<!-- 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</quote>.  In this chapter, we
   consistently use <firstterm>database user</firstterm> to mean <quote>role with the
   <literal>LOGIN</literal> 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</acronym> 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</literal> or <literal>replication</literal>) 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</quote> or
   <quote>backup</quote>: 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</literal>.
      </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</literal>;
       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</literal> specifies that the record
       matches if the requested database has the same name as the
       requested user.  The value <literal>samerole</literal> specifies that
       the requested user must be a member of the role with the same
       name as the requested database.  (<literal>samegroup</literal> is an
       obsolete but still accepted spelling of <literal>samerole</literal>.)
       Superusers are not considered to be members of a role for the
       purposes of <literal>samerole</literal> unless they are explicitly
       members of the role, directly or indirectly, and not just by
       virtue of being a superuser.
       The value <literal>replication</literal> 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>@</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>+</literal>.
       (Recall that there is no real distinction between users and groups
       in <productname>PostgreSQL</productname>; a <literal>+</literal> mark really means
       <quote>match any of the roles that are directly or indirectly members
       of this role</quote>, while a name without a <literal>+</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>@</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</acronym> 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</literal> records.
      </para>

      <note>
       <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>
      </note>
     </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</replaceable><literal>/</literal><replaceable>mask-length</replaceable>
       notation.  Instead of
       specifying the mask length, the actual mask is specified in a
       separate column. For example, <literal>255.0.0.0</literal> represents an IPv4
       CIDR mask length of 8, and <literal>255.255.255.255</literal> represents a
       CIDR mask length of 32.
      </para>

      <para>
       These fields only apply to <literal>host</literal>,
       <literal>hostssl</literal>, and <literal>hostnossl</literal> 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"/>.  All the options
       are lower case and treated case sensitively, so even acronyms like
       <literal>ldap</literal> must be specified as lower case.

       <variablelist>
        <varlistentry>
         <term><literal>trust</literal></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</literal></term>
        <listitem>
         <para>
          Reject the connection unconditionally. This is useful for
          <quote>filtering out</quote> certain hosts from a group, for example a
          <literal>reject</literal> 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</literal></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</literal></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</literal></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</literal></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</literal></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</literal></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</literal></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</literal></term>
        <listitem>
         <para>
          Authenticate using an <acronym>LDAP</acronym> server. See <xref
          linkend="auth-ldap"/> for details.
         </para>
        </listitem>
       </varlistentry>

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

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

       <varlistentry>
        <term><literal>pam</literal></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</literal></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</replaceable> field, there can be field(s) of
       the form <replaceable>name</replaceable><literal>=</literal><replaceable>value</replaceable> 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</literal>, which
       can be specified in any <literal>hostssl</literal> record.  When set
       to <literal>1</literal>, 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>@</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>@</literal> constructs are
   allowed.  Unless the file name following <literal>@</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</literal>
   authentication for local TCP/IP connections but require a password for
   remote TCP/IP connections. In this case a record specifying
   <literal>trust</literal> 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</literal>, calling the SQL function
   <function>pg_reload_conf()</function>, or using <literal>kill
   -HUP</literal>) 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</filename>
   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</literal> 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</literal> 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</filename><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> <replaceable>system-username</replaceable> <replaceable>database-username</replaceable>
</synopsis>
   Comments and whitespace are handled in the same way as in
   <filename>pg_hba.conf</filename>.  The
   <replaceable>map-name</replaceable> 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</replaceable> 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</replaceable> field starts with a slash (<literal>/</literal>),
   the remainder of the field is treated as a regular expression.
   (See <xref linkend="posix-syntax-details"/> for details of
   <productname>PostgreSQL</productname>'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</replaceable>
   field as <literal>\1</literal> (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</literal>, and allow any user whose system name ends with
   <literal>@otherdomain.com</literal> to log in as <literal>guest</literal>.
  </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>^</literal> and <literal>$</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</literal>, calling the SQL function
   <function>pg_reload_conf()</function>, or using <literal>kill
   -HUP</literal>) 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</filename> 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>, <literal>ann</literal>, or
   <literal>robert</literal> would not be granted access. Unix user
   <literal>robert</literal> would only be allowed access when he tries to
   connect as <productname>PostgreSQL</productname> user <literal>bob</literal>, not
   as <literal>robert</literal> or anyone else. <literal>ann</literal> would
   only be allowed to connect as <literal>ann</literal>. User
   <literal>bryanh</literal> would be allowed to connect as either
   <literal>bryanh</literal> or as <literal>guest1</literal>.
  </para>

  <example id="example-pg-ident.conf">
   <title>An Example <filename>pg_ident.conf</filename> 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>
   <productname>PostgreSQL</productname> provides various methods for
   authenticating users:

