PostgreSQL 9.0.13 Documentation
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# 19.3. Authentication methods

The following subsections describe the authentication methods in more detail.

## 19.3.1. Trust authentication

When `trust` authentication is specified, PostgreSQL assumes that anyone who can connect to the server is authorized to access the database with whatever database user name they specify (even superuser names). Of course, restrictions made in the `database` and `user` columns still apply. This method should only be used when there is adequate operating-system-level protection on connections to the server.

`trust` authentication is appropriate and very convenient for local connections on a single-user workstation. It is usually not appropriate by itself on a multiuser machine. However, you might be able to use `trust` even on a multiuser machine, if you restrict access to the server's Unix-domain socket file using file-system permissions. To do this, set the `unix_socket_permissions` (and possibly `unix_socket_group`) configuration parameters as described in Section 18.3. Or you could set the `unix_socket_directory` configuration parameter to place the socket file in a suitably restricted directory.

Setting file-system permissions only helps for Unix-socket connections. Local TCP/IP connections are not restricted by file-system permissions. Therefore, if you want to use file-system permissions for local security, remove the `host ... 127.0.0.1 ...` line from `pg_hba.conf`, or change it to a non-`trust` authentication method.

`trust` authentication is only suitable for TCP/IP connections if you trust every user on every machine that is allowed to connect to the server by the `pg_hba.conf` lines that specify `trust`. It is seldom reasonable to use `trust` for any TCP/IP connections other than those from localhost (127.0.0.1).

## 19.3.2. Password authentication

The password-based authentication methods are `md5` and `password`. These methods operate similarly except for the way that the password is sent across the connection, namely MD5-hashed and clear-text respectively.

If you are at all concerned about password "sniffing" attacks then `md5` is preferred. Plain `password` should always be avoided if possible. However, `md5` cannot be used with the db_user_namespace feature. If the connection is protected by SSL encryption then `password` can be used safely (though SSL certificate authentication might be a better choice if one is depending on using SSL).

PostgreSQL database passwords are separate from operating system user passwords. The password for each database user is stored in the `pg_authid` system catalog. Passwords can be managed with the SQL commands CREATE USER and ALTER USER, e.g., `CREATE USER foo WITH PASSWORD 'secret'`. If no password has been set up for a user, the stored password is null and password authentication will always fail for that user.

## 19.3.3. GSSAPI authentication

GSSAPI is an industry-standard protocol for secure authentication defined in RFC 2743. PostgreSQL supports GSSAPI with Kerberos authentication according to RFC 1964. GSSAPI provides automatic authentication (single sign-on) for systems that support it. The authentication itself is secure, but the data sent over the database connection will be sent unencrypted unless SSL is used.

When GSSAPI uses Kerberos, it uses a standard principal in the format ``servicename`/`hostname`@`realm``. For information about the parts of the principal, and how to set up the required keys, see Section 19.3.5.

GSSAPI support has to be enabled when PostgreSQL is built; see Chapter 15 for more information.

The following configuration options are supported for GSSAPI:

`include_realm`

  If set to `1`, the realm name from the authenticated user principal is included in the system user name that's passed through user name mapping (Section 19.2). This is useful for handling users from multiple realms.

`map`

  Allows for mapping between system and database user names. See Section 19.2 for details. For a Kerberos principal `username/hostbased@EXAMPLE.COM`, the user name used for mapping is `username/hostbased` if `include_realm` is disabled, and `username/hostbased@EXAMPLE.COM` if `include_realm` is enabled.

`krb_realm`

  Sets 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.

## 19.3.4. SSPI authentication

SSPI is a Windows technology for secure authentication with single sign-on. PostgreSQL will use SSPI in `negotiate` mode, which will use Kerberos when possible and automatically fall back to NTLM in other cases. SSPI authentication only works when both server and client are running Windows.

When using Kerberos authentication, SSPI works the same way GSSAPI does; see Section 19.3.3 for details.

The following configuration options are supported for SSPI:

`include_realm`

  If set to `1`, the realm name from the authenticated user principal is included in the system user name that's passed through user name mapping (Section 19.2). This is useful for handling users from multiple realms.

`map`

  Allows for mapping between system and database user names. See Section 19.2 for details.

`krb_realm`

  Sets 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.

## 19.3.5. Kerberos authentication

> Note: Native Kerberos authentication has been deprecated and should be used only for backward compatibility. New and upgraded installations are encouraged to use the industry-standard GSSAPI authentication method (see Section 19.3.3) instead.

Kerberos is an industry-standard secure authentication system suitable for distributed computing over a public network. A description of the Kerberos system is beyond the scope of this document; in full generality it can be quite complex (yet powerful). The Kerberos FAQ or MIT Kerberos page can be good starting points for exploration. Several sources for Kerberos distributions exist. Kerberos provides secure authentication but does not encrypt queries or data passed over the network; for that use SSL.

PostgreSQL supports Kerberos version 5. Kerberos support has to be enabled when PostgreSQL is built; see Chapter 15 for more information.

PostgreSQL operates like a normal Kerberos service. The name of the service principal is ``servicename`/`hostname`@`realm``.

