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Chapter 19. Client Authentication

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 17 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 database users who have no local operating system account, and in such cases there need be no connection between database user names and OS user names.

19.1. The pg_hba.conf file

Client authentication is controlled by a configuration file, which traditionally is named pg_hba.conf and is stored in the database cluster's data directory. (HBA stands for host-based authentication.) A default pg_hba.conf file is installed when the data directory is initialized by initdb. It is possible to place the authentication configuration file elsewhere, however; see the hba_file configuration parameter.

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 seven formats

local      database  user  authentication-method  [authentication-option]
host       database  user  CIDR-address  authentication-method  [authentication-option]
hostssl    database  user  CIDR-address  authentication-method  [authentication-option]
hostnossl  database  user  CIDR-address  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]
hostnossl  database  user  IP-address  IP-mask  authentication-method  [authentication-option]

The meaning of the fields is as follows:

local

This record matches connection attempts using Unix-domain sockets. Without a record of this type, Unix-domain socket connections are disallowed.

host

This record matches connection attempts made using TCP/IP. host records match either SSL or non-SSL connection attempts.

Note: Remote TCP/IP connections will not be possible unless the server is started with an appropriate value for the listen_addresses configuration parameter, since the default behavior is to listen for TCP/IP connections only on the local loopback address localhost.

hostssl

This record matches connection attempts made using TCP/IP, but only when the connection is made with SSL encryption.

To make use of this option the server must be built with SSL support. Furthermore, SSL must be enabled at server start time by setting the ssl configuration parameter (see Section 16.8 for more information).

hostnossl

This record type has the opposite logic to hostssl: it only matches connection attempts made over TCP/IP that do not use SSL.

database

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 be 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 @.

user

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 @.

CIDR-address

Specifies the client machine IP address range that this record matches. It contains an IP address in standard dotted decimal notation and a CIDR mask length. (IP addresses can only be specified numerically, not as domain or host names.) The mask length indicates the number of high-order bits of the client IP address that must match. Bits to the right of this must be zero in the given IP address. There must not be any white space between the IP address, the /, and the CIDR mask length.

A typical CIDR-address is 172.20.143.89/32 for a single host, or 172.20.143.0/24 for a network. To specify a single host, use a CIDR mask of 32 for IPv4 or 128 for IPv6.

An IP address given in IPv4 format will match IPv6 connections that have the corresponding address, for example 127.0.0.1 will match the IPv6 address ::ffff:127.0.0.1. An entry given in IPv6 format will match only IPv6 connections, even if the represented address is in the IPv4-in-IPv6 range. Note that entries in IPv6 format will be rejected if the system's C library does not have support for IPv6 addresses.

This field only applies to host, hostssl, and hostnossl records.

IP-address
IP-mask

These fields may be used as an alternative to the CIDR-address notation. Instead of specifying the mask length, the actual mask is specified in a separate column. For example, 255.0.0.0 represents an IPv4 CIDR mask length of 8, and 255.255.255.255 represents a CIDR mask length of 32.

These fields only apply to host, hostssl, and hostnossl records.

authentication-method

Specifies the authentication method to use when connecting via this record. The possible choices are summarized here; details are in Section 19.2.

trust

Allow the connection unconditionally. This method allows anyone that can connect to the PostgreSQL database server to login as any PostgreSQL user they like, without the need for a password. See Section 19.2.1 for details.

reject

Reject the connection unconditionally. This is useful for "filtering out" certain hosts from a group.

md5

Require the client to supply an MD5-encrypted password for authentication. See Section 19.2.2 for details.

crypt

Require the client to supply a crypt()-encrypted password for authentication. md5 is preferred for 7.2 and later clients, but pre-7.2 clients only support crypt. See Section 19.2.2 for details.

password

Require the client to supply an unencrypted password for authentication. Since the password is sent in clear text over the network, this should not be used on untrusted networks. See Section 19.2.2 for details.

krb4

Use Kerberos V4 to authenticate the user. This is only available for TCP/IP connections. See Section 19.2.3 for details.

krb5

Use Kerberos V5 to authenticate the user. This is only available for TCP/IP connections. See Section 19.2.3 for details.

ident

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. See Section 19.2.4 for details.

pam

Authenticate using the Pluggable Authentication Modules (PAM) service provided by the operating system. See Section 19.2.5 for details.

authentication-option

The meaning of this optional field depends on the chosen authentication method. Details appear below.

Files included by @ constructs are read as lists of names, which can be separated by either whitespace or commas. Comments are introduced by #, just as in pg_hba.conf, and nested @ constructs are allowed. Unless the file name following @ is an absolute path, it is taken to be relative to the directory containing the referencing file.

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/IP connections but require a password for remote TCP/IP 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.

The pg_hba.conf file is read on start-up and when the main server process (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.

Some examples of pg_hba.conf entries are shown in Example 19-1. See the next section for details on the different authentication methods.

Example 19-1. Example pg_hba.conf entries

# 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        CIDR-ADDRESS          METHOD
local   all         all                               trust

# The same using local loopback TCP/IP connections.
#
# TYPE  DATABASE    USER        CIDR-ADDRESS          METHOD
host    all         all         127.0.0.1/32          trust     

# The same as the last line but using a separate netmask column
#
# 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        CIDR-ADDRESS          METHOD
host    template1   all         192.168.93.0/24       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        CIDR-ADDRESS          METHOD
host    template1   all         192.168.12.10/32      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 5 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        CIDR-ADDRESS          METHOD
host    all         all         192.168.54.1/32       reject
host    all         all         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        CIDR-ADDRESS          METHOD
host    all         all         192.168.0.0/16        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        CIDR-ADDRESS          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
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