Alias_urlN MTA options

From Messaging Server Technical Reference Wiki
Jump to: navigation, search

Alias and address reversal MTA options: alias_urlN (URL)

The alias_urlN MTA options each specify a URL to query for alias lookups. If more than one of these options is set, then the URLs lookups specified are performed in the order specified by the alias_magic MTA option. The options are normally checked in numeric order, so alias_url0, if specified, will be the first URL queried. Note that with the usual value of alias_magic (8764), the alias_url3 option is not used.

Such alias lookups will be performed any time an envelope To address matches the local ("l") channel, or any channel marked with the aliaslocal channel option.

The URL (that is, the alias_urlN option value) must be specified using standard LDAP URL syntax as per RFC 4516, with the following exception and special interpretations:

  • The LDAP server and port are typically omitted, being instead specified via the ldap_host and ldap_port MTA options; (or if the MTA options are not explicitly set, the MTA will use more general options: in legacy configuration, the configutil parameters local.ugldaphost and local.ugldapport, or in Unified Configuration the ugldaphost and ugldapport base options). (Indeed, prior to Messaging Server 7.4-18.01, the host and port had to be omitted; but as of Messaging Server 7.4-18.01, specifying the host and port in the URL itself is supported.)
  • The MTA makes a distinction between a completely omitted attributes field, which as per RFC 2255 means to request the return of all attributes, and an attributes field consisting of the asterisk character, *, which the MTA instead interprets as meaning to request the return of all known-to-the-MTA attributes, that is, all the attributes listed in Table of MTA LDAP attribute name options. This distinction is available since for some directory setups, there may be a noticeable performance difference in LDAP directory response to one type of query (all attributes requested) vs. the other type of query (specific, though large, list of attributes requested).
  • Various substitution sequences of the form "$n" are available. A literal dollar sign must be represented by "$$".

The LDAP URL, before any substitutions, is limited to 256 characters in length (252 in iMS 5.2 and earlier); the substitutions may insert additional material and the length after such substitutions is limited to 1024 characters. Note that the substitution of known attributes when asterisk, *, is specified as the attribute to return, is not considered as part of the regular substitution; this substitution is performed at a later step and the length after this "known" attributes substitution is limited to 4096 characters.

alias_url0, if set, is normally looked up first (unless the alias_magic value has been changed). Next alias_url1, if set, etc. It is permissible to have "gaps" in the alias_urlN list; for instance, it is permissible to set alias_url0 and alias_url2 without setting alias_url1.

Since alias_url0 is normally looked up first, in a typical direct LDAP configuration it is used to perform the "main" user/group lookup, with alias_url1 optionally being used by those sites that need to do an additional, secondary lookup. In particular, alias_url1 is typically used by those sites that need to support vanity domains, or it could be used by sites that do not support vanity domains but that need to support "old-style" catch-all addresses, (that is, sites that use the deprecated approach of defining a catch-all address by means of a user mailAlternateAddress attribute with a wildcard, rather than using the preferred approach of defining a domain level mailDomainCatchallAddress attribute).

As of 7.0.5, the default value for alias_url0 is ldap:///$V?*?sub?$R. Previously, there was no default and alias_url0 had to be set explicitly. The other alias_urlN MTA options have no default value.

See also: