# 21.2. User Name Maps
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-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-name system-username database-username
Comments, whitespace and line continuations are handled in the same way as in
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.
system-username field starts with a slash (
/), the remainder of the field is treated as a regular expression. (See Section 220.127.116.11 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
Keep in mind that by default, a regular expression can match just part of a string. It's usually wise to use
$, as shown in the above example, to force the match to be to the entire system user name.
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.
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
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
bryanh would be allowed to connect as either
bryanh or as
Example 21.2. An Example
# 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