When using an external authentication system such as Ident or GSSAPI,
the name of the operating system user that initiated the connection
might not be the same as the database user (role) that is to be used.
In this case, a user name map can be applied to map the operating system
user name to a database user. To use user name mapping, specify
map=map-name
in the options field in pg_hba.conf. This option is
supported for all authentication methods that receive external user names.
Since different mappings might be needed for different connections,
the name of the map to be used is specified in the
map-name parameter in pg_hba.conf
to indicate which map to use for each individual connection.
User name maps are defined in the ident map file, which by default is named
pg_ident.conf
and is stored in the
cluster's data directory. (It is possible to place the map file
elsewhere, however; see the ident_file
configuration parameter.)
The ident map file contains lines of the general forms:
map-namesystem-usernamedatabase-usernameincludefileinclude_if_existsfileinclude_dirdirectory
Comments, whitespace and line continuations are handled in the same way as in
pg_hba.conf. The
map-name is an arbitrary name that will be used to
refer to this mapping in pg_hba.conf. The other
two fields specify an operating system user name and a matching
database user name. The same map-name can be
used repeatedly to specify multiple user-mappings within a single map.
As for pg_hba.conf, the lines in this file can
be include directives, following the same rules.
There is no restriction regarding how many database users a given
operating system user can correspond to, nor vice versa. Thus, entries
in a map should be thought of as meaning “this operating system
user is allowed to connect as this database user”, rather than
implying that they are equivalent. The connection will be allowed if
there is any map entry that pairs the user name obtained from the
external authentication system with the database user name that the
user has requested to connect as. The value all
can be used as the database-username to specify
that if the system-user matches, then this user
is allowed to log in as any of the existing database users. Quoting
all makes the keyword lose its special meaning.
If the database-username begins with a
+ character, then the operating system user can login as
any user belonging to that role, similarly to how user names beginning with
+ are treated in pg_hba.conf.
Thus, a + mark means “match any of the roles that
are directly or indirectly members of this role”, while a name
without a + mark matches only that specific role. Quoting
a username starting with a + makes the
+ lose its special meaning.
If the system-username field starts with a slash (/),
the remainder of the field is treated as a regular expression.
(See Section 9.7.3.1 for details of
PostgreSQL's regular expression syntax.) The regular
expression can include a single capture, or parenthesized subexpression,
which can then be referenced in the database-username
field as \1 (backslash-one). This allows the mapping of
multiple user names in a single line, which is particularly useful for
simple syntax substitutions. For example, these entries
mymap /^(.*)@mydomain\.com$ \1 mymap /^(.*)@otherdomain\.com$ guest
will remove the domain part for users with system user names that end with
@mydomain.com, and allow any user whose system name ends with
@otherdomain.com to log in as guest.
Quoting a database-username containing
\1 does not make
\1 lose its special meaning.
If the database-username field starts with
a slash (/), the remainder of the field is treated
as a regular expression (see Section 9.7.3.1
for details of PostgreSQL's regular
expression syntax). It is not possible to use \1
to use a capture from regular expression on
system-username for a regular expression
on database-username.
Keep in mind that by default, a regular expression can match just part of
a string. It's usually wise to use ^ and $, as
shown in the above example, to force the match to be to the entire
system user name.
The pg_ident.conf file is read on start-up and
when the main server process receives a
SIGHUP
signal. If you edit the file on an
active system, you will need to signal the postmaster
(using pg_ctl reload, calling the SQL function
pg_reload_conf(), or using kill
-HUP) to make it re-read the file.
The system view
pg_ident_file_mappings
can be helpful for pre-testing changes to the
pg_ident.conf file, or for diagnosing problems if
loading of the file did not have the desired effects. Rows in the view with
non-null error fields indicate problems in the
corresponding lines of the file.
A pg_ident.conf file that could be used in
conjunction with the pg_hba.conf file in Example 21.1 is shown in Example 21.2. In this example, anyone
logged in to a machine on the 192.168 network that does not have the
operating system user name bryanh, ann, or
robert would not be granted access. Unix user
robert would only be allowed access when he tries to
connect as PostgreSQL user bob, not
as robert or anyone else. ann would
only be allowed to connect as ann. User
bryanh would be allowed to connect as either
bryanh or as guest1.
Example 21.2. An Example pg_ident.conf File
# MAPNAME SYSTEM-USERNAME PG-USERNAME omicron bryanh bryanh omicron ann ann # bob has user name robert on these machines omicron robert bob # bryanh can also connect as guest1 omicron bryanh guest1