ipchains - IP firewall administration
ipchains -[ADC] chain rule-specification [options]
ipchains -[RI] chain rulenum rule-specification [options]
ipchains -D chain rulenum [options]
ipchains -[LFZNX] [chain] [options]
ipchains -P chain target [options]
ipchains -M [ -L | -S ] [options]
Ipchains is used to set up, maintain, and inspect the IP firewall rules in the Linux ker-
nel. These rules can be divided into 4 different categories: the IP input chain, the IP
output chain, the IP forwarding chain, and user defined chains.
For each of these categories, a separate table of rules is maintained, any of which might
refer to one of the user-defined chains. See ipfw(4) for more details.
A firewall rule specifies criteria for a packet, and a target. If the packet does not
match, the next rule in the chain is then examined; if it does match, then the next rule
is specified by the value of the target, which can be the name of a user-defined chain, or
one of the special values ACCEPT, DENY, REJECT, MASQ, REDIRECT, or RETURN.
ACCEPT means to let the packet through. DENY means to drop the packet on the floor.
REJECT means the same as drop, but is more polite and easier to debug, since an ICMP mes-
sage is sent back to the sender indicating that the packet was dropped. (Note that DENY
and REJECT are the same for ICMP packets). [Note: this is incorrect; setting ICMP to
REJECT will cause ICMP port unreachables to be sent!]
MASQ is only legal for the forward and user defined chains, and can only be used when the
kernel is compiled with CONFIG_IP_MASQUERADE defined. With this, packets will be masquer-
aded as if they originated from the local host. Furthermore, reverse packets will be rec-
ognized as such and they will be demasqueraded automatically, bypassing the forwarding
REDIRECT is only legal for the input and user-defined chains and can only be used when the
Linux kernel is compiled with CONFIG_IP_TRANSPARENT_PROXY defined. With this, packets
will be redirected to a local socket, even if they were sent to a remote host. If the
specified redirection port is 0, which is the default value, the destination port of a
packet will be used as the redirection port. When this target is used, an optional extra
argument (the port number) can be supplied.
If the end of a user-defined chain is reached, or a rule with target RETURN is matched,
then the next rule in the previous (calling) chain is examined. If the end of a builtin
chain is reached, or a rule in a builtin chain with target RETURN is matched, the target
specified by the chain policy determines the fate of the packet.
The options that are recognized by ipchains can be divided into several different groups.
These options specify the specific action to perform; only one of them can be specified on
the command line, unless otherwise specified below. For all the long versions of the com-
mand and option names, you only need to use enough letters to ensure that ipchains can
differentiate it from all other options.
Append one or more rules to the end of the selected chain. When the source and/or
destination names resolve to more than one address, a rule will be added for each
possible address combination.
Delete one or more rules from the selected chain. There are two versions of this
command: the rule can be specified as a number in the chain (starting at 1 for the
first rule) or a rule to match.
Replace a rule in the selected chain. If the source and/or destination names
resolve to multiple addresses, the command will fail. Rules are numbered starting
Insert one or more rules in the selected chain as the given rule number. So, if
the rule number is 1, the rule or rules are inserted at the head of the chain.
List all rules in the selected chain. If no chain is selected, all chains are
listed. It is legal to specify the -Z (zero) option as well, in which case no
chain may be specified. The exact output is affected by the other arguments given.
Flush the selected chain. This is equivalent to deleting all the rules one by one.
Zero the packet and byte counters in all chains. It is legal to specify the -L,
--list (list) option as well, to see the counters immediately before they are
cleared; if this is done, then no specific chain can be specified (they will all be
displayed and cleared).
Create a new user-defined chain of the given name. There must be no target of that
Delete the specified user-defined chain. There must be no references to the chain
(if there are you must delete or replace the referring rules before the chain can
be deleted). If no argument is given, it will attempt to delete every non-builtin
Set the policy for the chain to the given target. See the section TARGETS for the
legal targets. Only non-userdefined chains can have policies, and neither built-in
nor user-defined chains can be policy targets.
This option allows viewing of the currently masqueraded connections (in conjuction
with the -L option) or to set the kernel masquerading parameters (with the -S
-S, --set tcp tcpfin udp
Change the timeout values used for masquerading. This command always takes 3
parameters, representing the timeout values (in seconds) for TCP sessions, TCP ses-
sions after receiving a FIN packet, and UDP packets, respectively. A timeout value
0 means that the current timeout value of the corresponding entry is preserved.
This option is only allowed in combination with the -M flag.
Check the given packet against the selected chain. This is extremely useful for
testing, as the same kernel routines used to check "real" network packets are used
to check this packet. It can be used to check user-defined chains as well as the
builtin ones. The same arguments used to specify firewall rules are used to con-
struct the packet to be tested. In particular, the -s (source), -d (destination),
-p (protocol), and -i (interface) flags are compulsory.
