Linux 2.4 Packet Filtering HOWTO
Rusty Russell, mailing list netfilter@lists.samba.org
$Revision: 1.21 $ $Date: 2001/08/15 11:03:17 $

This document describes how to use iptables to filter out bad packets
for the 2.4 Linux kernels.
______________________________________________________________________

Table of Contents

1. Introduction

2. Where is the official Web Site? Is there a Mailing List?

3. So What's A Packet Filter?

3.1 Why Would I Want to Packet Filter?
3.2 How Do I Packet Filter Under Linux?
3.2.1 iptables
3.2.2 Making Rules Permanent

4. Who the hell are you, and why are you playing with my kernel?

5. Rusty's Really Quick Guide To Packet Filtering

6. How Packets Traverse The Filters

7. Using iptables

7.1 What You'll See When Your Computer Starts Up
7.2 Operations on a Single Rule
7.3 Filtering Specifications
7.3.1 Specifying Source and Destination IP Addresses
7.3.2 Specifying Inversion
7.3.3 Specifying Protocol
7.3.4 Specifying an Interface
7.3.5 Specifying Fragments
7.3.6 Extensions to iptables: New Matches
7.3.6.1 TCP Extensions
7.3.6.1.1 An Explanation of TCP Flags
7.3.6.2 UDP Extensions
7.3.6.3 ICMP Extensions
7.3.6.4 Other Match Extensions
7.3.6.5 The State Match
7.4 Target Specifications
7.4.1 User-defined chains
7.4.2 Extensions to iptables: New Targets
7.4.3 Special Built-In Targets
7.5 Operations on an Entire Chain
7.5.1 Creating a New Chain
7.5.2 Deleting a Chain
7.5.3 Flushing a Chain
7.5.4 Listing a Chain
7.5.5 Resetting (Zeroing) Counters
7.5.6 Setting Policy

8. Using ipchains and ipfwadm

9. Mixing NAT and Packet Filtering

10. Differences Between iptables and ipchains

11. Advice on Packet Filter Design

______________________________________________________________________

1. Introduction

Welcome, gentle reader.

It is assumed you know what an IP address, a network address, a
netmask, routing and DNS are. If not, I recommend that you read the
Network Concepts HOWTO.

This HOWTO flips between a gentle introduction (which will leave you
feeling warm and fuzzy now, but unprotected in the Real World) and raw
full-disclosure (which would leave all but the hardiest souls
confused, paranoid and seeking heavy weaponry).

Your network is not secure. The problem of allowing rapid, convenient
communication while restricting its use to good, and not evil intents
is congruent to other intractable problems such as allowing free
speech while disallowing a call of ``Fire!'' in a crowded theater. It
will not be solved in the space of this HOWTO.

So only you can decide where the compromise will be. I will try to
instruct you in the use of some of the tools available and some
vulnerabilities to be aware of, in the hope that you will use them for
good, and not evil purposes. Another equivalent problem.

2. Where is the official Web Site? Is there a Mailing List?

There are three official sites:

o Thanks to Filewatcher <http://netfilter.filewatcher.org/>.

o Thanks to The Samba Team and SGI <http://netfilter.samba.org/>.

o Thanks to Harald Welte <http://netfilter.gnumonks.org/>.

For the official netfilter mailing list, see Samba's Listserver
<http://lists.samba.org/>.

3. So What's A Packet Filter?

A packet filter is a piece of software which looks at the header of
packets as they pass through, and decides the fate of the entire
packet. It might decide to DROP the packet (i.e., discard the packet
as if it had never received it), ACCEPT the packet (i.e., let the
packet go through), or something more complicated.

Under Linux, packet filtering is built into the kernel (as a kernel
module, or built right in), and there are a few trickier things we can
do with packets, but the general principle of looking at the headers
and deciding the fate of the packet is still there.

3.1. Why Would I Want to Packet Filter?

Control. Security. Watchfulness.

Control:
when you are using a Linux box to connect your internal network
to another network (say, the Internet) you have an opportunity
to allow certain types of traffic, and disallow others. For
example, the header of a packet contains the destination address
of the packet, so you can prevent packets going to a certain
part of the outside network. As another example, I use Netscape
to access the Dilbert archives. There are advertisements from
doubleclick.net on the page, and Netscape wastes my time by
cheerfully downloading them. Telling the packet filter not to
allow any packets to or from the addresses owned by
doubleclick.net solves that problem (there are better ways of
doing this though: see Junkbuster).

Security:
when your Linux box is the only thing between the chaos of the
Internet and your nice, orderly network, it's nice to know you
can restrict what comes tromping in your door. For example, you
might allow anything to go out from your network, but you might
be worried about the well-known `Ping of Death' coming in from
malicious outsiders. As another example, you might not want
outsiders telnetting to your Linux box, even though all your
accounts have passwords. Maybe you want (like most people) to
be an observer on the Internet, and not a server (willing or
otherwise). Simply don't let anyone connect in, by having the
packet filter reject incoming packets used to set up
connections.

Watchfulness:
sometimes a badly configured machine on the local network will
decide to spew packets to the outside world. It's nice to tell
the packet filter to let you know if anything abnormal occurs;
maybe you can do something about it, or maybe you're just
curious by nature.

