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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: respectively, MD5-hashed and clear-text.

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';. By default, that is, if no password has been set up, 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 in clear 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:

map

Allows for mapping between system and database usernames. 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 username 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 username 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:

map

Allows for mapping between system and database usernames. 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 username 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 username 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 (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 far 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 30.1.) 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, to support multiple PostgreSQL installations on the same host it is necessary. Some Kerberos implementations might also require a different service name, such as Microsoft Active Directory which requires the service name to be in uppercase (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 username 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 username 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 whichever 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, both principal fred@EXAMPLE.COM and fred/users.example.com@EXAMPLE.COM could be used to authenticate to the database server.

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, no extra passwords required.

The following configuration options are supported for Kerberos:

map

Allows for mapping between system and database usernames. 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 username 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 username 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 username mapping). The determination of the client's user name is the security-critical point, and it works differently depending on the connection type.

The following configuration options are supported for ident:

map

Allows for mapping between system and database usernames. 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 could theoretically determine the operating system user for any given connection this way.

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. 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.

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.

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.

ldapprefix

String to prepend to the username when forming the DN to bind as.

ldapsuffix

String to append to the username when forming the DN to bind as.

ldapport

Port number on LDAP server to connect to. If no port is specified, the default port in the LDAP library 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.

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. 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 attribute of the certificate will be compared to the requested database username, and if they match the login will be allowed. Username mapping can be used to allow cn to be different from the database username.

The following configuration options are supported for SSL certificate authentication:

map

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

19.3.9. 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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