Give a (currently very brief) description of the command syntax. If followed by
the word icmp, then a list of ICMP names is listed.
Simply output the ipchains version number.
The following parameters make up a rule specification (as used in the add, delete,
replace, append and check commands).
-p, --protocol[!] protocol
The protocol of the rule or of the packet to check. The specified protocol can be
one of tcp, udp, icmp, or all, or it can be a numeric value, representing one of
these protocols or a different one. Also a protocol name from /etc/protocols is
allowed. A "!" argument before the protocol inverts the test. The number zero is
equivalent to all. Protocol all will match with all protocols and is taken as
default when this option is omitted. All may not be used in in combination with
the check command.
-s, --source, --src [!] address[/mask] [!] [port[:port]]
Source specification. Address can be either a hostname, a network name, or a plain
IP address. The mask can be either a network mask or a plain number, specifying
the number of 1's at the left side of the network mask. Thus, a mask of 24 is
equivalent to 255.255.255.0. A "!" argument before the address specification
inverts the sense of the address.
The source may include a port specification or ICMP type. This can either be a
service name, a port number, a numeric ICMP type, or one of the ICMP type names
shown by the command
ipchains -h icmp
Note that many of these ICMP names refer to both a type and code, meaning that an
ICMP code after the -d flag is illegal. In the rest of this paragraph, a port
means either a port specification or an ICMP type. An inclusive range can also be
specified, using the format port:port. If the first port is omitted, "0" is
assumed; if the last is omitted, "65535" is assumed.
Ports may only be specified in combination with the tcp, udp, or icmp protocols. A
"!" before the port specification inverts the sense. When the check command is
specified, exactly one port is required, and if the -f (fragment) flag is speci-
fied, no ports are allowed.
--source-port [!] [port[:port]]
This allows separate specification of the source port or port range. See the
description of the -s flag above for details.The flag --sport is an alias for this
-d, --destination, --dst [!] address[/mask] [!] [port[:port]]
Destination specification. See the desciption of the -s (source) flag for a
detailed description of the syntax. For ICMP, which does not have ports, a "desti-
nation port" refers to the numeric ICMP code.
--destination-port [!] [port[:port]]
This allows separate specification of the ports. See the description of the -s
flag for details. The flag --dport is an alias for this option.
--icmp-type [!] typename
This allows specification of the ICMP type (use the -h icmp option to see valid
ICMP type names). This is often more convenient than appending it to the destina-
-j, --jump target
This specifies the target of the rule; ie. what to do if the packet matches it.
The target can be a user-defined chain (not the one this rule is in) or one of the
special targets which decide the fate of the packet immediately. If this option is
omitted in a rule, then matching the rule will have no effect on the packet's fate,
but the counters on the rule will be incremented.
-i, --interface [!] name
Optional name of an interface via which a packet is received (for packets entering
the input chain), or via which is packet is going to be sent (for packets entering
the forward or output chains). When this option is omitted, the empty string is
assumed, which has a special meaning and will match with any interface name. When
the "!" argument is used before the interface name, the sense is inverted. If the
interface name ends in a "+", then any interface which begins with this name will
[!] -f, --fragment
This means that the rule only refers to second and further fragments of fragmented
packets. Since there is no way to tell the source or destination ports of such a
packet (or ICMP type), such a packet will not match any rules which specify them.
When the "!" argument precedes the "-f" flag, the sense is inverted.
The following additional options can be specified:
Bidirectional mode. The rule will match with IP packets in both directions; this
has the same effect as repeating the rule with the source & destination reversed.
Note that this does NOT mean that if you allow TCP syn packets out, the -b rule
will allow non-SYN packets back in: the reverse rule is exactly the same as the
rule you entered. This means that it's usually better to simply avoid the -b flag
and spell the rules out explicitly.
Verbose output. This option makes the list command show the interface address, the
rule options (if any), and the TOS masks. The packet and byte counters are also
listed, with the suffix 'K', 'M' or 'G' for 1000, 1,000,000 and 1,000,000,000 mul-
tipliers respectively (but see the -x flag to change this). When used in combina-
tion with -M, information related to delta sequence numbers will also be listed.
For appending, insertion, deletion and replacement, this causes detailed informa-
tion on the rule or rules to be printed.
Numeric output. IP addresses and port numbers will be printed in numeric format.
By default, the program will try to display them as host names, network names, or
services (whenever applicable).
Turn on kernel logging of matching packets. When this option is set for a rule,
the Linux kernel will print some information of all matching packets (like most IP
header fields) via printk().