   <itemizedlist>
    <listitem>
     <para>
      <link linkend="auth-trust">Trust authentication</link>, which
      simply trusts that users are who they say they are.
     </para>
    </listitem>
    <listitem>
     <para>
      <link linkend="auth-password">Password authentication</link>, which
      requires that users send a password.
     </para>
    </listitem>
    <listitem>
     <para>
      <link linkend="gssapi-auth">GSSAPI authentication</link>, which
      relies on a GSSAPI-compatible security library.  Typically this is
      used to access an authentication server such as a Kerberos or
      Microsoft Active Directory server.
     </para>
    </listitem>
    <listitem>
     <para>
      <link linkend="sspi-auth">SSPI authentication</link>, which
      uses a Windows-specific protocol similar to GSSAPI.
     </para>
    </listitem>
    <listitem>
     <para>
      <link linkend="auth-ident">Ident authentication</link>, which
      relies on an <quote>Identification Protocol</quote> (RFC 1413)
      service on the client's machine.  (On local Unix-socket connections,
      this is treated as peer authentication.)
     </para>
    </listitem>
    <listitem>
     <para>
      <link linkend="auth-peer">Peer authentication</link>, which
      relies on operating system facilities to identify the process at the
      other end of a local connection.  This is not supported for remote
      connections.
     </para>
    </listitem>
    <listitem>
     <para>
      <link linkend="auth-ldap">LDAP authentication</link>, which
      relies on an LDAP authentication server.
     </para>
    </listitem>
    <listitem>
     <para>
      <link linkend="auth-radius">RADIUS authentication</link>, which
      relies on a RADIUS authentication server.
     </para>
    </listitem>
    <listitem>
     <para>
      <link linkend="auth-cert">Certificate authentication</link>, which
      requires an SSL connection and authenticates users by checking the
      SSL certificate they send.
     </para>
    </listitem>
    <listitem>
     <para>
      <link linkend="auth-pam">PAM authentication</link>, which
      relies on a PAM (Pluggable Authentication Modules) library.
     </para>
    </listitem>
    <listitem>
     <para>
      <link linkend="auth-bsd">BSD authentication</link>, which
      relies on the BSD Authentication framework (currently available
      only on OpenBSD).
     </para>
    </listitem>
   </itemizedlist>
  </para>

  <para>
   Peer authentication is usually recommendable for local connections,
   though trust authentication might be sufficient in some circumstances.
   Password authentication is the easiest choice for remote connections.
   All the other options require some kind of external security
   infrastructure (usually an authentication server or a certificate
   authority for issuing SSL certificates), or are platform-specific.
  </para>

  <para>
   The following sections describe each of these authentication methods
   in more detail.
  </para>
 </sect1>

  <sect1 id="auth-trust">
   <title>Trust Authentication</title>

   <para>
    When <literal>trust</literal> 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</literal> and
    <literal>user</literal> 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</literal> authentication is appropriate and very
    convenient for local connections on a single-user workstation.  It
    is usually <emphasis>not</emphasis> appropriate by itself on a multiuser
    machine.  However, you might be able to use <literal>trust</literal> 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 ...</literal> line from
    <filename>pg_hba.conf</filename>, or change it to a
    non-<literal>trust</literal> authentication method.
   </para>

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

  </sect1>

  <sect1 id="auth-password">
   <title>Password Authentication</title>

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

   <para>
    There are several password-based authentication methods.  These methods
    operate similarly but differ in how the users' passwords are stored on the
    server and how the password provided by a client is sent across the
    connection.
   </para>

   <variablelist>
    <varlistentry>
     <term><literal>scram-sha-256</literal></term>
     <listitem>
      <para>
       The method <literal>scram-sha-256</literal> 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 and supports storing passwords on the server in a
       cryptographically hashed form that is thought to be secure.
      </para>

      <para>
       This is the most secure of the currently provided methods, but it is
       not supported by older client libraries.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>md5</literal></term>
     <listitem>
      <para>
       The method <literal>md5</literal> uses a custom less secure challenge-response
       mechanism.  It prevents password sniffing and avoids storing passwords
       on the server in plain text but provides no protection if an attacker
       manages to steal the password hash from the server.  Also, the MD5 hash
       algorithm is nowadays no longer considered secure against determined
       attacks.
      </para>

      <para>
       The <literal>md5</literal> method cannot be used with
       the <xref linkend="guc-db-user-namespace"/> feature.
      </para>

      <para>
       To ease transition from the <literal>md5</literal> method to the newer
       SCRAM method, if <literal>md5</literal> is specified as a method
       in <filename>pg_hba.conf</filename> but the user's password on the
       server is encrypted for SCRAM (see below), then SCRAM-based
       authentication will automatically be chosen instead.
      </para>
     </listitem>
    </varlistentry>

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

   <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</literal> system
    catalog. Passwords can be managed with the SQL commands
    <xref linkend="sql-createrole"/> and
    <xref linkend="sql-alterrole"/>,
    e.g., <userinput>CREATE ROLE foo WITH LOGIN PASSWORD 'secret'</userinput>,
    or the <application>psql</application>
    command <literal>\password</literal>.
    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>

   <para>
    The availability of the different password-based authentication methods
    depends on how a user's password on the server is encrypted (or hashed,
    more accurately).  This is controlled by the configuration
    parameter <xref linkend="guc-password-encryption"/> at the time the
    password is set.  If a password was encrypted using
    the <literal>scram-sha-256</literal> setting, then it can be used for the
    authentication methods <literal>scram-sha-256</literal>
    and <literal>password</literal> (but password transmission will be in
    plain text in the latter case).  The authentication method
    specification <literal>md5</literal> will automatically switch to using
    the <literal>scram-sha-256</literal> method in this case, as explained
    above, so it will also work.  If a password was encrypted using
    the <literal>md5</literal> setting, then it can be used only for
    the <literal>md5</literal> and <literal>password</literal> authentication
    method specifications (again, with the password transmitted in plain text
    in the latter case).  (Previous PostgreSQL releases supported storing the
    password on the server in plain text.  This is no longer possible.)  To
    check the currently stored password hashes, see the system
    catalog <literal>pg_authid</literal>.
   </para>

