Scanning the Rewrite Rules

Once the first host/domain specification has been extracted from the address, the IMTA consults the rewrite rules to find out what to do with it. The host/domain specification is compared with the pattern part of each rule (i.e., the left-hand side of each rule). The comparison is case insensitive. Case insensitivity is mandated by RFC 822, UUCP addresses notwithstanding. The IMTA is insensitive to case but preserves it whenever possible.

If the host/domain specification does not match any pattern, in which case it is said to "not match any rule", the first part of the host/domain specification--the part before the first period, usually the host name--is removed and replaced with an asterisk and another attempt is made to locate the resulting host/domain specification, but only in the configuration file rewrite rules (the domain database is not consulted). If this fails the first part is removed and the process is repeated. If this also fails the next part is removed (usually a subdomain) and the rewriter tries again, first with asterisks and then without. This process proceeds until either a match is found or the entire host/domain specification is exhausted. The effect of this procedure is to try to match the most specific domain first, working outward to less specific and more general domains.

A somewhat more algorithmic view of this matching procedure is:

The host/domain specification is used as the initial value for the comparison strings spec_1 and spec_2. (For example, spec_1 = spec_2 = a.b.c).

The comparison string spec_1 is compared with the pattern part of each rewrite rule in the configuration file and then the domain database until a match is found. The matching procedure is exited if a match is found.

If no match is found then the left-most, non-asterisk part of spec_2 is converted to an asterisk. For example, if spec_2 is a.b.c then it is changed to *.b.c; if spec_2 is *.b.c then it is changed to *.*.c. The matching procedure is exited if a match is found.

If no match is found then the first part, including any leading period, of the comparison string spec_1 is removed. In the case where spec_1 has only one part (for example., .c or c), the string is replaced with a single period, ".". If the resulting string spec_1 is of non-zero length, then we return to Step 1. If the resulting string has zero length (for example, was previously ".") then the lookup process has failed and we exit the matching procedure.

For example, suppose the address dan@sc.cs.cmu.edu is to be rewritten. This causes the rewriter to look for the following patterns in the given order:

sc.cs.cmu.edu   *.cs.cmu.edu   .cs.cmu.edu   *.*.cmu.edu   .cmu.edu   *.*.*.edu   .edu   *.*.*.*   .