3.2. How Do I Packet Filter Under Linux?

Linux kernels have had packet filtering since the 1.1 series. The
first generation, based on ipfw from BSD, was ported by Alan Cox in
late 1994. This was enhanced by Jos Vos and others for Linux 2.0; the
userspace tool `ipfwadm' controlled the kernel filtering rules. In
mid-1998, for Linux 2.2, I reworked the kernel quite heavily, with the
help of Michael Neuling, and introduced the userspace tool `ipchains'.
Finally, the fourth-generation tool, `iptables', and another kernel
rewrite occurred in mid-1999 for Linux 2.4. It is this iptables which
this HOWTO concentrates on.

You need a kernel which has the netfilter infrastructure in it:
netfilter is a general framework inside the Linux kernel which other
things (such as the iptables module) can plug into. This means you
need kernel 2.3.15 or beyond, and answer `Y' to CONFIG_NETFILTER in
the kernel configuration.

The tool iptables talks to the kernel and tells it what packets to
filter. Unless you are a programmer, or overly curious, this is how
you will control the packet filtering.

3.2.1. iptables

The iptables tool inserts and deletes rules from the kernel's packet
filtering table. This means that whatever you set up, it will be lost
upon reboot; see ``Making Rules Permanent'' for how to make sure they
are restored the next time Linux is booted.

iptables is a replacement for ipfwadm and ipchains: see ``Using
ipchains and ipfwadm'' for how to painlessly avoid using iptables if
you're using one of those tools.

3.2.2. Making Rules Permanent

Your current firewall setup is stored in the kernel, and thus will be
lost on reboot. You can try the iptables-save and iptables-restore
scripts to save them to, and restore them from a file.

The other way is to put the commands required to set up your rules in
an initialization script. Make sure you do something intelligent if
one of the commands should fail (usually `exec /sbin/sulogin').

4. Who the hell are you, and why are you playing with my kernel?

I'm Rusty Russell; the Linux IP Firewall maintainer and just another
working coder who happened to be in the right place at the right time.
I wrote ipchains (see ``How Do I Packet Filter Under Linux?'' above
for due credit to the people who did the actual work), and learnt
enough to get packet filtering right this time. I hope.

WatchGuard <http://www.watchguard.com>, an excellent firewall company
who sell the really nice plug-in Firebox, offered to pay me to do
nothing, so I could spend all my time writing this stuff, and
maintaining my previous stuff. I predicted 6 months, and it took 12,
but I felt by the end that it had been done Right. Many rewrites, a
hard-drive crash, a laptop being stolen, a couple of corrupted
filesystems and one broken screen later, here it is.

While I'm here, I want to clear up some people's misconceptions: I am
no kernel guru. I know this, because my kernel work has brought me
into contact with some of them: David S. Miller, Alexey Kuznetsov,
Andi Kleen, Alan Cox. However, they're all busy doing the deep magic,
leaving me to wade in the shallow end where it's safe.

5. Rusty's Really Quick Guide To Packet Filtering

Most people just have a single PPP connection to the Internet, and
don't want anyone coming back into their network, or the firewall:

## Insert connection-tracking modules (not needed if built into kernel).
# insmod ip_conntrack
# insmod ip_conntrack_ftp

## Create chain which blocks new connections, except if coming from inside.
# iptables -N block
# iptables -A block -m state --state ESTABLISHED,RELATED -j ACCEPT
# iptables -A block -m state --state NEW -i ! ppp0 -j ACCEPT
# iptables -A block -j DROP

## Jump to that chain from INPUT and FORWARD chains.
# iptables -A INPUT -j block
# iptables -A FORWARD -j block

6. How Packets Traverse The Filters

The kernel starts with three lists of rules in the `filter' table;
these lists are called firewall chains or just chains. The three
chains are called INPUT, OUTPUT and FORWARD.

For ASCII-art fans, the chains are arranged like so: (Note: this is a
very different arrangement from the 2.0 and 2.2 kernels!)

The three circles represent the three chains mentioned above. When a
packet reaches a circle in the diagram, that chain is examined to
decide the fate of the packet. If the chain says to DROP the packet,
it is killed there, but if the chain says to ACCEPT the packet, it
continues traversing the diagram.

A chain is a checklist of rules. Each rule says `if the packet header
looks like this, then here's what to do with the packet'. If the rule
doesn't match the packet, then the next rule in the chain is
consulted. Finally, if there are no more rules to consult, then the
kernel looks at the chain policy to decide what to do. In a security-
conscious system, this policy usually tells the kernel to DROP the
packet.

1. When a packet comes in (say, through the Ethernet card) the kernel
first looks at the destination of the packet: this is called
`routing'.

2. If it's destined for this box, the packet passes downwards in the
diagram, to the INPUT chain. If it passes this, any processes
waiting for that packet will receive it.

3. Otherwise, if the kernel does not have forwarding enabled, or it
doesn't know how to forward the packet, the packet is dropped. If
forwarding is enabled, and the packet is destined for another
network interface (if you have another one), then the packet goes
rightwards on our diagram to the FORWARD chain. If it is ACCEPTed,
it will be sent out.

