int setsockopt (int socket, IPPROTO_IP, int command, void *data, int length)
The IP firewall facilities in the Linux kernel provide mechanisms for
accounting IP packets, for building firewalls based on packet-level
filtering, for building firewalls using transparent proxy servers (by
redirecting packets to local sockets), and for masquerading forwarded
packets. The administration of these functions is maintained in the
kernel as a series of separate lists (hereafter referred to as
chains) each containing zero or more rules. There are three builtin
chains which are called input, forward and output which always exist.
All other chains are user defined. A chain is a sequence of rules;
each rule contains specific information about source and destination
addresses, protocols, port numbers, and some other characteristics.
Information about what to do if a packet matches the rule is also
contained. A packet will match with a rule when the characteristics
of the rule match those of the IP packet.
A packet always traverses a chain starting at rule number 1. Each
rule specifies what to do when a packet matches. If a packet does not
match a rule, the next rule in that chain is tried. If the end of a
builtin chain is reached the default policy for that chain is
returned. If the end of a user defined chain is reached then the rule
after the rule which branched to that chain is tried. The purpose of
the three builtin chains are
These rules regulate the acceptance of incoming IP packets. All
packets coming in via one of the local network interfaces are checked
against the input firewall rules (locally-generated packets are
considered to come from the loopback interface). A rule which matches
a packet will cause the rule's packet and byte counters to be
These rules define the permissions for forwarding IP packets. All
packets sent by a remote host having another remote host as
destination are checked against the forwarding firewall rules. A rule
which matches will cause the rule's packet and byte counters to be
These rules define the permissions for sending IP packets. All
packets that are ready to be be sent via one of the local network
interfaces are checked against the output firewall rules. A rule
which matches will cause the rule's packet and byte counters to be
Each of the firewall rules contains either a branch name or a policy,
which specifies what action has to be taken when a packet matches with
the rule. There are five different policies possible:
(let the packet pass the firewall),
(do not accept the packet and send an ICMP host unreachable message
back to the sender as notification),
(sometimes referred to as block; ignore the packet without sending
(redirected to a local socket - input rules only)
(pass the packet, but perform IP masquerading - forwarding rules only).
The last two are special; for
the packet will be received by a local process, even if it was sent to
another host and/or another port number. This function only applies
to TCP or UDP packets.
the sender address in the IP packets is replaced by the address of the
local host and the source port in the TCP or UDP header is replaced by
a locally generated (temporary) port number before being forwarded.
Because this administration is kept in the kernel, reverse packets
(sent to the temporary port number on the local host) are recognized
automatically. The destination address and port number of these
packets will be replaced by the original address and port number that
was saved when the first packet was masqueraded. This function only
applies to TCP or UDP packets.
There is also a special target
which is equivalent to falling off the end of the chain.
This paragraph describes the way a packet goes through the firewall.
Packets received via one of the local network interfaces will pass the
Here, the device (network interface) that is used when trying to
match a rule with an IP packet is listed between brackets.
After this step, a packet will optionally be redirected to a local socket.
When a packet has to be forwarded to a remote host, it will also pass
the next set of rules:
After this step, a packet will optionally be masqueraded.
Responses to masqueraded packets will never pass the forwarding firewall
(but they will pass both the input and output firewalls).
All packets sent via one of the local network interfaces, either
locally generated or being forwarded, will pass the following sets
When a packet enters one of the three above chains rules are traversed
from the first rule in order. When analysing a rule one of three
things may occur.
If a rule is unmatched then the next rule in that chain is analysed.
If there are no more rules for that chain the default policy for that
chain is returned (or traversal continues back at the calling chain,
in the case of a user-defined chain).
Rule matched (with branch to chain):
When a rule is matched by a packet and the rule contains a branch
field then a jump/branch to that chain is made. Jumps can only be
made to user defined chains. As described above, when the end of a
builtin chain is reached then a default policy is returned. If the
end of a used defined chain is reached then we return to the rule from
whence we came.
