Exim 4.10 Specification chapter 16

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# 16. The dnslookup router

The nslookup router looks up the hosts that handle mail for the given domain in the DNS. A transport must always be set for this router, unless `verify_only` is set.

MX records are looked up first, followed by address records if no MX records are found, unless the domain matches `mx_domains`. MX records of equal priority are sorted by Exim into a random order. Unless they have the highest priority (lowest MX value), MX records that point to the local host, or to any host name that matches `hosts_treat_as_local`, are discarded, together with any other MX records of equal or lower priority.

If the host pointed to by the highest priority MX record, or looked up as an address record, is the local host, or matches `hosts_treat_as_local`, what happens is controlled by the generic `self` option.

There are a number of private options that can be used to vary the way the DNS lookup is handled.

### check_secondary_mx

Type: boolean
Default: false

If this option is set, the router declines unless the local host is found in (and removed from) the list of hosts obtained by MX lookup. This can be used to process domains for which the local host is a secondary mail exchanger differently to other domains.

### mx_domains

Type: domain list, expanded
Default: unset

A domain which matches `mx_domains` is required to have an MX record in order to be recognised. For example, if all the mail hosts in fict.example are known to have MX records, except for those in discworld.fict.example, you could use this setting:

      mx_domains = ! *.discworld.fict.example : *.fict.example

This specifies that messages addressed to a domain that matches the list but has no MX record should be bounced immediately instead of being routed using the address record.

### qualify_single

Type: boolean
Default: true

When this option is true, the resolver option RES_DEFNAMES is set for DNS lookups. Typically, but not standardly, this causes the resolver to qualify single-component names with the default domain. For example, on a machine called dictionary.ref.example, the domain thesaurus would be changed to thesaurus.ref.example inside the resolver. For details of what your resolver actually does, consult your man pages for resolver and resolv.conf.

### rewrite_headers

Type: boolean
Default: true

An abbreviated name may be expanded to its full form by a DNS lookup, or as a result of the `widen_domains` option. For example, if an address is specified as dormouse@teaparty, the domain might get expanded to teaparty.wonderland.fict.example. If this option is true, all occurrences of the abbreviated name in the headers of the message are rewritten with the full name. This option should be turned off only when it is known that no message is ever going to be sent outside an environment where the abbreviation makes sense.

When an MX record is looked up in the DNS and matches a wildcard record, name servers normally return a record containing the name that has been looked up, making it impossible to detect whether a wildcard was present or not. However, some name servers have recently been seen to return the wildcard entry. If the name returned by a DNS lookup begins with an asterisk, it is not used for header rewriting.

### same_domain_copy_routing

Type: boolean
Default: false

Addresses with the same domain are normally routed by the dnslookup router to the same list of hosts. However, this cannot be presumed, because the router options and pre-conditions may refer to the local part of the address. By default, therefore, Exim routes each address in a message independently. DNS servers run caches, so repeated DNS lookups are not normally expensive, and in any case, personal messages rarely have more than a few recipients.

If you are running mailing lists with large numbers of subscribers at the same domain, and you are using a dnslookup router which is independent of the local part, you can set `same_domain_copy_routing` to bypass repeated DNS lookups for identical domains in one message. In this case, when dnslookup routes an address to a remote transport, any other unrouted addresses in the message that have the same domain are automatically given the same routing without processing them independently. However, this is only done if `headers_add` and `headers_remove` are unset, and the router does not ``widen'' the domain.

### search_parents

Type: boolean
Default: false

When this option is true, the resolver option RES_DNSRCH is set for DNS lookups. This is different from the `qualify_single` option in that it applies to domains containing dots. Typically, but not standardly, it causes the resolver to search for the name in the current domain and in parent domains. For example, on a machine in the fict.example domain, if looking up teaparty.wonderland failed, the resolver would try teaparty.wonderland.fict.example. For details of what your resolver actually does, consult your man pages for resolver and resolv.conf.

Setting this option true can cause problems in domains that have a wildcard MX record, because any domain that does not have its own MX record matches the local wildcard.

### widen_domains

Type: string list
Default: unset

If a DNS lookup fails and this option is set, each of its strings in turn is added onto the end of the domain, and the lookup is tried again. For example, if

      widen_domains = fict.example:ref.example

is set and a lookup of klingon.dictionary fails, klingon.dictionary.fict.example is looked up, and if this fails, klingon.dictionary.ref.example is tried. Note that the `qualify_single` and `search_parents` options can cause some widening to be undertaken inside the DNS resolver.

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