4. Finally, a program running on the box can send network packets.
These packets pass through the OUTPUT chain immediately: if it says
ACCEPT, then the packet continues out to whatever interface it is
destined for.

7. Using iptables

iptables has a fairly detailed manual page (man iptables), and if you
need more detail on particulars. Those of you familiar with ipchains
may simply want to look at ``Differences Between iptables and
ipchains''; they are very similar.

There are several different things you can do with iptables. You
start with three built-in chains INPUT, OUTPUT and FORWARD which you
can't delete. Let's look at the operations to manage whole chains:

1. Create a new chain (-N).

2. Delete an empty chain (-X).

3. Change the policy for a built-in chain. (-P).

4. List the rules in a chain (-L).

5. Flush the rules out of a chain (-F).

6. Zero the packet and byte counters on all rules in a chain (-Z).

There are several ways to manipulate rules inside a chain:

1. Append a new rule to a chain (-A).

2. Insert a new rule at some position in a chain (-I).

3. Replace a rule at some position in a chain (-R).

4. Delete a rule at some position in a chain (-D).

5. Delete the first rule that matches in a chain (-D).

7.1. What You'll See When Your Computer Starts Up

iptables may be a module, called (`iptable_filter.o'), which should be
automatically loaded when you first run iptables. It can also be
built into the kernel permenantly.

Before any iptables commands have been run (be careful: some
distributions will run iptables in their initialization scripts),
there will be no rules in any of the built-in chains (`INPUT',
`FORWARD' and `OUTPUT'), all the chains will have a policy of ACCEPT.
You can alter the default policy of the FORWARD chain by providing the
`forward=0' option to the iptable_filter module.

7.2. Operations on a Single Rule

This is the bread-and-butter of packet filtering; manipulating rules.
Most commonly, you will probably use the append (-A) and delete (-D)
commands. The others (-I for insert and -R for replace) are simple
extensions of these concepts.

Each rule specifies a set of conditions the packet must meet, and what
to do if it meets them (a `target'). For example, you might want to
drop all ICMP packets coming from the IP address 127.0.0.1. So in
this case our conditions are that the protocol must be ICMP and that
the source address must be 127.0.0.1. Our target is `DROP'.

127.0.0.1 is the `loopback' interface, which you will have even if you
have no real network connection. You can use the `ping' program to
generate such packets (it simply sends an ICMP type 8 (echo request)
which all cooperative hosts should obligingly respond to with an ICMP
type 0 (echo reply) packet). This makes it useful for testing.

# ping -c 1 127.0.0.1
PING 127.0.0.1 (127.0.0.1): 56 data bytes
64 bytes from 127.0.0.1: icmp_seq=0 ttl=64 time=0.2 ms

--- 127.0.0.1 ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max = 0.2/0.2/0.2 ms
# iptables -A INPUT -s 127.0.0.1 -p icmp -j DROP
# ping -c 1 127.0.0.1
PING 127.0.0.1 (127.0.0.1): 56 data bytes

--- 127.0.0.1 ping statistics ---
1 packets transmitted, 0 packets received, 100% packet loss
#

You can see here that the first ping succeeds (the `-c 1' tells ping
to only send a single packet).

Then we append (-A) to the `INPUT' chain, a rule specifying that for
packets from 127.0.0.1 (`-s 127.0.0.1') with protocol ICMP (`-p icmp')
we should jump to DROP (`-j DROP').

Then we test our rule, using the second ping. There will be a pause
before the program gives up waiting for a response that will never
come.

We can delete the rule in one of two ways. Firstly, since we know
that it is the only rule in the input chain, we can use a numbered
delete, as in:

# iptables -D INPUT 1
#

To delete rule number 1 in the INPUT chain.

The second way is to mirror the -A command, but replacing the -A with
-D. This is useful when you have a complex chain of rules and you
don't want to have to count them to figure out that it's rule 37 that
you want to get rid of. In this case, we would use:

# iptables -D INPUT -s 127.0.0.1 -p icmp -j DROP
#

The syntax of -D must have exactly the same options as the -A (or -I
or -R) command. If there are multiple identical rules in the same
chain, only the first will be deleted.

7.3. Filtering Specifications

We have seen the use of `-p' to specify protocol, and `-s' to specify
source address, but there are other options we can use to specify
packet characteristics. What follows is an exhaustive compendium.

7.3.1. Specifying Source and Destination IP Addresses

Source (`-s', `--source' or `--src') and destination (`-d',
`--destination' or `--dst') IP addresses can be specified in four
ways. The most common way is to use the full name, such as
`localhost' or `www.linuxhq.com'. The second way is to specify the IP
address such as `127.0.0.1'.

The third and fourth ways allow specification of a group of IP
addresses, such as `199.95.207.0/24' or `199.95.207.0/255.255.255.0'.
These both specify any IP address from 199.95.207.0 to 199.95.207.255
inclusive; the digits after the `/' tell which parts of the IP address
are significant. `/32' or `/255.255.255.255' is the default (match
all of the IP address). To specify any IP address at all `/0' can be
used, like so:

[ NOTE: `-s 0/0' is redundant here. ]
# iptables -A INPUT -s 0/0 -j DROP
#

This is rarely used, as the effect above is the same as not specifying
the `-s' option at all.

