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4.2. Authentication methods

The following describes the authentication methods in more detail.

4.2.1. Trust authentication

When trust authentication is specified, PostgreSQL assumes that anyone who can connect to the postmaster is authorized to access the database as whatever database user he specifies (including the database superuser). This method should only be used when there is adequate system-level protection on connections to the postmaster port.

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 may be able to use trust even on a multiuser machine, if you restrict access to the postmaster's socket file using file-system permissions. To do this, set the parameter unix_socket_permissions (and possibly unix_socket_group) in postgresql.conf, as described in Section 3.4.3. Or you could set unix_socket_directory to place the socket file in a suitably restricted directory.

Setting file-system permissions only helps for Unix-socket connections. Local TCP connections are not restricted by it; therefore, if you want to use 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 connections if you trust every user on every machine that is allowed to connect to the postmaster by the pg_hba.conf lines that specify trust. It is seldom reasonable to use trust for any TCP connections other than those from localhost (127.0.0.1).

4.2.2. Password authentication

Password-based authentication methods include md5, crypt, and password. These methods operate similarly except for the way that the password is sent across the connection. If you are at all concerned about password "sniffing" attacks then md5 is preferred, with crypt a second choice if you must support obsolete clients. Plain password should especially be avoided for connections over the open Internet (unless you use SSL, SSH, or other communications security wrappers around the connection).

PostgreSQL database passwords are separate from operating system user passwords. Ordinarily, the password for each database user is stored in the pg_shadow system catalog table. Passwords can be managed with the query language 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.

To restrict the set of users that are allowed to connect to certain databases, list the set of users in a separate file (one user name per line) in the same directory that pg_hba.conf is in, and mention the (base) name of the file after the password, md5, or crypt keyword, respectively, in pg_hba.conf. If you do not use this feature, then any user that is known to the database system can connect to any database (so long as he supplies the correct password, of course).

These files can also be used to apply a different set of passwords to a particular database or set thereof. In that case, the files have a format similar to the standard Unix password file /etc/passwd, that is,

username:password

Any extra colon-separated fields following the password are ignored. The password is expected to be encrypted using the system's crypt() function. The utility program pg_passwd that is installed with PostgreSQL can be used to manage these password files.

Lines with and without passwords can be mixed in secondary password files. Lines without password indicate use of the main password in pg_shadow that is managed by CREATE USER and ALTER USER. Lines with passwords will cause that password to be used. A password entry of "+" also means using the pg_shadow password.

Alternative passwords cannot be used when using the md5 or crypt methods. The file will be read as usual, but the password field will simply be ignored and the pg_shadow password will always be used.

Note that using alternative passwords like this means that one can no longer use ALTER USER to change one's password. It will appear to work but the password one is changing is not the password that the system will end up using.

4.2.3. Kerberos authentication

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 all generality it can be quite complex (yet powerful). The Kerberos FAQ or MIT Project Athena can be a good starting point for exploration. Several sources for Kerberos distributions exist.

In order to use Kerberos, support for it must be enabled at build time. Both Kerberos 4 and 5 are supported (./configure --with-krb4 or ./configure --with-krb5 respectively), although only one version can be supported in any one build.

PostgreSQL operates like a normal Kerberos service. The name of the service principal is servicename/hostname@realm, where servicename is postgres (unless a different service name was selected at configure time with ./configure --with-krb-srvnam=whatever). hostname is the fully qualified domain name of the server machine. The service principal's realm is the preferred realm of the server machine.

Client principals must have their PostgreSQL user name as their first component, for example pgusername/otherstuff@realm. At present the realm of the client is not checked by PostgreSQL; so if you have cross-realm authentication enabled, then any principal in any realm that can communicate with yours will be accepted.

Make sure that your server key file is readable (and preferably only readable) by the PostgreSQL server account (see Section 3.1). The location of the key file is specified with the krb_server_keyfile run time configuration parameter. (See also Section 3.4.) The default is /etc/srvtab if you are using Kerberos 4 and FILE:/usr/local/pgsql/etc/krb5.keytab (or whichever directory was specified as sysconfdir at build time) with Kerberos 5.

To generate the keytab file, use for example (with version 5)

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

Read the Kerberos documentation for details.

When connecting to the database make sure you have a ticket for a principal matching the requested database user name. An example: For database user name fred, both principal fred@EXAMPLE.COM and fred/users.example.com@EXAMPLE.COM can be used to authenticate to the database server.

If you use mod_auth_krb 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.

4.2.4. Ident-based authentication

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  

On systems supporting SO_PEERCRED requests for Unix-domain sockets, 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 a system.

When using ident-based authentication, after having determined the name of the operating system user that initiated the connection, PostgreSQL checks whether that user is allowed to connect as the database user he is requesting to connect as. This is controlled by the ident map argument that follows the ident keyword in the pg_hba.conf file. There is a predefined ident map sameuser, which allows any operating system user to connect as the database user of the same name (if the latter exists). Other maps must be created manually.

Ident maps other than sameuser are defined in the file pg_ident.conf in the data directory, which contains lines of the general form:

map-name ident-username database-username

Comments and whitespace are handled in the usual way. 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 which operating system user is allowed to connect as which database user. The same map-name can be used repeatedly to specify more user-mappings within a single map. There is no restriction regarding how many database users a given operating system user may correspond to and vice versa.

The pg_ident.conf file is read on startup and when the postmaster receives a SIGHUP signal. If you edit the file on an active system, you will need to signal the postmaster (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 4-1 is shown in Example 4-2. 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 4-2. An example pg_ident.conf file

#MAP           IDENT-NAME   POSTGRESQL-NAME

omicron        bryanh       bryanh
omicron        ann          ann
# bob has username robert on these machines
omicron        robert       bob
# bryanh can also connect as guest1
omicron        bryanh       guest1
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