-o, --output [maxsize]
Copy matching packets to the userspace device. This is currently mainly for devel-
opers who want to play with firewalling effects in userspace. The optional maxsize
argument can be used to limit the maximum number of bytes from the packet which are
to be copied. This option is only valid if the kernel has been compiled with CON-
-m, --mark markvalue
Mark matching packets. Packets can be marked with a 32-bit unsigned value which
may (one day) change how they are handled internally. If you are not a kernel
hacker you are unlikely to care about this. If the string markvalue begins with a
+ or -, then this value will be added or subtracted from the current marked value
of the packet (which starts at zero).
-t, --TOS andmask xormask
Masks used for modifying the TOS field in the IP header. When a packet matches a
rule, its TOS field is first bitwise and'ed with first mask and the result of this
will be bitwise xor'ed with the second mask. The masks should be specified as
hexadecimal 8-bit values. As the LSB of the TOS field must be unaltered (RFC
1349), TOS values which would cause it to be altered are rejected, as are any rules
which always set more than one TOS bit. Rules which might set multiple TOS bits
for certain packets result in warnings (sent to stdout) which can be ignored if you
know that packets with those TOS values will never reach that rule. Obviously,
manipulating the TOS is a meaningless gesture if the rule's target is DENY or
Expand numbers. Display the exact value of the packet and byte counters, instead
of only the rounded number in K's (multiples of 1000) M's (multiples of 1000K) or
G's (multiples of 1000M). This option is only relevant for the -L command.
[!] -y, --syn
Only match TCP packets with the SYN bit set and the ACK and FIN bits cleared. Such
packets are used to request TCP connection initiation; for example, blocking such
packets coming in an interface will prevent incoming TCP connections, but outgoing
TCP connections will be unaffected. This option is only meaningful when the proto-
col type is set to TCP. If the "!" flag precedes the "-y", the sense of the option
When listing rules, add line numbers to the beginning of each rule, corresponding
to that rule's position in the chain.
Disable all warnings.
Various error messages are printed to standard error. The exit code is 0 for correct
functioning. Errors which appear to be caused by invalid or abused command line parame-
ters cause an exit code of 2, and other errors cause an exit code of 1.
If input is a terminal, and a rule is inserted in, or appended to, the forward chain, and
IP forwarding does not seem to be enabled, and --no-warnings is not specified, a message
is printed to standard output, warning that no forwarding will occur until this is recti-
fied. This is to help users unaware of the requirement (which did not exist in the 2.0
There is no way to reset the packet and byte counters in one chain only. This is a kernel
Loop detection is not done in ipchains; packets in a loop get dropped and logged, but
that's the first you'll find out about it if you inadvertantly create a loop.
The explanation of what effect marking a packet has is intentionally vague until documen-
tation describing the new 2.1 kernel's packet scheduling routines is released.
There is no way to zero the policy counters (ie. those on the built-in chains).
This ipchains is very different from the ipfwadm by Jos Vos, as it uses the new IP fire-
wall trees. Its functionality is a superset of ipfwadm, and there is generally a 1:1 map-
ping of commands. I believe the new command names are more rational. There are, however,
a few changes of which you should be aware.
Fragments are handled differently. All fragments after the first used to be let through
(which is usually safe); they can now be filtered. This means that you should probably
add an explicit rule to accept fragments if you are converting over. Also, look for old
accounting rules which check for source and destination ports of 0xFFFF (0xFF for ICMP
packets) which was the old way of doing accounting on fragments.
Accounting rules are now simply integrated into the input and output chains; you can simu-
late the old behaviour like so:
ipchains -N acctin
ipchains -N acctout
ipchains -N acctio
ipchains -I input -j acctio
ipchains -I input -j acctin
ipchains -I output -j acctio
ipchains -I output -j acctout
This creates three user-defined chains, acctin, acctout and acctio, which are to contain
any accounting rules (these rules should be specified without a -j flag, so that the pack-
ets simply pass through them unscathed).
A MASQ or REDIRECT target encountered by the kernel out of place (ie. not during a forward
or input rule respectively) will cause a message to the syslog and the packet to be
The old behaviour of SYN and ACK matching (which was previously ignored for non-TCP pack-
ets) has changed; the SYN option is not valid for non-TCP-specific rules.
The ACK matching option (the -k flag) is no longer supported; the combination of ! and -y
will give the equivalent).
It is now illegal to specify a TOS mask which will set or alter the least significant TOS
bit; previously TOS masks were silently altered by the kernel if they tried to do this.
The -b flag is now handled by simply inserting or deleting a pair of rules, one with the
source and destination specifications reversed.
There is no way to specify an interface by address: use its name.
Rusty Russell <email@example.com>. Thanks also to Hans Persson for detailed proofread-
ing; I want him to read all my future documents!
February 8, 1998 IPCHAINS(8)