7.3.2. Specifying Inversion

Many flags, including the `-s' (or `--source') and `-d'
(`--destination') flags can have their arguments preceded by `!'
(pronounced `not') to match addresses NOT equal to the ones given.
For example. `-s ! localhost' matches any packet not coming from
localhost.

7.3.3. Specifying Protocol

The protocol can be specified with the `-p' (or `--protocol') flag.
Protocol can be a number (if you know the numeric protocol values for
IP) or a name for the special cases of `TCP', `UDP' or `ICMP'. Case
doesn't matter, so `tcp' works as well as `TCP'.

The protocol name can be prefixed by a `!', to invert it, such as `-p
! TCP' to specify packets which are not TCP.

7.3.4. Specifying an Interface

The `-i' (or `--in-interface') and `-o' (or `--out-interface') options
specify the name of an interface to match. An interface is the
physical device the packet came in on (`-i') or is going out on
(`-o'). You can use the ifconfig command to list the interfaces which
are `up' (i.e., working at the moment).

Packets traversing the INPUT chain don't have an output interface, so
any rule using `-o' in this chain will never match. Similarly,
packets traversing the OUTPUT chain don't have an input interface, so
any rule using `-i' in this chain will never match.

Only packets traversing the FORWARD chain have both an input and
output interface.

It is perfectly legal to specify an interface that currently does not
exist; the rule will not match anything until the interface comes up.
This is extremely useful for dial-up PPP links (usually interface
ppp0) and the like.

As a special case, an interface name ending with a `+' will match all
interfaces (whether they currently exist or not) which begin with that
string. For example, to specify a rule which matches all PPP
interfaces, the -i ppp+ option would be used.

The interface name can be preceded by a `!' with spaces around it, to
match a packet which does not match the specified interface(s), eg -i
! ppp+.

7.3.5. Specifying Fragments

Sometimes a packet is too large to fit down a wire all at once. When
this happens, the packet is divided into fragments, and sent as
multiple packets. The other end reassembles these fragments to
reconstruct the whole packet.

The problem with fragments is that the initial fragment has the
complete header fields (IP + TCP, UDP and ICMP) to examine, but
subsequent packets only have a subset of the headers (IP without the
additional protocol fields). Thus looking inside subsequent fragments
for protocol headers (such as is done by the TCP, UDP and ICMP
extensions) is not possible.

If you are doing connection tracking or NAT, then all fragments will
get merged back together before they reach the packet filtering code,
so you need never worry about fragments.

Otherwise, it is important to understand how fragments get treated by
the filtering rules. Any filtering rule that asks for information we
don't have will not match. This means that the first fragment is
treated like any other packet. Second and further fragments won't be.
Thus a rule -p TCP --sport www (specifying a source port of `www')
will never match a fragment (other than the first fragment). Neither
will the opposite rule -p TCP --sport ! www.

However, you can specify a rule specifically for second and further
fragments, using the `-f' (or `--fragment') flag. It is also legal to
specify that a rule does not apply to second and further fragments, by
preceding the `-f' with ` ! '.

Usually it is regarded as safe to let second and further fragments
through, since filtering will effect the first fragment, and thus
prevent reassembly on the target host; however, bugs have been known
to allow crashing of machines simply by sending fragments. Your call.

Note for network-heads: malformed packets (TCP, UDP and ICMP packets
too short for the firewalling code to read the ports or ICMP code and
type) are dropped when such examinations are attempted. So are TCP
fragments starting at position 8.

As an example, the following rule will drop any fragments going to
192.168.1.1:

# iptables -A OUTPUT -f -d 192.168.1.1 -j DROP
#

7.3.6. Extensions to iptables: New Matches

iptables is extensible, meaning that both the kernel and the iptables
tool can be extended to provide new features.

Some of these extensions are standard, and other are more exotic.
Extensions can be made by other people and distributed separately for
niche users.

Kernel extensions normally live in the kernel module subdirectory,
such as /lib/modules/2.4.0-test10/kernel/net/ipv4/netfilter. They are
demand loaded if your kernel was compiled with CONFIG_KMOD set, so you
should not need to manually insert them.

Extensions to the iptables program are shared libraries which usually
live in /usr/local/lib/iptables/, although a distribution would put
them in /lib/iptables or /usr/lib/iptables.

Extensions come in two types: new targets, and new matches (we'll talk
about new targets a little later). Some protocols automatically offer
new tests: currently these are TCP, UDP and ICMP as shown below.

For these you will be able to specify the new tests on the command
line after the `-p' option, which will load the extension. For
explicit new tests, use the `-m' option to load the extension, after
which the extended options will be available.

To get help on an extension, use the option to load it (`-p', `-j' or
`-m') followed by `-h' or `--help', eg:

# iptables -p tcp --help
#

7.3.6.1. TCP Extensions

The TCP extensions are automatically loaded if `-p tcp' is specified.
It provides the following options (none of which match fragments).

--tcp-flags
Followed by an optional `!', then two strings of flags, allows
you to filter on specific TCP flags. The first string of flags
is the mask: a list of flags you want to examine. The second
string of flags tells which one(s) should be set. For example,

# iptables -A INPUT --protocol tcp --tcp-flags ALL SYN,ACK -j DROP

This indicates that all flags should be examined (`ALL' is
synonymous with `SYN,ACK,FIN,RST,URG,PSH'), but only SYN and ACK
should be set. There is also an argument `NONE' meaning no flags.

