations to shape. Instead, it is a classful Token Bucket Filter - hence the name. It has only a few parameters, which are well documented on his site. As your HTB configuration gets more complex, your configuration scales well. With CBQ it is already complex even in simple cases! HTB is not yet a part of the standard kernel, but it should soon be! If you are in a position to patch your kernel, by all means consider HTB. 9.5.5.1. Sample configurationFunctionally almost identical to the CBQ sample configuration above:
The author then recommends SFQ for beneath these classes:
Add the filters which direct traffic to the right classes:
HTB certainly looks wonderful - if 10: and 20: both have their guaranteed bandwidth, and more is left to divide, they borrow in a 5:3 ratio, just as you would expect. Unclassified traffic gets routed to 30:, which has little bandwidth of its own but can borrow everything that is left over. Because we chose SFQ internally, we get fairness thrown in for free! 9.6. Classifying packets with filtersTo determine which class shall process a packet, the so-called 'classifier chain' is called each time a choice needs to be made. This chain consists of all filters attached to the classful qdisc that needs to decide. To reiterate the tree, which is not a tree:
When enqueueing a packet, at each branch the filter chain is consulted for a relevant instruction. A typical setup might be to have a filter in 1:1 that directs a packet to 12: and a filter on 12: that sends the packet to 12:2. You might also attach this latter rule to 1:1, but you can make efficiency gains by having more specific tests lower in the chain. You can't filter a packet 'upwards', by the way. Also, with HTB, you should attach all filters to the root! And again - packets are only enqueued downwards! When they are dequeued, they go up again, where the interface lives. They do NOT fall off the end of the tree to the network adaptor! 9.6.1. Some simple filtering examplesAs explained in the Classifier chapter, you can match on literally anything, using a very complicated syntax. To start, we will show how to do the obvious things, which luckily are quite easy. Let's say we have a PRIO qdisc called '10:' which contains three classes, and we want to assign all traffic from and to port 22 to the highest priority band, the filters would be:
What does this say? It says: attach to eth0, node 10: a priority 1 u32 filter that matches on IP destination port 22 *exactly* and send it to band 10:1. And it then repeats the same for source port 80. The last command says that anything unmatched so far should go to band 10:2, the next-highest priority. You need to add 'eth0', or whatever your interface is called, because each interface has a unique namespace of handles. To select on an IP address, use this:
This assigns traffic to 4.3.2.1 and traffic from 1.2.3.4 to the highest priority queue, and the rest to the next-highest one. You can concatenate matches, to match on traffic from 1.2.3.4 and from port 80, do this:
9.6.2. All the filtering commands you will normally needMost shaping commands presented here start with this preamble:
For more filtering commands, see the Advanced Filters chapter. 9.7. The Intermediate queueing device (IMQ)The Intermediate queueing device is not a qdisc but its usage is tightly bound to qdiscs. Within linux, qdiscs are attached to network devices and everything that is queued to the device is first queued to the qdisc. From this concept, two limitations arise: 1. Only egress shaping is possible (an ingress qdisc exists, but its possibilities are very limited compared to classful qdiscs). 2. A qdisc can only see traffic of one interface, global limitations can't be placed. IMQ is there to help solve those two limitations. In short, you can put everything you choose in a qdisc. Specially marked packets get intercepted in netfilter NF_IP_PRE_ROUTING and NF_IP_POST_ROUTING hooks and pass through the qdisc attached to an imq device. An iptables target is used for marking the packets. This enables you to do ingress shaping as you can just mark packets coming in from somewhere and/or treat interfaces as classes to set global limits. You can also do lots of other stuff like just putting your http traffic in a qdisc, put new connection requests in a qdisc, ... 9.7.1. Sample configurationThe first thing that might come to mind is use ingress shaping to give yourself a high guaranteed bandwidth. ;) Configuration is just like with any other interface:
The IMQ iptables targets is valid in the PREROUTING and POSTROUTING chains of the mangle table. It's syntax is
Please note traffic is not enqueued when the target is hit but afterwards. The exact location where traffic enters the imq device depends on the direction of the traffic (in/out). These are the predefined netfilter hooks used by iptables:
For ingress traffic, imq registers itself with NF_IP_PRI_MANGLE + 1 priority which means packets enter the imq device directly after the mangle PREROUTING chain has been passed. For egress imq uses NF_IP_PRI_LAST which honours the fact that packets dropped by the filter table won't occupy bandwidth. The patches and some more information can be found at the imq site. Chapter 10. Load sharing over multiple interfacesThere are several ways of doing this. One of the easiest and straightforward ways is 'TEQL' - "True" (or "trivial") link equalizer. Like most things having to do with queueing, load sharing goes both ways. Both ends of a link may need to participate for full effect. Imagine this situation:
A and B are routers, and for the moment we'll assume both run Linux. If traffic is going from network 1 to network 2, router A needs to distribute the packets over both links to B. Router B needs to be configured to accept this. Same goes the other way around, when packets go from network 2 to network 1, router B needs to send the packets over both eth1 and eth2. The distributing part is done by a 'TEQL' device, like this (it couldn't be easier):
Don't forget the 'ip link set up' command! This needs to be done on both hosts. The device teql0 is basically a roundrobbin distributor over eth1 and eth2, for sending packets. No data ever comes in over an teql device, that just appears on the 'raw' eth1 and eth2. But now we just have devices, we also need proper routing. One way to do this is to assign a /31 network to both links, and a /31 to the teql0 device as well: FIXME: does this need something like 'nobroadcast'? A /31 is too small to house a network address and a broadcast address - if this doesn't work as planned, try a /30, and adjust the ip addresses accordingly. You might even try to make eth1 and eth2 do without an IP address! On router A:
On router B:
Router A should now be able to ping 10.0.0.1, 10.0.0.3 and 10.0.0.5 over the 2 real links and the 1 equalized device. Router B should be able to ping 10.0.0.0, 10.0.0.2 and 10.0.0.4 over the links. If this works, Router A should make 10.0.0.5 its route for reaching network 2, and Router B should make 10.0.0.4 its route for reaching network 1. For the special case where network 1 is your network at home, and network 2 is the Internet, Router A should make 10.0.0.5 its default gateway. 10.1. CaveatsNothing is as easy as it seems. eth1 and eth2 on both router A and B need to have return path filtering turned off, because they will otherwise drop packets destined for ip addresses other than their own:
