There are three Ethernet frame tables with built-in chains in the Linux kernel. The kernel tables are 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 Ethernet frames. If a rule matches an Ethernet 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.

A firewall rule specifies criteria for an Ethernet 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.

As stated earlier, there are three Ethernet frame tables in the Linux kernel. The tables are filter, nat and broute. Of these three tables, the filter table is the default table that the ebtables command operates on. If you are working with the filter table, then you can drop the '-t filter' argument to the ebtables command. However, you will need to provide the -t argument for the other two tables. The -t argument must be the first argument on the ebtables command line, if used.

-t, --table

filter, is the default table and contains three built-in chains: INPUT (for frames destined for the bridge itself), OUTPUT (for locally-generated frames) and FORWARD (for frames being bridged).



nat, is used to change the mac addresses and contains three built-in chains: PREROUTING (for altering frames as soon as they come in), OUTPUT (for altering locally generated frames before they are bridged) and POSTROUTING (for altering frames as they are about to go out). A small note on the naming of chains POSTROUTING and PREROUTING: it would be more accurate to call them PREFORWARDING and POSTFORWARDING, but for all those who come from the iptables world to ebtables it is easier to have the same names.



broute, is used to make a brouter, it has one built-in chain: BROUTING. The targets DROP and ACCEPT have special meaning in the broute table. DROP actually means the frame has to be routed, while ACCEPT means the frame has to be bridged. The BROUTING chain is traversed very early. It is only traversed by frames entering on a bridge enslaved NIC that is in forwarding state. Normally those frames would be bridged, but you can decide otherwise here. The redirect target is very handy here.

The ebtables 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 both the -Z and --atomic-file commands, 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.

-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.

The following three options change the output of the -L list command:

--Ln

Places the rule number in front of every rule.

--Lc

Shows the counters at the end of each rule displayed by the -L command. Both a frame counter (pcnt) and a byte counter (bcnt) are displayed.

--Lx

The output of the --Lx option may be used to create a set of ebtables commands. You may use this set of commands in an ebtables boot or reload script. For example the output could be used at system startup. The --Lx option is incompatible with both of the other --Ln and --Lc chain listing options.

--Lmac2

Shows all MAC addresses with the same length, adding leading zeroes if necessary. The default representation omits zeroes in the addresses when they are not needed.

All necessary ebtables commands for making the current list of user-defined chains in the kernel and any commands issued by the user to rename the standard ebtables chains will be listed, when no chain name is supplied for the -L command while using the --Lx option.

-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 ebtables 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 ebtables chain names, please be sure to mention this fact should you post a question on the ebtables mailing lists. It would be wise to use the standard name in your post. Renaming a standard ebtables chain in this fashion has no effect on the structure or function of the ebtables kernel table.

--init-table

Replace the current table data by the initial table data.

--atomic-init

Copy the kernel's initial data of the table to the specified file. This can be used as the first action, after which rules are added to the file. The file can be specified using the --atomic-file command or through the EBTABLES_ATOMIC_FILE environment variable.

--atomic-save

Copy the kernel's current data of the table to the specified file. This can be used as the first action, after which rules are added to the file. The file can be specified using the --atomic-file command or through the EBTABLES_ATOMIC_FILE environment variable.

--atomic-commit

Replace the kernel table data with the data contained in the specified file. This is a useful command that allows you to load all your rules of a certain table into the kernel at once, saving the kernel a lot of precious time and allowing atomic updates of the tables. The file which contains the table data is constructed by using either the --atomic-init or the --atomic-save command to generate a starting file. After that, using the --atomic-file command when constructing rules or setting the EBTABLES_ATOMIC_FILE environment variable allows you to extend the file and build the complete table before committing it to the kernel.

--atomic-file -Z

The counters stored in a file with, say, --atomic-init can be optionally zeroed by supplying the -Z command. You may also zero the counters by setting the EBTABLES_ATOMIC_FILE environment variable.

-V, --version

Show the version of the ebtables userspace program.

-h, --help

Give a brief description of the command syntax. Here you can also specify names of extensions and ebtables will try to write help about those extensions. E.g. ebtables -h snat log ip arp. Specify list_extensions to list all extensions supported by the userspace utility.

-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.

--atomic-file file

Let the command operate on the specified file. The data of the table to operate on will be extracted from the file and the result of the operation will be saved back into the file. If specified, this option should come before the command specification. An alternative that should be preferred, is setting the EBTABLES_ATOMIC_FILE environment variable.

-M, --modprobe program

When talking to the kernel, use this program to try to automatically load missing kernel modules.

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. See both the MATCH-EXTENSIONS and the WATCHER-EXTENSION(S) below.

