The lookuphost router looks up the hosts that handle mail for the given domain either via the gethostbyname() function, or by using the DNS directly. A transport must always be set for this router, unless verify_only is set.
When the DNS is used, 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 the host looked up by gethostbyname() is the local host, or matches hosts_treat_as_local, then what happens is controlled by the generic self option.
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.
If this is true, the gethostbyname() function is used and the options relating to the DNS are ignored. Otherwise, the name is looked up directly in the DNS. Of course, gethostbyname() may do its own DNS lookup for an A record (no MX processing is involved), but it may also access other sources of information such as /etc/hosts.
When Exim is compiled with IPv6 support, if a host that is looked up in the DNS has both A and AAAA or A6 records, all the addresses are used.
Type: domain list
This option applies to domains that are looked up directly in the DNS (gethostbyname is not set). 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.book are known to have MX records, except for those in discworld.fict.book, options of the form
mx_domains = ! *.discworld.fict.book : *.fict.book
could be used. This would cause messages addressed to a machine that matched the option but had only an A record to be bounced immediately instead of sitting on the queue until the delivery timed out.
If domains are being looked up in the DNS (gethostbyname is false), the resolver option that causes it to qualify single-component names with the default domain (RES_DEFNAMES) is set. For example, on a machine called dictionary.ref.book, looking up the domain thesaurus would cause the name thesaurus.ref.book to be looked up internally in the resolver. Exim itself still looks up the single name.
An abbreviated name may be expanded to its full form by both gethostbyname() or by 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.book. 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, nameservers 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 nameservers 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.
If domains are being looked up in the DNS (gethostbyname is false), the resolver option that causes it to search parent domains (RES_DNSRCH) is set if this option is true. This is different from the qualify_single option in that it applies to domains containing dots. For example, on a machine in the fict.book domain, when looking up teaparty.wonderland initially fails, the resolver automatically tries teaparty.wonderland.fict.book if this option is set. The default setting of this option used to be true, but this causes problems in domains that have a wildcard MX record, because any domain that does not have its own MX record then matches the local wildcard. The default was changed to false in Exim 1.80.
Type: string list
If a 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.book:ref.book
is set and a lookup of klingon.dictionary fails, klingon.dictionary.fict.book is looked up, and if this fails, klingon.dictionary.ref.book is tried. This option applies to lookups using gethostbyname() as well as to DNS lookups. Note that when the DNS is being used for lookups, the qualify_single and search_parents options cause some widening to be undertaken inside the DNS resolver.
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