Then there is the nasty problem of packet reordering. Let's say 6 packets need to be sent from A to B - eth1 might get 1, 3 and 5. eth2 would then do 2, 4 and 6. In an ideal world, router B would receive this in order, 1, 2, 3, 4, 5, 6. But the possibility is very real that the kernel gets it like this: 2, 1, 4, 3, 6, 5. The problem is that this confuses TCP/IP. While not a problem for links carrying many different TCP/IP sessions, you won't be able to to a bundle multiple links and get to ftp a single file lots faster, except when your receiving or sending OS is Linux, which is not easily shaken by some simple reordering. However, for lots of applications, link load balancing is a great idea. 10.2. Other possibilitiesWilliam Stearns has used an advanced tunneling setup to achieve good use of multiple, unrelated, internet connections together. It can be found on his tunneling page. The HOWTO may feature more about this in the future. Chapter 11. Netfilter & iproute - marking packetsSo far we've seen how iproute works, and netfilter was mentioned a few times. This would be a good time to browse through Rusty's Remarkably Unreliable Guides. Netfilter itself can be found here. Netfilter allows us to filter packets, or mangle their headers. One special feature is that we can mark a packet with a number. This is done with the --set-mark facility. As an example, this command marks all packets destined for port 25, outgoing mail:
Let's say that we have multiple connections, one that is fast (and expensive, per megabyte) and one that is slower, but flat fee. We would most certainly like outgoing mail to go via the cheap route. We've already marked the packets with a '1', we now instruct the routing policy database to act on this:
Now we generate the mail.out table with a route to the slow but cheap link:
And we are done. Should we want to make exceptions, there are lots of ways to achieve this. We can modify the netfilter statement to exclude certain hosts, or we can insert a rule with a lower priority that points to the main table for our excepted hosts. We can also use this feature to honour TOS bits by marking packets with a different type of service with different numbers, and creating rules to act on that. This way you can even dedicate, say, an ISDN line to interactive sessions. Needless to say, this also works fine on a host that's doing NAT ('masquerading'). IMPORTANT: We received a report that MASQ and SNAT at least collide with marking packets. Rusty Russell explains it in this posting. Turn off the reverse path filter to make it work properly. Note: to mark packets, you need to have some options enabled in your kernel:
See also the Section 15.5 in the Cookbook. Chapter 12. Advanced filters for (re-)classifying packetsAs explained in the section on classful queueing disciplines, filters are needed to classify packets into any of the sub-queues. These filters are called from within the classful qdisc. Here is an incomplete list of classifiers available:
Note that in general there are many ways in which you can classify packet and that it generally comes down to preference as to which system you wish to use. Classifiers in general accept a few arguments in common. They are listed here for convenience:
All the following sections will assume you are trying to shape the traffic going to HostA. They will assume that the root class has been configured on 1: and that the class you want to send the selected traffic to is 1:1. 12.1. The u32 classifierThe U32 filter is the most advanced filter available in the current implementation. It entirely based on hashing tables, which make it robust when there are many filter rules. In its simplest form the U32 filter is a list of records, each consisting of two fields: a selector and an action. The selectors, described below, are compared with the currently processed IP packet until the first match occurs, and then the associated action is performed. The simplest type of action would be directing the packet into defined CBQ class. The command line of tc filter program, used to configure the filter, consists of three parts: filter specification, a selector and an action. The filter specification can be defined as:
The protocol field describes protocol that the filter will be applied to. We will only discuss case of ip protocol. The preference field (priority can be used alternatively) sets the priority of currently defined filter. This is important, since you can have several filters (lists of rules) with different priorities. Each list will be passed in the order the rules were added, then list with lower priority (higher preference number) will be processed. The parent field defines the CBQ tree top (e.g. 1:0), the filter should be attached to. The options described above apply to all filters, not only U32. 12.1.1. U32 selectorThe U32 selector contains definition of the pattern, that will be matched to the currently processed packet. Precisely, it defines which bits are to be matched in the packet header and nothing more, but this simple method is very powerful. Let's take a look at the following examples, taken directly from a pretty complex, real-world filter:
For now, leave the first line alone - all these parameters describe the filter's hash tables. Focus on the selector line, containing match keyword. This selector will match to IP headers, whose second byte will be 0x10 (0010). As you can guess, the 00ff number is the match mask, telling the filter exactly which bits to match. Here it's 0xff, so the byte will match if it's exactly 0x10. The at keyword means that the match is to be started at specified offset (in bytes) -- in this case it's beginning of the packet. Translating all that to human language, the packet will match if its Type of Service field will have `low delay' bits set. Let's analyze another rule:
The nexthdr option means next header encapsulated in the IP packet, i.e. header of upper-layer protocol. The match will also start here at the beginning of the next header. The match should occur in the second, 32-bit word of the header. In TCP and UDP protocols this field contains packet's destination port. The number is given in big-endian format, i.e. older bits first, so we simply read 0x0016 as 22 decimal, which stands for SSH service if this was TCP. As you guess, this match is ambiguous without a context, and we will discuss this later. Having understood all the above, we will find the following selector quite easy to read: match c0a80100 ffffff00 at 16. What we got here is a three byte match at 17-th byte, counting from the IP header start. This will match for packets with destination address anywhere in 192.168.1/24 network. After analyzing the examples, we can summarize what we have learned. 12.1.2. General selectorsGeneral selectors define the pattern, mask and offset the pattern will be matched to the packet contents. Using the general selectors you can match virtually any single bit in the IP (or upper layer) header. They are more difficult to write and read, though, than specific selectors that described below. The general selector syntax is:
One of the keywords u32, u16 or u8 specifies length of the pattern in bits. PATTERN and MASK should follow, of length defined by the previous keyword. The OFFSET parameter is the offset, in bytes, to start matching. If nexthdr+ keyword is given, the offset is relative to start of the upper layer header. Some examples:
Packet will match to this rule, if its time to live (TTL) is 64. TTL is the field starting just after 8-th byte of the IP header.
