client-auth.sgml

<!-- doc/src/sgml/client-auth.sgml -->

<chapter id="client-authentication">
 <title>Client Authentication</title>

 <indexterm zone="client-authentication">
  <primary>client authentication</primary>
 </indexterm>

 <para>
  When a client application connects to the database server, it
  specifies which <productname>PostgreSQL</productname> database 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 &mdash; see
  <xref linkend="user-manag"> for more information. Therefore, it is
  essential to restrict which database users can connect.
 </para>

 <note>
  <para>
   As explained in <xref linkend="user-manag">,
   <productname>PostgreSQL</productname> actually does privilege
   management in terms of <quote>roles</>.  In this chapter, we
   consistently use <firstterm>database user</> to mean <quote>role with the
   <literal>LOGIN</> privilege</quote>.
  </para>
 </note>

 <para>
  <firstterm>Authentication</firstterm> 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
  database user name that was requested.
 </para>

 <para>
  <productname>PostgreSQL</productname> 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.
 </para>

 <para>
  <productname>PostgreSQL</productname> database 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 might 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.
 </para>

 <sect1 id="auth-pg-hba-conf">
  <title>The <filename>pg_hba.conf</filename> File</title>

  <indexterm zone="auth-pg-hba-conf">
   <primary>pg_hba.conf</primary>
  </indexterm>

  <para>
   Client authentication is controlled by a configuration file,
   which traditionally is named
   <filename>pg_hba.conf</filename> and is stored in the database
   cluster's data directory.
   (<acronym>HBA</> stands for host-based authentication.) A default
   <filename>pg_hba.conf</filename> file is installed when the data
   directory is initialized by <command>initdb</command>.  It is
   possible to place the authentication configuration file elsewhere,
   however; see the <xref linkend="guc-hba-file"> configuration parameter.
  </para>

  <para>
   The general format of the <filename>pg_hba.conf</filename> file is
   a set of records, one per line. Blank lines are ignored, as is any
   text after the <literal>#</literal> comment character.
   Records cannot be continued across lines.
   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 double-quoted.
   Quoting one of the keywords in a database, user, or address field (e.g.,
   <literal>all</> or <literal>replication</>) makes the word lose its special
   meaning, and just match a database, user, or host with that name.
  </para>

  <para>
   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 <quote>fall-through</> or
   <quote>backup</>: if one record is chosen and the authentication
   fails, subsequent records are not considered. If no record matches,
   access is denied.
  </para>

  <para>
   A record can have one of the seven formats
<synopsis>
local      <replaceable>database</replaceable>  <replaceable>user</replaceable>  <replaceable>auth-method</replaceable>  <optional><replaceable>auth-options</replaceable></optional>
host       <replaceable>database</replaceable>  <replaceable>user</replaceable>  <replaceable>address</replaceable>  <replaceable>auth-method</replaceable>  <optional><replaceable>auth-options</replaceable></optional>
hostssl    <replaceable>database</replaceable>  <replaceable>user</replaceable>  <replaceable>address</replaceable>  <replaceable>auth-method</replaceable>  <optional><replaceable>auth-options</replaceable></optional>
hostnossl  <replaceable>database</replaceable>  <replaceable>user</replaceable>  <replaceable>address</replaceable>  <replaceable>auth-method</replaceable>  <optional><replaceable>auth-options</replaceable></optional>
host       <replaceable>database</replaceable>  <replaceable>user</replaceable>  <replaceable>IP-address</replaceable>  <replaceable>IP-mask</replaceable>  <replaceable>auth-method</replaceable>  <optional><replaceable>auth-options</replaceable></optional>
hostssl    <replaceable>database</replaceable>  <replaceable>user</replaceable>  <replaceable>IP-address</replaceable>  <replaceable>IP-mask</replaceable>  <replaceable>auth-method</replaceable>  <optional><replaceable>auth-options</replaceable></optional>
hostnossl  <replaceable>database</replaceable>  <replaceable>user</replaceable>  <replaceable>IP-address</replaceable>  <replaceable>IP-mask</replaceable>  <replaceable>auth-method</replaceable>  <optional><replaceable>auth-options</replaceable></optional>
</synopsis>
   The meaning of the fields is as follows:

   <variablelist>
    <varlistentry>
     <term><literal>local</literal></term>
     <listitem>
      <para>
       This record matches connection attempts using Unix-domain
       sockets.  Without a record of this type, Unix-domain socket
       connections are disallowed.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>host</literal></term>
     <listitem>
      <para>
       This record matches connection attempts made using TCP/IP.
       <literal>host</literal> records match either
       <acronym>SSL</acronym> or non-<acronym>SSL</acronym> connection
       attempts.
      </para>
     <note>
      <para>
       Remote TCP/IP connections will not be possible unless
       the server is started with an appropriate value for the
       <xref linkend="guc-listen-addresses"> configuration parameter,
       since the default behavior is to listen for TCP/IP connections
       only on the local loopback address <literal>localhost</>.
      </para>
     </note>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>hostssl</literal></term>
     <listitem>
      <para>
       This record matches connection attempts made using TCP/IP,
       but only when the connection is made with <acronym>SSL</acronym>
       encryption.
      </para>

      <para>
       To make use of this option the server must be built with
       <acronym>SSL</acronym> support. Furthermore,
       <acronym>SSL</acronym> must be enabled
       by setting the <xref linkend="guc-ssl"> configuration parameter (see
       <xref linkend="ssl-tcp"> for more information).
       Otherwise, the <literal>hostssl</literal> record is ignored except for
       logging a warning that it cannot match any connections.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>hostnossl</literal></term>
     <listitem>
      <para>
       This record type has the opposite behavior of <literal>hostssl</>;
       it only matches connection attempts made over
       TCP/IP that do not use <acronym>SSL</acronym>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable>database</replaceable></term>
     <listitem>
      <para>
       Specifies which database name(s) this record matches.  The value
       <literal>all</literal> specifies that it matches all databases.
       The value <literal>sameuser</> specifies that the record
       matches if the requested database has the same name as the
       requested user.  The value <literal>samerole</> specifies that
       the requested user must be a member of the role with the same
       name as the requested database.  (<literal>samegroup</> is an
       obsolete but still accepted spelling of <literal>samerole</>.)
       Superusers are not considered to be members of a role for the
       purposes of <literal>samerole</> unless they are explicitly
       members of the role, directly or indirectly, and not just by
       virtue of being a superuser.
       The value <literal>replication</> specifies that the record
       matches if a physical replication connection is requested (note that
       replication connections do not specify any particular database).
       Otherwise, this is the name of
       a specific <productname>PostgreSQL</productname> database.
       Multiple database names can be supplied by separating them with
       commas.  A separate file containing database names can be specified by
       preceding the file name with <literal>@</>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable>user</replaceable></term>
     <listitem>
      <para>
       Specifies which database user name(s) this record
       matches. The value <literal>all</literal> specifies that it
       matches all users.  Otherwise, this is either the name of a specific
       database user, or a group name preceded by <literal>+</>.
       (Recall that there is no real distinction between users and groups
       in <productname>PostgreSQL</>; a <literal>+</> mark really means
       <quote>match any of the roles that are directly or indirectly members
       of this role</>, while a name without a <literal>+</> mark matches
       only that specific role.) For this purpose, a superuser is only
       considered to be a member of a role if they are explicitly a member
       of the role, directly or indirectly, and not just by virtue of
       being a superuser.
       Multiple user names can be supplied by separating them with commas.
       A separate file containing user names can be specified by preceding the
       file name with <literal>@</>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable>address</replaceable></term>
     <listitem>
      <para>
       Specifies the client machine address(es) that this record
       matches.  This field can contain either a host name, an IP
       address range, or one of the special key words mentioned below.
      </para>

      <para>
       An IP address range is specified using standard numeric notation
       for the range's starting address, then a slash (<literal>/</literal>)
       and a <acronym>CIDR</> mask length.  The mask
       length indicates the number of high-order bits of the client
       IP address that must match.  Bits to the right of this should
       be zero in the given IP address.
       There must not be any white space between the IP address, the
       <literal>/</literal>, and the CIDR mask length.
      </para>

      <para>
       Typical examples of an IPv4 address range specified this way are
       <literal>172.20.143.89/32</literal> for a single host, or
       <literal>172.20.143.0/24</literal> for a small network, or
       <literal>10.6.0.0/16</literal> for a larger one.
       An IPv6 address range might look like <literal>::1/128</literal>
       for a single host (in this case the IPv6 loopback address) or
       <literal>fe80::7a31:c1ff:0000:0000/96</literal> for a small
       network.
       <literal>0.0.0.0/0</literal> represents all
       IPv4 addresses, and <literal>::0/0</literal> represents
       all IPv6 addresses.
       To specify a single host, use a mask length of 32 for IPv4 or
       128 for IPv6.  In a network address, do not omit trailing zeroes.
      </para>