There is a reference counter at the head of each chain which
determines the number of references to that chain. The reference
count of a chain must be zero before it can be deleted to ensure that
no branches are effected. To ensure the builtin chains are never
deleted their reference count is initialised to one. Also since no
branches to builtin chains can be made, their reference counts are
always one. The reference count on user defined chains are
initialised to zero and are changed accordingly when rules are
inserted, deleted etc.
Multiple jumps to different chains are possible which unfortunately
make loops possible. Loop detection is therefore provided. Loops are
detected when a packet tries to re-enter a chain it is already
traversing. An example of a simple loop that could be created is if
we set up two user defined chains called "test1" and "test2". We
firstly insert a rule in the "input" chain which jumps to "test1". We
then create a rule in the "test1" chain which points to "test2" and a
rule in "test2" which points to "test1". Here we have obviously
created a loop. When a packet then enters the input chain it will
branch to the "test1" chain and then to the "test2" chain. From here
it will try to branch back to the "test1" chain. A message in the
syslog will be recorded along with the path which the packet
traversed, to assist in debugging firewall rules.
Rule matched (special branch):
The special labels
can be given which specify the immediate fate of the packet as
discussed above. If no label is specified then the next
rule in the chain is analysed.
Using this last option (no label) an accounting chain can be created.
If each of the rules in this accounting chain have no branch or label
then the packet will always fall through to the end of the chain and
then return to the calling chain. Each rule that matches in the
accounting chain will have its byte and packet counters incremented as
expected. This accounting chain can be branched to from any other
chain (eg input, forward or output chain). This is a very neat way of
performing packet accounting.
The firewall administration can be changed via calls to
The existing rules can be inspected by looking at two files in
These two files are readable only by root.
The current administration related to masqueraded sessions can
be found in the file
in the same directory.
Command for changing and setting up chains and rules is
Most commands require some additional data to be passed.
A pointer to this data and the length of the data are passed
as option value and option length arguments to
The following commands are available:
This command allows a rule to be inserted in a chain at a given
position (where 1 is considered the start of the chain). If there is
already a rule in that position, it is moved one slot, as are any
following rules in that chain. The reference count of any chains
referenced by this inserted rule are incremented appropriately. The
data passed with this command is an
structure, defining the position, chain and contents of the new rule.
Remove the first rule matching the specification from the given chain.
The data passed with this command is an
structure, defining the rule to be deleted and its chain. The
reference count of any chains referenced by this deleted rule are
Note that the fw_mark field is currently ignored in rule comparisons
Remove a rule from one of the chains at a given rule number (where 1
means the first rule). The data passed with this command is an
structure, defining the rule number of the rule to be deleted and its
chain. The reference count of any chains referenced by this deleted
rule are decremented appropriately.
Reset the packet and byte counters in all rules of a chain. The data
passed with this command is an
which defines the chain which is to be operated on. See also the
description of the
files for a way to atomically list and reset the counters.
Remove all rules from a chain. The data passed with this command is an
which defines the chain to be operated on.
Replace a rule in a chain. The new rule overwrites the rule in the
given position. Any chains referenced by the new rule are incremented
and chains referenced by the overwritten rule are decremented. The
data passed with this command is an
structure, defining the contents of the new rule, the the chain name
and the position of the rule in that chain.
Insert a rule at the end of one of the chains. The data passed with
this command is an
structure, defining the contents of the new rule and the chain to
which it is to be appended. Any chains referenced by this new rule
have their refcount incremented.
Set the timeout values used for masquerading.
The data passed with this command is a structure containing three fields of type
representing the timeout values (in jiffies, 1/HZ second) for TCP sessions,
TCP sessions after receiving a FIN packet, and UDP packets,
A timeout value 0 means that the current timeout value of the
corresponding entry is preserved.
Check whether a packet would be accepted, denied, rejected, redirected
or masqueraded by a chain. The data passed with this command is an
structure, defining the packet to be tested and the chain which it is
to be test on. Both builtin and user defined chains can be tested.
Create a chain. The data passed with this command is an
defining the name of the chain to be created. Two chains can not have
the same name.
Delete a chain. The data passed with this command is an
defining the name of the chain to be deleted. The chain must not be
referenced by any rule (ie. refcount must be zero). The chain must
also be empty which can be achieved using IP_FW_FLUSH.
Changes the default policy on a builtin rule. The data passed with
this command is an
structure, defining the chain whose policy is to be changed and the
new policy. The chain must be a builtin chain as user-defined chains
don't have default policies.
structure contains the following relevant fields to be filled
in for adding or replacing a rule:
struct in_addr fw_src, fw_dst
Source and destination IP addresses.
struct in_addr fw_smsk, fw_dmsk
Masks for the source and destination IP addresses.
Note that a mask of 0.0.0.0 will result in a match for all hosts.
Name of the interface via which a packet is received by the system or is
going to be sent by the system.
If the option
is specified, then the fw_vianame need only match the packet interface
up to the first NUL character in fw_vianame. This allows
wildcard-like effects. The empty string has a special meaning: it
will match with all device names.
Flags for this rule.
The flags for the different options can be bitwise or'ed with each other.
The options are:
(only matches with TCP packets when the SYN bit is set and both the
ACK and RST bits are cleared in the TCP header, invalid with other
protocols), The option
is described under the fw_mark entry.
can be used to list some information about a matching packet via
can be used to specify a rule which applies only to second and
succeeding fragments (initial fragments can be treated like normal
packets for the sake of firewalling). Non-fragmented packets and
initial fragments will never match such a rule. Fragments do not
contain the complete information assumed for most firewall rules,
notably ICMP type and code, UDP/TCP port numbers, or TCP SYN or ACK
bits. Rules which try to match packets by these criteria will never
match a (non-first) fragment.
can be specified if the kernel has been compiled with
CONFIG_IP_FIREWALL_NETLINK enabled. This means that all matching
packets will be sent out the firewall netlink device (character
device, major number 36, minor number 3). The output of this device
is four bytes indicating the total length, four bytes indicating the
mark value of the packet (as described under fw_mark above), a string
of IFNAMSIZ characters containing the interface name for the packet,
and then the packet itself. The packet is truncated to
bytes if it is longer.
This field is a set of flags used to negate the meaning of other
fields, eg. to specify that a packet must NOT be on an interface. The
valid flags are
(invert the meaning of the fw_src field)
(invert the meaning of fw_dst)
(invert the meaning of fw_proto)
(invert the meaning of fw_spts)
(invert the meaning of fw_dpts)
(invert the meaning of fw_vianame)
(invert the meaning of fw_flg & IP_FW_F_TCPSYN)
(invert the meaning of fw_flg & IP_FW_F_FRAG). It is illegal (and
useless) to specify a rule that can never be matched, by inverting an
all-inclusive set. Note also, that a fragment will never pass any
test on ports or SYN, even an inverted one.
The protocol that this rule applies to. The protocol number 0 is used
to mean `any protocol'.
__u16 fw_spts, fw_dpts
These fields specify the range of source ports, and the range of
destination ports respectively. The first array element is the
inclusive minimum, and the second is the inclusive maximum. Unless
the rule specifies a protocol of TCP, UDP or ICMP, the port range must
be 0 to 65535. For ICMP, the
field is used to check the ICMP type, and the
field is used to check the ICMP code.
This field must be zero unless the target of the rule is "REDIRECT".
Otherwise, if this redirection port is 0, the destination port of a
packet will be used as the redirection port.
This field indicates a value to mark the skbuff with (which contains
the administration data for the matching packet). This is currently
unused, but could be used to control how individual packets are
treated. If the
flag is set then the value in
simply replaces the current mark in the skbuff, rather than being
added to the current mark value which is normally done. To subtract a
value, simply use a large number for
and 32-bit wrap-around will occur.
__u8 fw_tosand, fw_tosxor
These 8-bit masks define how the TOS field in the IP header should be
changed when a packet is accepted by the firewall rule.
