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

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

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

"Filter=" is checked when a user attempts to post or subscribe to a list (but not when the list owner issues an ADD command). 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 which LISTSERV prevents from posting to the list.  CAUTION: You should not use this option unless you also code "Safe= Yes", and even then you will want to ask your LISTSERV maintainer for permission. This option has been added mostly for LISTSERV maintainers 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 (1) a "minimal" one, which is used for safe lists;  and (2) 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 usernames 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 which were set up to allow them.

In all cases, LISTSERV extracts as many e-mail addresses as it can from the userid being checked and runs them all through the filter. For instance if your list receives mail from '!', LISTSERV will check '!', '' and 'mailer@searn' (via the ':internet.' tag in BITEARN NODES).

LISTSERV provides 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 follow:

Filter= Also,,*

Filter= Allow,

The first example stops everyone from from posting to the list, but since we've determined that is a considerate human being and 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,

The second example would invoke the "safe" filter mentioned above, but would allow 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 "safe" filter and transferred to the list owner as an error.

See also Safe= and Loopcheck=.