--syn
Optionally preceded by a `!', this is shorthand for `--tcp-flags
SYN,RST,ACK SYN'.

--source-port
followed by an optional `!', then either a single TCP port, or a
range of ports. Ports can be port names, as listed in
/etc/services, or numeric. Ranges are either two port names
separated by a `:', or (to specify greater than or equal to a
given port) a port with a `:' appended, or (to specify less than
or equal to a given port), a port preceded by a `:'.

--sport
is synonymous with `--source-port'.
--destination-port
and

--dport
are the same as above, only they specify the destination, rather
than source, port to match.

--tcp-option
followed by an optional `!' and a number, matches a packet with
a TCP option equaling that number. A packet which does not have
a complete TCP header is dropped automatically if an attempt is
made to examine its TCP options.

7.3.6.1.1. An Explanation of TCP Flags

It is sometimes useful to allow TCP connections in one direction, but
not the other. For example, you might want to allow connections to an
external WWW server, but not connections from that server.

The naive approach would be to block TCP packets coming from the
server. Unfortunately, TCP connections require packets going in both
directions to work at all.

The solution is to block only the packets used to request a
connection. These packets are called SYN packets (ok, technically
they're packets with the SYN flag set, and the RST and ACK flags
cleared, but we call them SYN packets for short). By disallowing only
these packets, we can stop attempted connections in their tracks.

The `--syn' flag is used for this: it is only valid for rules which
specify TCP as their protocol. For example, to specify TCP connection
attempts from 192.168.1.1:

-p TCP -s 192.168.1.1 --syn

This flag can be inverted by preceding it with a `!', which means
every packet other than the connection initiation.

7.3.6.2. UDP Extensions

These extensions are automatically loaded if `-p udp' is specified.
It provides the options `--source-port', `--sport', `--destination-
port' and `--dport' as detailed for TCP above.

7.3.6.3. ICMP Extensions

This extension is automatically loaded if `-p icmp' is specified. It
provides only one new option:

--icmp-type
followed by an optional `!', then either an icmp type name (eg
`host-unreachable'), or a numeric type (eg. `3'), or a numeric
type and code separated by a `/' (eg. `3/3'). A list of
available icmp type names is given using `-p icmp --help'.

7.3.6.4. Other Match Extensions

The other extensions in the netfilter package are demonstration
extensions, which (if installed) can be invoked with the `-m' option.

mac
This module must be explicitly specified with `-m mac' or
`--match mac'. It is used for matching incoming packet's source
Ethernet (MAC) address, and thus only useful for packets
traversing the PREROUTING and INPUT chains. It provides only
one option:

--mac-source
followed by an optional `!', then an ethernet address in
colon-separated hexbyte notation, eg `--mac-source
00:60:08:91:CC:B7'.

limit
This module must be explicitly specified with `-m limit' or
`--match limit'. It is used to restrict the rate of matches,
such as for suppressing log messages. It will only match a
given number of times per second (by default 3 matches per hour,
with a burst of 5). It takes two optional arguments:

--limit
followed by a number; specifies the maximum average number of
matches to allow per second. The number can specify units
explicitly, using `/second', `/minute', `/hour' or `/day', or
parts of them (so `5/second' is the same as `5/s').

--limit-burst
followed by a number, indicating the maximum burst before the
above limit kicks in.

This match can often be used with the LOG target to do rate-
limited logging. To understand how it works, let's look at the
following rule, which logs packets with the default limit
parameters:

# iptables -A FORWARD -m limit -j LOG

The first time this rule is reached, the packet will be logged; in
fact, since the default burst is 5, the first five packets will be
logged. After this, it will be twenty minutes before a packet will
be logged from this rule, regardless of how many packets reach it.
Also, every twenty minutes which passes without matching a packet,
one of the burst will be regained; if no packets hit the rule for
100 minutes, the burst will be fully recharged; back where we
started.

Note: you cannot currently create a rule with a recharge time
greater than about 59 hours, so if you set an average rate of one
per day, then your burst rate must be less than 3.

You can also use this module to avoid various denial of service
attacks (DoS) with a faster rate to increase responsiveness.

Syn-flood protection:

# iptables -A FORWARD -p tcp --syn -m limit --limit 1/s -j ACCEPT

Furtive port scanner:

# iptables -A FORWARD -p tcp --tcp-flags SYN,ACK,FIN,RST RST -m limit --limit 1/s -j ACCEPT

Ping of death:

# iptables -A FORWARD -p icmp --icmp-type echo-request -m limit --limit 1/s -j ACCEPT

This module works like a "hysteresis door", as shown in the graph
below.

Say we say match one packet per second with a five packet burst,
but packets start coming in at four per second, for three seconds,
then start again in another three seconds.

<--Flood 1--> <---Flood 2--->

You can see that the first five packets are allowed to exceed the
one packet per second, then the limiting kicks in. If there is a
pause, another burst is allowed but not past the maximum rate set
by the rule (1 packet per second after the burst is used).

owner
This module attempts to match various characteristics of the
packet creator, for locally-generated packets. It is only valid
in the OUTPUT chain, and even then some packets (such as ICMP
ping responses) may have no owner, and hence never match.