      <para>
       An entry given in IPv4 format will match only IPv4 connections,
       and 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.
      </para>

      <para>
       You can also write <literal>all</literal> to match any IP address,
       <literal>samehost</literal> to match any of the server's own IP
       addresses, or <literal>samenet</literal> to match any address in any
       subnet that the server is directly connected to.
      </para>

      <para>
       If a host name is specified (anything that is not an IP address
       range or a special key word is treated as a host name),
       that name is compared with the result of a reverse name
       resolution of the client's IP address (e.g., reverse DNS
       lookup, if DNS is used).  Host name comparisons are case
       insensitive.  If there is a match, then a forward name
       resolution (e.g., forward DNS lookup) is performed on the host
       name to check whether any of the addresses it resolves to are
       equal to the client's IP address.  If both directions match,
       then the entry is considered to match.  (The host name that is
       used in <filename>pg_hba.conf</filename> should be the one that
       address-to-name resolution of the client's IP address returns,
       otherwise the line won't be matched.  Some host name databases
       allow associating an IP address with multiple host names, but
       the operating system will only return one host name when asked
       to resolve an IP address.)
      </para>

      <para>
       A host name specification that starts with a dot
       (<literal>.</literal>) matches a suffix of the actual host
       name.  So <literal>.example.com</literal> would match
       <literal>foo.example.com</literal> (but not just
       <literal>example.com</literal>).
      </para>

      <para>
       When host names are specified
       in <filename>pg_hba.conf</filename>, you should make sure that
       name resolution is reasonably fast.  It can be of advantage to
       set up a local name resolution cache such
       as <command>nscd</command>.  Also, you may wish to enable the
       configuration parameter <varname>log_hostname</varname> to see
       the client's host name instead of the IP address in the log.
      </para>

      <para>
       This field only applies to <literal>host</literal>,
       <literal>hostssl</literal>, and <literal>hostnossl</> records.
      </para>

      <sidebar>
       <para>
        Users sometimes wonder why host names are handled
        in this seemingly complicated way, with two name resolutions
        including a reverse lookup of the client's IP address.  This
        complicates use of the feature in case the client's reverse DNS
        entry is not set up or yields some undesirable host name.
        It is done primarily for efficiency: this way, a connection attempt
        requires at most two resolver lookups, one reverse and one forward.
        If there is a resolver problem with some address, it becomes only
        that client's problem.  A hypothetical alternative
        implementation that only did forward lookups would have to
        resolve every host name mentioned in
        <filename>pg_hba.conf</filename> during every connection attempt.
        That could be quite slow if many names are listed.
        And if there is a resolver problem with one of the host names,
        it becomes everyone's problem.
       </para>

       <para>
        Also, a reverse lookup is necessary to implement the suffix
        matching feature, because the actual client host name needs to
        be known in order to match it against the pattern.
       </para>

       <para>
        Note that this behavior is consistent with other popular
        implementations of host name-based access control, such as the
        Apache HTTP Server and TCP Wrappers.
       </para>
      </sidebar>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable>IP-address</replaceable></term>
     <term><replaceable>IP-mask</replaceable></term>
     <listitem>
      <para>
       These two fields can be used as an alternative to the
       <replaceable>IP-address</><literal>/</><replaceable>mask-length</>
       notation.  Instead of
       specifying the mask length, the actual mask is specified in a
       separate column. For example, <literal>255.0.0.0</> represents an IPv4
       CIDR mask length of 8, and <literal>255.255.255.255</> represents a
       CIDR mask length of 32.
      </para>

      <para>
       These fields only apply to <literal>host</literal>,
       <literal>hostssl</literal>, and <literal>hostnossl</> records.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable>auth-method</replaceable></term>
     <listitem>
      <para>
       Specifies the authentication method to use when a connection matches
       this record. The possible choices are summarized here; details
       are in <xref linkend="auth-methods">.

