ARPTABLES

NAME

arptables - administration tool for arp packet filtering

SYNOPSIS

arptables [-t table] -[AD] chain rule-specification [options]
arptables [-t table] -I chain [rulenum] rule-specification [options]
arptables [-t table] -R chain rulenum rule-specification [options]
arptables [-t table] -D chain rulenum [options]
arptables [-t table] -[LFZ] [chain] [options]
arptables [-t table] -N chain
arptables [-t table] -X [chain]
arptables [-t table] -P chain target [options]
arptables [-t table] -E old-chain-name new-chain-name

DESCRIPTION

Arptables is used to set up, maintain, and inspect the tables of ARP packet filter rules in the Linux kernel. Several different tables may be defined. Each table contains a number of built-in chains and may also contain user-defined chains.

Each chain is a list of rules which can match a set of packets. Each rule specifies what to do with a packet that matches. This is called a `target', which may be a jump to a user-defined chain in the same table.

TARGETS

A firewall rule specifies criteria for a packet, and a target. If the packet does not match, the next rule in the chain is the 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, DROP, QUEUE, or RETURN.

ACCEPT means to let the packet through. DROP means to drop the packet on the floor. QUEUE means to pass the packet to userspace (if supported by the kernel). RETURN means stop traversing this chain and resume at the next rule in the previous (calling) chain. If the end of a built-in chain is reached or a rule in a built-in chain with target RETURN is matched, the target specified by the chain policy determines the fate of the packet.

TABLES

There is normally one table ("filter") included in the arptable_filter module. Which tables are present at any time depends on the kernel configuration options and which modules are present.

-t, --table table: This option specifies the packet matching table which the command should operate on. If the kernel is configured with automatic module loading, an attempt will be made to load the appropriate module for that table if it is not already there.
The tables are as follows:
filter: This is the default table (if no -t option is passed). It contains the built-in chains INPUT (for ARP packets entering the box), OUTPUT (for locally-generated ARP packets).

OPTIONS

The options that are recognized by arptables can be divided into several different groups.

COMMANDS

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 need to use only enough letters to ensure that arptables can differentiate it from all other options.

-A, --append chain rule-specification: 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 chain rule-specification
-D, --delete chain rulenum: 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.
-I, --insert chain [rulenum] rule-specification: 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. This is also the default if no rule number is specified.
-R, --replace chain rulenum rule-specification: 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.
-L, --list [chain]: List all rules in the selected chain. If no chain is selected, all chains are listed. As every other arptables command, it applies to the specified table (filter is the default).
Please note that it is often used with the -n option, in order to avoid long reverse DNS lookups. It is legal to specify the -Z (zero) option as well, in which case the chain(s) will be atomically listed and zeroed. The exact output is affected by the other arguments given. The exact rules are suppressed until you use

arptables -L -v
-F, --flush [chain]: Flush the selected chain (all the chains in the table if none is given). This is equivalent to deleting all the rules one by one.
-Z, --zero [chain]: 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. (See above.)
-N, --new-chain chain: Create a new user-defined chain by the given name. There must be no target of that name already.
-X, --delete-chain [chain]: Delete the optional user-defined chain specified. 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 in the table.
-P, --policy chain target: Set the policy for the chain to the given target. See the section TARGETS for the legal targets. Only built-in (non-user-defined) chains can have policies, and neither built-in nor user-defined chains can be policy targets.
-E, --rename-chain old-chain new-chain: Rename the user specified chain to the user supplied name. This is cosmetic, and has no effect on the structure of the table.
-h: Help. Give a (currently very brief) description of the command syntax.

PARAMETERS

The following parameters make up a rule specification (as used in the add, delete, insert, replace and append commands).