   <para>
    To upgrade an existing installation from <literal>md5</literal>
    to <literal>scram-sha-256</literal>, after having ensured that all client
    libraries in use are new enough to support SCRAM,
    set <literal>password_encryption = 'scram-sha-256'</literal>
    in <filename>postgresql.conf</filename>, make all users set new passwords,
    and change the authentication method specifications
    in <filename>pg_hba.conf</filename> to <literal>scram-sha-256</literal>.
   </para>
  </sect1>

  <sect1 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</productname> is built;
    see <xref linkend="installation"/> for more information.
   </para>

   <para>
    When <productname>GSSAPI</productname> uses
    <productname>Kerberos</productname>, it uses a standard principal
    in the format
    <literal><replaceable>servicename</replaceable>/<replaceable>hostname</replaceable>@<replaceable>realm</replaceable></literal>.
    The PostgreSQL server will accept any principal that is included in the keytab used by
    the server, but care needs to be taken to specify the correct principal details when
    making the connection from the client using the <literal>krbsrvname</literal> connection parameter. (See
    also <xref linkend="libpq-paramkeywords"/>.) The installation default can be
    changed from the default <literal>postgres</literal> at build time using
    <literal>./configure --with-krb-srvnam=</literal><replaceable>whatever</replaceable>.
    In most environments,
    this parameter never needs to be changed.
    Some Kerberos implementations might require a different service name,
    such as Microsoft Active Directory which requires the service name
    to be in upper case (<literal>POSTGRES</literal>).
   </para>
   <para>
    <replaceable>hostname</replaceable> is the fully qualified host name of the
    server machine. The service principal's realm is the preferred realm
    of the server machine.
   </para>

   <para>
    Client principals can be mapped to different <productname>PostgreSQL</productname>
    database user names with <filename>pg_ident.conf</filename>.  For example,
    <literal>pgusername@realm</literal> could be mapped to just <literal>pgusername</literal>.
    Alternatively, you can use the full <literal>username@realm</literal> principal as
    the role name in <productname>PostgreSQL</productname> without any mapping.
   </para>

   <para>
    <productname>PostgreSQL</productname> also supports a parameter to strip the realm from
    the principal.  This method is supported for backwards compatibility and is
    strongly discouraged as it is then impossible to distinguish different users
    with the same user name but coming from different realms.  To enable this,
    set <literal>include_realm</literal> to 0.  For simple single-realm
    installations, doing that combined with setting the
    <literal>krb_realm</literal> parameter (which checks that the principal's realm
    matches exactly what is in the <literal>krb_realm</literal> parameter)
    is still secure; but this is a
    less capable approach compared to specifying an explicit mapping in
    <filename>pg_ident.conf</filename>.
   </para>

   <para>
    Make sure that your server keytab file is readable (and preferably
    only readable, not writable) by the <productname>PostgreSQL</productname>
    server account.  (See also <xref linkend="postgres-user"/>.) The location
    of the key file is specified by the <xref
    linkend="guc-krb-server-keyfile"/> configuration
    parameter. The default is
    <filename>/usr/local/pgsql/etc/krb5.keytab</filename> (or whatever
    directory was specified as <varname>sysconfdir</varname> at build time).
    For security reasons, it is recommended to use a separate keytab
    just for the <productname>PostgreSQL</productname> server rather
    than opening up permissions on the system keytab file.
   </para>
   <para>
    The keytab file is generated by the Kerberos software; see the
    Kerberos documentation for details. The following example is
   for MIT-compatible Kerberos 5 implementations:
<screen>
<prompt>kadmin% </prompt><userinput>ank -randkey postgres/server.my.domain.org</userinput>
<prompt>kadmin% </prompt><userinput>ktadd -k krb5.keytab postgres/server.my.domain.org</userinput>
</screen>
   </para>

   <para>
    When connecting to the database make sure you have a ticket for a
    principal matching the requested database user name. For example, for
    database user name <literal>fred</literal>, principal
    <literal>fred@EXAMPLE.COM</literal> would be able to connect. To also allow
    principal <literal>fred/users.example.com@EXAMPLE.COM</literal>, use a user name
    map, as described in <xref linkend="auth-username-maps"/>.
   </para>