       <variablelist>
        <varlistentry>
         <term><literal>trust</></term>
         <listitem>
         <para>
          Allow the connection unconditionally. This method
          allows anyone that can connect to the
          <productname>PostgreSQL</productname> database server to login as
          any <productname>PostgreSQL</productname> user they wish,
          without the need for a password or any other authentication.  See <xref
          linkend="auth-trust"> for details.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>reject</></term>
        <listitem>
         <para>
          Reject the connection unconditionally. This is useful for
          <quote>filtering out</> certain hosts from a group, for example a
          <literal>reject</> line could block a specific host from connecting,
          while a later line allows the remaining hosts in a specific
          network to connect.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>scram-sha-256</></term>
        <listitem>
         <para>
          Perform SCRAM-SHA-256 authentication to verify the user's
          password. See <xref linkend="auth-password"> for details.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>md5</></term>
        <listitem>
         <para>
          Perform SCRAM-SHA-256 or MD5 authentication to verify the
          user's password. See <xref linkend="auth-password">
          for details.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>password</></term>
        <listitem>
         <para>
          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 <xref linkend="auth-password"> for details.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>gss</></term>
        <listitem>
         <para>
          Use GSSAPI to authenticate the user. This is only
          available for TCP/IP connections. See <xref
          linkend="gssapi-auth"> for details.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>sspi</></term>
        <listitem>
         <para>
          Use SSPI to authenticate the user. This is only
          available on Windows. See <xref
          linkend="sspi-auth"> for details.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>ident</></term>
        <listitem>
         <para>
          Obtain the operating system user name of the client
          by contacting the ident server on the client
          and check if it matches the requested database user name.
          Ident authentication can only be used on TCP/IP
          connections. When specified for local connections, peer
          authentication will be used instead.
          See <xref linkend="auth-ident"> for details.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>peer</></term>
        <listitem>
         <para>
          Obtain the client's operating system user name from the operating
          system and check if it matches the requested database user name.
          This is only available for local connections.
          See <xref linkend="auth-peer"> for details.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>ldap</></term>
        <listitem>
         <para>
          Authenticate using an <acronym>LDAP</> server. See <xref
          linkend="auth-ldap"> for details.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>radius</></term>
        <listitem>
         <para>
          Authenticate using a RADIUS server. See <xref
          linkend="auth-radius"> for details.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>cert</></term>
        <listitem>
         <para>
          Authenticate using SSL client certificates. See
          <xref linkend="auth-cert"> for details.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>pam</></term>
        <listitem>
         <para>
          Authenticate using the Pluggable Authentication Modules
          (PAM) service provided by the operating system.  See <xref
          linkend="auth-pam"> for details.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>bsd</></term>
        <listitem>
         <para>
          Authenticate using the BSD Authentication service provided by the
          operating system. See <xref linkend="auth-bsd"> for details.
         </para>
        </listitem>
       </varlistentry>
      </variablelist>

      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable>auth-options</replaceable></term>
     <listitem>
      <para>
       After the <replaceable>auth-method</> field, there can be field(s) of
       the form <replaceable>name</><literal>=</><replaceable>value</> that
       specify options for the authentication method. Details about which
       options are available for which authentication methods appear below.
      </para>

      <para>
       In addition to the method-specific options listed below, there is one
       method-independent authentication option <literal>clientcert</>, which
       can be specified in any <literal>hostssl</> record.  When set
       to <literal>1</>, this option requires the client to present a valid
       (trusted) SSL certificate, in addition to the other requirements of the
       authentication method.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

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

  <para>
   Since the <filename>pg_hba.conf</filename> 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 <literal>trust</>
   authentication for local TCP/IP connections but require a password for
   remote TCP/IP connections. In this case a record specifying
   <literal>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.
  </para>

  <para>
   The <filename>pg_hba.conf</filename> file is read on start-up and when
   the main server process receives a
   <systemitem>SIGHUP</systemitem><indexterm><primary>SIGHUP</primary></indexterm>
   signal. If you edit the file on an
   active system, you will need to signal the postmaster
   (using <literal>pg_ctl reload</> or <literal>kill -HUP</>) to make it
   re-read the file.
  </para>

  <note>
   <para>
    The preceding statement is not true on Microsoft Windows: there, any
    changes in the <filename>pg_hba.conf</filename> file are immediately
    applied by subsequent new connections.
   </para>
  </note>

