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27. Additional generic options for routers

The following additional generic options apply to all routers, in addition to the common generic options for both directors and routers which are described in chapter 20. Routers are concerned with addresses whose domains do not match something in local_domains.

ignore_target_hosts (router)

Type: host list, expanded
Default: unset

Although this option is a host list, it would normally contain IP address entries rather than names. If any host that is looked up by the router matches an item in this list, Exim behaves as if the host did not exist. This option allows you to cope with rogue DNS entries like

some.remote.domain  A

by setting

ignore_target_hosts =

on the relevant router. Attempts to mail to such a domain then receive the `unrouteable domain' error, and verifications fail. This option may also be useful for ignoring link local IPv6 addresses. The string value of ignore_target_hosts is expanded before use as a list, so it is possible to make it dependent on the domain that is being routed.

pass_on_timeout (router)

Type: boolean
Default: false

If a router times out during a host lookup, it normally causes deferral of the address. If pass_on_timeout is set, the address is passed on to the next router, overriding no_more. This may be helpful for systems that are intermittently connected to the Internet, or those that want to pass to a smart host any messages that cannot immediately be delivered.

There are occasional other temporary errors that can occur while doing DNS lookups. They are treated in the same way as a timeout, and this option applies to all of them.

self (router)

Type: string
Default: "freeze"

This option specifies what is to happen if routing a remote address ends up pointing at the local host (checked by comparing IP addresses), or at a host whose name matches hosts_treat_as_local. Normally this indicates either an error in Exim's configuration (for example, the domain should be listed as local), or an error in the DNS (for example, the MX shouldn't point at this host). However, this situation is not confined to the use of MX records, and the self option can be used on any router.

The default action is to freeze the message. The following alternatives are provided for use in special cases:

When a router just rewrites, that is, does not set up IP addresses, the self option is not relevant.

translate_ip_address (router)

Type: string, expanded
Default: unset

There exist some rare networking situations (for example, packet radio) where it is helpful to be able to translate IP addresses generated by normal routing mechanisms into other IP addresses, thus performing a kind of manual IP routing. This should be done only if the normal IP routing of the TCP/IP stack is inadequate or broken. Because this is an extremely uncommon requirement, the code to support this option is not included in the Exim binary unless SUPPORT_TRANSLATE_IP_ADDRESS=yes is set in `Local/Makefile'.

The translate_ip_address string is expanded for every IP address generated by the router, with the generated address set in $host_address. If the expansion is forced to fail, no action is taken. If it returns an IP address, that replaces the original address; otherwise the result is assumed to be a host name -- this is looked up using gethostbyname() to produce one or more replacement IP addresses. For example, to subvert all IP addresses in some specific networks, this could be added to a router:

$smc{translate_ip_address = \

The file would contain lines like    some.host

You should not make use of this facility unless you really understand what you are doing.

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