The TOS field is first bitwise and'ed with
and the result of this will be bitwise xor'ed with
Obviously, only packets which match the rule have their TOS effected.
It is the responsibility of the user that packets with invalid TOS
bits are not created using this option.
structure, used when calling some of the above commands contains the following fields:
struct ip_fw ipfw
This is the label of the chain which is to be operated on.
structure, used when checking a packet,
contains the following fields:
struct iphdr fwp_iph
The IP header. See
for a detailed description of the
The TCP, UDP, or ICMP header, combined in a union named
for a detailed description of the respective structures.
struct in_addr fwp_via
The interface address via which the packet is pretended to be
received or sent.
The ability to add in extra chains other than just the standard input,
output and forward chains is very powerful. The ability to branch to
any chain makes the replication of rules unnecessary. Accounting
becomes automatic as a single chain can be referenced by all builtin
chains to do the accounting.
Fragments must now be handled explicitly; previously second and
succeeding fragments were passed automatically.
The lowest TOS bit (MBZ) could not be effected previously; the kernel
used to silently mask out any attempted manipulation of the lowest
TOS bit. (``So now you know how to do it - DON'T.'').
The packet and byte counters are now 64-bit on 32-bit machines
(actually presented as two 32-bit values).
The ability to specify an interface by an IP address was obsoleted by
the ability to specify it by name; the combination of the two was
error-prone and so only an interface name can now be used.
flag was made obsolete by the ability to invert
flag made the kernel code complex and is no longer supported.
The ability to specify several ports in one rule was messy and didn't
win much, so has been removed.
On success (or a straightforward packet accept for the CHECK options), zero is returned.
On error, -1 is returned and
is set appropriately.
for a list of possible error values.
indicates that the given chain name doesn't exist.
When the check packet command is used, zero is returned
when the packet would be accepted without redirection or masquerading.
Otherwise, -1 is returned and
is set to
(packet would be accepted using redirection),
(packet would be accepted using masquerading),
(packet would be denied),
(packet would be rejected),
(packet got into a loop),
(packet fell off end of chain; only occurs for user defined chains).
In the directory
there are two entries to list the currently defined rules and chains:
(for IP firewall chain names) One line per chain. Each line contains
the chain name, policy, the number of references to that chain and the
packet and byte counters which have matched the policy (represented as
two pairs of 32-bit numbers; most significant 32-bits first).
(for IP firewall chains)
One line per rule; rules are listed one chain at a time (from
first to last as they appear in
and in order from first to last down each chain.
The fields are: the chain name for that rule, source address and mask,
destination address and mask, interface name (or "-"), the fw_flg
field, the fw_invflg field, protocol number, packet and byte counters,
the source and destination port ranges, the TOS and-mask, the TOS
xor-mask, the fw_redirpt field, the fw_mark field, the fw_outputsize
field, and the target (label). The IP addresses and masks are listed
as eight hexadecimal digits, the TOS masks are listed as two hexadecimal
digits preceded by the letters A and X, respectively, the fw_mark,
fw_flg and fw_invflg fields are listed in hex, and the other values
are represented in decimal format. The packet and bytes counters are
represented as two space-separated 32-bit numbers, representing the
most and least significant words respectively. Individual fields are
separated by white space, by a "/" (the address and the corresponding
mask), by "->" (the source and destination address/mask pairs), or "-"
(the ranges for source and destination ports).
These files may also be opened in read/write mode. In that case, the
packet and byte counters in all the rules of that category will be
reset to zero after listing their current values.
contains the kernel administration related to masquerading.
After a header line, each masqueraded session is described on a
separate line with the following entries, separated by white space or by ':'
(the address/port number pairs):
protocol name ("TCP" or "UDP"), source IP address and port number,
destination IP address and port number,
the new port number, the initial sequence number
for adding a delta value, the delta value, the previous delta value,
and the expire time in jiffies (1/HZ second).
All addresses and numeric values are in hexadecimal format, except the last
three entries, being represented in decimal format.