Linux 2.4 Advanced Routing HOWTO
Netherlabs BV (bert hubert <
[email protected]>)
Gregory Maxwell <
[email protected]>
Remco van Mook <
[email protected]>
Martijn van Oosterhout <
[email protected]>
Paul B Schroeder <
[email protected]>
[email protected]
v0.1.0 $Date: 2000/05/26 15:42:43 $
A very hands-on approach to iproute2, traffic shaping and a bit of
netfilter
______________________________________________________________________
Table of Contents
1. Dedication
2. Introduction
2.1 Disclaimer & License
2.2 Prior knowledge
2.3 What Linux can do for you
2.4 Housekeeping notes
2.5 Access, CVS & submitting updates
2.6 Layout of this document
3. Introduction to iproute2
3.1 Why iproute2?
3.2 Iproute2 tour
3.3 Prerequisites
3.4 Exploring your current configuration
3.4.1 (TT
3.4.2 (TT
3.4.3 (TT
3.5 ARP
4. Rules - routing policy database
4.1 Simple source routing
5. GRE and other tunnels
5.1 A few general remarks about tunnels:
5.2 IP in IP tunneling
5.3 GRE tunneling
5.3.1 IPv4 Tunneling
5.3.2 IPv6 Tunneling
5.4 Userland tunnels
6. IPsec: secure IP over the internet
7. Multicast routing
8. Using Class Based Queueing for bandwidth management
8.1 What is queueing?
8.2 First attempt at bandwidth division
8.3 What to do with excess bandwidth
8.4 Class subdivisions
8.5 Loadsharing over multiple interfaces
9. More queueing disciplines
9.1 pfifo_fast
9.2 Stochastic Fairness Queueing
9.3 Token Bucket Filter
9.4 Random Early Detect
9.5 Ingress policer qdisc
10. Netfilter & iproute - marking packets
11. More classifiers
11.1 The "fw" classifier
11.2 The "u32" classifier
11.2.1 U32 selector
11.2.2 General selectors
11.2.3 Specific selectors
11.3 The "route" classifier
11.4 The "rsvp" classifier
11.5 The "tcindex" classifier
12. Kernel network parameters
12.1 Reverse Path Filtering
12.2 Obscure settings
12.2.1 Generic ipv4
12.2.2 Per device settings
12.2.3 Neighbor pollicy
12.2.4 Routing settings
13. Backbone applications of traffic control
13.1 Router queues
14. Shaping Cookbook
14.1 Running multiple sites with different SLAs
14.2 Protecting your host from SYN floods
14.3 Ratelimit ICMP to prevent dDoS
14.4 Prioritising interactive traffic
15. Advanced Linux Routing
15.1 How does packet queueing really work?
15.2 Advanced uses of the packet queueing system
15.3 Other packet shaping systems
16. Dynamic routing - OSPF and BGP
17. Further reading
18. Acknowledgements
______________________________________________________________________
1. Dedication
This document is dedicated to lots of people, and is my attempt to do
something back. To list but a few:
� Rusty Russel
� Alexey N. Kuznetsov
� The good folks from Google
� The staff of Casema Internet
2. Introduction
Welcome, 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
Russel of (amongst other things) netfilter fame.
You can always reach us by writing the HOWTO team
<mailto:
[email protected]>.
2.1. Disclaimer & License
This 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) 2000 by bert hubert, Gregory Maxwell and Martijn van
Oosterhout
Please freely copy and distribute (sell or give away) this document in
any format. It's requested that corrections and/or comments be
fowarded to the document maintainer. You may create a derivative work
and distribute it provided that you:
1. Send your derivative work (in the most suitable format such as
sgml) to the LDP (Linux Documentation Project) or the like for
posting on the Internet. If not the LDP, then let the LDP know
where it is available.
2. License the derivative work with this same license or use GPL.
Include a copyright notice and at least a pointer to the license
used.
3. Give due credit to previous authors and major contributors.
If you're considering making a derived work other than a
translation, it's requested that you discuss your plans with the
current 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 knowledge
As the title implies, this is the 'Advanced' HOWTO. While by no means
rocket science, some prior knowledge is assumed. This document is
meant as a sequel to the Linux 2.4 Networking HOWTO
<
http://www.ds9a.nl/2.4Networking/> by the same authors. You should
probably read that first.
Here are some orther references which might help learn you more:
Rusty Russels networking-concepts-HOWTO <
http://netfilter.kernel�
notes.org/unreliable-guides/networking-concepts-HOWTO.html>
Very nice introduction, explaining what a network is, and how it
is connected to other networks
Linux Networking-HOWTO (Previously the Net-3 HOWTO)
Great stuff, although very verbose. It learns you a lot of stuff
that's already configured if you are able to connect to the
internet. Should be located in /usr/doc/HOWTO/NET3-4-HOWTO.txt
but can be also be found online
<
http://www.linuxports.com/howto/networking>
2.3. What Linux can do for you
A small list of things that are possible:
� Throttle bandwidth for certain computers
� Throttle bandwidth to certain computers
� Help you to fairly share your bandwidth
� Protect your network from DoS attacks
� Protect the internet from your customers
� Multiplex several servers as one, for load balancing or enhanced
availability
� Restrict access to your computers
� Limit access of your users to other hosts
� Do routing based on user id (yes!), MAC address, source IP address,
port, type of service, time of day or content
Currently not many people are using these advanced features. This has
several reasons. While the provided documentation is verbose, it is
not very hands on. Traffic control is almost undocumented.
2.4. Housekeeping notes
There 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 realise 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 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 updates
The canonical location for the HOWTO is here
<
http://www.ds9a.nl/2.4Routing>.
We now have anonymous CVS access available for the world at large.
This is good in several 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.
$ export CVSROOT=:pserver:
[email protected]:/var/cvsroot
$ cvs login
CVS password: [enter 'cvs' (without 's)]
$ cvs co 2.4routing
cvs server: Updating 2.4routing
U 2.4routing/2.4routing.sgml
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 sgml-tools,
ghostscript and tetex to get all formats.
2.6. Layout of this document
We 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:
� Rusty's Remarkably Unreliable Guides
<
http://netfilter.kernelnotes.org/unreliable-guides/>
We will be focusing mostly on what is possible by combining netfilter
and iproute2.
3. Introduction to iproute2
3.1. 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 then that 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 has made it possible to clearly express features
previously not possible.
3.2. Iproute2 tour
Linux has a sophisticated system for bandwidth provisioning called
Traffic Control. This system supports various method for classifying,
prioritising, sharing, and limiting both inbound and outbound traffic.
We'll start off with a tiny tour of iproute2 possibilities.
3.3. Prerequisites
You 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". Some parts of iproute
require you to have certain kernel options enabled.
FIXME: We should mention <
ftp://ftp.inr.ac.ru/ip-
routing/iproute2-current.tar.gz> is always the latest
3.4. Exploring your current configuration
This 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 do display our interfaces for
us.
