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.
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
means to let the packet through.
means to drop the packet on the floor.
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
setting ICMP to
will cause ICMP port unreachables to be sent.)
is only legal for the forward and user defined chains, and can only be
used when the kernel is compiled with
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
is only legal for the input and user-defined chains and can only be
used when the Linux kernel is compiled with
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
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
is matched, the target specified by the chain policy determines the
fate of the packet.
The options that are recognized by
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
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 at 1.
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
(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
(list) option as well, to see the counters immediately before they are
cleared; if this is done, then no specific chain can be specified
be displayed and cleared).
Create a new user-defined chain of the given name. There must be no
target of that name already.
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.
Set the policy for the chain to the given target. See the section
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
option) or to set the kernel masquerading parameters (with the
-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
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
(interface) flags are compulsory.
Give a (currently very brief) description of the command syntax. If followed by the word
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
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
will match with all protocols and is taken as default when this
option is omitted.
may not be used in in combination with the check command.
can be either a hostname, a network name, or a plain IP address.
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
is equivalent to
A "!" argument before the address specification inverts the sense of
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
flag is illegal. In the rest of this paragraph, a
means either a port specification or an ICMP type.
An inclusive range can also be specified, using the format
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
protocols. A "!" before the port specification inverts the sense.
When the check command is specified, exactly one port is required, and
(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
flag above for details.The flag
is an alias for this option.
See the desciption of the
(source) flag for a detailed description of the syntax. For ICMP,
which does not have ports, a "destination port" refers to the numeric
--destination-port [!] [port[:port]]
This allows separate specification of the ports. See the description of
flag for details. The flag
is an alias for this option.
--icmp-type [!] typename
This allows specification of the ICMP type (use the
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:
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 multipliers respectively (but see
flag to change this). When used in combination with
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.
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
of all matching packets (like most IP header fields) via
-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
-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
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
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
[!] -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.
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
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,
which are to contain any accounting rules (these rules should be
specified without a
flag, so that the packets simply pass through them unscathed).
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
flag) is no longer supported; the combination of
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.
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.