When a client application connects to the database server, it specifies which PostgreSQL user name it wants to connect as, much the same way one logs into a Unix computer as a particular user. Within the SQL environment the active database user name determines access privileges to database objects -- see Chapter 4 for more information. Therefore, it is essential to restrict which database users can connect.
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 who runs the client application) is permitted to connect with the user name that was requested.
PostgreSQL offers a number of different client authentication methods. The method used to authenticate a particular client connection can be selected on the basis of (client) host address, database, and user.
PostgreSQL user names are logically separate from user names of the operating system in which the server runs. If all the users of a particular server also have accounts on the server's machine, it makes sense to assign database user names that match their operating system user names. However, a server that accepts remote connections may have many users who have no local account, and in such cases there need be no connection between database user names and OS user names.
Client authentication is controlled by the file pg_hba.conf in the data directory, e.g., /usr/local/pgsql/data/pg_hba.conf. (HBA stands for host-based authentication.) A default pg_hba.conf file is installed when the data directory is initialized by initdb.
The general format of the pg_hba.conf file is a set of records, one per line. Blank lines are ignored, as is any text after the "#" comment character. A record is made up of a number of fields which are separated by spaces and/or tabs. Fields can contain white space if the field value is quoted. Records cannot be continued across lines.
Each record specifies a connection type, a client IP address range (if relevant for the connection type), a database name, a user name, and the authentication method to be used for connections matching these parameters. The first record with a matching connection type, client address, requested database, and user name is used to perform authentication. There is no "fall-through" or "backup": if one record is chosen and the authentication fails, subsequent records are not considered. If no record matches, access is denied.
A record may have one of the three formats
local database user authentication-method [authentication-option] host database user IP-address IP-mask authentication-method [authentication-option] hostssl database user IP-address IP-mask authentication-method [authentication-option]
The meaning of the fields is as follows:
This record matches connection attempts using Unix domain sockets. Without a record of this type, Unix-domain socket connections are disallowed
This record matches connection attempts using TCP/IP
networks. Note that TCP/IP connections are disabled
unless the server is started with the -i option or the tcpip_socket postgresql.conf configuration parameter
is enabled.
This record matches connection attempts using SSL over TCP/IP. host records will match either SSL or non-SSL connection attempts, but hostssl records require SSL connections.
To be able make use of this option the server must be built with SSL support enabled. Furthermore, SSL must be enabled by enabling the option ssl in postgresql.conf (see Section 3.4).
Specifies which databases this record matches. The value all specifies that it matches all databases. The value sameuser specifies that the record matches if the requested database has the same name as the requested user. The value samegroup specifies that the requested user must a member of the group with the same name as the requested database. Otherwise, this is the name of a specific PostgreSQL database. Multiple database names can be supplied by separating them with commas. A file containing database names can be specified by preceding the file name with @. The file must be in the same directory as pg_hba.conf.
Specifies which PostgreSQL users this record matches. The value all specifies that it matches all users. Otherwise, this is the name of a specific PostgreSQL user. Multiple user names can be supplied by separating them with commas. Group names can be specified by preceding the group name with +. A file containing user names can be specified by preceding the file name with @. The file must be in the same directory as pg_hba.conf.
These two fields contain IP address/mask values in standard dotted decimal notation. (IP addresses can only be specified numerically, not as domain or host names.) Taken together they specify the client machine IP addresses that this record matches. The precise logic is that
must be zero for the record to match. (Of course IP addresses can be spoofed but this consideration is beyond the scope of PostgreSQL.)
These fields only apply to host and hostssl records.
Specifies the authentication method to use when connecting via this record. The possible choices are summarized here; details are in Section 6.2.
The connection is allowed unconditionally. This method allows anyone that can connect to the PostgreSQL database to login as any PostgreSQL user they like, without the need for a password. See Section 6.2.1 for details.
The connection is rejected unconditionally. This is useful for "filtering out" certain hosts from a group.
Requires the client to supply an MD5 encrypted password for authentication. This is the only method that allows encrypted passwords to be stored in pg_shadow. See Section 6.2.2 for details.
Like md5 method but uses older crypt encryption, which is needed for pre-7.2 clients. md5 is preferred for 7.2 and later clients. See Section 6.2.2 for details.
