The User Database is a special database file that you create for use by sendmail. It causes sender and recipient addresses to be rewritten under control of an external database file. Ordinarily, any local address is first looked up in the aliases database. If it is not found there, that user's ~/.forward is next examined. If the User Database is enabled, the address is looked up in that database after aliasing and before forwarding.
Lookup can be local via a database file, remote via a User Database server, or via a Hesiod network service. Here, we describe the database file form. The others are described in Section 34.8.75, UserDatabaseSpec (U).
The User Database is automatically enabled when you compile
sendmail if you include support for NEWDB or HESIOD
(see Section 18.8.54, USERDB).
To see whether a precompiled version of sendmail includes User Database
support, run it with the
/usr/lib/sendmail -d0.1 -bt < /dev/nullVersion 8.8.4 Compiled with: LOG MIME8TO7 NETINET NETUNIX NEWDB SCANF
If USERDB is listed, User Database support is included.
Next you must declare the location of
its database file with the
(see Section 34.8.75):
OU/etc/userdb.db in your cf file (V8) O UserDatabaseSpec=/etc/userdb.db in your cf file (V8.7) define(`confUSERDB_SPEC',/etc/userdb.db) in your m4 file
Here, the location of the database file is set to be /etc/userdb.db.
You can also enable a default location for the database file that will
take effect should the
U) option be
missing by defining that location with UDB_DEFAULT_SPEC in
compiling (see Section 18.8.53, UDB-DEFAULT-SPEC).
The User Database is a
btree class (see Section 33.8.1)
database file created from a source text file using the makemap program:
makemap btree /etc/userdb.db < /etc/userdb
Here, /etc/userdb is the source-text file that is input, and /etc/userdb.db
is the database we are creating (the one defined by the
U option in the
.dbis added automatically if it is missing. We include it here for clarity.
The source-text file is composed of key and value pairs, one pair per line:
key is a user's login name, a colon, and one of two
The keyword that is chosen determines the nature of the
value is the official delivery address for this user.
If there are multiple official addresses, they should either be listed
as a single compound value, with separating commas, as,
or be listed on individual lines:
root:maildrop firstname.lastname@example.org root:maildrop email@example.com
This latter form requires you to use the
-d command-line switch
with the makemap(1) program (see Section 188.8.131.52) when
creating the database but has the advantage of being a simpler source
file to manage.
mailname keyword causes a "reverse alias" transformation,
wherein the login name in the key is changed into the address
in the value for outgoing mail. For example:
This causes mail sent by
bob to go out addressed as though
it is from
This transformation occurs in the header and envelope.
But note that the sender-envelope is not rewritten by UDB unless
F=i flag (see Section 30.8.24, F=i) is present in the
delivery agent that is selected for the sender.
Also note that the recipient headers are not rewritten by UDB
F=k flag (see Section 30.8.26, F=j)
delivery agent is selected for the recipient.
 Using full names in outgoing mail is probably not a good idea. Unlike login names, full names are not guaranteed to be unique. If current users expect to be able to receive mail under full names, future users with the same full name may be out of luck. Always weigh convenience against maintainable uniqueness when designing your mail setup.
mailname keywords should
occur in pairs. Each outgoing address that is created with
should have a corresponding
maildrop entry so that return
mail can be delivered. In the above example a reasonable pair
might look like this:
bob:mailname Bob.Roberts@Here.US.EDU Bob.Roberts:maildrop bob
Here, outgoing mail from the user named
bob will be addressed
as though it is from
Bob.Roberts@Here.US.EDU. Incoming mail
(whether it is original or in reply to the outgoing) will be addressed
as though it is to the name
Bob.Roberts, which will be
transformed into and delivered to the local user
mailname keyword allows the host part of outgoing
addresses to mask the real hostname of the originating machine.
This property can, for example, be used to convert the hostname into a
Here, the canonical name of
bob's machine is
mailname keyword causes outgoing mail from
to appear as though it is from the firewall machine
Ordinarily, this transformation is not automatic. Each username
that is to appear to be from the firewall machine will
need an entry like that above in the User Database.
To automate this process, you can use the special username
:default in a
maildrop entry is found for a particular name,
but no corresponding
mailname record is found, the
outgoing address is ordinarily unchanged. If, however, a default
hostname has been defined with
:default, that hostname
replaces the local hostname for all addresses that lack their own
:default:mailname Firewall.US.EDU bob:maildrop firstname.lastname@example.org
In this example the user
bob has a
but lacks a
mailname entry. Outgoing mail from this user
will have the
:default hostname used instead of the
local hostname. The user
sally, on the other hand, has
maildrop entry nor a
and so will not have her outgoing address rewritten.