9.1.1.1 Rewrite Rule Machinery

A facility has been added to the MTA rewrite rule machinery to check a given string to see if it is a domain we need to handle locally. This new facility is activated by a $V or $Z metacharacter. These new metacharacters are syntactically similar to the existing $N, $M, $Q, and $C metacharacters, that is, they are followed by a pattern string. In the case of $N, $M, $Q, and $C the pattern is matched against either the source or destination channel. In the case of $V and $Z the pattern is a domain and the check is to see whether or not it is local. $V causes a rule failure for a nonlocal domain and $Z causes a rule failure for a local domain.

The handling of these metacharacters is implemented as the following procedure:

1. Messaging Server checks to see if the current domain matches a valid domain entry in the directory. Go to step 3 if no entry exists.

2. If the domain has an entry in the directory, the attribute specified by the LDAP_DOMAIN_ATTR_ROUTING_HOSTS MTA option (default mailRoutingHosts) is retrieved from the domain entry. If this attribute is present, it lists the set of hosts able to handle users in this domain. This list is compared against the host specified by the local.hostname configutil parameter and the list of hosts specified by the local.imta.hostnamealiases configutil parameter. These options can be overridden by the LDAP_LOCAL_HOST and LDAP_HOST_ALIAS_LIST MTA options, respectively. If there is a match or the attribute is not present on the domain, the domain is local. If no match occurs, the domain is nonlocal.

   The handling of domains considered to be nonlocal because of the mailRoutingHosts attribute depends on the setting of the ROUTE_TO_ROUTING_HOST MTA option. If the option is set to 0 (the default), the address is simply treated as nonlocal and MTA rewrite rules are used to determine routing. If the option is set to 1, a source route consisting of the first value listed in the LDAP_DOMAIN_ATTR_ROUTING_HOSTS MTA option is prepended to the address.

3. If no domain entry can be found, remove a component from the left hand side of the domain and go to Step 1. If no components remain continue to Step 4.

   A consequence of this backtracking up the domain tree is that if siroe.com is recognized as local, any subdomain of siroe.com will be recognized as local. Situations may arise where this is undesirable, so an MTA option, DOMAIN_UPLEVEL, is provided to control this behavior. Specifically, bit 0 (value = 1) of DOMAIN_UPLEVEL, if clear, disables retries with domain components removed. The default value of DOMAIN_UPLEVEL is 0.

4. Vanity domain checking now needs to be performed. Vanity domains do not have domain entries, rather, they are specified by attaching special domain attributes to one or more user entries. The vanity domain check is done by instituting an LDAP search using the LDAP URL specified by the DOMAIN_MATCH_URL MTA option. The value of this option should be set to:

   ldap:///$B?msgVanityDomain?sub?(msgVanityDomain=$D)

   $B substitutes the value of the local.ugldapbasedn configutil parameter; this is the base of the user tree in the directory. The LDAP_USER_ROOT MTA option can be used to override the value of this configutil option specifically for the MTA.

   The actual return value from this search does not matter. What matters is if there is a value to return. If there is a return value the domain is considered to be local, if not it is considered to be nonlocal.