FIXME: it has been pointed out that this syntax does not work currently. Use this to match ACKs on packets smaller than 64 bytes:
This rule will only match TCP packets with ACK bit set, and no further payload. Here we can see an example of using two selectors, the final result will be logical AND of their results. If we take a look at TCP header diagram, we can see that the ACK bit is second older bit (0x10) in the 14-th byte of the TCP header (at nexthdr+13). As for the second selector, if we'd like to make our life harder, we could write match u8 0x06 0xff at 9 instead of using the specific selector protocol tcp, because 6 is the number of TCP protocol, present in 10-th byte of the IP header. On the other hand, in this example we couldn't use any specific selector for the first match - simply because there's no specific selector to match TCP ACK bits. 12.1.3. Specific selectorsThe following table contains a list of all specific selectors the author of this section has found in the tc program source code. They simply make your life easier and increase readability of your filter's configuration. FIXME: table placeholder - the table is in separate file ,,selector.html'' FIXME: it's also still in Polish :-( FIXME: must be sgml'ized Some examples:
FIXME: tcp dst match does not work as described below: The above rule will match packets which have the TOS field set to 0x10. The TOS field starts at second byte of the packet and is one byte big, so we could write an equivalent general selector: match u8 0x10 0xff at 1. This gives us hint to the internals of U32 filter -- the specific rules are always translated to general ones, and in this form they are stored in the kernel memory. This leads to another conclusion -- the tcp and udp selectors are exactly the same and this is why you can't use single match tcp dst 53 0xffff selector to match TCP packets sent to given port -- they will also match UDP packets sent to this port. You must remember to also specify the protocol and end up with the following rule:
12.2. The route classifierThis classifier filters based on the results of the routing tables. When a packet that is traversing through the classes reaches one that is marked with the "route" filter, it splits the packets up based on information in the routing table.
Here we add a route classifier onto the parent node 1:0 with priority 100. When a packet reaches this node (which, since it is the root, will happen immediately) it will consult the routing table and if one matches will send it to the given class and give it a priority of 100. Then, to finally kick it into action, you add the appropriate routing entry: The trick here is to define 'realm' based on either destination or source. The way to do it is like this:
For instance, we can define our destination network 192.168.10.0 with a realm number 10:
When adding route filters, we can use realm numbers to represent the networks or hosts and specify how the routes match the filters.
The above rule says packets going to the network 192.168.10.0 match class id 1:10. Route filter can also be used to match source routes. For example, there is a subnetwork attached to the Linux router on eth2.
Here the filter specifies that packets from the subnetwork 192.168.2.0 (realm 2) will match class id 1:2. 12.3. Policing filtersTo make even more complicated setups possible, you can have filters that only match up to a certain bandwidth. You can declare a filter to entirely cease matching above a certain rate, or only to not match only the bandwidth exceeding a certain rate. So if you decided to police at 4mbit/s, but 5mbit/s of traffic is present, you can stop matching either the entire 5mbit/s, or only not match 1mbit/s, and do send 4mbit/s to the configured class. If bandwidth exceeds the configured rate, you can drop a packet, reclassify it, or see if another filter will match it. 12.3.1. Ways to policeThere are basically two ways to police. If you compiled the kernel with 'Estimators', the kernel can measure for each filter how much traffic it is passing, more or less. These estimators are very easy on the CPU, as they simply count 25 times per second how many data has been passed, and calculate the bitrate from that. The other way works again via a Token Bucket Filter, this time living within your filter. The TBF only matches traffic UP TO your configured bandwidth, if more is offered, only the excess is subject to the configured overlimit action. 12.3.1.1. With the kernel estimatorThis is very simple and has only one parameter: avrate. Either the flow remains below avrate, and the filter classifies the traffic to the classid configured, or your rate exceeds it in which case the specified action is taken, which is 'reclassify' by default. The kernel uses an Exponential Weighted Moving Average for your bandwidth which makes it less sensitive to short bursts. 12.3.1.2. With Token Bucket FilterUses the following parameters:
Which behave mostly identical to those described in the Token Bucket Filter section. Please note however that if you set the mtu of a TBF policer too low, *no* packets will pass, whereas the egress TBF qdisc will just pass them slower. Another difference is that a policer can only let a packet pass, or drop it. It cannot delay hold on to it in order to delay it. 12.3.2. Overlimit actionsIf your filter decides that it is overlimit, it can take 'actions'. Currently, three actions are available:
12.4. Hashing filters for very fast massive filteringIf you have a need for thousands of rules, for example if you have a lot of clients or computers, all with different QoS specifications, you may find that the kernel spends a lot of time matching all those rules. By default, all filters reside in one big chain which is matched in descending order of priority. If you have 1000 rules, 1000 checks may be needed to determine what to do with a packet. Matching would go much quicker if you would have 256 chains with each four rules - if you could divide packets over those 256 chains, so that the right rule will be there. Hashing makes this possible. Let's say you have 1024 cable modem customers in your network, with IP addresses ranging from 1.2.0.0 to 1.2.3.255, and each has to go in another bin, for example 'lite', 'regular' and 'premium'. You would then have 1024 rules like this:
To speed this up, we can use the last part of the IP address as a 'hash key'. We then get 256 tables, the first of which looks like this:
The next one starts like this:
This way, only four checks are needed at most, two on average. Configuration is pretty complicated, but very worth it by the time you have this many rules. First we make a filter root, then we create a table with 256 entries:
Now we add some rules to entries in the created table:
Next create a 'hashing filter' that directs traffic to the right entry in the hashing table:
It is quite complicated, but it does work in practice and performance will be staggering. Note that this example could be improved to the ideal case where each chain contains 1 filter! Chapter 13. Kernel network parametersThe kernel has lots of parameters which can be tuned for different circumstances. While, as usual, the default parameters serve 99% of installations very well, we don't call this the Advanced HOWTO for the fun of it! The interesting bits are in /proc/sys/net, take a look there. Not everything will be documented here initially, but we're working on it. In the meantime you may want to have a look at the Linux-Kernel sources; read the file Documentation/filesystems/proc.txt. Most of the features are explained there. (FIXME) 13.1. Reverse Path FilteringBy default, routers route everything, even packets which 'obviously' don't belong on your network. A common example is private IP space escaping onto the Internet. If you have an interface with a route of 195.96.96.0/24 to it, you do not expect packets from 212.64.94.1 to arrive there. Lots of people will want to turn this feature off, so the kernel hackers have made it easy. There are files in /proc where you can tell the kernel to do this for you. The method is called "Reverse Path Filtering". Basically, if the reply to this packet wouldn't go out the interface this packet came in, then this is a bogus packet and should be ignored. The following fragment will turn this on for all current and future interfaces.
