3.9 HELO Domain

Some of the mail filtering conditions may depend on the value of helo domain name, i.e. the argument to the SMTP EHLO (or HELO) command. If you ever need such conditions, take into account the following caveats. Firstly, although Sendmail passes the helo domain in $s macro, it does not do this consistently. In fact, the $s macro is available only to the helo handler, all other handlers won’t see it, no matter what the value of the corresponding Milter.macros.handler statement. So, if you wish to access its value from any handler, other than helo, you will have to store it in a variable in the helo handler and then use this variable value in the other handler. This approach is also recommended for another MTAs. This brings us to the concept of variables in mailfromd scripts.

A variable is declared using the following syntax:

type name

where variable is the variable name and type is ‘string’, if the variable is to hold a string value, and ‘number’, if it is supposed to have a numeric value.

A variable is assigned a value using the set statement:

set name expr

where expr is any valid MFL expression.

The set statement can occur within handler or function declarations as well as outside of them.

There are two kinds of Mailfromd variables: global variables, that are visible to all handlers and functions, and automatic variables, that are available only within the handler or function where they are declared. For our purpose we need a global variable (See Variable classes, for detailed descriptions of both kinds of variables).

The following example illustrates an approach that allows to use the HELO domain name in any handler:

# Declare the helohost variable
string helohost

prog helo
do
  # Save the host name for further use
  set helohost $s
done

prog envfrom
do
  # Reject hosts claiming to be localhost
  if helohost = "localhost"
    reject 570 "Please specify real host name"
  fi
done

Notice, that for this approach to work, your MTA must export the ‘s’ macro (e.g., in case of Sendmail, the Milter.macros.helo statement in the sendmail.cf file must contain ‘s’. see Sendmail). This requirement can be removed by using the handler argument of helo. Each mailfromd handler is given one or several arguments. The exact number of arguments and their meaning are handler-specific and are described in Handlers, and Figure 3.1. The arguments are referenced by their ordinal number, using the notation $n. The helo handler takes one argument, whose value is the helo domain. Using this information, the helo handler from the example above can be rewritten as follows:

prog helo
do
  # Save the host name for further use
  set helohost $1
done