  <para>
   The system view
   <link linkend="view-pg-hba-file-rules"><structname>pg_hba_file_rules</structname></link>
   can be helpful for pre-testing changes to the <filename>pg_hba.conf</>
   file, or for diagnosing problems if loading of the file did not have the
   desired effects.  Rows in the view with
   non-null <structfield>error</structfield> fields indicate problems in the
   corresponding lines of the file.
  </para>
 
  <tip>
   <para>
    To connect to a particular database, a user must not only pass the
    <filename>pg_hba.conf</filename> checks, but must have the
    <literal>CONNECT</> privilege for the database.  If you wish to
    restrict which users can connect to which databases, it's usually
    easier to control this by granting/revoking <literal>CONNECT</> privilege
    than to put the rules in <filename>pg_hba.conf</filename> entries.
   </para>
  </tip>

  <para>
   Some examples of <filename>pg_hba.conf</filename> entries are shown in
   <xref linkend="example-pg-hba.conf">. See the next section for details on the
   different authentication methods.
  </para>

   <example id="example-pg-hba.conf">
    <title>Example <filename>pg_hba.conf</filename> Entries</title>
<programlisting>
# Allow any user on the local system to connect to any database with
# any database user name using Unix-domain sockets (the default for local
# connections).
#
# TYPE  DATABASE        USER            ADDRESS                 METHOD
local   all             all                                     trust

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

# The same as the previous 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

# The same over IPv6.
#
# TYPE  DATABASE        USER            ADDRESS                 METHOD
host    all             all             ::1/128                 trust

# The same using a host name (would typically cover both IPv4 and IPv6).
#
# TYPE  DATABASE        USER            ADDRESS                 METHOD
host    all             all             localhost               trust

# Allow any user from any host with IP address 192.168.93.x to connect
# to database "postgres" as the same user name that ident reports for
# the connection (typically the operating system user name).
#
# TYPE  DATABASE        USER            ADDRESS                 METHOD
host    postgres        all             192.168.93.0/24         ident

# Allow any user from host 192.168.12.10 to connect to database
# "postgres" if the user's password is correctly supplied.
#
# TYPE  DATABASE        USER            ADDRESS                 METHOD
host    postgres        all             192.168.12.10/32        scram-sha-256

# Allow any user from hosts in the example.com domain to connect to
# any database if the user's password is correctly supplied.
#
# Require SCRAM authentication for most users, but make an exception
# for user 'mike', who uses an older client that doesn't support SCRAM
# authentication.
#
# TYPE  DATABASE        USER            ADDRESS                 METHOD
host    all             mike            .example.com            md5
host    all             all             .example.com            scram-sha-256

# In the absence of preceding "host" lines, these two lines will
# reject all connections from 192.168.54.1 (since that entry will be
# matched first), but allow GSSAPI connections from anywhere else
# on the Internet.  The zero mask causes no bits of the host IP
# address to be considered, so it matches any host.
#
# TYPE  DATABASE        USER            ADDRESS                 METHOD
host    all             all             192.168.54.1/32         reject
host    all             all             0.0.0.0/0               gss

# 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            ADDRESS                 METHOD
host    all             all             192.168.0.0/16          ident map=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 database user name) except for administrators
# and members of role "support", who can connect to all databases.  The file
# $PGDATA/admins contains a list of names of administrators.  Passwords
# are required in all cases.
#
# TYPE  DATABASE        USER            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:
local   db1,db2,@demodbs  all                                   md5
</programlisting>
   </example>
 </sect1>

 <sect1 id="auth-username-maps">
  <title>User Name Maps</title>

  <indexterm zone="auth-username-maps">
   <primary>User name maps</primary>
  </indexterm>

  <para>
   When using an external authentication system such as Ident or GSSAPI,
   the name of the operating system user that initiated the connection
   might not be the same as the database user (role) that is to be used.
   In this case, a user name map can be applied to map the operating system
   user name to a database user.  To use user name mapping, specify
   <literal>map</literal>=<replaceable>map-name</replaceable>
   in the options field in <filename>pg_hba.conf</filename>. This option is
   supported for all authentication methods that receive external user names.
   Since different mappings might be needed for different connections,
   the name of the map to be used is specified in the
   <replaceable>map-name</replaceable> parameter in <filename>pg_hba.conf</filename>
   to indicate which map to use for each individual connection.
  </para>

