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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 kernel. 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 message 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 masqueraded as if they originated from the local host. Furthermore, reverse packets will be recognized as such and they will be demasqueraded automatically, bypassing the forwarding chain.

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 command and option names, you only need to use enough letters to ensure that ipchains can differentiate it from all other options.
-A, --append
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.
-D, --delete
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.
-R, --replace
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 at 1.
-I, --insert
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.
-L, --list
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.
-F, --flush
Flush the selected chain. This is equivalent to deleting all the rules one by one.
-Z, --zero
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).
-N, --new-chain
Create a new user-defined chain of the given name. There must be no target of that name already.
-X, --delete-chain
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 chain.
-P, --policy
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.
-M, --masquerading
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 option).
-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 sessions 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.
-C, --check
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 construct the packet to be tested. In particular, the -s (source), -d (destination), -p (protocol), and -i (interface) flags are compulsory.
-h, --help
Give a (currently very brief) description of the command syntax. If followed by the word icmp, then a list of ICMP names is listed.
-V, --version
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 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 specified, 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 option.
-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 "destination 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 destination specification.
-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 match.
[!] -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:
-b, --bidirectional
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.
-v, --verbose
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 multipliers respectively (but see the -x flag to change this). When used in combination with -M, information related to delta sequence numbers will also be listed. For appending, insertion, deletion and replacement, this causes detailed information on the rule or rules to be printed.
-n, --numeric
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).
-l, --log
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 developers 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 CONFIG_IP_FIREWALL_NETLINK set.
-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 REJECT.
-x, --exact
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 protocol type is set to TCP. If the "!" flag precedes the "-y", the sense of the option is inverted.
When listing rules, add line numbers to the beginning of each rule, corresponding to that rule's position in the chain.
Disable all warnings.


/proc/net/ip_fwchains /proc/net/ip_masquerade


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 parameters 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 rectified. This is to help users unaware of the requirement (which did not exist in the 2.0 kernels).

There is no way to reset the packet and byte counters in one chain only. This is a kernel limitation.

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 documentation 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 firewall trees. Its functionality is a superset of ipfwadm, and there is generally a 1:1 mapping 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 simulate 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 packets 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 dropped.

The old behaviour of SYN and ACK matching (which was previously ignored for non-TCP packets) 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 <rusty@linuxcare.com>. Thanks also to Hans Persson for detailed proofreading; I want him to read all my future documents!


brctl(8), ifconfig(8), ipchains-restore(8), ipchains-save(8), ipfwadm-wrapper(8), ldirectord(8), netstat(8), psad(8)