CHAINS

The kernel table is used to divide functionality into different sets of rules. Each set of rules is called a chain. Each chain is an ordered list of rules that can match ARP frames. If a rule matches an ARP frame, then a processing specification tells what to do with that matching frame. The processing specification is called a 'target'. However, if the frame does not match the current rule in the chain, then the next rule in the chain is examined and so forth. The user can create new (user-defined) chains which can be used as the 'target' of a rule.

TARGETS

A firewall rule specifies criteria for an ARP frame and a frame processing specification called a target. When a frame matches a rule, then the next action performed by the kernel is specified by the target. The target can be one of these values: ACCEPT, DROP, CONTINUE, RETURN, an 'extension' (see below) or a user-defined chain.

ACCEPT means to let the frame through. DROP means the frame has to be dropped. CONTINUE means the next rule has to be checked. This can be handy to know how many frames pass a certain point in the chain or to log those frames. RETURN means stop traversing this chain and resume at the next rule in the previous (calling) chain. For the extension targets please see the TARGET EXTENSIONS section of this man page.

TABLES

There is only one ARP table in the Linux kernel. The table is filter. You can drop the '-t filter' argument to the arptables command. The -t argument must be the first argument on the arptables command line, if used.

-t, --table

filter, is the only table and contains two built-in chains: INPUT (for frames destined for the host) and OUTPUT (for locally-generated frames).

COMMANDS

The arptables command arguments specify the actions to perform on the table defined with the -t argument. If you do not use the -t argument to name a table, the commands apply to the default filter table. With the exception of the -Z command, only one command may be used on the command line at a time.

-A, --append

Append a rule to the end of the selected chain.

-D, --delete

Delete the specified rule from the selected chain. There are two ways to use this command. The first is by specifying an interval of rule numbers to delete, syntax: start_nr[:end_nr]. Using negative numbers is allowed, for more details about using negative numbers, see the -I command. The second usage is by specifying the complete rule as it would have been specified when it was added.

-I, --insert

Insert the specified rule into the selected chain at the specified rule number. If the current number of rules equals N, then the specified number can be between -N and N+1. For a positive number i, it holds that i and i-N-1 specify the same place in the chain where the rule should be inserted. The number 0 specifies the place past the last rule in the chain and using this number is therefore equivalent with using the -A command.

-R, --replace

Replaces the specified rule into the selected chain at the specified rule number. If the current number of rules equals N, then the specified number can be between 1 and N. i specifies the place in the chain where the rule should be replaced.

-P, --policy

Set the policy for the chain to the given target. The policy can be ACCEPT, DROP or RETURN.

-F, --flush

Flush the selected chain. If no chain is selected, then every chain will be flushed. Flushing the chain does not change the policy of the chain, however.

-Z, --zero

Set the counters of the selected chain to zero. If no chain is selected, all the counters are set to zero. The -Z command can be used in conjunction with the -L command. When both the -Z and -L commands are used together in this way, the rule counters are printed on the screen before they are set to zero.

-L, --list

List all rules in the selected chain. If no chain is selected, all chains are listed.

-N, --new-chain

Create a new user-defined chain with the given name. The number of user-defined chains is unlimited. A user-defined chain name has maximum length of 31 characters.

-X, --delete-chain

Delete the specified user-defined chain. There must be no remaining references to the specified chain, otherwise arptables will refuse to delete it. If no chain is specified, all user-defined chains that aren't referenced will be removed.

-E, --rename-chain

Rename the specified chain to a new name. Besides renaming a user-defined chain, you may rename a standard chain name to a name that suits your taste. For example, if you like PREBRIDGING more than PREROUTING, then you can use the -E command to rename the PREROUTING chain. If you do rename one of the standard arptables chain names, please be sure to mention this fact should you post a question on the arptables mailing lists. It would be wise to use the standard name in your post. Renaming a standard arptables chain in this fashion has no effect on the structure or function of the arptables kernel table.

MISCELLANOUS COMMANDS

-V, --version

Show the version of the arptables userspace program.

-h, --help

Give a brief description of the command syntax.

-j, --jump target

The target of the rule. This is one of the following values: ACCEPT, DROP, CONTINUE, RETURN, a target extension (see TARGET EXTENSIONS) or a user-defined chain name.

-c, --set-counters PKTS BYTES

This enables the administrator to initialize the packet and byte counters of a rule (during INSERT, APPEND, REPLACE operations).

RULE-SPECIFICATIONS

The following command line arguments make up a rule specification (as used in the add and delete commands). A "!" option before the specification inverts the test for that specification. Apart from these standard rule specifications there are some other command line arguments of interest.

-s, --source-ip [!] address[/mask]

The Source IP specification.

-d, --destination-ip [!] address[/mask]

The Destination IP specification.

--source-mac [!] address[/mask]

The source mac address. Both mask and address are written as 6 hexadecimal numbers separated by colons.

--destination-mac [!] address[/mask]

The destination mac address. Both mask and address are written as 6 hexadecimal numbers separated by colons.

-i, --in-interface [!] name

The interface via which a frame is received (for the INPUT chain). The flag --in-if is an alias for this option.

-o, --out-interface [!] name

The interface via which a frame is going to be sent (for the OUTPUT chain). The flag --out-if is an alias for this option.

-l, --h-length length[/mask]

The hardware length (nr of bytes)

--opcode code[/mask]

The operation code (2 bytes). Available values are: 1=Request 2=Reply 3=Request_Reverse 4=Reply_Reverse 5=DRARP_Request 6=DRARP_Reply 7=DRARP_Error 8=InARP_Request 9=ARP_NAK.

--h-type type[/mask]

The hardware type (2 bytes, hexadecimal). Available values are: 1=Ethernet.

--proto-type type[/mask]

The protocol type (2 bytes). Available values are: 0x800=IPv4.

TARGET-EXTENSIONS

arptables extensions are precompiled into the userspace tool. So there is no need to explicitly load them with a -m option like in iptables. However, these extensions deal with functionality supported by supplemental kernel modules.

mangle

--mangle-ip-s IP address

Mangles Source IP Address to given value.

--mangle-ip-d IP address

Mangles Destination IP Address to given value.

--mangle-mac-s MAC address

Mangles Source MAC Address to given value.

--mangle-mac-d MAC address

Mangles Destination MAC Address to given value.

--mangle-target target

Target of ARP mangle operation (DROP, CONTINUE or ACCEPT -- default is ACCEPT).

CLASSIFY

This module allows you to set the skb->priority value (and thus clas- sify the packet into a specific CBQ class).

--set-class major:minor

Set the major and minor class value. The values are always interpreted as hexadecimal even if no 0x prefix is given.

MARK

This module allows you to set the skb->mark value (and thus classify the packet by the mark in u32)

--set-mark mark

Set the mark value. The values are always interpreted as hexadecimal even if no 0x prefix is given

--and-mark mark

Binary AND the mark with bits.

--or-mark mark

Binary OR the mark with bits.