   <para>
    The following configuration options are supported for <productname>GSSAPI</productname>:
    <variablelist>
     <varlistentry>
      <term><literal>include_realm</literal></term>
      <listitem>
       <para>
        If set to 0, the realm name from the authenticated user principal is
        stripped off before being passed through the user name mapping
        (<xref linkend="auth-username-maps"/>). This is discouraged and is
        primarily available for backwards compatibility, as it is not secure
        in multi-realm environments unless <literal>krb_realm</literal> is
        also used.  It is recommended to
        leave <literal>include_realm</literal> set to the default (1) and to
        provide an explicit mapping in <filename>pg_ident.conf</filename> to convert
        principal names to <productname>PostgreSQL</productname> user names.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>map</literal></term>
      <listitem>
       <para>
        Allows for mapping between system and database user names. See
        <xref linkend="auth-username-maps"/> for details.  For a GSSAPI/Kerberos
        principal, such as <literal>username@EXAMPLE.COM</literal> (or, less
        commonly, <literal>username/hostbased@EXAMPLE.COM</literal>), the
        user name used for mapping is
        <literal>username@EXAMPLE.COM</literal> (or
        <literal>username/hostbased@EXAMPLE.COM</literal>, respectively),
        unless <literal>include_realm</literal> has been set to 0, in which case
        <literal>username</literal> (or <literal>username/hostbased</literal>)
        is what is seen as the system user name when mapping.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>krb_realm</literal></term>
      <listitem>
       <para>
        Sets the realm to match user principal names against. If this parameter
        is set, only users of that realm will be accepted.  If it is not set,
        users of any realm can connect, subject to whatever user name mapping
        is done.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </sect1>

  <sect1 id="sspi-auth">
   <title>SSPI Authentication</title>

   <indexterm zone="sspi-auth">
    <primary>SSPI</primary>
   </indexterm>

   <para>
    <productname>SSPI</productname> is a <productname>Windows</productname>
    technology for secure authentication with single sign-on.
    <productname>PostgreSQL</productname> will use SSPI in
    <literal>negotiate</literal> mode, which will use
    <productname>Kerberos</productname> when possible and automatically
    fall back to <productname>NTLM</productname> in other cases.
    <productname>SSPI</productname> authentication only works when both
    server and client are running <productname>Windows</productname>,
    or, on non-Windows platforms, when <productname>GSSAPI</productname>
    is available.
   </para>

   <para>
    When using <productname>Kerberos</productname> authentication,
    <productname>SSPI</productname> works the same way
    <productname>GSSAPI</productname> does; see <xref linkend="gssapi-auth"/>
    for details.
   </para>

   <para>
    The following configuration options are supported for <productname>SSPI</productname>:
    <variablelist>

     <varlistentry>
      <term><literal>include_realm</literal></term>
      <listitem>
       <para>
        If set to 0, the realm name from the authenticated user principal is
        stripped off before being passed through the user name mapping
        (<xref linkend="auth-username-maps"/>). This is discouraged and is
        primarily available for backwards compatibility, as it is not secure
        in multi-realm environments unless <literal>krb_realm</literal> is
        also used.  It is recommended to
        leave <literal>include_realm</literal> set to the default (1) and to
        provide an explicit mapping in <filename>pg_ident.conf</filename> to convert
        principal names to <productname>PostgreSQL</productname> user names.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>compat_realm</literal></term>
      <listitem>
       <para>
        If set to 1, the domain's SAM-compatible name (also known as the
        NetBIOS name) is used for the <literal>include_realm</literal>
        option. This is the default. If set to 0, the true realm name from
        the Kerberos user principal name is used.
       </para>
       <para>
        Do not disable this option unless your server runs under a domain
        account (this includes virtual service accounts on a domain member
        system) and all clients authenticating through SSPI are also using
        domain accounts, or authentication will fail.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>upn_username</literal></term>
      <listitem>
       <para>
        If this option is enabled along with <literal>compat_realm</literal>,
        the user name from the Kerberos UPN is used for authentication. If
        it is disabled (the default), the SAM-compatible user name is used.
        By default, these two names are identical for new user accounts.
       </para>
       <para>
        Note that <application>libpq</application> uses the SAM-compatible name if no
        explicit user name is specified. If you use
        <application>libpq</application> or a driver based on it, you should
        leave this option disabled or explicitly specify user name in the
        connection string.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>map</literal></term>
      <listitem>
       <para>
        Allows for mapping between system and database user names. See
        <xref linkend="auth-username-maps"/> for details.  For a SSPI/Kerberos
        principal, such as <literal>username@EXAMPLE.COM</literal> (or, less
        commonly, <literal>username/hostbased@EXAMPLE.COM</literal>), the
        user name used for mapping is
        <literal>username@EXAMPLE.COM</literal> (or
        <literal>username/hostbased@EXAMPLE.COM</literal>, respectively),
        unless <literal>include_realm</literal> has been set to 0, in which case
        <literal>username</literal> (or <literal>username/hostbased</literal>)
        is what is seen as the system user name when mapping.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>krb_realm</literal></term>
      <listitem>
       <para>
        Sets the realm to match user principal names against. If this parameter
        is set, only users of that realm will be accepted.  If it is not set,
        users of any realm can connect, subject to whatever user name mapping
        is done.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </sect1>

  <sect1 id="auth-ident">
   <title>Ident Authentication</title>

   <indexterm>
    <primary>ident</primary>
   </indexterm>

   <para>
    The ident authentication method works by obtaining the client's
    operating system user name from an ident server and using it as
    the allowed database user name (with an optional user name mapping).
    This is only supported on TCP/IP connections.
   </para>

   <note>
    <para>
     When ident is specified for a local (non-TCP/IP) connection,
     peer authentication (see <xref linkend="auth-peer"/>) will be
     used instead.
    </para>
   </note>

   <para>
    The following configuration options are supported for <productname>ident</productname>:
    <variablelist>
     <varlistentry>
      <term><literal>map</literal></term>
      <listitem>
       <para>
        Allows for mapping between system and database user names. See
        <xref linkend="auth-username-maps"/> for details.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>

