Filter Keyword

Filter= Only | Also | Safe,net-address1,net-address2,...[Allow],net-addressn,...

This keyword is checked when a user attempts to post or subscribe to a list (but not when the list owner adds a subscriber). The first parameter of this keyword is either "Only", "Also" or "Safe", and it is followed by a list of patterns such as "X400MAIL@*" or "*@*.XYZ.EDU" (without the quotes).

• If "Filter= Also..." is specified, your filter is used in addition to the standard LISTSERV filter. This is useful to register additional looping mailers to prevent users behind broken gateways from subscribing until the problem is addressed, or to ban anonymous posters.
• If "Filter= Only..." is specified, the addresses you specify are the only ones that LISTSERV prevents from posting to the list. Note that you should not use this option unless you also code "Safe= Yes", and even then you will want to ask your LISTSERV administrator for permission. This option has been added mostly for LISTSERV administrators with very specific problems to solve. The minimal filter is very small and you should never need to override it.
• If "Filter= Safe" is specified, LISTSERV uses the "more safe" of its two built-in filters. These built-in filters are a "minimal" one, which is used for safe lists; and a "safe" one which is used for lists running with "Safe= No". That is, the unsafe lists need a safe filter to avoid mailing loops; safe lists only need the minimal filter, but can be made even safer by selecting "Filter= Safe". This, however, prevents user names such as 'root' from posting to the list because they are included in the safe filter.
  
  Messages sent to the LISTSERV userid for execution are always checked with the minimal filter, as people with userids such as 'root' would otherwise not be allowed to subscribe to lists that were set up to allow them.

LISTSERV has the ability to "exempt" certain addresses (or wildcards) from the default filters. You can combine "Filter= ...,Allow,..." with the forms documented above as long as you put the "allow" information last. That is, the first word must be Only, Safe or Also as before, and you can then have Allow anywhere (including as the second word) followed by a list of addresses that should be allowed even if present in the filter. The default for Allow is the empty string. Examples of Allow usage include:

• Filter= Also,userid@host1.com,*@host2.edu
• Filter= Allow,niceguy@host2.edu

The first example stops everyone from host2.edu from posting to the list, but since we've determined that niceguy@host2.edu should be allowed to post, we've defined him as an exception to the general rule by including him in the "Allow" part of the filter.

• Filter= Safe,Allow,root@host1.edu

The second example would invoke the "safe" filter mentioned above, but would allow root@host1.edu to subscribe to and post to the list, instead of bouncing his mail because it matches one of the entries in the "safe" filter. All other "root" users' mail will be caught by the filter and transferred to the list owner as an error.