3.4.1. ip shows us our links
[ahu@home ahu]$ ip link list
1: lo: <LOOPBACK,UP> mtu 3924 qdisc noqueue
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: dummy: <BROADCAST,NOARP> mtu 1500 qdisc noop
link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff
3: eth0: <BROADCAST,MULTICAST,PROMISC,UP> mtu 1400 qdisc pfifo_fast qlen 100
link/ether 48:54:e8:2a:47:16 brd ff:ff:ff:ff:ff:ff
4: eth1: <BROADCAST,MULTICAST,PROMISC,UP> mtu 1500 qdisc pfifo_fast qlen 100
link/ether 00:e0:4c:39:24:78 brd ff:ff:ff:ff:ff:ff
3764: ppp0: <POINTOPOINT,MULTICAST,NOARP,UP> mtu 1492 qdisc pfifo_fast qlen 10
link/ppp
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 octects, and it is not supposed to queue. Which
makes sense because the loopback interface is a figment of your
kernels imagination.
I'll skip the dummy interface for now, and it may not be present on
your computer. Then there are my two network interfaces, one at the
side of my cable modem, the other 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
[ahu@home ahu]$ ip address show
1: lo: <LOOPBACK,UP> mtu 3924 qdisc noqueue
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 brd 127.255.255.255 scope host lo
2: dummy: <BROADCAST,NOARP> mtu 1500 qdisc noop
link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff
3: eth0: <BROADCAST,MULTICAST,PROMISC,UP> mtu 1400 qdisc pfifo_fast qlen 100
link/ether 48:54:e8:2a:47:16 brd ff:ff:ff:ff:ff:ff
inet 10.0.0.1/8 brd 10.255.255.255 scope global eth0
4: eth1: <BROADCAST,MULTICAST,PROMISC,UP> mtu 1500 qdisc pfifo_fast qlen 100
link/ether 00:e0:4c:39:24:78 brd ff:ff:ff:ff:ff:ff
3764: ppp0: <POINTOPOINT,MULTICAST,NOARP,UP> mtu 1492 qdisc pfifo_fast qlen 10
link/ppp
inet 212.64.94.251 peer 212.64.94.1/32 scope global ppp0
This contains more information. It shows all our addresses, and to
which cards they belong. 'inet' stands for Internet. There are lots of
other address families, but these don't concern us right now.
Lets 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.
It's 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 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 routes
Well, 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.
[ahu@home ahu]$ ip route show
212.64.94.1 dev ppp0 proto kernel scope link src 212.64.94.251
10.0.0.0/8 dev eth0 proto kernel scope link src 10.0.0.1
127.0.0.0/8 dev lo scope link
default via 212.64.94.1 dev ppp0
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:
[ahu@home ahu]$ route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use
Iface
212.64.94.1 0.0.0.0 255.255.255.255 UH 0 0 0 ppp0
10.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 eth0
127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
0.0.0.0 212.64.94.1 0.0.0.0 UG 0 0 0 ppp0
3.5. ARP
ARP is the Address Resolution Protocol as described in RFC 826
<
http://www.faqs.org/rfcs/rfc826.html>. 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 used to locate his friend on the network foo is able to
communicate with bar until he (his arp cache) forgets where bar is.
Now let's see how this works. You can view your machines current
arp/neighbor cache/table like so:
[root@espa041 /home/src/iputils]# ip neigh show
9.3.76.42 dev eth0 lladdr 00:60:08:3f:e9:f9 nud reachable
9.3.76.1 dev eth0 lladdr 00:06:29:21:73:c8 nud reachable
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.
[root@espa041 /home/paulsch/.gnome-desktop]# ping -c 1 espa043
PING espa043.austin.ibm.com (9.3.76.43) from 9.3.76.41 : 56(84) bytes of data.
64 bytes from 9.3.76.43: icmp_seq=0 ttl=255 time=0.9 ms
--- espa043.austin.ibm.com ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max = 0.9/0.9/0.9 ms
[root@espa041 /home/src/iputils]# ip neigh show
9.3.76.43 dev eth0 lladdr 00:06:29:21:80:20 nud reachable
9.3.76.42 dev eth0 lladdr 00:60:08:3f:e9:f9 nud reachable
9.3.76.1 dev eth0 lladdr 00:06:29:21:73:c8 nud reachable
As a result of espa041 trying to contact espa043, espa043's hardware
address/location has now been added to the arp/nieghbor 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:
[root@espa041 /home/src/iputils]# ip neigh delete 9.3.76.43 dev eth0
[root@espa041 /home/src/iputils]# ip neigh show
9.3.76.43 dev eth0 nud failed
9.3.76.42 dev eth0 lladdr 00:60:08:3f:e9:f9 nud reachable
9.3.76.1 dev eth0 lladdr 00:06:29:21:73:c8 nud stale
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.
4. Rules - routing policy database
If 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: policy routing" feature.
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:
[ahu@home ahu]$ ip rule list
0: from all lookup local
32766: from all lookup main
32767: from all lookup default
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's
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-cfref documentation.
4.1. Simple source routing
Let'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 you'll end up using the low-end
cable modem.
The 'fast' cable modem is known as 212.64.94.251 and is an 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:
[ahu@home ahu]$ ip route list table local
broadcast 127.255.255.255 dev lo proto kernel scope link src 127.0.0.1
local 10.0.0.1 dev eth0 proto kernel scope host src 10.0.0.1
broadcast 10.0.0.0 dev eth0 proto kernel scope link src 10.0.0.1
local 212.64.94.251 dev ppp0 proto kernel scope host src 212.64.94.251
broadcast 10.255.255.255 dev eth0 proto kernel scope link src 10.0.0.1
broadcast 127.0.0.0 dev lo proto kernel scope link src 127.0.0.1
local 212.64.78.148 dev ppp2 proto kernel scope host src 212.64.78.148
local 127.0.0.1 dev lo proto kernel scope host src 127.0.0.1
local 127.0.0.0/8 dev lo proto kernel scope host src 127.0.0.1
Lots of obvious things, but things that need to specified somewhere.
Well, here they are. The default table is empty.
Let's view the 'main' table:
[ahu@home ahu]$ ip route list table main
195.96.98.253 dev ppp2 proto kernel scope link src 212.64.78.148
212.64.94.1 dev ppp0 proto kernel scope link src 212.64.94.251
10.0.0.0/8 dev eth0 proto kernel scope link src 10.0.0.1
127.0.0.0/8 dev lo scope link
default via 212.64.94.1 dev ppp0
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.
# echo 200 John >> /etc/iproute2/rt_tables
# ip rule add from 10.0.0.10 table John
# ip rule ls
0: from all lookup local
32765: from 10.0.0.10 lookup John
32766: from all lookup main
32767: from all lookup default
Now all that is left is to generate Johns table, and flush the route
cache:
# ip route add default via 195.96.98.253 dev ppp2 table John
# ip route flush cache
And we are done. It is left as an exercise for the reader to implement
this in ip-up.
5. GRE and other tunnels
There 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 tunneling
This 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:
network 10.0.1.0
netmask 255.255.255.0
router 10.0.1.1
The router has address 172.16.17.18 on network C.
and network B:
network 10.0.2.0
netmask 255.255.255.0
router 10.0.2.1
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:
insmod ipip.o
insmod new_tunnel.o
Then, on the router of network A, you do the following:
ifconfig tunl0 10.0.1.1 pointopoint 172.19.20.21
route add -net 10.0.2.0 netmask 255.255.255.0 dev tunl0
And on the router of network B:
ifconfig tunl0 10.0.2.1 pointopoint 172.16.17.18
route add -net 10.0.1.0 netmask 255.255.255.0 dev tunl0
And if you're finished with your tunnel:
ifconfig tunl0 down
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 tun�
neling 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 tunneling
GRE 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 module.
5.3.1. IPv4 Tunneling
Let'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:
network 10.0.1.0
netmask 255.255.255.0
router 10.0.1.1
The router has address 172.16.17.18 on network C. Let's call this
network neta (ok, hardly original)
and network B:
network 10.0.2.0
netmask 255.255.255.0
router 10.0.2.1
The router has address 172.19.20.21 on network C. Let's call this
network netb (still not original)
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:
ip tunnel add netb mode gre remote 172.19.20.21 local 172.16.17.18 ttl 255
ip addr add 10.0.1.1 dev netb
ip route add 10.0.2.0/24 dev netb
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).
In the second 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 third 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.
ip tunnel add neta mode gre remote 172.16.17.18 local 172.19.20.21 ttl 255
ip addr add 10.0.2.1 dev neta
ip route add 10.0.1.0/24 dev neta
And when you want to remove the tunnelon router A:
ip link set netb down
ip tunnel del netb
Of course, you can replace netb with neta for router B.
5.3.2. IPv6 Tunneling
BIG FAT WARNING !!
The following is untested and might therefore be completely and utter
BOLLOCKS. Proceed at your own risk. Don't say I didn't warn you.
FIXME: check & try all this
A short bit about IPv6 addresses:
IPv6 addresses are, compared to IPv4 addresses, monstrously big. An
example:
3ffe:2502:200:40:281:48fe:dcfe:d9bc
So, to make writing them down easier, there are a few rules:
� Don't use leading zeroes. Same as in IPv4.
� Use colons to separate every 16 bits or two bytes.
� When you have lots of consecutive zeroes, you can write this down
as ::. You can only do this once in an address and only for
quantities of 16 bits, though.
Using these rules, the address
3ffe:0000:0000:0000:0000:0020:34A1:F32C can be written down as
3ffe::20:34A1:F32C, which is a lot shorter.
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.
Network 3ffe:406:5:1:5:a:2:1/96
Your IPv4 address is 172.16.17.18, and the 6bone router has IPv4
address 172.22.23.24.
ip tunnel add sixbone mode sit remote 172.22.23.24 local 172.16.17.18 ttl 255
ip link set sixbone up
ip addr add 3ffe:406:5:1:5:a:2:1/96 dev sixbone
ip route add 3ffe::/15 dev sixbone
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's also widely adopted outside the Linux community and
therefore a Good Thing.
5.4. Userland tunnels
There are literally dozens of implementations of tunneling outside the
kernel. Best known are of course PPP and PPTP, but there are lots more
(some proprietary, some secure, some that don't even use IP) and that
is really beyond the scope of this HOWTO.
6. IPsec: secure IP over the internet
FIXME: Waiting for our feature editor Stefan to finish his stuf
7. Multicast routing
FIXME: Editor Vacancy!
8. Using Class Based Queueing for bandwidth management
Now, when I discovered this, it *really* blew me away. Linux 2.2 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.
The two basic units of Traffic Control are filters and queues. Filters
place traffic into queues, and queues gather traffic and decide what
to send first, send later, or drop. There are several flavours of
filters and queues.
The most common filters are fwmark and u32, the first lets you use the
Linux netfilter code to select traffic, and the second allows you to
select traffic based on ANY header. The most notable queue is Class
Based Queue. CBQ is a super-queue, in that it contains other queues
(even other CBQs).
It may not be immediately clear what queueing has to do with bandwidth
management, but it really does work.
For our frame of reference, I have modelled this section on an ISP
where I learned the ropes, so to speak, Casema Internet in The
Netherlands. Casema, which is actually a cable company, has internet
needs both for their customers and for their own office. Most
corporate computers there have access to the internet. In reality,
they have lots of money to spend and do not use Linux for bandwidth
management.
We will explore how our ISP could have used Linux to manage their
bandwidth.
8.1. What is queueing?
With queueing we determine the order in which data is *sent*. It it
important to realise this, we can only shape data that we transmit.
How this changing the order determine the speed of transmission?
Imagine a cash register which is able to process 3 customers per
minute.
People wishing to pay go stand in line at the 'tail end' of the queue.
This is 'fifo queueing'. Let's suppose however that we let certain
people always join in the middle of the queue, in stead of at the end.
These people spend a lot less time in the queue and are therefore able
to shop faster.
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.
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 :-)
FIXME: explain that normally, ACKs are used to determine speed
[The Internet] ---<E3, T3, whatever>--- [Linux router] --- [Office+ISP]
eth1 eth0
Now, our Linux router has two interfaces which I shall dub eth0 and
eth1. Eth1 is connected to our router which moves packets from to and
from our fibre link.
Eth0 is connected to a subnet which contains both the corporate
firewall and our network head ends, through which we can connect to
our customers.
Because we can only limit what we send, we need two separate but
possibly very similar sets of rules. By modifying queueing on eth0, we
determine how fast data gets sent to our customers, and therefor how
much downstream bandwidth is available for them. Their 'download
speed' in short.
On eth1, we determine how fast we send data to The Internet, how fast
our users, both corporate and commercial can upload data.
8.2. First attempt at bandwidth division
CBQ enables us to generate several classes, and even classes within
classes. The larger devisions might be called 'agencies'. Within
these classes may be things like 'bulk' or 'interactive'.
For example, we may have a 10 megabit internet connection to 'the
internet' which is to be shared by our customers, and our corporate
needs. We should not allow a few people at the office to steal away
large amounts of bandwidth which we should sell to our customers.
On the other hand, or customers should not be able to drown out the
traffic from our field offices to the customer database.
Previously, one way to solve this was either to use Frame relay/ATM
and create virtual circuits. This works, but frame is not very fine
grained, ATM is terribly inefficient at carrying IP traffic, and
neither have standardised ways to segregate different types of traffic
into different VCs.
Hover, if you do use ATM, Linux can also happily perform deft acts of
fancy traffic classification for you too. Another way is to order
separate connections, but this is not very practical and also not very
elegant, and still does not solve all your problems.
CBQ to the rescue!
Clearly we have two main classes, 'ISP' and 'Office'. Initially, we
really don't care what the divisions do with their bandwidth, so we
don't further subdivide their classes.
We decide that the customers should always be guaranteed 8 megabits of
downstream traffic, and our office 2 megabits.
Setting up traffic control is done with the iproute2 tool tc.
# tc qdisc add dev eth0 root handle 10: cbq bandwidth 10Mbit avpkt 1000
Ok, lots of numbers here. What has happened? We have configured the
'queueing discipline' of eth0. With 'root' we denote that this is the
root discipline. We have given it the handle '10:'. We want to do CBQ,
so we mention that on the command line as well. We tell the kernel
that it can allocate 10Mbit and that the average packet size is
somewhere around 1000 octets.