   <para>
    The <quote>Identification Protocol</quote> 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 <quote>What user initiated the
    connection that goes out of your port <replaceable>X</replaceable>
    and connects to my port <replaceable>Y</replaceable>?</quote>.
    Since <productname>PostgreSQL</productname> knows both <replaceable>X</replaceable> and
    <replaceable>Y</replaceable> when a physical connection is established, it
    can interrogate the ident server on the host of the connecting
    client and can theoretically determine the operating system user
    for any given connection.
   </para>

   <para>
    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 they choose. 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:
    <blockquote>
     <attribution>RFC 1413</attribution>
     <para>
      The Identification Protocol is not intended as an authorization
      or access control protocol.
     </para>
    </blockquote>
   </para>

   <para>
    Some ident servers have a nonstandard option that causes the returned
    user name to be encrypted, using a key that only the originating
    machine's administrator knows.  This option <emphasis>must not</emphasis> be
    used when using the ident server with <productname>PostgreSQL</productname>,
    since <productname>PostgreSQL</productname> does not have any way to decrypt the
    returned string to determine the actual user name.
   </para>
  </sect1>

  <sect1 id="auth-peer">
   <title>Peer Authentication</title>

   <indexterm>
    <primary>peer</primary>
   </indexterm>

   <para>
    The peer authentication method works by obtaining the client's
    operating system user name from the kernel and using it as the
    allowed database user name (with optional user name mapping). This
    method is only supported on local connections.
   </para>

   <para>
    The following configuration options are supported for <productname>peer</productname>:
    <variablelist>
     <varlistentry>
      <term><literal>map</literal></term>
      <listitem>
       <para>
        Allows for mapping between system and database user names. See
        <xref linkend="auth-username-maps"/> for details.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>

   <para>
    Peer authentication is only available on operating systems providing
    the <function>getpeereid()</function> function, the <symbol>SO_PEERCRED</symbol>
    socket parameter, or similar mechanisms.  Currently that includes
    <systemitem class="osname">Linux</systemitem>,
    most flavors of <systemitem class="osname">BSD</systemitem> including
    <systemitem class="osname">macOS</systemitem>,
    and <systemitem class="osname">Solaris</systemitem>.
   </para>

  </sect1>

  <sect1 id="auth-ldap">
   <title>LDAP Authentication</title>

   <indexterm zone="auth-ldap">
    <primary>LDAP</primary>
   </indexterm>

   <para>
    This authentication method operates similarly to
    <literal>password</literal> except that it uses LDAP
    as the password verification method. LDAP is used only to validate
    the user name/password pairs. Therefore the user must already
    exist in the database before LDAP can be used for
    authentication.
   </para>

   <para>
    LDAP authentication can operate in two modes. In the first mode,
    which we will call the simple bind mode,
    the server will bind to the distinguished name constructed as
    <replaceable>prefix</replaceable> <replaceable>username</replaceable> <replaceable>suffix</replaceable>.
    Typically, the <replaceable>prefix</replaceable> parameter is used to specify
    <literal>cn=</literal>, or <replaceable>DOMAIN</replaceable><literal>\</literal> in an Active
    Directory environment.  <replaceable>suffix</replaceable> is used to specify the
    remaining part of the DN in a non-Active Directory environment.
   </para>

   <para>
    In the second mode, which we will call the search+bind mode,
    the server first binds to the LDAP directory with
    a fixed user name and password, specified with <replaceable>ldapbinddn</replaceable>
    and <replaceable>ldapbindpasswd</replaceable>, and performs a search for the user trying
    to log in to the database. If no user and password is configured, an
    anonymous bind will be attempted to the directory. The search will be
    performed over the subtree at <replaceable>ldapbasedn</replaceable>, and will try to
    do an exact match of the attribute specified in
    <replaceable>ldapsearchattribute</replaceable>.
    Once the user has been found in
    this search, the server disconnects and re-binds to the directory as
    this user, using the password specified by the client, to verify that the
    login is correct. This mode is the same as that used by LDAP authentication
    schemes in other software, such as Apache <literal>mod_authnz_ldap</literal> and <literal>pam_ldap</literal>.
    This method allows for significantly more flexibility
    in where the user objects are located in the directory, but will cause
    two separate connections to the LDAP server to be made.
   </para>

   <para>
    The following configuration options are used in both modes:
    <variablelist>
     <varlistentry>
      <term><literal>ldapserver</literal></term>
      <listitem>
       <para>
        Names or IP addresses of LDAP servers to connect to. Multiple
        servers may be specified, separated by spaces.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><literal>ldapport</literal></term>
      <listitem>
       <para>
        Port number on LDAP server to connect to. If no port is specified,
        the LDAP library's default port setting will be used.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><literal>ldapscheme</literal></term>
      <listitem>
       <para>
        Set to <literal>ldaps</literal> to use LDAPS.  This is a non-standard
        way of using LDAP over SSL, supported by some LDAP server
        implementations.  See also the <literal>ldaptls</literal> option for
        an alternative.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><literal>ldaptls</literal></term>
      <listitem>
       <para>
        Set to 1 to make the connection between PostgreSQL and the LDAP server
        use TLS encryption.  This uses the <literal>StartTLS</literal>
        operation per RFC 4513.  See also the <literal>ldapscheme</literal>
        option for an alternative.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>

