Exim 4.40 Specification chapter 17

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# 17. The dnslookup router

The dnslookup 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.

If SRV support is configured (see `check_srv` below), Exim first searches for SRV records. If none are found, or if SRV support is not configured, MX records are looked up. If no MX records exist, address records are sought. However, `mx_domains` can be set to disable the direct use of address records.

MX records of equal priority are sorted by Exim into a random order. Exim then looks for address records for the host names obtained from MX or SRV records. When a host has more than one IP address, they are sorted into a random order, except that IPv6 addresses are always sorted before IPv4 addresses. If all the IP addresses found are discarded by a setting of the `ignore_target_hosts` generic option, the router declines.

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. The way in which Exim decides whether a host is the local host is described in section 13.6.

### check_srv

Type: string, expanded
Default: unset

The dnslookup router supports the use of SRV records (see RFC 2782) in addition to MX and address records. The support is disabled by default. To enable SRV support, set the `check_srv` option to the name of the service required. For example,

      check_srv = smtp

looks for SRV records that refer to the normal smtp service. The option is expanded, so the service name can vary from message to message or address to address. This might be helpful if SRV records are being used for a submission service. If the expansion is forced to fail, the `check_srv` option is ignored, and the router proceeds to look for MX records in the normal way.

When the expansion succeeds, the router searches first for SRV records for the given service (it assumes TCP protocol). A single SRV record with the host name `.` indicates “no such service for this domain”; if this is encountered, the router declines. If other kinds of SRV record are found, they are used to construct a host list for delivery according to the rules of RFC 2782. MX records are not sought in this case.

However, when no SRV records are found, MX records (and address records) are sought in the traditional way. In other words, SRV records take precedence over MX records, just as MX records take precedence over address records. Note that this behaviour is not sanctioned by RFC 2782, though a previous draft RFC defined it. It is apparently believed that MX records are sufficient for email and that SRV records should not be used for this purpose. However, SRV records have an additional “weight” feature which some people might find useful when trying to split an SMTP load between hosts of different power.

### mx_domains

Type: domain list, expanded
Default: unset

A domain that matches `mx_domains` is required to have either an MX or an SRV record in order to be recognised. (The name of this option could be improved.) 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

If the domain name in the address that is being processed is not fully qualified, it may be expanded to its full form by a DNS lookup. For example, if an address is specified as dormouse@teaparty, the domain might be expanded to teaparty.wonderland.fict.example. Domain expansion can also occur as a result of setting the `widen_domains` option. If `rewrite_headers` is true, all occurrences of the abbreviated domain name in any Bcc:, Cc:, From:, Reply-to:, Sender:, and To: header lines of the message are rewritten with the full domain 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 preconditions 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, provided the following conditions are met:

- No router that processed the address specified `headers_add` or `headers_remove`.

- The router did not change the address in any way, for example, by “widening” 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.

## 17.1. Effect of qualify_single and search_parents

When a domain from an envelope recipient is changed by the resolver as a result of the `qualify_single` or `search_parents` options, Exim rewrites the corresponding address in the message's header lines unless `rewrite_headers` is set false. Exim then re-routes the address, using the full domain.

These two options affect only the DNS lookup that takes place inside the router for the domain of the address that is being routed. They do not affect lookups such as that implied by

      domains = @mx_any

that may happen while processing a router precondition before the router is entered. No widening ever takes place for these lookups.

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