FIXME: Double check with Alexey the the built in cell calculation is
sufficient.
FIXME: With a 1500 mtu, the default cell is calculated same as the old
example.
FIXME: I checked the sources (userspace and kernel), so we should be
safe omitting it.
Now we need to generate our root class, from which all others descend:
# tc class add dev eth0 parent 10:0 classid 10:1 cbq bandwidth 10Mbit rate \
10Mbit allot 1514 weight 1Mbit prio 8 maxburst 20 avpkt 1000
Even more numbers to worry about - the Linux CBQ implementation is
very generic. With 'parent 10:0' we indicate that this class descends
from the root of qdisc handle '10:' we generated earlier. With
'classid 10:1' we name this class.
We really don't tell the kernel a lot more, we just generate a class
that completely fills the available device. We also specify that the
MTU (plus some overhead) is 1514 octets. We also 'weigh' this class
with 1Mbit - a tuning parameter.
We now generate our ISP class:
# tc class add dev eth0 parent 10:1 classid 10:100 cbq bandwidth 10Mbit rate \
8Mbit allot 1514 weight 800Kbit prio 5 maxburst 20 avpkt 1000 \
bounded
We allocate 8Mbit, and indicate that this class must not exceed this
by adding the 'bounded' parameter. Otherwise this class would have
started borrowing bandwidth from other classes, something we will
discuss later on.
To top it off, we generate the root Office class:
# tc class add dev eth0 parent 10:1 classid 10:200 cbq bandwidth 10Mbit rate \
2Mbit allot 1514 weight 200Kbit prio 5 maxburst 20 avpkt 1000 \
bounded
To make this a bit clearer, a diagram which shows our classes:
+-------------[10: 10Mbit]----------------------+
|+-------------[10:1 root 10Mbit]--------------+|
|| ||
|| +-[10:100 8Mbit]-+ +--[10:200 2Mbit]-----+ ||
|| | | | | ||
|| | ISP | | Office | ||
|| | | | | ||
|| +----------------+ +---------------------+ ||
|| ||
|+---------------------------------------------+|
+-----------------------------------------------+
Ok, now we have told the kernel what our classes are, but not yet how
to manage the queues. We do this presently, in one fell swoop for both
classes.
# tc qdisc add dev eth0 parent 10:100 sfq quantum 1514b perturb 15
# tc qdisc add dev eth0 parent 10:200 sfq quantum 1514b perturb 15
In this case we install the Stochastic Fairness Queueing discipline
(sfq), which is not quite fair, but works well up to high bandwidths
without burning up CPU cycles. There are other queueing disciplines
available which are better, but need more CPU. The Token Bucket Filter
is often used.
Now there is only one thing left to do and that is to explain to the
kernel which packets belong to which class. Initially we will do this
natively with iproute2, but more interesting applications are possible
in combination with netfilter.
# tc filter add dev eth0 parent 10:0 protocol ip prio 100 u32 match ip dst \
150.151.23.24 flowid 10:200
# tc filter add dev eth0 parent 10:0 protocol ip prio 25 u32 match ip dst \
150.151.0.0/16 flowid 10:100
Here is is assumed that our office hides behind a firewall with IP
address 150.151.23.24 and that all our other IP addresses should be
considered to be part of the ISP.
The u32 match is a very simple one - more sophisticated matching rules
are possible when using netfilter to mark our packets, which we can
than match on in tc.
Now we have fairly divided the downstream bandwidth, we need to do the
same for the upstream. For brevity's sake, all in one go:
# tc qdisc add dev eth1 root handle 20: cbq bandwidth 10Mbit avpkt 1000
# tc class add dev eth1 parent 20:0 classid 20:1 cbq bandwidth 10Mbit rate \
10Mbit allot 1514 weight 1Mbit prio 8 maxburst 20 avpkt 1000
# tc class add dev eth1 parent 20:1 classid 20:100 cbq bandwidth 10Mbit rate \
8Mbit allot 1514 weight 800Kbit prio 5 maxburst 20 avpkt 1000 \
bounded
# tc class add dev eth1 parent 20:1 classid 20:200 cbq bandwidth 10Mbit rate \
2Mbit allot 1514 weight 200Kbit prio 5 maxburst 20 avpkt 1000 \
bounded
# tc qdisc add dev eth1 parent 20:100 sfq quantum 1514b perturb 15
# tc qdisc add dev eth1 parent 20:200 sfq quantum 1514b perturb 15
# tc filter add dev eth1 parent 20:0 protocol ip prio 100 u32 match ip src \
150.151.23.24 flowid 20:200
# tc filter add dev eth1 parent 20:0 protocol ip prio 25 u32 match ip src \
150.151.0.0/16 flowid 20:100
8.3. What to do with excess bandwidth
In our hypothetical case, we will find that even when the ISP
customers are mostly offline (say, at 8AM), our office still gets only
2Mbit, which is rather wasteful.
By removing the 'bounded' statements, classes will be able to borrow
bandwidth from each other.
Some classes may not wish to borrow their bandwidth to other classes.
Two rival ISPs on a single link may not want to offer each other
freebees. In such a case, you can add the keyword 'isolated' at the
end of your 'tc class add' lines.
8.4. Class subdivisions
FIXME: completely untested suppositions! Try this!
We can go further than this. Should the employees at the office decide
to all fire up their 'napster' clients, it is still possible that our
database runs out of bandwidth. Therefore, we create two subclasses,
'Human' and 'Database'.
Our database always needs 500Kbit, so we have 1.5Mbit left for Human
consumption.
We now need to create two new classes, within our Office class:
# tc class add dev eth0 parent 10:200 classid 10:250 cbq bandwidth 10Mbit rate \
500Kbit allot 1514 weight 50Kbit prio 5 maxburst 20 avpkt 1000 \
bounded
# tc class add dev eth0 parent 10:200 classid 10:251 cbq bandwidth 10Mbit rate \
1500Kbit allot 1514 weight 150Kbit prio 5 maxburst 20 avpkt 1000 \
bounded
FIXME: Finish this example!
8.5. Loadsharing over multiple interfaces
FIXME: document TEQL
9. More queueing disciplines
The Linux kernel offers us lots of queueing disciplines. By far the
most widely used is the pfifo_fast queue - 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.
9.1. pfifo_fast
This 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.
9.2. Stochastic Fairness Queueing
SFQ, as said earlier, is not quite deterministic, but works (on
average). Its main benefits are that it requires little CPU and
memory. 'Real' fair queueing requires that the kernel keep track of
all running sessions.
Stochastic Fairness Queueing (SFQ) is a simple implementation of 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), being a sequence of
data packets having enough common parameters to distinguish it from
other conversations. The parameters used in case of IP packets are
source and destination address, and the protocol number.
SFQ consists of dynamically allocated number of FIFO queues, one queue
for one conversation. The discipline runs in round-robin, sending one
packet from each FIFO in one turn, and this is why it's called fair.
The main advantage of SFQ is that it allows fair sharing the link
between several applications and prevent bandwidth take-over by one
client. SFQ however cannot determine interactive flows from bulk ones
-- one usually needs to do the selection with CBQ before, and then
direct the bulk traffic into SFQ.