   <para>
    Note that using <literal>ldapscheme</literal> or
    <literal>ldaptls</literal> only encrypts the traffic between the
    PostgreSQL server and the LDAP server.  The connection between the
    PostgreSQL server and the PostgreSQL client will still be unencrypted
    unless SSL is used there as well.
   </para>

   <para>
    The following options are used in simple bind mode only:
    <variablelist>
     <varlistentry>
      <term><literal>ldapprefix</literal></term>
      <listitem>
       <para>
        String to prepend to the user name when forming the DN to bind as,
        when doing simple bind authentication.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><literal>ldapsuffix</literal></term>
      <listitem>
       <para>
        String to append to the user name when forming the DN to bind as,
        when doing simple bind authentication.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>

   <para>
    The following options are used in search+bind mode only:
    <variablelist>
     <varlistentry>
      <term><literal>ldapbasedn</literal></term>
      <listitem>
       <para>
        Root DN to begin the search for the user in, when doing search+bind
        authentication.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><literal>ldapbinddn</literal></term>
      <listitem>
       <para>
        DN of user to bind to the directory with to perform the search when
        doing search+bind authentication.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><literal>ldapbindpasswd</literal></term>
      <listitem>
       <para>
        Password for user to bind to the directory with to perform the search
        when doing search+bind authentication.
       </para>
      </listitem>
      </varlistentry>
      <varlistentry>
       <term><literal>ldapsearchattribute</literal></term>
       <listitem>
        <para>
         Attribute to match against the user name in the search when doing
         search+bind authentication.  If no attribute is specified, the
         <literal>uid</literal> attribute will be used.
        </para>
       </listitem>
      </varlistentry>
      <varlistentry>
       <term><literal>ldapsearchfilter</literal></term>
       <listitem>
        <para>
         The search filter to use when doing search+bind authentication.
         Occurrences of <literal>$username</literal> will be replaced with the
         user name.  This allows for more flexible search filters than
         <literal>ldapsearchattribute</literal>.
        </para>
       </listitem>
      </varlistentry>
      <varlistentry>
       <term><literal>ldapurl</literal></term>
       <listitem>
        <para>
         An RFC 4516 LDAP URL.  This is an alternative way to write some of the
         other LDAP options in a more compact and standard form.  The format is
<synopsis>
ldap[s]://<replaceable>host</replaceable>[:<replaceable>port</replaceable>]/<replaceable>basedn</replaceable>[?[<replaceable>attribute</replaceable>][?[<replaceable>scope</replaceable>][?[<replaceable>filter</replaceable>]]]]
</synopsis>
         <replaceable>scope</replaceable> must be one
         of <literal>base</literal>, <literal>one</literal>, <literal>sub</literal>,
         typically the last.  (The default is <literal>base</literal>, which
         is normally not useful in this application.)  <replaceable>attribute</replaceable> can
         nominate a single attribute, in which case it is used as a value for
         <literal>ldapsearchattribute</literal>.  If
         <replaceable>attribute</replaceable> is empty then
         <replaceable>filter</replaceable> can be used as a value for
         <literal>ldapsearchfilter</literal>.
        </para>

        <para>
         The URL scheme <literal>ldaps</literal> chooses the LDAPS method for
         making LDAP connections over SSL, equivalent to using
         <literal>ldapscheme=ldaps</literal>.  To use encrypted LDAP
         connections using the <literal>StartTLS</literal> operation, use the
         normal URL scheme <literal>ldap</literal> and specify the
         <literal>ldaptls</literal> option in addition to
         <literal>ldapurl</literal>.
        </para>

        <para>
         For non-anonymous binds, <literal>ldapbinddn</literal>
         and <literal>ldapbindpasswd</literal> must be specified as separate
         options.
        </para>

        <para>
         LDAP URLs are currently only supported with OpenLDAP, not on Windows.
        </para>
       </listitem>
      </varlistentry>
    </variablelist>
   </para>

   <para>
    It is an error to mix configuration options for simple bind with options
    for search+bind.
   </para>

   <para>
    When using search+bind mode, the search can be performed using a single
    attribute specified with <literal>ldapsearchattribute</literal>, or using
    a custom search filter specified with
    <literal>ldapsearchfilter</literal>.
    Specifying <literal>ldapsearchattribute=foo</literal> is equivalent to
    specifying <literal>ldapsearchfilter="(foo=$username)"</literal>.  If neither
    option is specified the default is
    <literal>ldapsearchattribute=uid</literal>.
   </para>

   <para>
    Here is an example for a simple-bind LDAP configuration:
<programlisting>
host ... ldap ldapserver=ldap.example.net ldapprefix="cn=" ldapsuffix=", dc=example, dc=net"
</programlisting>
    When a connection to the database server as database
    user <literal>someuser</literal> is requested, PostgreSQL will attempt to
    bind to the LDAP server using the DN <literal>cn=someuser, dc=example,
    dc=net</literal> and the password provided by the client.  If that connection
    succeeds, the database access is granted.
   </para>