Going by the example above, if a packet arrived on the Linux router on eth1 claiming to come from the Office+ISP subnet, it would be dropped. Similarly, if a packet came from the Office subnet, claiming to be from somewhere outside your firewall, it would be dropped also. The above is full reverse path filtering. The default is to only filter based on IPs that are on directly connected networks. This is because the full filtering breaks in the case of asymmetric routing (where packets come in one way and go out another, like satellite traffic, or if you have dynamic (bgp, ospf, rip) routes in your network. The data comes down through the satellite dish and replies go back through normal land-lines). If this exception applies to you (and you'll probably know if it does) you can simply turn off the rp_filter on the interface where the satellite data comes in. If you want to see if any packets are being dropped, the log_martians file in the same directory will tell the kernel to log them to your syslog.
FIXME: is setting the conf/{default,all}/* files enough? - martijn 13.2. Obscure settingsOk, there are a lot of parameters which can be modified. We try to list them all. Also documented (partly) in Documentation/ip-sysctl.txt. Some of these settings have different defaults based on whether you answered 'Yes' to 'Configure as router and not host' while compiling your kernel. 13.2.1. Generic ipv4As a generic note, most rate limiting features don't work on loopback, so don't test them locally. The limits are supplied in 'jiffies', and are enforced using the earlier mentioned token bucket filter. The kernel has an internal clock which runs at 'HZ' ticks (or 'jiffies') per second. On Intel, 'HZ' is mostly 100. So setting a *_rate file to, say 50, would allow for 2 packets per second. The token bucket filter is also configured to allow for a burst of at most 6 packets, if enough tokens have been earned. Several entries in the following list have been copied from /usr/src/linux/Documentation/networking/ip-sysctl.txt, written by Alexey Kuznetsov <kuznet@ms2.inr.ac.ru> and Andi Kleen <ak@muc.de>
13.2.2. Per device settingsDEV can either stand for a real interface, or for 'all' or 'default'. Default also changes settings for interfaces yet to be created.
13.2.3. Neighbor policyDev can either stand for a real interface, or for 'all' or 'default'. Default also changes settings for interfaces yet to be created.
13.2.4. Routing settings
Chapter 14. Advanced & less common queueing disciplinesShould you find that you have needs not addressed by the queues mentioned earlier, the kernel contains some other more specialized queues mentioned here. 14.1. bfifo/pfifoThese classless queues are even simpler than pfifo_fast in that they lack the internal bands - all traffic is really equal. They have one important benefit though, they have some statistics. So even if you don't need shaping or prioritizing, you can use this qdisc to determine the backlog on your interface. pfifo has a length measured in packets, bfifo in bytes. 14.2. Clark-Shenker-Zhang algorithm (CSZ)This is so theoretical that not even Alexey (the main CBQ author) claims to understand it. From his source:
14.3. DSMARKSource documents:
This chapter was written by Esteve Camps <esteve@hades.udg.es>. 14.3.1. IntroductionFirst of all, first of all, it would be a great idea for you to read RFCs written about this (RFC2474, RFC2475, RFC2597 and RFC2598) at IETF DiffServ working Group web site and Werner Almesberger web site (he wrote the code to support Differentiated Services on Linux). 14.3.2. What is Dsmark related to?Dsmark is a queueing discipline that offers the capabilities needed in Differentiated Services (also called DiffServ or, simply, DS). DiffServ is one of two actual QoS architectures (the other one is called Integrated Services) that is based on a value carried by packets in the DS field of the IP header. One of the first solutions in IP designed to offer some QoS level was the Type of Service field (TOS byte) in IP header. By changing that value, we could choose a high/low level of throughput, delay or reliability. But this didn't provide sufficient flexibility to the needs of new services (such as real-time applications, interactive applications and others). After this, new architectures appeared. One of these was DiffServ which kept TOS bits and renamed DS field. 14.3.3. Differentiated Services guidelinesDifferentiated Services is group-oriented. I mean, we don't know anything about flows (this will be the Integrated Services purpose); we know about flow aggregations and we will apply different behaviours depending on which aggregation a packet belongs to. When a packet arrives to an edge node (entry node to a DiffServ domain) entering to a DiffServ Domain we'll have to policy, shape and/or mark those packets (marking refers to assigning a value to the DS field. It's just like the cows :-) ). This will be the mark/value that the internal/core nodes on our DiffServ Domain will look at to determine which behaviour or QoS level apply. As you can deduce, Differentiated Services involves a domain on which all DS rules will have to be applied. In fact you can think I will classify all the packets entering my domain. Once they enter my domain they will be subjected to the rules that my classification dictates and every traversed node will apply that QoS level. In fact, you can apply your own policies into your local domains, but some Service Level Agreements should be considered when connecting to other DS domains. At this point, you maybe have a lot of questions. DiffServ is more than I've explained. In fact, you can understand that I can not resume more than 3 RFCs in just 50 lines :-). 14.3.4. Working with DsmarkAs the DiffServ bibliography specifies, we differentiate boundary nodes and interior nodes. These are two important points in the traffic path. Both types perform a classification when the packets arrive. Its result may be used in different places along the DS process before the packet is released to the network. It's just because of this that the diffserv code supplies an structure called sk_buff, including a new field called skb->tc_index where we'll store the result of initial classification that may be used in several points in DS treatment. The skb->tc_index value will be initially set by the DSMARK qdisc, retrieving it from the DS field in IP header of every received packet. Besides, cls_tcindex classifier will read all or part of skb->tcindex value and use it to select classes. But, first of all, take a look at DSMARK qdisc command and its parameters:
14.3.5. How SCH_DSMARK works.This qdisc will apply the next steps:
How to do marking? Just change the mask and value of the class you want to remark. See next line of code:
Now, we'll explain how TC_INDEX filter works and how fits into this. Besides, TCINDEX filter can be used in other configurations rather than those including DS services. 14.3.6. TC_INDEX FilterThis is the basic command to declare a TC_INDEX filter:
First of all, suppose we receive a packet marked as EF . If you read RFC2598, you'll see that DSCP recommended value for EF traffic is 101110. This means that DS field will be 10111000 (remember that less significant bits in TOS byte are not used in DS) or 0xb8 in hexadecimal codification.