9.3. Token Bucket Filter
The Token Bucket Filter (TBF) is a simple queue, that only passes
packets arriving at rate in bounds of some administratively set limit,
with possibility to buffer short bursts.
The TBF implementation consists of a buffer (bucket), constatly filled
by some virtual pieces of information called tokens, at specific rate
(token rate). The most important parameter of the bucket is its size,
that is number of tokens it can store.
Each arriving token lets one incoming data packet of out the queue and
is then deleted from the bucket. Associating this algorithm with the
two flows -- token and data, gives us three possible scenarios:
� The data arrives into TBF at rate equal the rate of incoming
tokens. In this case each incoming packet has its matching token
and passes the queue without delay.
� The data arrives into TBF at rate smaller than the token rate.
Only some tokens are deleted at output of each data packet sent out
the queue, so the tokens accumulate, up to the bucket size. The
saved tokens can be then used to send data over the token rate, if
short data burst occurs.
� The data arrives into TBF at rate bigger than the token rate. In
this case filter overrun occurs -- incoming data can be only sent
out without loss until all accumulated tokens are used. After that,
overlimit packets are dropped.
The last scenario is very important, because it allows to
administratively shape the bandwidth available to data, passing the
filter. The accumulation of tokens allows short burst of overlimit
data to be still passed without loss, but any lasting overload will
cause packets to be constantly dropped.
The Linux kernel seems to go beyond this specification, and also
allows us to limit the speed of the burst transmission. However,
Alexey warns us:
Note that the peak rate TBF is much more tough: with MTU 1500 P_crit =
150Kbytes/sec. So, if you need greater peak rates, use alpha with
HZ=1000 :-)
FIXME: is this still true with TSC (pentium+)? Well sort of
FIXME: if not, add section on raising HZ
9.4. Random Early Detect
RED has some extra smartness built in. When a TCP/IP session starts,
neither end knows the amount of bandwidth available. So TCP/IP starts
to transmit slowly and goes faster and faster, though limited by the
latency at which ACKs return.
Once a link is filling up, RED starts dropping packets, which indicate
to TCP/IP that the link is congested, and that it should slow down.
The smart bit is that RED simulates real congestion, and starts to
drop some packets some time before the link is entirely filled up.
Once the link is completely saturated, it behaves like a normal
policer.
For more information on this, see the Backbone chapter.
9.5. Ingress policer qdisc
The Ingress qdisc comes in handy if you need to ratelimit a host
without help from routers or other Linux boxes. You can police
incoming bandwidth and drop packets when this bandwidth exceeds your
desired rate. This can save your host from a SYN flood, for example,
and also works to slow down TCP/IP, which responds to dropped packets
by reducing speed.
FIXME: instead of dropping, can we also assign it to a real queue?
FIXME: shaping by dropping packets seems less desirable than using,
for example, a token bucket filter. Not sure though, Cisco CAR works
this way, and people appear happy with it.
See the reference to ``IOS Committed Access Rate'' at the end of this
document.
In short: you can use this to limit how fast your computer downloads
files, thus leaving more of the available bandwidth for others.
See the section on protecting your host from SYN floods for an example
on how this works.
10. Netfilter & iproute - marking packets
So 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 <
http://netfilter.kernelnotes.org/unreliable-
guides/>. Netfilter itself can be found here
<
http://antarctica.penguincomputing.com/~netfilter/>.
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:
# iptables -A PREROUTING -i eth0 -t mangle -p tcp --dport 25 \
-j MARK --set-mark 1
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:
# echo 201 mail.out >> /etc/iproute2/rt_tables
# ip rule add fwmark 1 table mail.out
# ip rule ls
0: from all lookup local
32764: from all fwmark 1 lookup mail.out
32766: from all lookup main
32767: from all lookup default
Now we generate the mail.out table with a route to the slow but cheap
link:
# /sbin/ip route add default via 195.96.98.253 dev ppp0 table mail.out
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').
Note: for this to work, you need to have some options enabled in your
kernel:
IP: advanced router (CONFIG_IP_ADVANCED_ROUTER) [Y/n/?]
IP: policy routing (CONFIG_IP_MULTIPLE_TABLES) [Y/n/?]
IP: use netfilter MARK value as routing key (CONFIG_IP_ROUTE_FWMARK) [Y/n/?]
11. More classifiers
Classifiers are the way by which the kernel decides which queue a
packet should be placed into. There are various different classifiers,
each of which can be used for different purposes.
fw Bases the decision on how the firewall has marked the packet.
u32
Bases the decision on fields within the packet (i.e. source IP
address, etc)
route
Bases the decision on which route the packet will be routed by.
rsvp, rsvp6
Bases the decision on the target (destination,protocol) and
optionally the source as well. (I think)
tcindex
FIXME: Fill me in
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:
protocol
The protocol this classifier will accept. Generally you will
only be accepting only IP traffic. Required.
parent
The handle this classifier is to be attached to. This handle
must be an already existing class. Required.
prio
The priority of this classifier. Higher numbers get tested
first.
handle
This handle means different things to different filters.
FIXME: Add more
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.
11.1. The "fw" classifier
The "fw" classifier relies on the firewall tagging the packets to be
shaped. So, first we will setup the firewall to tag them:
# iptables -I PREROUTING -t mangle -p tcp -d HostA \
-j MARK --set-mark 1
Now all packets to that machine are tagged with the mark 1. Now we
build the packet shaping rules to actually shape the packets. Now we
just need to indicate that we want the packets that are tagged with
the mark 1 to go to class 1:1. This is accomplished with the command:
# tc filter add dev eth1 protocol ip parent 1:0 prio 1 handle 1 fw classid 1:1
This should be fairly self-explanatory. Attach to the 1:0 class a
filter with priority 1 to filter all packet marked with 1 in the
firewall to class 1:1. Note how the handle here is used to indicate
what the mark should be.
That's all there is to it! This is the (IMHO) easy way, the other ways
are I think harder to understand. Note that you can apply the full
power of the firewalling code with this classifier, including matching
MAC addresses, user IDs and anything else the firewall can match.
11.2. The "u32" classifier
The 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 and the associated action is performed. The
simplest type of action would be directing the packet into defined CBQ
class.
The commandline 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:
tc filter add dev IF [ protocol PROTO ]
[ (preference|priority) PRIO ]
[ parent CBQ ]
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 decribed apply to all filters, not only U32.
11.2.1. U32 selector
The 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
examplesm taken directly from a pretty complex, real-world filter:
# filter parent 1: protocol ip pref 10 u32 fh 800::800 order 2048 key ht 800 bkt 0 flowid 1:3 \
match 00100000/00ff0000 at 0
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:
# filter parent 1: protocol ip pref 10 u32 fh 800::803 order 2051 key ht 800 bkt 0 flowid 1:3 \
match 00000016/0000ffff at nexthdr+0
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 ambigous 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 learnt.
