Sendmail, 3rd Edition

The R Configuration Command

Rules are declared in the configuration file with the R configuration command. Like all configuration commands, the R rule configuration command must begin a line. The general form consists of an R command followed by three parts:

Rlhs    rhs   comment
   ↑        ↑  tabs     tabs 

The lhs stands for lefthand side and is most commonly expressed as LHS. The rhs stands for righthand side and is expressed as RHS. The LHS and RHS are mandatory. The third part (the comment) is optional. The three parts must be separated from each other by one or more tab characters (space characters will not work).

Space characters between the R and the LHS are optional. If there is a tab between the R and the LHS, sendmail prints and logs the following error:

configfile: line number: R line: null LHS

Space characters can be used inside any of the three parts: the LHS, RHS, or comment. They are often used in those parts to make rules clearer and easier to parse visually.

The tabs leading to the comment and the comment itself are optional and can be omitted. If the RHS is absent, sendmail prints the following warning and ignores that R line:

invalid rewrite line "bad rule here" (tab expected)

This error is printed when the RHS is absent, even if there are tabs following the LHS. (This warning is usually the result of tabs being converted to spaces when text is copied from one window to another in a windowing system using cut and paste.)

Macros in Rules

Each noncomment part of a rule is expanded as ...