`servicename` can be set on the server side using the krb_srvname configuration parameter, and on the client side using the `krbsrvname` connection parameter. (See also Section 31.1.) The installation default can be changed from the default `postgres` at build time using `./configure --with-krb-srvnam=``whatever`. In most environments, this parameter never needs to be changed. However, it is necessary when supporting multiple PostgreSQL installations on the same host. Some Kerberos implementations might also require a different service name, such as Microsoft Active Directory which requires the service name to be in upper case (`POSTGRES`).

`hostname` is the fully qualified host name of the server machine. The service principal's realm is the preferred realm of the server machine.

Client principals must have their PostgreSQL database user name as their first component, for example `pgusername@realm`. Alternatively, you can use a user name mapping to map from the first component of the principal name to the database user name. By default, the realm of the client is not checked by PostgreSQL. If you have cross-realm authentication enabled and need to verify the realm, use the `krb_realm` parameter, or enable `include_realm` and use user name mapping to check the realm.

Make sure that your server keytab file is readable (and preferably only readable) by the PostgreSQL server account. (See also Section 17.1.) The location of the key file is specified by the krb_server_keyfile configuration parameter. The default is `/usr/local/pgsql/etc/krb5.keytab` (or whatever directory was specified as `sysconfdir` at build time).

The keytab file is generated by the Kerberos software; see the Kerberos documentation for details. The following example is for MIT-compatible Kerberos 5 implementations:

    kadmin% ank -randkey postgres/server.my.domain.org
    kadmin% ktadd -k krb5.keytab postgres/server.my.domain.org

When connecting to the database make sure you have a ticket for a principal matching the requested database user name. For example, for database user name `fred`, principal `fred@EXAMPLE.COM` would be able to connect. To also allow principal `fred/users.example.com@EXAMPLE.COM`, use a user name map, as described in Section 19.2.

If you use mod_auth_kerb and mod_perl on your Apache web server, you can use `AuthType KerberosV5SaveCredentials` with a mod_perl script. This gives secure database access over the web, with no additional passwords required.

The following configuration options are supported for Kerberos:

`map`

  Allows for mapping between system and database user names. See Section 19.2 for details.

`include_realm`

  If set to `1`, the realm name from the authenticated user principal is included in the system user name that's passed through user name mapping (Section 19.2). This is useful for handling users from multiple realms.

`krb_realm`

  Sets 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.

`krb_server_hostname`

  Sets the host name part of the service principal. This, combined with `krb_srvname`, is used to generate the complete service principal, that is `krb_srvname``/``krb_server_hostname``@`REALM. If not set, the default is the server host name.

## 19.3.6. Ident-based authentication

The ident authentication method works by obtaining the client's operating system user name and using it as the allowed database user name (with an optional user name mapping). The determination of the client's user name is the security-critical point, and it works differently depending on the connection type, as described below.

The following configuration options are supported for ident:

`map`

  Allows for mapping between system and database user names. See Section 19.2 for details.

### 19.3.6.1. Ident Authentication over TCP/IP

The "Identification Protocol" is described in RFC 1413. Virtually every Unix-like operating system ships with an ident server that listens on TCP port 113 by default. The basic functionality of an ident server is to answer questions like "What user initiated the connection that goes out of your port `X` and connects to my port `Y`?". Since PostgreSQL knows both `X` and `Y` when a physical connection is established, it can interrogate the ident server on the host of the connecting client and can theoretically determine the operating system user for any given connection.

The drawback of this procedure is that it depends on the integrity of the client: if the client machine is untrusted or compromised, an attacker could run just about any program on port 113 and return any user name he chooses. This authentication method is therefore only appropriate for closed networks where each client machine is under tight control and where the database and system administrators operate in close contact. In other words, you must trust the machine running the ident server. Heed the warning:

| The Identification Protocol is not intended as an authorization or access control protocol. |
--RFC 1413 |

Some ident servers have a nonstandard option that causes the returned user name to be encrypted, using a key that only the originating machine's administrator knows. This option must not be used when using the ident server with PostgreSQL, since PostgreSQL does not have any way to decrypt the returned string to determine the actual user name.

### 19.3.6.2. Ident Authentication over Local Sockets

On systems supporting `SO_PEERCRED` requests for Unix-domain sockets (currently Linux, FreeBSD, NetBSD, OpenBSD, BSD/OS, and Solaris), ident authentication can also be applied to local connections. PostgreSQL uses `SO_PEERCRED` to find out the operating system name of the connected client process. In this case, no security risk is added by using ident authentication; indeed it is a preferable choice for local connections on such systems.

On systems without `SO_PEERCRED` requests, ident authentication is only available for TCP/IP connections. As a work-around, it is possible to specify the localhost address 127.0.0.1 and make connections to this address. This method is trustworthy to the extent that you trust the local ident server.

## 19.3.7. LDAP authentication

This authentication method operates similarly to `password` except that it uses LDAP as the password verification method. LDAP is used only to validate the user name/password pairs. Therefore the user must already exist in the database before LDAP can be used for authentication.