  <para>
   User name maps are defined in the ident map file, which by default is named
   <filename>pg_ident.conf</><indexterm><primary>pg_ident.conf</primary></indexterm>
   and is stored in the
   cluster's data directory.  (It is possible to place the map file
   elsewhere, however; see the <xref linkend="guc-ident-file">
   configuration parameter.)
   The ident map file contains lines of the general form:
<synopsis>
<replaceable>map-name</> <replaceable>system-username</> <replaceable>database-username</>
</synopsis>
   Comments and whitespace are handled in the same way as in
   <filename>pg_hba.conf</>.  The
   <replaceable>map-name</> is an arbitrary name that will be used to
   refer to this mapping in <filename>pg_hba.conf</filename>. The other
   two fields specify an operating system user name and a matching
   database user name. The same <replaceable>map-name</> can be
   used repeatedly to specify multiple user-mappings within a single map.
  </para>
  <para>
   There is no restriction regarding how many database users a given
   operating system user can correspond to, nor vice versa.  Thus, entries
   in a map should be thought of as meaning <quote>this operating system
   user is allowed to connect as this database user</quote>, rather than
   implying that they are equivalent.  The connection will be allowed if
   there is any map entry that pairs the user name obtained from the
   external authentication system with the database user name that the
   user has requested to connect as.
  </para>
  <para>
   If the <replaceable>system-username</> field starts with a slash (<literal>/</>),
   the remainder of the field is treated as a regular expression.
   (See <xref linkend="posix-syntax-details"> for details of
   <productname>PostgreSQL</>'s regular expression syntax.)  The regular
   expression can include a single capture, or parenthesized subexpression,
   which can then be referenced in the <replaceable>database-username</>
   field as <literal>\1</> (backslash-one).  This allows the mapping of
   multiple user names in a single line, which is particularly useful for
   simple syntax substitutions.  For example, these entries
<programlisting>
mymap   /^(.*)@mydomain\.com$      \1
mymap   /^(.*)@otherdomain\.com$   guest
</programlisting>
   will remove the domain part for users with system user names that end with
   <literal>@mydomain.com</>, and allow any user whose system name ends with
   <literal>@otherdomain.com</> to log in as <literal>guest</>.
  </para>

  <tip>
   <para>
    Keep in mind that by default, a regular expression can match just part of
    a string.  It's usually wise to use <literal>^</> and <literal>$</>, as
    shown in the above example, to force the match to be to the entire
    system user name.
   </para>
  </tip>

  <para>
   The <filename>pg_ident.conf</filename> file is read on start-up and
   when the main server process receives a
   <systemitem>SIGHUP</systemitem><indexterm><primary>SIGHUP</primary></indexterm>
   signal. If you edit the file on an
   active system, you will need to signal the postmaster
   (using <literal>pg_ctl reload</> or <literal>kill -HUP</>) to make it
   re-read the file.
  </para>

  <para>
   A <filename>pg_ident.conf</filename> file that could be used in
   conjunction with the <filename>pg_hba.conf</> file in <xref
   linkend="example-pg-hba.conf"> is shown in <xref
   linkend="example-pg-ident.conf">. In this example, anyone
   logged in to a machine on the 192.168 network that does not have the
   operating system user name <literal>bryanh</>, <literal>ann</>, or
   <literal>robert</> would not be granted access. Unix user
   <literal>robert</> would only be allowed access when he tries to
   connect as <productname>PostgreSQL</> user <literal>bob</>, not
   as <literal>robert</> or anyone else. <literal>ann</> would
   only be allowed to connect as <literal>ann</>. User
   <literal>bryanh</> would be allowed to connect as either
   <literal>bryanh</> or as <literal>guest1</>.
  </para>

  <example id="example-pg-ident.conf">
   <title>An Example <filename>pg_ident.conf</> File</title>
<programlisting>
# MAPNAME       SYSTEM-USERNAME         PG-USERNAME

omicron         bryanh                  bryanh
omicron         ann                     ann
# bob has user name robert on these machines
omicron         robert                  bob
# bryanh can also connect as guest1
omicron         bryanh                  guest1
</programlisting>
  </example>
 </sect1>

 <sect1 id="auth-methods">
  <title>Authentication Methods</title>
  <para>
   The following subsections describe the authentication methods in more detail.
  </para>

