PostgreSQL 8.4.4 Documentation
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# 19.2. Username maps

When using an external authentication system like Ident or GSSAPI, the name of the operating system user that initiated the connection might not be the same as the database user he needs to connect as. In this case, a user name map can be applied to map the operating system username to a database user. To use username mapping, specify `map`=`map-name` in the options field in `pg_hba.conf`. This option is supported for all authentication methods that receive external usernames. 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.

Username 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 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 matches the user name obtained from the external authentication system to 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. Regular expressions in username maps are always treated as being "advanced" flavor.) 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 usernames 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 usernames that end with `@mydomain.com`, and allow any user whose system name ends with `@otherdomain.com` to log in as `guest`.

> Tip: 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 username.

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 server (using `pg_ctl reload` or `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 19-1 is shown in Example 19-2. In this example setup, anyone logged in to a machine on the 192.168 network that does not have the Unix 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` himself or as `guest1`.

Example 19-2. 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

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