LDAP authentication can operate in two modes. In the first mode, the server will bind to the distinguished name constructed as `prefix` `username` `suffix`. Typically, the `prefix` parameter is used to specify `cn=`, or `DOMAIN``\` in an Active Directory environment. `suffix` is used to specify the remaining part of the DN in a non-Active Directory environment.

In the second mode, the server first binds to the LDAP directory with a fixed user name and password, specified with `ldapbinddn` and `ldapbindpasswd`, and performs a search for the user trying to log in to the database. If no user and password is configured, an anonymous bind will be attempted to the directory. The search will be performed over the subtree at `ldapbasedn`, and will try to do an exact match of the attribute specified in `ldapsearchattribute`. If no attribute is specified, the `uid` attribute will be used. Once the user has been found in this search, the server disconnects and re-binds to the directory as this user, using the password specified by the client, to verify that the login is correct. This method allows for significantly more flexibility in where the user objects are located in the directory, but will cause two separate connections to the LDAP server to be made.

The following configuration options are supported for LDAP:

`ldapserver`

  Names or IP addresses of LDAP servers to connect to. Multiple servers may be specified, separated by spaces.

`ldapport`

  Port number on LDAP server to connect to. If no port is specified, the LDAP library's default port setting will be used.

`ldaptls`

  Set to `1` to make the connection between PostgreSQL and the LDAP server use TLS encryption. Note that this only encrypts the traffic to the LDAP server — the connection to the client will still be unencrypted unless SSL is used.

`ldapprefix`

  String to prepend to the user name when forming the DN to bind as, when doing simple bind authentication.

`ldapsuffix`

  String to append to the user name when forming the DN to bind as, when doing simple bind authentication.

`ldapbasedn`

  Root DN to begin the search for the user in, when doing search+bind authentication.

`ldapbinddn`

  DN of user to bind to the directory with to perform the search when doing search+bind authentication.

`ldapbindpasswd`

  Password for user to bind to the directory with to perform the search when doing search+bind authentication.

`ldapsearchattribute`

  Attribute to match against the user name in the search when doing search+bind authentication.

> Note: Since LDAP often uses commas and spaces to separate the different parts of a DN, it is often necessary to use double-quoted parameter values when configuring LDAP options, for example:

>     ldapserver=ldap.example.net ldapprefix="cn=" ldapsuffix=", dc=example, dc=net"

## 19.3.8. RADIUS authentication

This authentication method operates similarly to `password` except that it uses RADIUS as the password verification method. RADIUS is used only to validate the user name/password pairs. Therefore the user must already exist in the database before RADIUS can be used for authentication.

When using RADIUS authentication, an Access Request message will be sent to the configured RADIUS server. This request will be of type `Authenticate Only`, and include parameters for `user name`, `password` (encrypted) and `NAS Identifier`. The request will be encrypted using a secret shared with the server. The RADIUS server will respond to this server with either `Access Accept` or `Access Reject`. There is no support for RADIUS accounting.

The following configuration options are supported for RADIUS:

`radiusserver`

  The name or IP address of the RADIUS server to connect to. This parameter is required.

`radiussecret`

  The shared secret used when talking securely to the RADIUS server. This must have exactly the same value on the PostgreSQL and RADIUS servers. It is recommended that this be a string of at least 16 characters. This parameter is required.

  > Note: The encryption vector used will only be cryptographically strong if PostgreSQL is built with support for OpenSSL. In other cases, the transmission to the RADIUS server should only be considered obfuscated, not secured, and external security measures should be applied if necessary.

`radiusport`

  The port number on the RADIUS server to connect to. If no port is specified, the default port `1812` will be used.

`radiusidentifier`

  The string used as `NAS Identifier` in the RADIUS requests. This parameter can be used as a second parameter identifying for example which database user the user is attempting to authenticate as, which can be used for policy matching on the RADIUS server. If no identifier is specified, the default `postgresql` will be used.

## 19.3.9. Certificate authentication

This authentication method uses SSL client certificates to perform authentication. It is therefore only available for SSL connections. When using this authentication method, the server will require that the client provide a valid certificate. No password prompt will be sent to the client. The `cn` (Common Name) attribute of the certificate will be compared to the requested database user name, and if they match the login will be allowed. User name mapping can be used to allow `cn` to be different from the database user name.

The following configuration options are supported for SSL certificate authentication:

`map`

  Allows for mapping between system and database user names. See Section 19.2 for details.

## 19.3.10. PAM authentication

This authentication method operates similarly to `password` except that it uses PAM (Pluggable Authentication Modules) as the authentication mechanism. The default PAM service name is `postgresql`. PAM is used only to validate user name/password pairs. Therefore the user must already exist in the database before PAM can be used for authentication. For more information about PAM, please read the Linux-PAM Page and the Solaris PAM Page.

The following configuration options are supported for PAM:

`pamservice`

  PAM service name.

> Note: If PAM is set up to read `/etc/shadow`, authentication will fail because the PostgreSQL server is started by a non-root user. However, this is not an issue when PAM is configured to use LDAP or other authentication methods.

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