--uid-owner userid
Matches if the packet was created by a process with the given
effective (numerical) user id.

--gid-owner groupid
Matches if the packet was created by a process with the given
effective (numerical) group id.

--pid-owner processid
Matches if the packet was created by a process with the given
process id.

--sid-owner sessionid
Matches if the packet was created by a process in the given
session group.

unclean
This experimental module must be explicitly specified with `-m
unclean or `--match unclean'. It does various random sanity
checks on packets. This module has not been audited, and should
not be used as a security device (it probably makes things
worse, since it may well have bugs itself). It provides no
options.

7.3.6.5. The State Match

The most useful match criterion is supplied by the `state' extension,
which interprets the connection-tracking analysis of the
`ip_conntrack' module. This is highly recommended.

Specifying `-m state' allows an additional `--state' option, which is
a comma-separated list of states to match (the `!' flag indicates not
to match those states). These states are:

NEW
A packet which creates a new connection.

ESTABLISHED
A packet which belongs to an existing connection (i.e., a reply
packet, or outgoing packet on a connection which has seen
replies).

RELATED
A packet which is related to, but not part of, an existing
connection, such as an ICMP error, or (with the FTP module
inserted), a packet establishing an ftp data connection.

INVALID
A packet which could not be identified for some reason: this
includes running out of memory and ICMP errors which don't
correspond to any known connection. Generally these packets
should be dropped.

7.4. Target Specifications

Now we know what examinations we can do on a packet, we need a way of
saying what to do to the packets which match our tests. This is
called a rule's target.

There are two very simple built-in targets: DROP and ACCEPT. We've
already met them. If a rule matches a packet and its target is one of
these two, no further rules are consulted: the packet's fate has been
decided.

There are two types of targets other than the built-in ones:
extensions and user-defined chains.

7.4.1. User-defined chains

One powerful feature which iptables inherits from ipchains is the
ability for the user to create new chains, in addition to the three
built-in ones (INPUT, FORWARD and OUTPUT). By convention, user-
defined chains are lower-case to distinguish them (we'll describe how
to create new user-defined chains below in ``Operations on an Entire
Chain'').

When a packet matches a rule whose target is a user-defined chain, the
packet begins traversing the rules in that user-defined chain. If
that chain doesn't decide the fate of the packet, then once traversal
on that chain has finished, traversal resumes on the next rule in the
current chain.

Time for more ASCII art. Consider two (silly) chains: INPUT (the
built-in chain) and test (a user-defined chain).

`INPUT' `test'
---------------------------- ----------------------------
| Rule1: -p ICMP -j DROP | | Rule1: -s 192.168.1.1 |
|--------------------------| |--------------------------|
| Rule2: -p TCP -j test | | Rule2: -d 192.168.1.1 |
|--------------------------| ----------------------------
| Rule3: -p UDP -j DROP |
----------------------------

Consider a TCP packet coming from 192.168.1.1, going to 1.2.3.4. It
enters the INPUT chain, and gets tested against Rule1 - no match.
Rule2 matches, and its target is test, so the next rule examined is
the start of test. Rule1 in test matches, but doesn't specify a
target, so the next rule is examined, Rule2. This doesn't match, so
we have reached the end of the chain. We return to the INPUT chain,
where we had just examined Rule2, so we now examine Rule3, which
doesn't match either.

So the packet path is:

v __________________________
`INPUT' | / `test' v
------------------------|--/ -----------------------|----
| Rule1 | /| | Rule1 | |
|-----------------------|/-| |----------------------|---|
| Rule2 / | | Rule2 | |
|--------------------------| -----------------------v----
| Rule3 /--+___________________________/
------------------------|---
v

User-defined chains can jump to other user-defined chains (but don't
make loops: your packets will be dropped if they're found to be in a
loop).

7.4.2. Extensions to iptables: New Targets

The other type of target is an extension. A target extension consists
of a kernel module, and an optional extension to iptables to provide
new command line options. There are several extensions in the default
netfilter distribution:

LOG
This module provides kernel logging of matching packets. It
provides these additional options:

--log-level
Followed by a level number or name. Valid names are (case-
insensitive) `debug', `info', `notice', `warning', `err',
`crit', `alert' and `emerg', corresponding to numbers 7
through 0. See the man page for syslog.conf for an
explanation of these levels.

--log-prefix
Followed by a string of up to 29 characters, this message is
sent at the start of the log message, to allow it to be
uniquely identified.

This module is most useful after a limit match, so you don't
flood your logs.

REJECT
This module has the same effect as `DROP', except that the
sender is sent an ICMP `port unreachable' error message. Note
that the ICMP error message is not sent if (see RFC 1122):

o The packet being filtered was an ICMP error message in the first
place, or some unknown ICMP type.

o The packet being filtered was a non-head fragment.

o We've sent too many ICMP error messages to that destination
recently.

REJECT also takes a `--reject-with' optional argument which
alters the reply packet used: see the manual page.

7.4.3. Special Built-In Targets

There are two special built-in targets: RETURN and QUEUE.