-p, --protocol [!] protocol

The protocol that was responsible for creating the frame. This can be a hexadecimal number, above 0x0600, a name (e.g. ARP ) or LENGTH. The protocol field of the Ethernet frame can be used to denote the length of the header (802.2/802.3 networks). When the value of that field is below (or equals) 0x0600, the value equals the size of the header and shouldn't be used as a protocol number. Instead, all frames where the protocol field is used as the length field are assumed to be of the same 'protocol'. The protocol name used in ebtables for these frames is LENGTH.

The file /etc/ethertypes can be used to show readable characters instead of hexadecimal numbers for the protocols. For example, 0x0800 will be represented by IPV4. The use of this file is not case sensitive. See that file for more information. The flag --proto is an alias for this option.

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

The interface via which a frame is received (for the INPUT, FORWARD, PREROUTING and BROUTING chains). The flag --in-if is an alias for this option.

--logical-in [!] name

The (logical) bridge interface via which a frame is received (for the INPUT, FORWARD, PREROUTING and BROUTING chains).

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

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

--logical-out [!] name

The (logical) bridge interface via which a frame is going to be sent (for the OUTPUT, FORWARD and POSTROUTING chains).

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

The source mac address. Both mask and address are written as 6 hexadecimal numbers separated by colons. Alternatively one can specify Unicast, Multicast, Broadcast or BGA (Bridge Group Address).

Unicast = 00:00:00:00:00:00/01:00:00:00:00:00, Multicast = 01:00:00:00:00:00/01:00:00:00:00:00, Broadcast = ff:ff:ff:ff:ff:ff/ff:ff:ff:ff:ff:ff or BGA = 01:80:c2:00:00:00/ff:ff:ff:ff:ff:ff. Note that a broadcast address will also match the multicast specification. The flag --src is an alias for this option.

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

The destination mac address. See -s (above) for more details. The flag --dst is an alias for this option.

ebtables 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.

Specify 802.3 DSAP/SSAP fields or SNAP type. The protocol must be specified as LENGTH (see protocol above).

--802_3-sap [!] sap

DSAP and SSAP are two one byte 802.3 fields. The bytes are always equal, so only one byte (hexadecimal) is needed as an argument.

--802_3-type [!] type

If the 802.3 DSAP and SSAP values are 0xaa then the SNAP type field must be consulted to determine the payload protocol. This is a two byte (hexadecimal) argument. Only 802.3 frames with DSAP/SSAP 0xaa are checked for type.

Specify arp fields. The protocol must be specified as ARP or RARP.

--arp-opcode [!] opcode

The (r)arp opcode (decimal or a string, for more details see ebtables -h arp).

--arp-htype [!] hardware type

The hardware type, this can be a decimal or the string "Ethernet". This is normally Ethernet (value 1).

--arp-ptype [!] protocol type

The protocol type for which the (r)arp is used (hexadecimal or the string "IPv4"). This is normally IPv4 (0x0800).

--arp-ip-src [!] address[/mask]

The ARP IP source address specification.

--arp-ip-dst [!] address[/mask]

The ARP IP destination address specification.

--arp-mac-src [!] address[/mask]

The ARP MAC source address specification.

--arp-mac-dst [!] address[/mask]

The ARP MAC destination address specification.

Specify ip fields. The protocol must be specified as IPv4.

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

The source ip address. The flag --ip-src is an alias for this option.

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

The destination ip address. The flag --ip-dst is an alias for this option.

--ip-tos [!] tos

The ip type of service, in hexadecimal numbers. IPv4.

--ip-protocol [!] protocol

The ip protocol. The flag --ip-proto is an alias for this option.

--ip-source-port [!] port[:port]

The source port or port range for the ip protocols 6 (TCP) and 17 (UDP). If the first port is omitted, "0" is assumed; if the last is omitted, "65535" is assumed. The flag --ip-sport is an alias for this option.

--ip-destination-port [!] port[:port]

The destination port or port range for ip protocols 6 (TCP) and 17 (UDP). The flag --ip-dport is an alias for this option.

--mark [!] [value][/mask]

Matches frames with the given unsigned mark value. If a mark value and mask is specified, the logical AND of the mark value of the frame and the user-specified mask is taken before comparing it with the user-specified mark value. If only a mask is specified (start with '/') the logical AND of the mark value of the frame and the user-specified mark is taken and the result is compared with zero.

--pkttype-type [!] type

Matches on the Ethernet "class" of the frame, which is determined by the generic networking code. Possible values: broadcast (MAC destination is broadcast address), multicast (MAC destination is multicast address), host (MAC destination is the receiving network device) or otherhost (none of the above).

Specify stp BPDU (bridge protocol data unit) fields. The destination address must be specified as the bridge group address (BGA).

--stp-type [!] type

The BPDU type (0-255), special recognized types: config: configuration BPDU (=0) and tcn: topology change notification BPDU (=128).

--stp-flags [!] flag

The BPDU flag (0-255), special recognized flags: topology-change: the topology change flag (=1) topology-change-ack: the topology change acknowledgement flag (=128).

