The following flags are used to select the category of rules to which the given command applies:

-A [direction]

IP accounting rules. Optionally, a direction can be specified (in, out, or both), indicating whether only incoming or outgoing packets should be counted. The default direction is both.

-I

IP input firewall rules.

-O

IP output firewall rules.

-F

IP forwarding firewall rules.

-M

IP masquerading administration. This category can only be used in combination with the -l (list) or -s (set timeout values) command.

Exactly one of these options has to be specified.

The next options specify the specific action to perform. Only one of them can be specified on the command line, unless something else is listed in the description.

-a [policy]

Append one or more rules to the end of the selected list. For the accounting chain, no policy should be specified. For firewall chains, it is required to specify one of the following policies: accept, deny, reject, or masquerade. When the source and/or destination names resolve to more than one address, a rule will be added for each possible address combination.

-i [policy]

Insert one or more rules at the beginning of the selected list. See the description of the -a command for more details.

-d [policy]

Delete one or more entries from the selected list of rules. The semantics are equal to those of the append/insert commands. The specified parameters should exactly match the parameters given with an append or insert command, otherwise no match will be found and the rule will not be removed from the list. Only the first matching rule in the list will be deleted.

-l

List all the rules in the selected list. This command may be combined with the -z (reset counters to zero) command. In that case, the packet and byte counters will be reset immediately after listing their current values. Unless the -x option is present, packet and byte counters (if listed) will be shown as numberK or numberM, where 1K means 1000 and 1M means 1000K (rounded to the nearest integer value). See also the -e and -x flags for more capabilities.

-z

Reset the packet and byte counters of all the rules in selected list. This command may be combined with the -l (list) command.

-f

Flush the selected list of rules.

-p policy

Change the default policy for the selected type of firewall. The given policy has to be one of accept, deny, reject, or masquerade. The default policy is used when no matching rule is found. This operation is only valid for IP firewalls, that is, in combination with the -I, -O, or -F flag.

-s 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 operation is only allowed in combination with the -M flag.

-c

Check whether this IP packet would be accepted, denied, or rejected by the selected type of firewall. This operation is only valid for IP firewalls, that is, in combination with the -I, -O, or -F flag.

-h

Help. Give a (currently very brief) description of the command syntax.

The following parameters can be used in combination with the append, insert, delete, or check commands:

-P protocol

The protocol of the rule or of the packet to check. The specified protocol can be one of tcp, udp, icmp, or 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 address[/mask] [port ...]

Source specification (optional). 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 with 255.255.255.0. The source may include one or more port specifications or ICMP types. Each of them can either be a service name, a port number, or a (numeric) ICMP type. In the rest of this paragraph, a port means either a port specification or an ICMP type. One of these specifications may be a range of ports, in the format port:port. Furthermore, the total number of ports specified with the source and destination addresses should not be greater than IP_FW_MAX_PORTS (currently 10). Here a port range counts as 2 ports. Packets not being the first fragment of a TCP, UDP, or ICMP packet are always accepted by the firewall. For accounting purposes, these second and further fragments are treated special, to be able to count them in some way. The port number 0xFFFF (65535) is used for a match with the second and further fragments of TCP or UDP packets. These packets will be treated for accounting purposes as if both their port numbers are 0xFFFF. The number 0xFF (255) is used for a match with the second and further fragments of ICMP packets. These packets will be treated for acounting purposes as if their ICMP types are 0xFF. Note that the specified command and protocol may imply restrictions on the ports to be specified. Ports may only be specified in combination with the tcp, udp, or icmp protocol. When this option is omitted, the default address/mask 0.0.0.0/0 (matching with any address) is used as source address. This option is required in combination with the check command, in which case also exactly one port has to be specified.

-D "address[/mask][port...]

Destination specification (optional). See the desciption of the -S (source) flag for a detailed description of the syntax, default values, and other requirements. Note that ICMP types are not allowed in combination with the -D flag: ICMP types can only be specified after the the -S flag.

-V address

Optional address of an interface via which a packet is received, or via which is packet is going to be sent. Address can be either a hostname or a plain IP address. When a hostname is specified, it should resolve to exactly one IP address. When this option is omitted, the address 0.0.0.0 is assumed, which has a special meaning and will match with any interface address. For the check command, this option is mandatory.

-W name

Optional name of an interface via which a packet is received, or via which is packet is going to be sent. When this option is omitted, the empty string is assumed, which has a special meaning and will match with any interface name. For the check command, this option is mandatory.

The following additional options can be specified:

-b

Bidirectional mode. The rule will match with IP packets in both directions. This option is only valid in combination with the append, insert, or delete commands.

-e

Extended output. This option makes the list command also show the interface address and the rule options (if any). For firewall lists, also the packet and byte counters (the default is to only show these counters for the accounting rules) and the TOS masks will be listed. When used in combination with -M, information related to delta sequence numbers will also be listed. This option is only valid in combination with the list command.

-k

Only match TCP packets with the ACK bit set (this option will be ignored for packets of other protocols). This option is only valid in combination with the append, insert, or delete command.

-m

Masquerade packets accepted for forwarding. When this option is set, packets accepted by this rule 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 firewall. This option is only valid in forwarding firewall rules with policy accept (or when specifying accept as default policy) and can only be used when the kernel is compiled with CONFIG_IP_MASQUERADE defined.

-n

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).

-o

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(). This option will only be effective when the Linux kernel is compiled with CONFIG_IP_FIREWALL_VERBOSE defined. This option is only valid in combination with the append, insert or delete command.

-r [port]

Redirect packets to a local socket. When this option is set, packets accepted by this rule 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. This option is only valid in input firewall rules with policy accept and can only be used when the Linux kernel is compiled with CONFIG_IP_TRANSPARENT_PROXY defined.

-t andmask xormask

Masks used for modifying the TOS field in the IP header. When a packet is accepted (with or without masquerading) by a firewall 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. This option is only valid in combination with the append, insert or delete command and will have no effect when used in combination with accounting rules or firewall rules for rejecting or denying a packet.

-v

Verbose output. Print detailed information of the rule or packet to be added, deleted, or checked. This option will only have effect with the append, insert, delete, or check command.

-x

Expand numbers. Display the exact value of the packet and byte counters, instead of only the rounded number in K's (multiples of 1000) or M's (multiples of 1000K). This option will only have effect when the counters are listed anyway (see also the -e option).

-y

Only match TCP packets with the SYN bit set and the ACK bit cleared (this option will be ignored for packets of other protocols). This option is only valid in combination with the append, insert, or delete command.