The GECOS field is the portion of a passwd(5)
file line that contains a user's full name. Typical
passwd file lines are illustrated here with the
GECOS field of each highlighted in bold type:
tim:Fss9UdQl55cde:103:45:& Plenty (Jr):/usr/tim:/bin/csh
When sendmail attempts to deliver through a
delivery agent that has the F=w flag set (F=w) it looks up the recipient's
name in the passwd file so that it can locate
the user's home directory. That lookup tries to
match the login name, the leftmost field in the
passwd file. If that lookup fails, and
sendmail has been compiled with MATCHGECOS
defined (MATCHGECOS) and this
MatchGECOS option is true,
sendmail also tries to match the recipient name
to the GECOS field.
First, sendmail converts any underscore
characters in the address into spaces and, if the
BlankSub option is set (BlankSub), any characters that match that space
substitution character into spaces. This makes the recipient name
look like a normal full name.
Second, sendmail normalizes each GECOS entry by
throwing away everything following and including the first comma,
semicolon, and percent characters. It also converts the
& to the login name wherever one is found.
After each GECOS name is normalized, it's compared
in a case-insensitive manner to the recipient. If they match, the
passwd entry for that user is used.
This feature allows users to receive mail addressed to their full
name as given in the GECOS field of the passwd
file. The usual form is to replace spaces in the full name with dots
or underscores, so email addresses could be:
Full names in gecos fields that contain
characters with special meaning to sendmail,
such as the last one in the previous example, must be quoted when
used as addresses.
You should not enable this option if your site lets users edit their
own GECOS fields with the chfn(1) program. For
one thing, they change their name in a way that can cause mail to
start failing. Worse, they can change their name to match another
user's and begin to capture that other
user's mail. Even if the GECOS field is secure, you
should avoid this option if your passwd file is
large. The sendmail program performs a
sequential read of the passwd file, which could
be very slow.
The forms of the MatchGECOS option are as follows:
O MatchGECOS=bool configuration file (V8.7 and later)
-OMatchGECOS=bool command line (V8.7 and later)
define(`confMATCH_GECOS',bool) mc configuration (V8.7 and later)
OGbool configuration file (deprecated)
-oGbool command line (deprecated)
If you are running DEC OSF/1 V3.2 or earlier, you will need to
compile sendmail with the
DEC_OSF_BROKEN_GETPWENT compile-time macro defined (see ...IS_BROKEN).
The MatchGECOS option is not safe. If it is
specified from the command line, it can cause
sendmail to give up any special privileges.