   <para>
    Here is an example for a search+bind configuration:
<programlisting>
host ... ldap ldapserver=ldap.example.net ldapbasedn="dc=example, dc=net" ldapsearchattribute=uid
</programlisting>
    When a connection to the database server as database
    user <literal>someuser</literal> is requested, PostgreSQL will attempt to
    bind anonymously (since <literal>ldapbinddn</literal> was not specified) to
    the LDAP server, perform a search for <literal>(uid=someuser)</literal>
    under the specified base DN.  If an entry is found, it will then attempt to
    bind using that found information and the password supplied by the client.
    If that second connection succeeds, the database access is granted.
   </para>

   <para>
    Here is the same search+bind configuration written as a URL:
<programlisting>
host ... ldap ldapurl="ldap://ldap.example.net/dc=example,dc=net?uid?sub"
</programlisting>
    Some other software that supports authentication against LDAP uses the
    same URL format, so it will be easier to share the configuration.
   </para>

   <para>
    Here is an example for a search+bind configuration that uses
    <literal>ldapsearchfilter</literal> instead of
    <literal>ldapsearchattribute</literal> to allow authentication by
    user ID or email address:
<programlisting>
host ... ldap ldapserver=ldap.example.net ldapbasedn="dc=example, dc=net" ldapsearchfilter="(|(uid=$username)(mail=$username))"
</programlisting>
   </para>

   <tip>
    <para>
     Since LDAP often uses commas and spaces to separate the different
     parts of a DN, it is often necessary to use double-quoted parameter
     values when configuring LDAP options, as shown in the examples.
    </para>
   </tip>

  </sect1>

  <sect1 id="auth-radius">
   <title>RADIUS Authentication</title>

   <indexterm zone="auth-radius">
    <primary>RADIUS</primary>
   </indexterm>

   <para>
    This authentication method operates similarly to
    <literal>password</literal> except that it uses RADIUS
    as the password verification method. RADIUS is used only to validate
    the user name/password pairs. Therefore the user must already
    exist in the database before RADIUS can be used for
    authentication.
   </para>

   <para>
    When using RADIUS authentication, an Access Request message will be sent
    to the configured RADIUS server. This request will be of type
    <literal>Authenticate Only</literal>, and include parameters for
    <literal>user name</literal>, <literal>password</literal> (encrypted) and
    <literal>NAS Identifier</literal>. The request will be encrypted using
    a secret shared with the server. The RADIUS server will respond to
    this request with either <literal>Access Accept</literal> or
    <literal>Access Reject</literal>. There is no support for RADIUS accounting.
   </para>

   <para>
    Multiple RADIUS servers can be specified, in which case they will
    be tried sequentially. If a negative response is received from
    a server, the authentication will fail. If no response is received,
    the next server in the list will be tried. To specify multiple
    servers, separate the server names with commas and surround the list
    with double quotes. If multiple servers are specified, the other
    RADIUS options can also be given as comma-separated lists, to provide
    individual values for each server. They can also be specified as
    a single value, in which case that value will apply to all servers.
   </para>

   <para>
    The following configuration options are supported for RADIUS:
     <variablelist>
      <varlistentry>
       <term><literal>radiusservers</literal></term>
       <listitem>
        <para>
         The DNS names or IP addresses of the RADIUS servers to connect to.
         This parameter is required.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><literal>radiussecrets</literal></term>
       <listitem>
        <para>
         The shared secrets used when talking securely to the RADIUS
         servers. This must have exactly the same value on the PostgreSQL
         and RADIUS servers. It is recommended that this be a string of
         at least 16 characters. This parameter is required.
         <note>
         <para>
          The encryption vector used will only be cryptographically
          strong if <productname>PostgreSQL</productname> is built with support for
          <productname>OpenSSL</productname>. In other cases, the transmission to the
          RADIUS server should only be considered obfuscated, not secured, and
          external security measures should be applied if necessary.
         </para>
         </note>
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><literal>radiusports</literal></term>
       <listitem>
        <para>
         The port numbers to connect to on the RADIUS servers. If no port
         is specified, the default RADIUS port (<literal>1812</literal>)
         will be used.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><literal>radiusidentifiers</literal></term>
       <listitem>
        <para>
         The strings to be used as <literal>NAS Identifier</literal> in the
         RADIUS requests. This parameter can be used, for example, to
         identify which database cluster the user is attempting to connect
         to, which can be useful for policy matching on
         the RADIUS server. If no identifier is specified, the default
         <literal>postgresql</literal> will be used.
        </para>
       </listitem>
      </varlistentry>

     </variablelist>
   </para>

   <para>
    If it is necessary to have a comma or whitespace in a RADIUS parameter
    value, that can be done by putting double quotes around the value, but
    it is tedious because two layers of double-quoting are now required.
    An example of putting whitespace into RADIUS secret strings is:
<programlisting>
host ... radius radiusservers="server1,server2" radiussecrets="""secret one"",""secret two"""
</programlisting>
   </para>
  </sect1>

  <sect1 id="auth-cert">
   <title>Certificate Authentication</title>

   <indexterm zone="auth-cert">
    <primary>Certificate</primary>
   </indexterm>

   <para>
    This authentication method uses SSL client certificates to perform
    authentication. It is therefore only available for SSL connections.
    When using this authentication method, the server will require that
    the client provide a valid, trusted certificate.  No password prompt
    will be sent to the client.  The <literal>cn</literal> (Common Name)
    attribute of the certificate
    will be compared to the requested database user name, and if they match
    the login will be allowed.  User name mapping can be used to allow
    <literal>cn</literal> to be different from the database user name.
   </para>