RETURN has the same effect of falling off the end of a chain: for a
rule in a built-in chain, the policy of the chain is executed. For a
rule in a user-defined chain, the traversal continues at the previous
chain, just after the rule which jumped to this chain.

QUEUE is a special target, which queues the packet for userspace
processing. For this to be useful, two further components are
required:

o a "queue handler", which deals with the actual mechanics of passing
packets between the kernel and userspace; and

o a userspace application to receive, possibly manipulate, and issue
verdicts on packets.

The standard queue handler for IPv4 iptables is the ip_queue
module, which is distributed with the kernel and marked as
experimental.

The following is a quick example of how to use iptables to queue
packets for userspace processing:

# modprobe iptable_filter
# modprobe ip_queue
# iptables -A OUTPUT -p icmp -j QUEUE

With this rule, locally generated outgoing ICMP packets (as created
with, say, ping) are passed to the ip_queue module, which then
attempts to deliver the packets to a userspace application. If no
userspace application is waiting, the packets are dropped.

To write a userspace application, use the libipq API. This is
distributed with iptables. Example code may be found in the testsuite
tools (e.g. redirect.c) in CVS.

The status of ip_queue may be checked via:

/proc/net/ip_queue

The maximum length of the queue (i.e. the number packets delivered to
userspace with no verdict issued back) may be controlled via:

/proc/sys/net/ipv4/ip_queue_maxlen

The default value for the maximum queue length is 1024. Once this
limit is reached, new packets will be dropped until the length of the
queue falls below the limit again. Nice protocols such as TCP inter-
pret dropped packets as congestion, and will hopefully back off when
the queue fills up. However, it may take some experimenting to deter-
mine an ideal maximum queue length for a given situation if the
default value is too small.

7.5. Operations on an Entire Chain

A very useful feature of iptables is the ability to group related
rules into chains. You can call the chains whatever you want, but I
recommend using lower-case letters to avoid confusion with the built-
in chains and targets. Chain names can be up to 31 letters long.

7.5.1. Creating a New Chain

Let's create a new chain. Because I am such an imaginative fellow,
I'll call it test. We use the `-N' or `--new-chain' options:

# iptables -N test
#

It's that simple. Now you can put rules in it as detailed above.

7.5.2. Deleting a Chain

Deleting a chain is simple as well, using the `-X' or `--delete-chain'
options. Why `-X'? Well, all the good letters were taken.

# iptables -X test
#

There are a couple of restrictions to deleting chains: they must be
empty (see ``Flushing a Chain'' below) and they must not be the target
of any rule. You can't delete any of the three built-in chains.

If you don't specify a chain, then all user-defined chains will be
deleted, if possible.

7.5.3. Flushing a Chain

There is a simple way of emptying all rules out of a chain, using the
`-F' (or `--flush') commands.

# iptables -F FORWARD
#

If you don't specify a chain, then all chains will be flushed.

7.5.4. Listing a Chain

You can list all the rules in a chain by using the `-L' (or `--list')
command.

The `refcnt' listed for each user-defined chain is the number of rules
which have that chain as their target. This must be zero (and the
chain be empty) before this chain can be deleted.

If the chain name is omitted, all chains are listed, even empty ones.

There are three options which can accompany `-L'. The `-n' (numeric)
option is very useful as it prevents iptables from trying to lookup
the IP addresses, which (if you are using DNS like most people) will
cause large delays if your DNS is not set up properly, or you have
filtered out DNS requests. It also causes TCP and UDP ports to be
printed out as numbers rather than names.

The `-v' options shows you all the details of the rules, such as the
the packet and byte counters, the TOS comparisons, and the interfaces.
Otherwise these values are omitted.

Note that the packet and byte counters are printed out using the
suffixes `K', `M' or `G' for 1000, 1,000,000 and 1,000,000,000
respectively. Using the `-x' (expand numbers) flag as well prints the
full numbers, no matter how large they are.

7.5.5. Resetting (Zeroing) Counters

It is useful to be able to reset the counters. This can be done with
the `-Z' (or `--zero') option.

Consider the following:

# iptables -L FORWARD
# iptables -Z FORWARD
#

In the above example, some packets could pass through between the `-L'
and `-Z' commands. For this reason, you can use the `-L' and `-Z'
together, to reset the counters while reading them.

7.5.6. Setting Policy

We glossed over what happens when a packet hits the end of a built-in
chain when we discussed how a packet walks through chains earlier. In
this case, the policy of the chain determines the fate of the packet.
Only built-in chains (INPUT, OUTPUT and FORWARD) have policies,
because if a packet falls off the end of a user-defined chain,
traversal resumes at the previous chain.

The policy can be either ACCEPT or DROP, for example:

# iptables -P FORWARD DROP
#

8. Using ipchains and ipfwadm

There are modules in the netfilter distribution called ipchains.o and
ipfwadm.o. Insert one of these in your kernel (NOTE: they are
incompatible with ip_tables.o!). Then you can use ipchains or ipfwadm
just like the good old days.

This will be supported for some time yet. I think a reasonable
formula is 2 * [notice of replacement - initial stable release],
beyond the date that a stable release of the replacement is available.
This means that support will probably be dropped in Linux 2.6 or 2.8.