-s, --source [!] address[/mask]: Source specification. Address can be either a network name, a hostname (please note that specifying any name to be resolved with a remote query such as DNS is a really bad idea), a network IP address (with /mask), 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 255.255.255.0. A "!" argument before the address specification inverts the sense of the address. The flag --src is an alias for this option.
-d, --destination [!] address[/mask]: Destination specification. See the description of the -s (source) flag for a detailed description of the syntax. The flags --dst , --tgt and --target are aliases for this option.
-z, --source-hw [!] hwaddr[mask]: Specify the source hardware (MAC) address of the packet. hwaddr (and mask, if specified) must consist of one or more 8-bit hexidecimal numbers, separated by ':' characters. If the mask is not specified, it defaults to a number of 0xff octets equal to the length of the hwaddr specified, then 0s. The flags --source-mac , --src-hw , and --src-mac are aliases for this option.
-y, --target-hw [!] hwaddr[mask]: Specify the target hardware (MAC) address of the packet. This is similar to the --src-hw option. The flags --target-mac , --tgt-hw , --tgt-mac , --dst-hw , and --dst-mac are all aliases for this option.
-i, --in-interface [!] name: Name of an interface via which a packet is going to be received (only for packets entering the INPUT chain). 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. If this option is omitted, any interface name will match.
-o, --out-interface [!] name: Name of an interface via which a packet is going to be sent (for packets entering the OUTPUT chain). 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. If this option is omitted, any interface name will match.
-a, --arhln [!] value[mask]: Specify the hardware address length of the packet. Both the value and mask must be 8-bit hexidecimal numbers. Note that packets with an incorrect hardware address length field may be dropped by the lower-level layers of the network stack, which may limit the usefulness of this option.
-p, --arpop [!] value[mask]: Specify the arp operation field of the packet. The value may be either a 16-bit hexidecimal number or one of the names "Request", "Reply", "Request_Reverse", "Reply_Reverse", "DRARP_Request", "DRARP_Reply", "DRARP_Error", "InARP_Request", or "ARP_NAK". The mask (if specified) must be a 16-bit hexidecicmal number.
-H, --arhrd [!] value[mask]: Specify the hardware type field of the packet. The value may be either a 16-bit hexidecimal number or the name "Ethernet". The mask (if specified) must be a 16-bit hexidecimal number.
-w, --arpro [!] value[value]: Specify the protocol type field of the packet. The value may be eithe a 16-bit hexidecimal numebr or the name "IPV4". The mask (if specified) must be a 16-bit hexidecimal number.
-j, --jump target: This specifies the target of the rule; i.e., what to do if the packet matches it. The target can be a user-defined chain (other than the one this rule is in), or one of the special builtin targets which decide the fate of the packet immediately. Unlike iptables, extensions are not yet implemented. 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.
-c, --set-counters PKTS BYTES: This enables the administrator to initialize the packet and byte counters of a rule (during INSERT, APPEND, REPLACE operations).

OTHER OPTIONS

The following additional options can be specified:

-v, --verbose: Verbose output. This option makes the list command show the interface name, 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). 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).
-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.
--line-numbers: When listing rules, add line numbers to the beginning of each rule, corresponding to that rule's position in the chain.
--modprobe=command: When adding or inserting rules into a chain, use command to load any necessary modules (targets, match extensions, etc).

MANGLE OPTIONS

The kernel mangle module supports the following options

--mangle-ip-s IP address: Change the source IP address of the packet to the specified value.
--mangle-ip-d IP address: Change the destination IP address of the packet to the specified value.
--mangle-hw-s hardware address: CHange the source hardware (MAC) address of the packet to the specified value.
--mangle-hw-d hardware address: Change the destination hardware (MAC) address of the packet to the specified value.
--mangle-target target: Disposition of the packet. Valid targets are DROP, CONTINUE, or ACCEPT. If no --mangle-target option is specified, the default is ACCEPT.

EXAMPLES

Let's say you have a machine with two ip addresses aaaa and bbbb. Address aaaa is only for the use of machine cccc. No other machine should be allowed to connect to it. Iptables rules are configured to enforce this requirement.

# Configure iptables to NAT any attempt to use aaaa on
# outgoing packets to machines other than cccc to use
# bbbb instead
iptables -t nat -A POSTROUTING -s aaaa ! -d cccc \
    -j SNAT --to bbbb

# Ignore arp requests from machines other than cccc for
# address aaaa.
arptables -A IN ! -s cccc -d aaaa -j DROP

# Mangle any outgoing requests from address aaaa to any
# machine but cccc to use address bbbb instead.
arptables -A OUT -s aaaa ! -d cccc -j mangle \
    --mangle-ip-s bbbb

DIAGNOSTICS

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.

BUGS

The -L -v output is excessively wide.

The short option names were chosen at random.

Well... the counters are not reliable on sparc64.

AUTHORS

Jay Fenlason <fenlason@redhat.com> wrote arptables, which was based on the iptables code by Rusty Russell, in early consultation with Michael Neuling.

The iptables man page was written by Herve Eychenne <rv@wallfire.org>, Jay Fenlason <fenlason@redhat.com> adapted it for arptables.