GSSAPI is an industry-standard protocol for secure authentication defined in RFC 2743. PostgreSQL supports GSSAPI for authentication, communications encryption, or both. GSSAPI provides automatic authentication (single sign-on) for systems that support it. The authentication itself is secure. If GSSAPI encryption or SSL encryption is used, the data sent along the database connection will be encrypted; otherwise, it will not.
GSSAPI support has to be enabled when PostgreSQL is built; see Chapter 16 for more information.
When GSSAPI uses
Kerberos, it uses a standard service
principal (authentication identity) name in the format
servicename/hostname@realm
When connecting, the client must know the principal name of the server
it intends to connect to. The servicename
part of the principal is ordinarily postgres,
but another value can be selected via libpq's
krbsrvname connection parameter.
The hostname part is the fully qualified
host name that libpq is told to connect to.
The realm name is the preferred realm specified in the Kerberos
configuration file(s) accessible to the client.
The client will also have a principal name for its own identity
(and it must have a valid ticket for this principal). To
use GSSAPI for authentication, the client
principal must be associated with
a PostgreSQL database user name.
The pg_ident.conf configuration file can be used
to map principals to user names; for example,
pgusername@realm could be mapped to just pgusername.
Alternatively, you can use the full username@realm principal as
the role name in PostgreSQL without any mapping.
PostgreSQL also supports mapping
client principals to user names by just stripping the realm from
the principal. This method is supported for backwards compatibility and is
strongly discouraged as it is then impossible to distinguish different users
with the same user name but coming from different realms. To enable this,
set include_realm to 0. For simple single-realm
installations, doing that combined with setting the
krb_realm parameter (which checks that the principal's realm
matches exactly what is in the krb_realm parameter)
is still secure; but this is a
less capable approach compared to specifying an explicit mapping in
pg_ident.conf.
The location of the server's keytab file is specified by the krb_server_keyfile configuration parameter. For security reasons, it is recommended to use a separate keytab just for the PostgreSQL server rather than allowing the server to read the system keytab file. Make sure that your server keytab file is readable (and preferably only readable, not writable) by the PostgreSQL server account. (See also Section 18.1.)
The keytab file is generated using the Kerberos software; see the Kerberos documentation for details. The following example shows doing this using the kadmin tool of MIT-compatible Kerberos 5 implementations:
kadmin%addprinc -randkey postgres/server.my.domain.orgkadmin%ktadd -k krb5.keytab postgres/server.my.domain.org
The following authentication options are supported for the GSSAPI authentication method:
include_realm
If set to 0, the realm name from the authenticated user principal is
stripped off before being passed through the user name mapping
(Section 20.2). This is discouraged and is
primarily available for backwards compatibility, as it is not secure
in multi-realm environments unless krb_realm is
also used. It is recommended to
leave include_realm set to the default (1) and to
provide an explicit mapping in pg_ident.conf to convert
principal names to PostgreSQL user names.
map
Allows mapping from client principals to database user names. See
Section 20.2 for details. For a GSSAPI/Kerberos
principal, such as username@EXAMPLE.COM (or, less
commonly, username/hostbased@EXAMPLE.COM), the
user name used for mapping is
username@EXAMPLE.COM (or
username/hostbased@EXAMPLE.COM, respectively),
unless include_realm has been set to 0, in which case
username (or username/hostbased)
is what is seen as the system user name when mapping.
krb_realmSets the realm to match user principal names against. If this parameter is set, only users of that realm will be accepted. If it is not set, users of any realm can connect, subject to whatever user name mapping is done.
In addition to these settings, which can be different for
different pg_hba.conf entries, there is the
server-wide krb_caseins_users configuration
parameter. If that is set to true, client principals are matched to
user map entries case-insensitively. krb_realm, if
set, is also matched case-insensitively.