9. Mixing NAT and Packet Filtering

It's common to want to do Network Address Translation (see the NAT
HOWTO) and packet filtering. The good news is that they mix extremely
well.

You design your packet filtering completely ignoring any NAT you are
doing. The sources and destinations seen by the packet filter will be
the `real' sources and destinations. For example, if you are doing
DNAT to send any connections to 1.2.3.4 port 80 through to 10.1.1.1
port 8080, the packet filter would see packets going to 10.1.1.1 port
8080 (the real destination), not 1.2.3.4 port 80. Similarly, you can
ignore masquerading: packets will seem to come from their real
internal IP addresses (say 10.1.1.1), and replies will seem to go back
there.

You can use the `state' match extension without making the packet
filter do any extra work, since NAT requires connection tracking
anyway. To enhance the simple masquerading example in the NAT HOWTO
to disallow any new connections from coming in the ppp0 interface, you
would do this:

# Masquerade out ppp0
iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE

# Disallow NEW and INVALID incoming or forwarded packets from ppp0.
iptables -A INPUT -i ppp0 -m state --state NEW,INVALID -j DROP
iptables -A FORWARD -i ppp0 -m state --state NEW,INVALID -j DROP

# Turn on IP forwarding
echo 1 > /proc/sys/net/ipv4/ip_forward

10. Differences Between iptables and ipchains

o Firstly, the names of the built-in chains have changed from lower
case to UPPER case, because the INPUT and OUTPUT chains now only
get locally-destined and locally-generated packets. They used to
see all incoming and all outgoing packets respectively.

o The `-i' flag now means the incoming interface, and only works in
the INPUT and FORWARD chains. Rules in the FORWARD or OUTPUT
chains that used `-i' should be changed to `-o'.

o TCP and UDP ports now need to be spelled out with the --source-port
or --sport (or --destination-port/--dport) options, and must be
placed after the `-p tcp' or `-p udp' options, as this loads the
TCP or UDP extensions respectively.

o The TCP -y flag is now --syn, and must be after `-p tcp'.

o The DENY target is now DROP, finally.

o Zeroing single chains while listing them works.

o Zeroing built-in chains also clears policy counters.

o Listing chains gives you the counters as an atomic snapshot.

o REJECT and LOG are now extended targets, meaning they are separate
kernel modules.

o Chain names can be up to 31 characters.

o MASQ is now MASQUERADE and uses a different syntax. REDIRECT,
while keeping the same name, has also undergone a syntax change.
See the NAT-HOWTO for more information on how to configure both of
these.

o The -o option is no longer used to direct packets to the userspace
device (see -i above). Packets are now sent to userspace via the
QUEUE target.

o Probably heaps of other things I forgot.

11. Advice on Packet Filter Design

Common wisdom in the computer security arena is to block everything,
then open up holes as neccessary. This is usually phrased `that which
is not explicitly allowed is prohibited'. I recommend this approach
if security is your maximal concern.

Do not run any services you do not need to, even if you think you have
blocked access to them.

If you are creating a dedicated firewall, start by running nothing,
and blocking all packets, then add services and let packets through as
required.

I recommend security in depth: combine tcp-wrappers (for connections
to the packet filter itself), proxies (for connections passing through
the packet filter), route verification and packet filtering. Route
verification is where a packet which comes from an unexpected
interface is dropped: for example, if your internal network has
addresses 10.1.1.0/24, and a packet with that source address comes in
your external interface, it will be dropped. This can be enabled for
one interface (ppp0) like so:

# echo 1 > /proc/sys/net/ipv4/conf/ppp0/rp_filter
#

Or for all existing and future interfaces like this:

# for f in /proc/sys/net/ipv4/conf/*/rp_filter; do
# echo 1 > $f
# done
#

Debian does this by default where possible. If you have asymmetric
routing (ie. you expect packets coming in from strange directions),
you will want to disable this filtering on those interfaces.

Logging is useful when setting up a firewall if something isn't
working, but on a production firewall, always combine it with the
`limit' match, to prevent someone from flooding your logs.

I highly recommend connection tracking for secure systems: it
introduces some overhead, as all connections are tracked, but is very
useful for controlling access to your networks. You may need to load
the `ip_conntrack.o' module if your kernel does not load modules
automatically, and it's not built into the kernel. If you want to
accurately track complex protocols, you'll need to load the
appropriate helper module (eg. `ip_conntrack_ftp.o').

# iptables -N no-conns-from-ppp0
# iptables -A no-conns-from-ppp0 -m state --state ESTABLISHED,RELATED -j ACCEPT
# iptables -A no-conns-from-ppp0 -m state --state NEW -i ! ppp0 -j ACCEPT
# iptables -A no-conns-from-ppp0 -i ppp0 -m limit -j LOG --log-prefix "Bad packet from ppp0:"
# iptables -A no-conns-from-ppp0 -i ! ppp0 -m limit -j LOG --log-prefix "Bad packet not from ppp0:"
# iptables -A no-conns-from-ppp0 -j DROP

# iptables -A INPUT -j no-conns-from-ppp0
# iptables -A FORWARD -j no-conns-from-ppp0

Building a good firewall is beyond the scope of this HOWTO, but my
advice is `always be minimalist'. See the Security HOWTO for more
information on testing and probing your box.