   <para>
    The following configuration options are supported for SSL certificate
    authentication:
    <variablelist>
     <varlistentry>
      <term><literal>map</literal></term>
      <listitem>
       <para>
        Allows for mapping between system and database user names. See
        <xref linkend="auth-username-maps"/> for details.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>

   <para>
    In a <filename>pg_hba.conf</filename> record specifying certificate
    authentication, the authentication option <literal>clientcert</literal> is
    assumed to be <literal>1</literal>, and it cannot be turned off since a client
    certificate is necessary for this method.  What the <literal>cert</literal>
    method adds to the basic <literal>clientcert</literal> certificate validity test
    is a check that the <literal>cn</literal> attribute matches the database
    user name.
   </para>
  </sect1>

  <sect1 id="auth-pam">
   <title>PAM Authentication</title>

   <indexterm zone="auth-pam">
    <primary>PAM</primary>
   </indexterm>

   <para>
    This authentication method operates similarly to
    <literal>password</literal> except that it uses PAM (Pluggable
    Authentication Modules) as the authentication mechanism. The
    default PAM service name is <literal>postgresql</literal>.
    PAM is used only to validate user name/password pairs and optionally the
    connected remote host name or IP address. Therefore the user must already
    exist in the database before PAM can be used for authentication.  For more
    information about PAM, please read the
    <ulink url="https://www.kernel.org/pub/linux/libs/pam/">
    <productname>Linux-PAM</productname> Page</ulink>.
   </para>

   <para>
    The following configuration options are supported for PAM:
    <variablelist>
     <varlistentry>
      <term><literal>pamservice</literal></term>
      <listitem>
       <para>
        PAM service name.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><literal>pam_use_hostname</literal></term>
      <listitem>
       <para>
        Determines whether the remote IP address or the host name is provided
        to PAM modules through the <symbol>PAM_RHOST</symbol> item.  By
        default, the IP address is used.  Set this option to 1 to use the
        resolved host name instead.  Host name resolution can lead to login
        delays.  (Most PAM configurations don't use this information, so it is
        only necessary to consider this setting if a PAM configuration was
        specifically created to make use of it.)
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>

   <note>
    <para>
     If PAM is set up to read <filename>/etc/shadow</filename>, authentication
     will fail because the PostgreSQL server is started by a non-root
     user.  However, this is not an issue when PAM is configured to use
     LDAP or other authentication methods.
    </para>
   </note>
  </sect1>

  <sect1 id="auth-bsd">
   <title>BSD Authentication</title>

   <indexterm zone="auth-bsd">
    <primary>BSD Authentication</primary>
   </indexterm>

   <para>
    This authentication method operates similarly to
    <literal>password</literal> except that it uses BSD Authentication
    to verify the password. BSD Authentication is used only
    to validate user name/password pairs. Therefore the user's role must
    already exist in the database before BSD Authentication can be used
    for authentication. The BSD Authentication framework is currently
    only available on OpenBSD.
   </para>

   <para>
    BSD Authentication in <productname>PostgreSQL</productname> uses
    the <literal>auth-postgresql</literal> login type and authenticates with
    the <literal>postgresql</literal> login class if that's defined
    in <filename>login.conf</filename>. By default that login class does not
    exist, and <productname>PostgreSQL</productname> will use the default login class.
   </para>

   <note>
    <para>
     To use BSD Authentication, the PostgreSQL user account (that is, the
     operating system user running the server) must first be added to
     the <literal>auth</literal> group.  The <literal>auth</literal> group
     exists by default on OpenBSD systems.
    </para>
   </note>
  </sect1>

  <sect1 id="client-authentication-problems">
   <title>Authentication Problems</title>

   <para>
    Authentication failures and related problems generally
    manifest themselves through error messages like the following:
   </para>

   <para>
<programlisting>
FATAL:  no pg_hba.conf entry for host "123.123.123.123", user "andym", database "testdb"
</programlisting>
    This is what you are most likely to get if you succeed in contacting
    the server, but it does not want to talk to you. As the message
    suggests, the server refused the connection request because it found
    no matching entry in its <filename>pg_hba.conf</filename>
    configuration file.
   </para>

   <para>
<programlisting>
FATAL:  password authentication failed for user "andym"
</programlisting>
    Messages like this indicate that you contacted the server, and it is
    willing to talk to you, but not until you pass the authorization
    method specified in the <filename>pg_hba.conf</filename> file. Check
    the password you are providing, or check your Kerberos or ident
    software if the complaint mentions one of those authentication
    types.
   </para>

   <para>
<programlisting>
FATAL:  user "andym" does not exist
</programlisting>
    The indicated database user name was not found.
   </para>

   <para>
<programlisting>
FATAL:  database "testdb" does not exist
</programlisting>
    The database you are trying to connect to does not exist. Note that
    if you do not specify a database name, it defaults to the database
    user name, which might or might not be the right thing.
   </para>

   <tip>
   <para>
    The server log might contain more information about an
    authentication failure than is reported to the client. If you are
    confused about the reason for a failure, check the server log.
   </para>
   </tip>
  </sect1>

 </chapter>