11.2.2. General selectors
General 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:
match [ u32 | u16 | u8 ] PATTERN MASK [ at OFFSET | nexthdr+OFFSET]
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:
# tc filter add dev ppp14 parent 1:0 prio 10 u32 \
match u8 64 0xff at 8 \
flowid 1:4
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.
# tc filter add dev ppp14 parent 1:0 prio 10 u32 \
match u8 0x10 0xff at nexthdr+13 \
protocol tcp \
flowid 1:3 \
This rule will only match TCP packets with ACK bit set. 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 if 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.
11.2.3. Specific selectors
The 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:
# tc filter add dev ppp0 parent 1:0 prio 10 u32 \
match ip tos 0x10 0xff \
flowid 1:4
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 coul 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:
# tc filter add dev ppp0 parent 1:0 prio 10 u32 \
match tcp dst 53 0xffff \
match ip protocol 0x6 0xff \
flowid 1:2
11.3. The "route" classifier
This 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.
# tc filter add dev eth1 parent 1:0 protocol ip prio 100 route
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:
# ip route add Host/Network via Gateway dev Device realm RealmNumber
For instance, we can define our destination network 192.168.10.0 with
a realm number 10:
# ip route add 192.168.10.0/24 via 192.168.10.1 dev eth1 realm 10
When adding route filters, we can use realm numbers to represent the
networks or hosts and specify how the routes match the filters.
# tc filter add dev eth1 parent 1:0 protocol ip prio 100 \
route to 10 classid 1:10
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.
# ip route add 192.168.2.0/24 dev eth2 realm 2
# tc filter add dev eth1 parent 1:0 protocol ip prio 100 \
route from 2 classid 1:2
Here the filter specifies that packets from the subnetwork 192.168.2.0
(realm 2) will match class id 1:2.
11.4. The "rsvp" classifier
FIXME: Fill me in
11.5. The "tcindex" classifier
FIXME: Fill me in
12. Kernel network parameters
The 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.
12.1. Reverse Path Filtering
By 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.
# for i in /proc/sys/net/ipv4/conf/*/rp_filter ; do
> echo 2 > $i
> done
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.
# echo 1 >/proc/sys/net/ipv4/conf/<interfacename>/log_martians
FIXME: is setting the conf/{default,all}/* files enough? - martijn
12.2. Obscure settings
Ok, 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 wether you
answered 'Yes' to 'Configure as router and not host' while compiling
your kernel.
12.2.1. Generic ipv4
As a generic note, most rate limiting features don't work on loopback,
so don't test them locally.
/proc/sys/net/ipv4/icmp_destunreach_rate
FIXME: fill this in
/proc/sys/net/ipv4/icmp_echo_ignore_all
FIXME: fill this in
/proc/sys/net/ipv4/icmp_echo_ignore_broadcasts [Useful]
If you ping the broadcast address of a network, all hosts are
supposed to respond. This makes for a dandy denial-of-service
tool. Set this to 1 to ignore these broadcast messages.
/proc/sys/net/ipv4/icmp_echoreply_rate
FIXME: fill this in
/proc/sys/net/ipv4/icmp_ignore_bogus_error_responses
FIXME: fill this in
/proc/sys/net/ipv4/icmp_paramprob_rate
FIXME: fill this in
/proc/sys/net/ipv4/icmp_timeexceed_rate
This the famous cause of the 'Solaris middle star' in
traceroutes. Limits number of ICMP Time Exceeded messages sent.
FIXME: Units of these rates - either I'm stupid, or this just
doesn't work
/proc/sys/net/ipv4/igmp_max_memberships
FIXME: fill this in
/proc/sys/net/ipv4/inet_peer_gc_maxtime
FIXME: fill this in
/proc/sys/net/ipv4/inet_peer_gc_mintime
FIXME: fill this in
/proc/sys/net/ipv4/inet_peer_maxttl
FIXME: fill this in
/proc/sys/net/ipv4/inet_peer_minttl
FIXME: fill this in
/proc/sys/net/ipv4/inet_peer_threshold
FIXME: fill this in
/proc/sys/net/ipv4/ip_autoconfig
FIXME: fill this in
/proc/sys/net/ipv4/ip_default_ttl
Time To Live of packets. Set to a safe 64. Raise it if you have
a huge network. Don't do so for fun - routing loops cause much
more damage that way. You might even consider lowering it in
some circumstances.
/proc/sys/net/ipv4/ip_dynaddr
You need to set this if you use dial-on-demand with a dynamic
interface address. Once your demand interface comes up, any
queued packets will be rebranded to have the right address. This
solves the problem that the connection that brings up your
interface itself does not work, but the second try does.
/proc/sys/net/ipv4/ip_forward
If the kernel should attempt to forward packets. Off by default
for hosts, on by default when configured as a router.
/proc/sys/net/ipv4/ip_local_port_range
Range of local ports for outgoing connections. Actually quite
small by default, 1024 to 4999.
/proc/sys/net/ipv4/ip_no_pmtu_disc
Set this if you want to disable Path MTU discovery - a technique
to determince the largest Maximum Transfer Unit possible on you
path.
/proc/sys/net/ipv4/ipfrag_high_thresh
FIXME: fill this in
/proc/sys/net/ipv4/ipfrag_low_thresh
FIXME: fill this in
/proc/sys/net/ipv4/ipfrag_time
FIXME: fill this in
/proc/sys/net/ipv4/tcp_abort_on_overflow
FIXME: fill this in
/proc/sys/net/ipv4/tcp_fin_timeout
FIXME: fill this in
/proc/sys/net/ipv4/tcp_keepalive_intvl
FIXME: fill this in
/proc/sys/net/ipv4/tcp_keepalive_probes
FIXME: fill this in
/proc/sys/net/ipv4/tcp_keepalive_time
FIXME: fill this in
/proc/sys/net/ipv4/tcp_max_orphans
FIXME: fill this in
/proc/sys/net/ipv4/tcp_max_syn_backlog
FIXME: fill this in
/proc/sys/net/ipv4/tcp_max_tw_buckets
FIXME: fill this in
/proc/sys/net/ipv4/tcp_orphan_retries
FIXME: fill this in
/proc/sys/net/ipv4/tcp_retrans_collapse
FIXME: fill this in
/proc/sys/net/ipv4/tcp_retries1
FIXME: fill this in
/proc/sys/net/ipv4/tcp_retries2
FIXME: fill this in
/proc/sys/net/ipv4/tcp_rfc1337
FIXME: fill this in
/proc/sys/net/ipv4/tcp_sack
Use Selective ACK which can be used to signify that only a
single packet is missing - therefore helping fast recovery.
/proc/sys/net/ipv4/tcp_stdurg
FIXME: fill this in
/proc/sys/net/ipv4/tcp_syn_retries
FIXME: fill this in
/proc/sys/net/ipv4/tcp_synack_retries
FIXME: fill this in
/proc/sys/net/ipv4/tcp_timestamps
FIXME: fill this in
/proc/sys/net/ipv4/tcp_tw_recycle
FIXME: fill this in
/proc/sys/net/ipv4/tcp_window_scaling
TCP/IP normally allows windows up to 65535 bytes big. For really
fast networks, this may not be enough. The window scaling
options allows for almost gigabyte windows, which is good for
high bandwidth*delay products.
