Name

interfaces — Shorewall interfaces file

Synopsis

/etc/shorewall/interfaces

Description

The interfaces file serves to define the firewall's network interfaces to Shorewall. The order of entries in this file is not significant in determining zone composition.

The columns in the file are as follows.

ZONE - zone-name

Zone for this interface. Must match the name of a zone declared in /etc/shorewall/zones. You may not list the firewall zone in this column.

If the interface serves multiple zones that will be defined in the shorewall-hosts(5) file, you should place "-" in this column.

If there are multiple interfaces to the same zone, you must list them in separate entries.

Example:

#ZONE   INTERFACE       BROADCAST
loc     eth1            -
loc     eth2            -
INTERFACE - interface[:port]

Name of interface. Each interface may be listed only once in this file. You may NOT specify the name of a "virtual" interface (e.g., eth0:0) here; see http://www.shorewall.net/FAQ.htm#faq18

You may use wildcards here by specifying a prefix followed by the plus sign ("+"). For example, if you want to make an entry that applies to all PPP interfaces, use 'ppp+'; that would match ppp0, ppp1, ppp2, …

When using Shorewall versions before 4.1.4, care must be exercised when using wildcards where there is another zone that uses a matching specific interface. See shorewall-nesting(5) for a discussion of this problem.

Beginning with Shorewall 4.2.3, Shorewall-perl allows '+' as an interface name.

There is no need to define the loopback interface (lo) in this file.

(Shorewall-perl only) If a port is given, then the interface must have been defined previously with the bridge option. The OPTIONS column may not contain the following options when a port is given.

arp_filter
arp_ignore
bridge
log_martians
mss
optional
proxyarp
routefilter
sourceroute
upnp
BROADCAST (Optional) - {-|detect|address[,address]...}

The broadcast address(es) for the network(s) to which the interface belongs. For P-T-P interfaces, this column is left blank. If the interface has multiple addresses on multiple subnets then list the broadcast addresses as a comma-separated list.

If you use the special value detect, Shorewall will detect the broadcast address(es) for you. If you select this option, the interface must be up before the firewall is started.

If you don't want to give a value for this column but you want to enter a value in the OPTIONS column, enter - in this column.

Note to Shorewall-perl users: Shorewall-perl only supports detect or - in this column. If you specify addresses, a compilation warning will be issued.

OPTIONS (Optional) - [option[,option]...]

A comma-separated list of options from the following list. The order in which you list the options is not significant but the list should have no embedded white space.

arp_filter[={0|1}]

If specified, this interface will only respond to ARP who-has requests for IP addresses configured on the interface. If not specified, the interface can respond to ARP who-has requests for IP addresses on any of the firewall's interface. The interface must be up when Shorewall is started.

The option value (0 or 1) may only be specified if you are using Shorewall-perl. With Shorewall-perl, only those interfaces with the arp_filter option will have their setting changes; the value assigned to the setting will be the value specified (if any) or 1 if no value is given.

Note

This option does not work with a wild-card interface name (e.g., eth0.+) in the INTERFACE column.

arp_ignore[=number]

If specified, this interface will respond to arp requests based on the value of number (defaults to 1).

1 - reply only if the target IP address is local address configured on the incoming interface

2 - reply only if the target IP address is local address configured on the incoming interface and the sender's IP address is part from same subnet on this interface

3 - do not reply for local addresses configured with scope host, only resolutions for global and link

4-7 - reserved

8 - do not reply for all local addresses

Note

This option does not work with a wild-card interface name (e.g., eth0.+) in the INTERFACE column.

Warning

Do not specify arp_ignore for any interface involved in Proxy ARP.

blacklist

Check packets arriving on this interface against the shorewall-blacklist(5) file.

bridge

(Shorewall-perl only) Designates the interface as a bridge.

detectnets (Deprecated and not supported by Shorewall-perl)

Automatically tailors the zone named in the ZONE column to include only those hosts routed through the interface.

Warning

Do not set the detectnets option on your internet interface.

Support for this option will be removed in a future release of Shorewall-perl. Better to use the routefilter option together with the logmartians option.

dhcp

Specify this option when any of the following are true:

  1. the interface gets its IP address via DHCP

  2. the interface is used by a DHCP server running on the firewall

  3. the interface has a static IP but is on a LAN segment with lots of DHCP clients.

  4. the interface is a simple bridge with a DHCP server on one port and DHCP clients on another port.

    Note

    If you use Shorewall-perl for firewall/bridging, then you need to include DHCP-specific rules in shorewall-rules(8). DHCP uses UDP ports 67 and 68.

This option allows DHCP datagrams to enter and leave the interface.

logmartians[={0|1}]

Turn on kernel martian logging (logging of packets with impossible source addresses. It is strongly suggested that if you set routefilter on an interface that you also set logmartians. Even if you do not specify the routefilter option, it is a good idea to specify logmartians because your distribution may be enabling route filtering without you knowing it.

The option value (0 or 1) may only be specified if you are using Shorewall-perl. With Shorewall-perl, only those interfaces with the logmartians option will have their setting changes; the value assigned to the setting will be the value specified (if any) or 1 if no value is given.

To find out if route filtering is set on a given interface, check the contents of /proc/sys/net/ipv4/conf/interface/rp_filter - a non-zero value indicates that route filtering is enabled.

Example:

        teastep@lists:~$ cat /proc/sys/net/ipv4/conf/eth0/rp_filter 
        1
        teastep@lists:~$

Note

This option does not work with a wild-card interface name (e.g., eth0.+) in the INTERFACE column.

