Client Authentication User names from the operating system and from a Postgres database installation are logically separate. When a client application connects, it specifies which database user name it wants to connect as, similar to how one logs into a Unix computer. Within the SQL environment the active database user name determines various access privileges to database objects -- see for more information about that. It is therefore obviously essential to restrict what database user name a given client can connect as. Authentication is the process by which the database server establishes the identity of the client, and by extension determines whether the client application (or the user which runs the client application) is permitted to connect with the user name that was requested. Postgres offers client authentication by (client) host and by database, with a number of different authentication methods available. The <filename>pg_hba.conf</filename> file Client authentication is controlled by the file pg_hba.conf in the data directory, e.g., /usr/local/pgsql/data/pg_hba.conf. (HBA = host-based authentication) A default file is installed when the data area is initialized by initdb. The general format of the pg_hba.conf file is of a set of records, one per line. Blank lines and lines beginning with a hash character (#) are ignored. A record is made up of a number of fields which are separated by spaces and/or tabs. A record may have one of the two formats local database authentication-method [ authentication-option ] host database IP-address IP-mask authentication-method [ authentication-option ] The meaning of the fields is as follows: local This record pertains to connection attempts over Unix domain sockets. host This record pertains to connection attempts over TCP/IP networks. Note that TCP/IP connections are completely disabled unless the server is started with the or the equivalent configuration parameter is set. database Specifies the database that this record applies to. The value all specifies that it applies to all databases. IP address IP mask These two fields control to which hosts a host record applies, based on their IP address. (Of course IP addresses can be spoofed but this consideration is beyond the scope of Postgres.) The precise logic is that
(actual-IP-address xor IP-address-field) and IP-mask-field
must be zero for the record to match. authentication method Specifies the method a user must use to authenticate themselves when connecting to that database. authentication option This field is interpreted differently depending on the authentication method. The first record that matches a connection attempt is used. Note that there is no fall-through or backup, that is, if one record is chosen and the authentication fails, the following records are not considered. If no record matches, the access will be denied. The pg_hba.conf file is re-read before each connection attempt. It is therefore easily possible to modify access permissions while the server is running. An example of a pg_hba.conf file is shown in . See below for details on the different authentication methods. An example <filename>pg_hba.conf</filename> file # Trust any connection via Unix domain sockets. local trust # Trust any connection via TCP/IP from this machine. host all 127.0.0.1 255.255.255.255 trust # We don't like this machine. host all 192.168.0.10 255.255.255.0 reject # This machine can't encrypt so we ask for passwords in clear. host all 192.168.0.3 255.255.255.0 password # The rest of this group of machines should provide encrypted passwords. host all 192.168.0.0 255.255.255.0 crypt # Authenticate these networks using ident host all 192.168.1.0 255.255.255.0 ident usermap host all 192.168.2.0 255.255.255.0 ident othermap Authentication methods The following authentication methods are supported. They are descibed in detail below. trust The connection is allowed unconditionally. This method allows any user that has login access to the client host to connect as any user whatsoever. Use with care. reject The connection is rejected unconditionally. This is mostly useful to filter out certain hosts from a group. password The client is required to supply a password with the connection attempt which is required to match the password that was set up for the user. An optional file name may be specified after the password keyword. This file is expected to contain a list of users that this record pertains to, and optionally alternative passwords. The password is sent over the wire in clear text. For better protection, use the crypt method. crypt Like the password method, but the password is sent over the wire encrypted using a simple challenge-response protocol. This is still not cryptographically secure but it protects against incidental wire-sniffing. The name of a file may follow the crypt keyword that contains a list of users that this record pertains to. krb4 Kerberos V4 is used to authenticate the user. This is only available for TCP/IP connections. krb5 Kerberos V5 is used to authenticate the user. This is only available for TCP/IP connections. ident The ident server on the client host is asked for the identity of the connecting user. Postgres then verifies whether the so identified operating system user is allowed to connect as the database user that is requested. The authentication option following the ident keyword specifies the name of an ident map that specifies which operating system users equate with which database users. See below for details. Password authentication Postgres database passwords are separate from any 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 explicitly 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 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 (as long as he passes password authentication, of course). These files can also be used a 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 Postgres 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 crypt method. The file will still be evaluated as usual but the password field will simply be ignored and the pg_shadow password will 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 still appear to work but the password one is actually changing is not the password that the system will end up using. 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 enable at build time. Both Kerberos 4 and 5 are supported (./configure --with-krb4 or ./configure --with-krb5 respectively). Postgres should operate like a normal Kerberos service. The name of the service principal is normally postgres, unless it was changed during the build. Make sure that your server keytab file is readable (and preferrably only readable) by the Postgres server account (see ). The location of the keytab file is specified at build time; by default it is /etc/srvtab in Kerberos 4 and FILE:/usr/local/pgsql/etc/krb5.keytab in 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 defails. In the Kerberos 5 hooks, the following assumptions are made about user and service naming: User principal names (anames) are assumed to contain the actual Unix/Postgres user name in the first component. The Postgres service is assumed to be have two components, the service name and a hostname, canonicalized as in Version 4 (i.e., with all domain suffixes removed). Parameter Example user frew@S2K.ORG user aoki/HOST=miyu.S2K.Berkeley.EDU@S2K.ORG host postgres_dbms/ucbvax@S2K.ORG 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. Ident-based authentication The Identification Protocol is described in RFC 1413. Virtually every Unix-like operating systems 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 Postgres 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. Heed the warning:
RFC 1413 The Identification Protocol is not intended as an authorization or access control protocol.
When using ident-based authentication, after having determined the operating system user that initiated the connection, Postgres determines as what database system user he may connect. This is controlled by the ident map argument that follows the ident keyword in the pg_hba.conf file. The simplest ident map is 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 are held 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. There is also no restriction regarding how many database users a given operating system may correspond to and vice versa. A pg_ident.conf file that could be used in conjunction with the pg_hba.conf file in is shown in . In that example setup, anyone logged in to a machine on the 192.168.1 network that does not have the a user name joe, robert, or ann would not be granted access. Unix user robert would only be allowed access when he tries to connect as bob, not as robert or anyone else. ann and joe would only be allowed to connect as themselves. On the 192.168.2 network, however, a user ann would not be allowed to connect at all, only the user bob can connect as bob and some user karl can connect as joe as well. An example <filename>pg_ident.conf</> file usermap joe joe # bob has username robert on these machines usermap robert bob usermap ann ann othermap joe joe othermap bob bob othermap karl joe Authentication problems Genuine authentication failures and related problems generally manifest themselves through error messages like the following. No pg_hba.conf entry for host 123.123.123.123, user joeblow, database testdb This is what you are most likely to get if you succeed in contacting the server, but it doesn't want to talk to you. As the message suggests, the server refused the connection request because it found no authorizing entry in its pg_hba.conf configuration file. Password authentication failed for user 'joeblow' Messages like this indicate that you contacted the server, and it's willing to talk to you, but not until you pass the authorization method specified in the pg_hba.conf file. Check the password you're providing, or check your Kerberos or IDENT software if the complaint mentions one of those authentication types. FATAL 1: SetUserId: user 'joeblow' is not in 'pg_shadow' This is the fancy way of saying that the user doesn't exist at all. FATAL 1: Database testdb does not exist in pg_database The database you're trying to connect to doesn't exist. Note that if you don't specify a database name, it defaults to the database user name, which may or may not be the right thing.