12.2.2. Per device settings
DEV can either stand for a real interface, or for 'all' or 'default'.
Default also changes settings for interfaces yet to be created.
/proc/sys/net/ipv4/conf/DEV/accept_redirects
If a router decides that you are using it for a wrong purpose
(ie, it needs to resend your packet on the same interface), it
will send us a ICMP Redirect. This is a slight security risk
however, so you may want to turn it off, or use secure
redirects.
/proc/sys/net/ipv4/conf/DEV/accept_source_route
Not used very much anymore. You used to be able to give a packet
a list of IP addresses it should visit on its way. Linux can be
made to honor this IP option.
/proc/sys/net/ipv4/conf/DEV/bootp_relay
FIXME: fill this in
/proc/sys/net/ipv4/conf/DEV/forwarding
FIXME:
/proc/sys/net/ipv4/conf/DEV/log_martians
See the section on reverse path filters.
/proc/sys/net/ipv4/conf/DEV/mc_forwarding
If we do multicast forwarding on this interface
/proc/sys/net/ipv4/conf/DEV/proxy_arp
FIXME: fill this in
/proc/sys/net/ipv4/conf/DEV/rp_filter
See the section on reverse path filters.
/proc/sys/net/ipv4/conf/DEV/secure_redirects
FIXME: fill this in
/proc/sys/net/ipv4/conf/DEV/send_redirects
If we send the above mentioned redirects.
/proc/sys/net/ipv4/conf/DEV/shared_media
FIXME: fill this in
/proc/sys/net/ipv4/conf/DEV/tag
FIXME: fill this in
12.2.3. Neighbor pollicy
Dev can either stand for a real interface, or for 'all' or 'default'.
Default also changes settings for interfaces yet to be created.
/proc/sys/net/ipv4/neigh/DEV/anycast_delay
FIXME: fill this in
/proc/sys/net/ipv4/neigh/DEV/app_solicit
FIXME: fill this in
/proc/sys/net/ipv4/neigh/DEV/base_reachable_time
FIXME: fill this in
/proc/sys/net/ipv4/neigh/DEV/delay_first_probe_time
FIXME: fill this in
/proc/sys/net/ipv4/neigh/DEV/gc_stale_time
FIXME: fill this in
/proc/sys/net/ipv4/neigh/DEV/locktime
FIXME: fill this in
/proc/sys/net/ipv4/neigh/DEV/mcast_solicit
FIXME: fill this in
/proc/sys/net/ipv4/neigh/DEV/proxy_delay
FIXME: fill this in
/proc/sys/net/ipv4/neigh/DEV/proxy_qlen
FIXME: fill this in
/proc/sys/net/ipv4/neigh/DEV/retrans_time
FIXME: fill this in
/proc/sys/net/ipv4/neigh/DEV/ucast_solicit
FIXME: fill this in
/proc/sys/net/ipv4/neigh/DEV/unres_qlen
FIXME: fill this in
12.2.4. Routing settings
/proc/sys/net/ipv4/route/error_burst
FIXME: fill this in
/proc/sys/net/ipv4/route/error_cost
FIXME: fill this in
/proc/sys/net/ipv4/route/flush
FIXME: fill this in
/proc/sys/net/ipv4/route/gc_elasticity
FIXME: fill this in
/proc/sys/net/ipv4/route/gc_interval
FIXME: fill this in
/proc/sys/net/ipv4/route/gc_min_interval
FIXME: fill this in
/proc/sys/net/ipv4/route/gc_thresh
FIXME: fill this in
/proc/sys/net/ipv4/route/gc_timeout
FIXME: fill this in
/proc/sys/net/ipv4/route/max_delay
FIXME: fill this in
/proc/sys/net/ipv4/route/max_size
FIXME: fill this in
/proc/sys/net/ipv4/route/min_adv_mss
FIXME: fill this in
/proc/sys/net/ipv4/route/min_delay
FIXME: fill this in
/proc/sys/net/ipv4/route/min_pmtu
FIXME: fill this in
/proc/sys/net/ipv4/route/mtu_expires
FIXME: fill this in
/proc/sys/net/ipv4/route/redirect_load
FIXME: fill this in
/proc/sys/net/ipv4/route/redirect_number
FIXME: fill this in
/proc/sys/net/ipv4/route/redirect_silence
FIXME: fill this in
13. Backbone applications of traffic control
This chapter is meant as an introduction to backbone routing, which
often involves >100 megabit bandwidths, which requires a different
approach then your ADSL modem at home.
13.1. Router queues
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 synchronisation. When retransmit synchronisation
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 bursty 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.
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 synchronisation. 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 then 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 synchronisation. 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 large then min/avpkt. Experimentally, I've found
(min+min+max)/(3*avpkt) to work okay.
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 okay on high speed Internet links
with a 1500byte MTU.
Read the paper on RED queueing
<
http://www.aciri.org/floyd/papers/red/red.html> by Sally Floyd and
Van Jacobson for technical information.
FIXME: more needed. This means *you* greg :-) - ahu
14. Shaping Cookbook
This 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.
14.1. Running multiple sites with different SLAs
You 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, cusomer B has paid for 5
megabits. We separate our customers by creating virtual IP addresses
on our server.
# ip address add 188.177.166.1 dev eth0
# ip address add 188.177.166.2 dev eth0
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:
# tc qdisc add dev eth0 root handle 1: bandwidth 10Mbit cell 8 avpkt 1000 \
mpu 64
We then create classes for our customers:
# tc class add dev eth0 parent 1:0 classid 1:1 cbq bandwidth 10Mbit rate \
2MBit avpkt 1000 prio 5 bounded isolated allot 1514 weight 1 maxburst 21
# tc class add dev eth0 parent 1:0 classid 1:2 cbq bandwidth 10Mbit rate \
5Mbit avpkt 1000 prio 5 bounded isolated allot 1514 weight 1 maxburst 21
Then we add filters for our two classes:
##FIXME: Why this line, what does it do?, what is a divisor?:
##FIXME: A divisor has something to do with a hash table, and the number of
## buckets - ahu
# tc filter add dev eth0 parent 1:0 protocol ip prio 5 handle 1: u32 divisor 1
# tc filter add dev eth0 parent 1:0 prio 5 u32 match ip src 188.177.166.1
flowid 1:1
# tc filter add dev eth0 parent 1:0 prio 5 u32 match ip src 188.177.166.2
flowid 1:2
And we're done.
FIXME: why no token bucket filter? is there a default pfifo_fast
fallback somewhere?
14.2. Protecting your host from SYN floods
From Alexeys 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.
#! /bin/sh -x
#
# sample script on using the ingress capabilities
# this script shows how one can rate limit incoming SYNs
# Useful for TCP-SYN attack protection. You can use
# IPchains to have more powerful additions to the SYN (eg
# in addition the subnet)
#
#path to various utilities;
#change to reflect yours.
