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 form:
map-namesystem-usernamedatabase-username
Comments and whitespace 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.
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.
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.
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.
A pg_ident.conf file that could be used in
conjunction with the pg_hba.conf file in Example 20.1 is shown in Example 20.2. 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 20.2. 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