  <sect2 id="auth-trust">
   <title>Trust Authentication</title>

   <para>
    When <literal>trust</> authentication is specified,
    <productname>PostgreSQL</productname> 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 <literal>database</> and
    <literal>user</> columns still apply.
    This method should only be used when there is adequate
    operating-system-level protection on connections to the server.
   </para>

   <para>
    <literal>trust</> authentication is appropriate and very
    convenient for local connections on a single-user workstation.  It
    is usually <emphasis>not</> appropriate by itself on a multiuser
    machine.  However, you might be able to use <literal>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
    <varname>unix_socket_permissions</varname> (and possibly
    <varname>unix_socket_group</varname>) configuration parameters as
    described in <xref linkend="runtime-config-connection">.  Or you
    could set the <varname>unix_socket_directories</varname>
    configuration parameter to place the socket file in a suitably
    restricted directory.
   </para>

   <para>
    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 <literal>host ... 127.0.0.1 ...</> line from
    <filename>pg_hba.conf</>, or change it to a
    non-<literal>trust</> authentication method.
   </para>

   <para>
    <literal>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 <filename>pg_hba.conf</> lines that specify
    <literal>trust</>.  It is seldom reasonable to use <literal>trust</>
    for any TCP/IP connections other than those from <systemitem>localhost</> (127.0.0.1).
   </para>

  </sect2>

  <sect2 id="auth-password">
   <title>Password Authentication</title>

   <indexterm>
    <primary>MD5</>
   </indexterm>
   <indexterm>
    <primary>password</primary>
    <secondary>authentication</secondary>
   </indexterm>

   <para>
    The password-based authentication methods are <literal>scram-sha-256</>,
    <literal>md5</>, and <literal>password</>. These methods operate
    similarly except for the way that the password is sent across the
    connection.
   </para>

   <para>
    Plain <literal>password</> sends the password in clear-text, and is
    therefore vulnerable to password <quote>sniffing</> attacks. It should
    always be avoided if possible. If the connection is protected by SSL
    encryption then <literal>password</> can be used safely, though.
    (Though SSL certificate authentication might be a better choice if one
    is depending on using SSL).
   </para>


   <para>
    <literal>scram-sha-256</> performs SCRAM-SHA-256 authentication, as
    described in
    <ulink url="https://tools.ietf.org/html/rfc7677">RFC 7677</ulink>. It
    is a challenge-response scheme, that prevents password sniffing on
    untrusted connections. It is more secure than the <literal>md5</>
    method, but might not be supported by older clients.
   </para>

   <para>
    <literal>md5</> allows falling back to a less secure challenge-response
    mechanism for those users with an MD5 hashed password.
    The fallback mechanism also prevents password sniffing, but provides no
    protection if an attacker manages to steal the password hash from the
    server, and it cannot be used with the <xref
    linkend="guc-db-user-namespace"> feature. For all other users,
    <literal>md5</> works the same as <literal>scram-sha-256</>.
   </para>

   <para>
    <productname>PostgreSQL</productname> database passwords are
    separate from operating system user passwords. The password for
    each database user is stored in the <literal>pg_authid</> system
    catalog. Passwords can be managed with the SQL commands
    <xref linkend="sql-createuser"> and
    <xref linkend="sql-alterrole">,
    e.g., <userinput>CREATE USER foo WITH PASSWORD 'secret'</userinput>.
    If no password has been set up for a user, the stored password
    is null and password authentication will always fail for that user.
   </para>

  </sect2>

  <sect2 id="gssapi-auth">
   <title>GSSAPI Authentication</title>

   <indexterm zone="gssapi-auth">
    <primary>GSSAPI</primary>
   </indexterm>

   <para>
    <productname>GSSAPI</productname> is an industry-standard protocol
    for secure authentication defined in RFC 2743.
    <productname>PostgreSQL</productname> supports
    <productname>GSSAPI</productname> with <productname>Kerberos</productname>
    authentication according to RFC 1964. <productname>GSSAPI</productname>
    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 sent unencrypted unless
    <acronym>SSL</acronym> is used.
   </para>

   <para>
    GSSAPI support has to be enabled when <productname>PostgreSQL</> is built;
    see <xref linkend="installation"> for more information.
   </para>

   <para>
    When <productname>GSSAPI</productname> uses
    <productname>Kerberos</productname>, it uses a standard principal