#
TC=/sbin/tc
IP=/sbin/ip
IPTABLES=/sbin/iptables
INDEV=eth2
#
# tag all incoming SYN packets through $INDEV as mark value 1
############################################################
$iptables -A PREROUTING -i $INDEV -t mangle -p tcp --syn \
-j MARK --set-mark 1
############################################################
#
# install the ingress qdisc on the ingress interface
############################################################
$TC qdisc add dev $INDEV handle ffff: ingress
############################################################
#
#
# SYN packets are 40 bytes (320 bits) so three SYNs equals
# 960 bits (approximately 1kbit); so we rate limit below
# the incoming SYNs to 3/sec (not very sueful really; but
#serves to show the point - JHS
############################################################
$TC filter add dev $INDEV parent ffff: protocol ip prio 50 handle 1 fw \
police rate 1kbit burst 40 mtu 9k drop flowid :1
############################################################
#
echo "---- qdisc parameters Ingress ----------"
$TC qdisc ls dev $INDEV
echo "---- Class parameters Ingress ----------"
$TC class ls dev $INDEV
echo "---- filter parameters Ingress ----------"
$TC filter ls dev $INDEV parent ffff:
#deleting the ingress qdisc
#$TC qdisc del $INDEV ingress
14.3. Ratelimit ICMP to prevent dDoS
Recently, distributed denial of service attacks have become a major
nuisance on the internet. By properly filtering and ratelimiting 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:
[The Internet] ---<E3, T3, whatever>--- [Linux router] --- [Office+ISP]
eth1 eth0
We first set up the prerequisite parts:
# tc qdisc add dev eth0 root handle 10: cbq bandwidth 10Mbit avpkt 1000
# tc class add dev eth0 parent 10:0 classid 10:1 cbq bandwidth 10Mbit rate \
10Mbit allot 1514 prio 5 maxburst 20 avpkt 1000
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:
# tc class add dev eth0 parent 10:1 classid 10:100 cbq bandwidth 10Mbit rate \
100Kbit allot 1514 weight 800Kbit prio 5 maxburst 20 avpkt 250 \
bounded
This limits at 100Kbit. Now we need a filter to assign ICMP traffic to
this class:
# tc filter add dev eth0 parent 10:0 protocol ip prio 100 u32 match ip
protocol 1 0xFF flowid 10:100
14.4. Prioritising interactive traffic
If 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:
Especially the "Minimum Delay" is important for me. I switch it on for
"interactive" packets in my upstream (Linux) router. I'm behind a 33k6
modem link. Linux prioritises packets in 3 queues. This way I get
acceptable interactive performance while doing bulk downloads at the
same time.
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:
# iptables -A PREROUTING -t mangle -p tcp --sport telnet \
-j TOS --set-tos Minimize-Delay
# iptables -A PREROUTING -t mangle -p tcp --sport ftp \
-j TOS --set-tos Minimize-Delay
# iptables -A PREROUTING -t mangle -p tcp --sport ftp-data \
-j TOS --set-tos Maximize-Throughput
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 a client that does not do this, you can always do it
with netfilter. On your local box:
# iptables -A OUTPUT -t mangle -p tcp --dport telnet \
-j TOS --set-tos Minimize-Delay
# iptables -A OUTPUT -t mangle -p tcp --dport ftp \
-j TOS --set-tos Minimize-Delay
# iptables -A OUTPUT -t mangle -p tcp --dport ftp-data \
-j TOS --set-tos Maximize-Throughput
15. Advanced Linux Routing
This section is for all you people who either want to understand why
the whole system works or have a configuration that's so bizarre that
you need the low down to make it work.
This section is completely optional. It's quite possible that this
section will be quite complex and really not intended for normal
users. You have been warned.
FIXME: Decide what really need to go in here.
15.1. How does packet queueing really work?
This is the low-down on how the packet queueing system really works.
Lists the steps the kernel takes to classify a packet, etc...
FIXME: Write this.
15.2. Advanced uses of the packet queueing system
Go through Alexeys extremely tricky example involving the unused bits
in the TOS field.
FIXME: Write this.
15.3. Other packet shaping systems
I'd like to include a brief description of other packet shaping
systems in other operating systems and how they compare to the Linux
one. Since Linux is one of the few OSes that has a completely original
(non-BSD derived) TCP/IP stack, I think it would be useful to see how
other people do it.
Unfortunately I have no experiene with other systems so cannot write
this.
FIXME: Anyone? - Martijn
16. Dynamic routing - OSPF and BGP
Once 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 standardised 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
<
http://www.cisco.com/univercd/cc/td/doc/cisintwk/idg4/nd2003.htm>
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
<
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ics/icsbgp4.htm>
Although the examples are Cisco-specific, they are remarkably similar
to the configuration language in Zebra :-)
17. Further reading
http://snafu.freedom.org/linux2.2/iproute-notes.html
<
http://snafu.freedom.org/linux2.2/iproute-notes.html>
Contains lots of technical information, comments from the kernel
http://www.davin.ottawa.on.ca/ols/
<
http://www.davin.ottawa.on.ca/ols/>
Slides by Jamal Hadi, one of the authors of Linux traffic
control
http://defiant.coinet.com/iproute2/ip-cref/ <
http://defi�
ant.coinet.com/iproute2/ip-cref/>
HTML version of Alexeys LaTeX documentation - explains part of
iproute2 in great detail
http://www.aciri.org/floyd/cbq.html
<
http://www.aciri.org/floyd/cbq.html>
Sally Floyd has a good page on CBQ, including her original
papers. None of it is Linux specific, but it does a fair job
discussing the theory and uses of CBQ. Very technical stuff,
but good reading for those so inclined.
http://ceti.pl/%7ekravietz/cbq/NET4_tc.html
<
http://ceti.pl/%7ekravietz/cbq/NET4_tc.html>
Yet another HOWTO, this time in Polish! You can copy/paste
command lines however, they work just the same in every
language. The author is cooperating with us and may soon author
sections of this HOWTO.
Differentiated Services on Linux <
http://snafu.free�
dom.org/linux2.2/docs/draft-almesberger-wajhak-diffserv-
linux-00.txt>
Discussion on how to use Linux in a diffserv compliant
environment. Pretty far removed from your everyday routing
needs, but very interesting none the less. We may include a
section on this at a later date.
IOS Committed Access Rate <
http://www.cisco.com/uni�
vercd/cc/td/doc/product/software/ios111/cc111/car.htm>
>From the helpful folks of Cisco who have the laudable habit of
putting their documentation online. Cisco syntax is different
but the concepts are the same, except that we can do more and do
it without routers the price of cars :-)
TCP/IP Illustrated, volume 1, W. Richard Stevens, ISBN
0-201-63346-9
Required reading if you truly want to understand TCP/IP.
Entertaining as well.
18. Acknowledgements
It is our goal to list everybody who has contributed to this HOWTO, or
helped us demistify how things work. While there are currently no
plans for a Netfilter type scoreboard, we do like to recognise the
people who are helping.
� Jamal Hadi <hadi%cyberus.ca>
� Nadeem Hasan <
[email protected]>
� Jason Lunz <
[email protected]>
� Alexey Mahotkin <
[email protected]>
� Pawel Krawczyk <kravietz%alfa.ceti.pl>
� Wim van der Most
� Glen Turner <glen.turner%aarnet.edu.au>
� Song Wang <
[email protected]>