Same as "md5", but the password is sent in clear text over the network. This should not be used on untrusted networks. See Section 6.2.2 for details.
Kerberos V4 is used to authenticate the user. This is only available for TCP/IP connections. See Section 6.2.3 for details.
Kerberos V5 is used to authenticate the user. This is only available for TCP/IP connections. See Section 6.2.3 for details.
Obtain the operating system user name of the client (for TCP/IP connections by contacting the ident server on the client, for local connections by getting it from the operating system) and check if the user is allowed to connect as the requested database user by consulting the map specified after the ident key word.
If you use the map sameuser, the user names are assumed to be identical. If not, the map name is looked up in the file pg_ident.conf in the same directory as pg_hba.conf. The connection is accepted if that file contains an entry for this map name with the ident-supplied user name and the requested PostgreSQL user name.
For local connections, this only works on machines that support Unix-domain socket credentials (currently Linux, FreeBSD, NetBSD, and BSD/OS).
See Section 6.2.4 below for details.
Authenticate using the Pluggable Authentication Modules (PAM) service provided by the operating system. See Section 6.2.5 for details.
The meaning of this optional field depends on the chosen authentication method and is described in the next section.
Since the pg_hba.conf records are examined sequentially for each connection attempt, the order of the records is significant. Typically, earlier records will have tight connection match parameters and weaker authentication methods, while later records will have looser match parameters and stronger authentication methods. For example, one might wish to use trust authentication for local TCP connections but require a password for remote TCP connections. In this case a record specifying trust authentication for connections from 127.0.0.1 would appear before a record specifying password authentication for a wider range of allowed client IP addresses.
Important: Do not prevent the superuser from accessing the template1 database. Various utility commands need access to template1.
The pg_hba.conf file is read on start-up 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.
An example of a pg_hba.conf file is shown in Example 6-1. See below for details on the different authentication methods.
Example 6-1. An example pg_hba.conf file
# Allow any user on the local system to connect to any database under # any user name using Unix-domain sockets (the default for local # connections). # # TYPE DATABASE USER IP-ADDRESS IP-MASK METHOD local all all trust # The same using local loopback TCP/IP connections. # # TYPE DATABASE USER IP-ADDRESS IP-MASK METHOD host all all 127.0.0.1 255.255.255.255 trust # Allow any user from any host with IP address 192.168.93.x to connect # to database "template1" as the same user name that ident reports for # the connection (typically the Unix user name). # # TYPE DATABASE USER IP-ADDRESS IP-MASK METHOD host template1 all 192.168.93.0 255.255.255.0 ident sameuser # Allow a user from host 192.168.12.10 to connect to database # "template1" if the user's password is correctly supplied. # # TYPE DATABASE USER IP-ADDRESS IP-MASK METHOD host template1 all 192.168.12.10 255.255.255.255 md5 # In the absence of preceding "host" lines, these two lines will # reject all connection from 192.168.54.1 (since that entry will be # matched first), but allow Kerberos V connections from anywhere else # on the Internet. The zero mask means that no bits of the host IP # address are considered so it matches any host. # # TYPE DATABASE USER IP-ADDRESS IP-MASK METHOD host all all 192.168.54.1 255.255.255.255 reject host all all 0.0.0.0 0.0.0.0 krb5 # Allow users from 192.168.x.x hosts to connect to any database, if # they pass the ident check. If, for example, ident says the user is # "bryanh" and he requests to connect as PostgreSQL user "guest1", the # connection is allowed if there is an entry in pg_ident.conf for map # "omicron" that says "bryanh" is allowed to connect as "guest1". # # TYPE DATABASE USER IP-ADDRESS IP-MASK METHOD host all all 192.168.0.0 255.255.0.0 ident omicron # If these are the only three lines for local connections, they will # allow local users to connect only to their own databases (databases # with the same name as their user name) except for administrators and # members of group "support" who may connect to all databases. The file # $PGDATA/admins contains a list of user names. Passwords are required in # all cases. # # TYPE DATABASE USER IP-ADDRESS IP-MASK METHOD local sameuser all md5 local all @admins md5 local all +support md5 # The last two lines above can be combined into a single line: local all @admins,+support md5 # The database column can also use lists and file names, but not groups: local db1,db2,@demodbs all md5