The packet arrives, then, set with 0xb8 value at DS field. As we explained before, dsmark qdisc identified by 1:0 id in the example, retrieves DS field and store it in skb->tc_index variable. Next step in the example will correspond to the filter associated to this qdisc (second line in the example). This will perform next operations:
In the example, MASK=0xFC i SHIFT=2.
The returned value will correspond to a qdisc internal filter handle (in the example, identifier 2:0). If a filter with this id exists, policing and metering conditions will be verified (in case that filter includes this) and the classid will be returned (in our example, classid 2:1) and stored in skb->tc_index variable. But if any filter with that identifier is found, the result will depend on fall_through flag declaration. If so, value key is returned as classid. If not, an error is returned and process continues with the rest filters. Be careful if you use fall_through flag; this can be done if a simple relation exists between values of skb->tc_index variable and class id's. The latest parameters to comment on are hash and pass_on. The first one relates to hash table size. Pass_on will be used to indicate that if no classid equal to the result of this filter is found, try next filter. The default action is fall_through (look at next table). Finally, let's see which possible values can be set to all this TCINDEX parameters:
This kind of filter is very powerful. It's necessary to explore all possibilities. Besides, this filter is not only used in DiffServ configurations. You can use it as any other kind of filter. I recommend you to look at all DiffServ examples included in iproute2 distribution. I promise I will try to complement this text as soon as I can. Besides, all I have explained is the result of a lot of tests. I would thank you tell me if I'm wrong in any point. 14.4. Ingress qdiscAll qdiscs discussed so far are egress qdiscs. Each interface however can also have an ingress qdisc which is not used to send packets out to the network adaptor. Instead, it allows you to apply tc filters to packets coming in over the interface, regardless of whether they have a local destination or are to be forwarded. As the tc filters contain a full Token Bucket Filter implementation, and are also able to match on the kernel flow estimator, there is a lot of functionality available. This effectively allows you to police incoming traffic, before it even enters the IP stack. 14.4.1. Parameters & usageThe ingress qdisc itself does not require any parameters. It differs from other qdiscs in that it does not occupy the root of a device. Attach it like this:
For a contrived example how the ingress qdisc could be used, see the Cookbook. 14.5. Random Early Detection (RED)This section is meant as an introduction to backbone routing, which often involves <100 megabit bandwidths, which requires a different approach than your ADSL modem at home. The normal behaviour of router queues on the Internet is called tail-drop. Tail-drop works by queueing up to a certain amount, then dropping all traffic that 'spills over'. This is very unfair, and also leads to retransmit synchronization. When retransmit synchronization occurs, the sudden burst of drops from a router that has reached its fill will cause a delayed burst of retransmits, which will over fill the congested router again. In order to cope with transient congestion on links, backbone routers will often implement large queues. Unfortunately, while these queues are good for throughput, they can substantially increase latency and cause TCP connections to behave very burstily during congestion. These issues with tail-drop are becoming increasingly troublesome on the Internet because the use of network unfriendly applications is increasing. The Linux kernel offers us RED, short for Random Early Detect, also called Random Early Drop, as that is how it works. RED isn't a cure-all for this, applications which inappropriately fail to implement exponential backoff still get an unfair share of the bandwidth, however, with RED they do not cause as much harm to the throughput and latency of other connections. RED statistically drops packets from flows before it reaches its hard limit. This causes a congested backbone link to slow more gracefully, and prevents retransmit synchronization. This also helps TCP find its 'fair' speed faster by allowing some packets to get dropped sooner keeping queue sizes low and latency under control. The probability of a packet being dropped from a particular connection is proportional to its bandwidth usage rather than the number of packets it transmits. RED is a good queue for backbones, where you can't afford the complexity of per-session state tracking needed by fairness queueing. In order to use RED, you must decide on three parameters: Min, Max, and burst. Min sets the minimum queue size in bytes before dropping will begin, Max is a soft maximum that the algorithm will attempt to stay under, and burst sets the maximum number of packets that can 'burst through'. You should set the min by calculating that highest acceptable base queueing latency you wish, and multiply it by your bandwidth. For instance, on my 64kbit/s ISDN link, I might want a base queueing latency of 200ms so I set min to 1600 bytes. Setting min too small will degrade throughput and too large will degrade latency. Setting a small min is not a replacement for reducing the MTU on a slow link to improve interactive response. You should make max at least twice min to prevent synchronization. On slow links with small Min's it might be wise to make max perhaps four or more times large then min. Burst controls how the RED algorithm responds to bursts. Burst must be set larger then min/avpkt. Experimentally, I've found (min+min+max)/(3*avpkt) to work ok. Additionally, you need to set limit and avpkt. Limit is a safety value, after there are limit bytes in the queue, RED 'turns into' tail-drop. I typical set limit to eight times max. Avpkt should be your average packet size. 1000 works OK on high speed Internet links with a 1500byte MTU. Read the paper on RED queueing by Sally Floyd and Van Jacobson for technical information. 14.6. Generic Random Early DetectionNot a lot is known about GRED. It looks like GRED with several internal queues, whereby the internal queue is chosen based on the Diffserv tcindex field. According to a slide found here, it contains the capabilities of Cisco's 'Distributed Weighted RED', as well as Dave Clark's RIO. Each virtual queue can have its own Drop Parameters specified. FIXME: get Jamal or Werner to tell us more 14.7. VC/ATM emulationThis is quite a major effort by Werner Almesberger to allow you to build Virtual Circuits over TCP/IP sockets. A Virtual Circuit is a concept from ATM network theory. For more information, see the ATM on Linux homepage. 14.8. Weighted Round Robin (WRR)This qdisc is not included in the standard kernels but can be downloaded from �. Currently the qdisc is only tested with Linux 2.2 kernels but it will probably work with 2.4/2.5 kernels too. The WRR qdisc distributes bandwidth between its classes using the weighted round robin scheme. That is, like the CBQ qdisc it contains classes into which arbitrary qdiscs can be plugged. All classes which have sufficient demand will get bandwidth proportional to the weights associated with the classes. The weights can be set manually using the tc program. But they can also be made automatically decreasing for classes transferring much data. The qdisc has a built-in classifier which assigns packets coming from or sent to different machines to different classes. Either the MAC or IP and either source or destination addresses can be used. The MAC address can only be used when the Linux box is acting as an ethernet bridge, however. The classes are automatically assigned to machines based on the packets seen. The qdisc can be very useful at sites such as dorms where a lot of unrelated individuals share an Internet connection. A set of scripts setting up a relevant behavior for such a site is a central part of the WRR distribution. Chapter 15. CookbookThis section contains 'cookbook' entries which may help you solve problems. A cookbook is no replacement for understanding however, so try and comprehend what is going on. 15.1. Running multiple sites with different SLAsYou can do this in several ways. Apache has some support for this with a module, but we'll show how Linux can do this for you, and do so for other services as well. These commands are stolen from a presentation by Jamal Hadi that's referenced below. Let's say we have two customers, with http, ftp and streaming audio, and we want to sell them a limited amount of bandwidth. We do so on the server itself. Customer A should have at most 2 megabits, customer B has paid for 5 megabits. We separate our customers by creating virtual IP addresses on our server.