--stp-root-prio [!] [prio][:prio]

The root priority (0-65535) range.

--stp-root-addr [!] [address][/mask]

The root mac address, see the option -s for more details.

--stp-root-cost [!] [cost][:cost]

The root path cost (0-4294967295) range.

--stp-sender-prio [!] [prio][:prio]

The BPDU's sender priority (0-65535) range.

--stp-sender-addr [!] [address][/mask]

The BPDU's sender mac address, see the option -s for more details.

--stp-port [!] [port][:port]

The port identifier (0-65535) range.

--stp-msg-age [!] [age][:age]

The message age timer (0-65535) range.

--stp-max-age [!] [age][:age]

The max age timer (0-65535) range.

--stp-hello-time [!] [time][:time]

The hello time timer (0-65535) range.

--stp-forward-delay [!] [delay][:delay]

The forward delay timer (0-65535) range.

Specify 802.1Q Tag Control Information fields. The protocol must be specified as 802_1Q (0x8100).

--vlan-id [!] id

The VLAN identifier field (VID). Decimal number from 0 to 4095.

--vlan-prio [!] prio

The user_priority field. Decimal number from 0 to 7. The VID should be set to 0 ("null VID") or unspecified (for this case the VID is deliberately set to 0).

--vlan-encap [!] type

The encapsulated Ethernet frame type/length. Specified as hexadecimal number from 0x0000 to 0xFFFF or as a symbolic name from /etc/ethertypes.

Watchers are things that only look at frames passing by. These watchers only see the frame if the frame matches the rule.

The fact that the log module is a watcher lets us log stuff while giving a target by choice. Note that the log module therefore is not a target.

--log

Use this if you won't specify any other log options, so if you want to use the default settings: log-prefix="", no arp logging, no ip logging, log-level=info.

--log-level level

defines the logging level. For the possible values: ebtables -h log. The default level is info.

--log-prefix text

defines the prefix to be printed before the logging information.

--log-ip

will log the ip information when a frame made by the ip protocol matches the rule. The default is no ip information logging.

--log-arp

will log the (r)arp information when a frame made by the (r)arp protocols matches the rule. The default is no (r)arp information logging.

The arpreply target can be used in the PREROUTING chain of the nat table. If this target sees an arp request it will automatically reply with an arp reply. The used MAC address for the reply can be specified. When the arp message is not an arp request, it is ignored by this target.

--arpreply-mac address

Specifies the MAC address to reply with: the Ethernet source MAC and the ARP payload source MAC will be filled in with this address.

--arpreply-target target

Specifies the standard target. After sending the arp reply, the rule still has to give a standard target so ebtables knows what to do. The default target is DROP.

The dnat target can only be used in the BROUTING chain of the broute table and the PREROUTING and OUTPUT chains of the nat table. It specifies that the destination mac address has to be changed.

--to-destination address

The flag --to-dst is an alias for this option.

--dnat-target target

Specifies the standard target. After doing the dnat, the rule still has to give a standard target so ebtables knows what to do. The default target is ACCEPT. Making it CONTINUE could let you use multiple target extensions on the same frame. Making it DROP only makes sense in the BROUTING chain but using the redirect target is more logical there. RETURN is also allowed. Note that using RETURN in a base chain is not allowed.

The mark target can be used in every chain of every table. It is possible to use the marking of a frame/packet in both ebtables and iptables, if the br-nf code is compiled into the kernel. Both put the marking at the same place. So, you can consider this fact as a feature, or as something to watch out for.

--set-mark value

Mark the frame with the specified unsigned value.

--mark-target target

Specifies the standard target. After marking the frame, the rule still has to give a standard target so ebtables knows what to do. The default target is ACCEPT. Making it CONTINUE can let you do other things with the frame in other rules of the chain.

The redirect target will change the MAC target address to that of the bridge device the frame arrived on. This target can only be used in the BROUTING chain of the broute table and the PREROUTING chain of the nat table.

--redirect-target target

Specifies the standard target. After doing the MAC redirect, the rule still has to give a standard target so ebtables knows what to do. The default target is ACCEPT. Making it CONTINUE could let you use multiple target extensions on the same frame. Making it DROP in the BROUTING chain will let the frames be routed. RETURN is also allowed. Note that using RETURN in a base chain is not allowed.

The snat target can only be used in the POSTROUTING chain of the nat table. It specifies that the source mac address has to be changed.

--to-source address

The flag --to-src is an alias for this option.

--snat-target target

Specifies the standard target. After doing the snat, the rule still has to give a standard target so ebtables knows what to do. The default target is ACCEPT. Making it CONTINUE could let you use multiple target extensions on the same frame. Making it DROP doesn't make sense, but you could do that too. RETURN is also allowed. Note that using RETURN in a base chain is not allowed.