This option may also be enabled globally in the shorewall.conf(5) file.

maclist

Connection requests from this interface are compared against the contents of shorewall-maclist(5). If this option is specified, the interface must be an ethernet NIC and must be up before Shorewall is started.

mss=number

Added in Shorewall 4.0.3. Causes forwarded TCP SYN packets entering or leaving on this interface to have their MSS field set to the specified number.

norfc1918

This interface should not receive any packets whose source is in one of the ranges reserved by RFC 1918 (i.e., private or "non-routable" addresses). If packet mangling or connection-tracking match is enabled in your kernel, packets whose destination addresses are reserved by RFC 1918 are also rejected.

nosmurfs

Filter packets for smurfs (packets with a broadcast address as the source).

Smurfs will be optionally logged based on the setting of SMURF_LOG_LEVEL in shorewall.conf(5). After logging, the packets are dropped.

optional

Only supported by Shorewall-perl. When optional is specified for an interface, Shorewall will be silent when:

  • a /proc/sys/net/ipv4/conf/ entry for the interface cannot be modified (including for proxy ARP).

  • The first address of the interface cannot be obtained.

I specify optional on interfaces to Xen virtual machines that may or may not be running when Shorewall is [re]started.

Caution

Use optional at your own risk. If you [re]start Shorewall when an 'optional' interface is not available and then do a shorewall save, subsequent shorewall restore and shorewall -f start operations will instantiate a ruleset that does not support that interface, even if it is available at the time of the restore/start.

proxyarp[={0|1}]

Sets /proc/sys/net/ipv4/conf/interface/proxy_arp. Do NOT use this option if you are employing Proxy ARP through entries in shorewall-proxyarp(5). This option is intended solely for use with Proxy ARP sub-networking as described at: http://tldp.org/HOWTO/Proxy-ARP-Subnet/index.html.

Note: This option does not work with a wild-card interface name (e.g., eth0.+) in the INTERFACE column.

The option value (0 or 1) may only be specified if you are using Shorewall-perl. With Shorewall-perl, only those interfaces with the proxyarp option will have their setting changed; the value assigned to the setting will be the value specified (if any) or 1 if no value is given.

routeback

If specified, indicates that Shorewall should include rules that allow traffic arriving on this interface to be routed back out that same interface. This option is also required when you have used a wildcard in the INTERFACE column if you want to allow traffic between the interfaces that match the wildcard.

routefilter[={0|1}]

Turn on kernel route filtering for this interface (anti-spoofing measure).

The option value (0 or 1) may only be specified if you are using Shorewall-perl. With Shorewall-perl, only those interfaces with the routefilter option will have their setting changes; the value assigned to the setting will be the value specified (if any) or 1 if no value is given.

Note

This option does not work with a wild-card interface name (e.g., eth0.+) in the INTERFACE column.

This option can also be enabled globally in the shorewall.conf(5) file.

sourceroute[={0|1}]

If this option is not specified for an interface, then source-routed packets will not be accepted from that interface (sets /proc/sys/net/ipv4/conf/interface/accept_source_route to 1). Only set this option if you know what you are doing. This might represent a security risk and is not usually needed.

The option value (0 or 1) may only be specified if you are using Shorewall-perl. With Shorewall-perl, only those interfaces with the sourceroute option will have their setting changes; the value assigned to the setting will be the value specified (if any) or 1 if no value is given.

Note

This option does not work with a wild-card interface name (e.g., eth0.+) in the INTERFACE column.

tcpflags

Packets arriving on this interface are checked for certain illegal combinations of TCP flags. Packets found to have such a combination of flags are handled according to the setting of TCP_FLAGS_DISPOSITION after having been logged according to the setting of TCP_FLAGS_LOG_LEVEL.

upnp

Incoming requests from this interface may be remapped via UPNP (upnpd). See http://www.shorewall.net/UPnP.html.

Example

Example 1:

Suppose you have eth0 connected to a DSL modem and eth1 connected to your local network and that your local subnet is 192.168.1.0/24. The interface gets it's IP address via DHCP from subnet 206.191.149.192/27. You have a DMZ with subnet 192.168.2.0/24 using eth2.

Your entries for this setup would look like:

#ZONE   INTERFACE BROADCAST        OPTIONS
net     eth0      206.191.149.223  dhcp
loc     eth1      192.168.1.255
dmz     eth2      192.168.2.255
Example 2:

The same configuration without specifying broadcast addresses is:

#ZONE   INTERFACE BROADCAST        OPTIONS
net     eth0      detect           dhcp
loc     eth1      detect
dmz     eth2      detect
Example 3:

You have a simple dial-in system with no ethernet connections.

#ZONE   INTERFACE BROADCAST        OPTIONS
net     ppp0      -

FILES

/etc/shorewall/interfaces

See ALSO

shorewall(8), shorewall-accounting(5), shorewall-actions(5), shorewall-blacklist(5), shorewall-hosts(5), shorewall-ipsec(5), shorewall-maclist(5), shorewall-masq(5), shorewall-nat(5), shorewall-netmap(5), shorewall-params(5), shorewall-policy(5), shorewall-providers(5), shorewall-proxyarp(5), shorewall-route_rules(5), shorewall-routestopped(5), shorewall-rules(5), shorewall.conf(5), shorewall-tcclasses(5), shorewall-tcdevices(5), shorewall-tcrules(5), shorewall-tos(5), shorewall-tunnels(5), shorewall-zones(5)