It is up to you to attach the different servers to the right IP address. All popular daemons have support for this. We first attach a CBQ qdisc to eth0:
We then create classes for our customers:
Then we add filters for our two classes:
And we're done. FIXME: why no token bucket filter? is there a default pfifo_fast fallback somewhere? 15.2. Protecting your host from SYN floods>From Alexey's iproute documentation, adapted to netfilter and with more plausible paths. If you use this, take care to adjust the numbers to reasonable values for your system. If you want to protect an entire network, skip this script, which is best suited for a single host. It appears that you need the very latest version of the iproute2 tools to get this to work with 2.4.0.
15.3. Rate limit ICMP to prevent dDoSRecently, distributed denial of service attacks have become a major nuisance on the Internet. By properly filtering and rate limiting your network, you can both prevent becoming a casualty or the cause of these attacks. You should filter your networks so that you do not allow non-local IP source addressed packets to leave your network. This stops people from anonymously sending junk to the Internet. Rate limiting goes much as shown earlier. To refresh your memory, our ASCIIgram again:
We first set up the prerequisite parts:
If you have 100Mbit, or more, interfaces, adjust these numbers. Now you need to determine how much ICMP traffic you want to allow. You can perform measurements with tcpdump, by having it write to a file for a while, and seeing how much ICMP passes your network. Do not forget to raise the snapshot length! If measurement is impractical, you might want to choose 5% of your available bandwidth. Let's set up our class:
This limits at 100Kbit. Now we need a filter to assign ICMP traffic to this class:
15.4. Prioritizing interactive trafficIf lots of data is coming down your link, or going up for that matter, and you are trying to do some maintenance via telnet or ssh, this may not go too well. Other packets are blocking your keystrokes. Wouldn't it be great if there were a way for your interactive packets to sneak past the bulk traffic? Linux can do this for you! As before, we need to handle traffic going both ways. Evidently, this works best if there are Linux boxes on both ends of your link, although other UNIX's are able to do this. Consult your local Solaris/BSD guru for this. The standard pfifo_fast scheduler has 3 different 'bands'. Traffic in band 0 is transmitted first, after which traffic in band 1 and 2 gets considered. It is vital that our interactive traffic be in band 0! We blatantly adapt from the (soon to be obsolete) ipchains HOWTO: There are four seldom-used bits in the IP header, called the Type of Service (TOS) bits. They effect the way packets are treated; the four bits are "Minimum Delay", "Maximum Throughput", "Maximum Reliability" and "Minimum Cost". Only one of these bits is allowed to be set. Rob van Nieuwkerk, the author of the ipchains TOS-mangling code, puts it as follows:
The most common use is to set telnet & ftp control connections to "Minimum Delay" and FTP data to "Maximum Throughput". This would be done as follows, on your upstream router:
Now, this only works for data going from your telnet foreign host to your local computer. The other way around appears to be done for you, ie, telnet, ssh & friends all set the TOS field on outgoing packets automatically. Should you have an application that does not do this, you can always do it with netfilter. On your local box:
15.5. Transparent web-caching using netfilter, iproute2, ipchains and squidThis section was sent in by reader Ram Narula from Internet for Education (Thailand). The regular technique in accomplishing this in Linux is probably with use of ipchains AFTER making sure that the "outgoing" port 80(web) traffic gets routed through the server running squid. There are 3 common methods to make sure "outgoing" port 80 traffic gets routed to the server running squid and 4th one is being introduced here.
(all servers on my network had 10.0.0.1 as the default gateway which was the former IP address of donmuang router so what I did was changed the IP address of donmuang to 10.0.0.3 and gave naret ip address of 10.0.0.1)
Setup Squid server on silom, make sure it does support transparent caching/proxying, the default port is usually 3128, so all traffic for port 80 has to be redirected to port 3128 locally. This can be done by using ipchains with the following:
Or, in netfilter lingo:
(note: you might have other entries as well) For more information on setting Squid server please refer to Squid FAQ page on http://squid.nlanr.net). Make sure ip forwarding is enabled on this server and the default gateway for this server is donmuang router (NOT naret).
The setup is complete, check the configuration
15.5.1. Traffic flow diagram after implementation
Note that the network is asymmetric as there is one extra hop on general outgoing path.
15.6. Circumventing Path MTU Discovery issues with per route MTU settingsFor sending bulk data, the Internet generally works better when using larger packets. Each packet implies a routing decision, when sending a 1 megabyte file, this can either mean around 700 packets when using packets that are as large as possible, or 4000 if using the smallest default. However, not all parts of the Internet support full 1460 bytes of payload per packet. It is therefore necessary to try and find the largest packet that will 'fit', in order to optimize a connection. This process is called 'Path MTU Discovery', where MTU stands for 'Maximum Transfer Unit.' When a router encounters a packet that's too big too send in one piece, AND it has been flagged with the "Don't Fragment" bit, it returns an ICMP message stating that it was forced to drop a packet because of this. The sending host acts on this hint by sending smaller packets, and by iterating it can find the optimum packet size for a connection over a certain path. This used to work well until the Internet was discovered by hooligans who do their best to disrupt communications. This in turn lead administrators to either block or shape ICMP traffic in a misguided attempt to improve security or robustness of their Internet service. What has happened now is that Path MTU Discovery is working less and less well and fails for certain routes, which leads to strange TCP/IP sessions which die after a while. Although I have no proof for this, two sites who I used to have this problem with both run Alteon Acedirectors before the affected systems - perhaps somebody more knowledgeable can provide clues as to why this happens. 15.6.1. SolutionWhen you encounter sites that suffer from this problem, you can disable Path MTU discovery by setting it manually. Koos van den Hout, slightly edited, writes:
In general, it is possible to override PMTU Discovery by setting specific routes. For example, if only a certain subnet is giving problems, this should help:
15.7. Circumventing Path MTU Discovery issues with MSS Clamping (for ADSL, cable, PPPoE & PPtP users)As explained above, Path MTU Discovery doesn't work as well as it should anymore. If you know for a fact that a hop somewhere in your network has a limited (<1500) MTU, you cannot rely on PMTU Discovery finding this out. Besides MTU, there is yet another way to set the maximum packet size, the so called Maximum Segment Size. This is a field in the TCP Options part of a SYN packet. Recent Linux kernels, and a few PPPoE drivers (notably, the excellent Roaring Penguin one), feature the possibility to 'clamp the MSS'. The good thing about this is that by setting the MSS value, you are telling the remote side unequivocally 'do not ever try to send me packets bigger than this value'. No ICMP traffic is needed to get this to work. The bad thing is that it's an obvious hack - it breaks 'end to end' by modifying packets. Having said that, we use this trick in many places and it works like a charm. In order for this to work you need at least iptables-1.2.1a and Linux 2.4.3 or higher. The basic command line is:
This calculates the proper MSS for your link. If you are feeling brave, or think that you know best, you can also do something like this:
This sets the MSS of passing SYN packets to 128. Use this if you have VoIP with tiny packets, and huge http packets which are causing chopping in your voice calls. 15.8. The Ultimate Traffic Conditioner: Low Latency, Fast Up & DownloadsNote: This script has recently been upgraded and previously only worked for Linux clients in your network! So you might want to update if you have Windows machines or Macs in your network and noticed that they were not able to download faster while others were uploading. I attempted to create the holy grail:
The next section explains in depth what causes the delays, and how we can fix them. You can safely skip it and head straight for the script if you don't care how the magic is performed. 15.8.1. Why it doesn't work well by defaultISPs know that they are benchmarked solely on how fast people can download. Besides available bandwidth, download speed is influenced heavily by packet loss, which seriously hampers TCP/IP performance. Large queues can help prevent packet loss, and speed up downloads. So ISPs configure large queues. These large queues however damage interactivity. A keystroke must first travel the upstream queue, which may be seconds (!) long and go to your remote host. It is then displayed, which leads to a packet coming back, which must then traverse the downstream queue, located at your ISP, before it appears on your screen. This HOWTO teaches you how to mangle and process the queue in many ways, but sadly, not all queues are accessible to us. The queue over at the ISP is completely off-limits, whereas the upstream queue probably lives inside your cable modem or DSL device. You may or may not be able to configure it. Most probably not. So, what next? As we can't control either of those queues, they must be eliminated, and moved to your Linux router. Luckily this is possible.
Now, once we have done this, we have eliminated the downstream queue totally (except for short bursts), and gain the ability to manage the upstream queue with all the power Linux offers. What remains to be done is to make sure interactive traffic jumps to the front of the upstream queue. To make sure that uploads don't hurt downloads, we also move ACK packets to the front of the queue. This is what normally causes the huge slowdown observed when generating bulk traffic both ways. The ACKnowledgements for downstream traffic must compete with upstream traffic, and get delayed in the process. If we do all this we get the following measurements using an excellent ADSL connection from xs4all in the Netherlands:
What you can expect from this script depends a lot on your actual uplink speed. When uploading at full speed, there will always be a single packet ahead of your keystroke. That is the lower limit to the latency you can achieve - divide your MTU by your upstream speed to calculate. Typical values will be somewhat higher than that. Lower your MTU for better effects! Next, two versions of this script, one with Devik's excellent HTB, the other with CBQ which is in each Linux kernel, unlike HTB. Both are tested and work well. 15.8.2. The actual script (CBQ)Works on all kernels. Within the CBQ qdisc we place two Stochastic Fairness Queues that make sure that multiple bulk streams don't drown each other out. Downstream traffic is policed using a tc filter containing a Token Bucket Filter. You might improve on this script by adding 'bounded' to the line that starts with 'tc class add .. classid 1:20'. If you lowered your MTU, also lower the allot & avpkt numbers!
If the last two lines give an error, update your tc tool to a newer version! 15.8.3. The actual script (HTB)The following script achieves all goals using the wonderful HTB queue, see the relevant chapter. Well worth patching your kernel for!
If you want this script to be run by ppp on connect, copy it to /etc/ppp/ip-up.d. If the last two lines give an error, update your tc tool to a newer version! 15.9. Rate limiting a single host or netmaskAlthough this is described in stupendous details elsewhere and in our manpages, this question gets asked a lot and happily there is a simple answer that does not need full comprehension of traffic control. This three line script does the trick:
The first line installs a class based queue on your interface, and tells the kernel that for calculations, it can be assumed to be a 10mbit interface. If you get this wrong, no real harm is done. But getting it right will make everything more precise. The second line creates a 512kbit class with some reasonable defaults. For details, see the cbq manpages and Chapter 9. The last line tells which traffic should go to the shaped class. Traffic not matched by this rule is NOT shaped. To make more complicated matches (subnets, source ports, destination ports), see Section 9.6.2. If you changed anything and want to reload the script, execute 'tc qdisc del dev $DEV root' to clean up your existing configuration. The script can further be improved by adding a last optional line 'tc qdisc add dev $DEV parent 1:1 sfq perturb 10'. See Section 9.2.3 for details on what this does. Chapter 16. Building bridges, and pseudo-bridges with Proxy ARPBridges are devices which can be installed in a network without any reconfiguration. A network switch is basically a many-port bridge. A bridge is often a 2-port switch. Linux does however support multiple interfaces in a bridge, making it a true switch. Bridges are often deployed when confronted with a broken network that needs to be fixed without any alterations. Because the bridge is a layer-2 device, one layer below IP, routers and servers are not aware of its existence. This means that you can transparently block or modify certain packets, or do shaping. Another good thing is that a bridge can often be replaced by a cross cable or a hub, should it break down. The bad news is that a bridge can cause great confusion unless it is very well documented. It does not appear in traceroutes, but somehow packets disappear or get changed from point A to point B ('this network is HAUNTED!'). You should also wonder if an organization that 'does not want to change anything' is doing the right thing. The Linux 2.4/2.5 bridge is documented on this page. 16.1. State of bridging and iptablesAs of Linux 2.4.14, bridging and iptables do not 'see' each other without help. If you bridge packets from eth0 to eth1, they do not 'pass' by iptables. This means that you cannot do filtering, or NAT or mangling or whatever. There are several projects going on to fix this, the truly right one is by the author of the Linux 2.4 bridging code, Lennert Buytenhek. He recently informed us that as of bridge-nf 0.0.2 (see the url above), the code is stable and usable in production environments. He is now asking the kernel people if and how the patch can be merged, stay tuned! 16.2. Bridging and shapingThis does work as advertised. Be sure to figure out which side each interface is on, otherwise you might be shaping outbound traffic in your internal interface, which won't work. Use tcpdump if needed. 16.3. Pseudo-bridges with Proxy-ARPIf you just want to implement a Pseudo-bridge, skip down a few sections to 'Implementing it', but it is wise to read a bit about how it works in practice. A Pseudo-bridge works a bit differently. By default, a bridge passes packets unaltered from one interface to the other. It only looks at the hardware address of packets to determine what goes where. This in turn means that you can bridge traffic that Linux does not understand, as long as it has an hardware address it does. A 'Pseudo-bridge' works differently and looks more like a hidden router than a bridge, but like a bridge, it has little impact on network design. An advantage of the fact that it is not a bridge lies in the fact that packets really pass through the kernel, and can be filtered, changed, redirected or rerouted. A real bridge can also be made to perform these feats, but it needs special code, like the Ethernet Frame Diverter, or the above mentioned patch. Another advantage of a pseudo-bridge is that it does not pass packets it does not understand - thus cleaning your network of a lot of cruft. In cases where you need this cruft (like SAP packets, or Netbeui), use a real bridge. 16.3.1. ARP & Proxy-ARPWhen a host wants to talk to another host on the same physical network segment, it sends out an Address Resolution Protocol packet, which, somewhat simplified, reads like this 'who has 10.0.0.1, tell 10.0.0.7'. In response to this, 10.0.0.1 replies with a short 'here' packet. 10.0.0.7 then sends packets to the hardware address mentioned in the 'here' packet. It caches this hardware address for a relatively long time, and after the cache expires, it re-asks the question. When building a Pseudo-bridge, we instruct the bridge to reply to these ARP packets, which causes the hosts in the network to send its packets to the bridge. The bridge then processes these packets, and sends them to the relevant interface. So, in short, whenever a host on one side of the bridge asks for the hardware address of a host on the other, the bridge replies with a packet that says 'hand it to me'. This way, all data traffic gets transmitted to the right place, and always passes through the bridge. 16.3.2. Implementing itIn the bad old days, it used to be possible to instruct the Linux Kernel to perform 'proxy-ARP' for just any subnet. So, to configure a pseudo-bridge, you would have to specify both the proper routes to both sides of the bridge AND create matching proxy-ARP rules. This is bad in that it requires a lot of typing, but also because it easily allows you to make mistakes which make your bridge respond to ARP queries for networks it does not know how to route. With Linux 2.4/2.5 (and possibly 2.2), this possibility has been withdrawn and has been replaced by a flag in the /proc directory, called 'proxy_arp'. The procedure for building a pseudo-bridge is then:
Also, do not forget to turn on the ip_forwarding flag! When converting from a true bridge, you may find that this flag was turned off as it is not needed when bridging. Another thing you might note when converting is that you need to clear the arp cache of computers in the network - the arp cache might contain old pre-bridge hardware addresses which are no longer correct. On a Cisco, this is done using the command 'clear arp-cache', under Linux, use 'arp -d ip.address'. You can also wait for the cache to expire manually, which can take rather long. You can speed this up using the wonderful 'arping' tool, which on many distributions is part of the 'iputils' package. Using 'arping' you can send out unsolicited ARP messages so as to update remote arp caches. This is a very powerful technique that is also used by 'black hats' to subvert your routing!
You may also discover that your network was misconfigured if you are/were of the habit of specifying routes without netmasks. To explain, some versions of route may have guessed your netmask right in the past, or guessed wrong without you noticing. When doing surgical routing like described above, it is *vital* that you check your netmasks! Chapter 17. Dynamic routing - OSPF and BGPOnce your network starts to get really big, or you start to consider 'the internet' as your network, you need tools which dynamically route your data. Sites are often connected to each other with multiple links, and more are popping up all the time. The Internet has mostly standardized on OSPF and BGP4 (rfc1771). Linux supports both, by way of gated and zebra While currently not within the scope of this document, we would like to point you to the definitive works: Overview: Cisco Systems Designing large-scale IP Internetworks For OSPF: Moy, John T. "OSPF. The anatomy of an Internet routing protocol" Addison Wesley. Reading, MA. 1998. Halabi has also written a good guide to OSPF routing design, but this appears to have been dropped from the Cisco web site. For BGP: Halabi, Bassam "Internet routing architectures" Cisco Press (New Riders Publishing). Indianapolis, IN. 1997. also Cisco Systems Using the Border Gateway Protocol for interdomain routing Although the examples are Cisco-specific, they are remarkably similar to the configuration language in Zebra :-) Chapter 18. Other possibilitiesThis chapter is a list of projects having to do with advanced Linux routing & traffic shaping. Some of these links may deserve chapters of their own, some are documented very well of themselves, and don't need more HOWTO.
Chapter 19. Further reading
Chapter 20. AcknowledgementsIt is our goal to list everybody who has contributed to this HOWTO, or helped us demystify how things work. While there are currently no plans for a Netfilter type scoreboard, we do like to recognize the people who are helping.
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