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<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> user name it
  wants to connect as, much the same way one logs into a Unix computer
  as a particular user. Within the SQL environment the active database
  user name determines access privileges to database objects -- see
  <xref linkend="user-manag"> for more information. Therefore, it is
  essential to restrict which database users can connect.
 </para>

 <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
  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> user names are logically
  separate from user names of the operating system in which the server
  runs. If all the users of a particular server also have accounts on
  the server's machine, it makes sense to assign database user names
  that match their operating system user names. However, a server that
  accepts remote connections may have many database users who have no local operating system
  account, and in such cases there need be no connection between
  database user names and OS user names.
 </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 the file
   <filename>pg_hba.conf</filename> in the data directory, e.g.,
   <filename>/usr/local/pgsql/data/pg_hba.conf</filename>.
   (<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>.
  </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. A record is made
   up of a number of fields which are separated by spaces and/or tabs.
   Fields can contain white space if the field value is quoted. Records
   cannot be continued across lines.
  </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 may have one of the seven formats
<synopsis>
local   <replaceable>database</replaceable>  <replaceable>user</replaceable>  <replaceable>authentication-method</replaceable>  <optional><replaceable>authentication-option</replaceable></optional>
host    <replaceable>database</replaceable>  <replaceable>user</replaceable>  <replaceable>IP-address</replaceable>  <replaceable>IP-mask</replaceable>  <replaceable>authentication-method</replaceable>  <optional><replaceable>authentication-option</replaceable></optional>
hostssl  <replaceable>database</replaceable>  <replaceable>user</replaceable>  <replaceable>IP-address</replaceable>  <replaceable>IP-mask</replaceable>  <replaceable>authentication-method</replaceable>  <optional><replaceable>authentication-option</replaceable></optional>
hostnossl  <replaceable>database</replaceable>  <replaceable>user</replaceable>  <replaceable>IP-address</replaceable>  <replaceable>IP-mask</replaceable>  <replaceable>authentication-method</replaceable>  <optional><replaceable>authentication-option</replaceable></optional>
host    <replaceable>database</replaceable>  <replaceable>user</replaceable>  <replaceable>IP-address</replaceable>/<replaceable>CIDR-mask</replaceable>  <replaceable>authentication-method</replaceable>  <optional><replaceable>authentication-option</replaceable></optional>
hostssl  <replaceable>database</replaceable>  <replaceable>user</replaceable>  <replaceable>IP-address</replaceable>/<replaceable>CIDR-mask</replaceable>  <replaceable>authentication-method</replaceable>  <optional><replaceable>authentication-option</replaceable></optional>
hostnossl  <replaceable>database</replaceable>  <replaceable>user</replaceable>  <replaceable>IP-address</replaceable>/<replaceable>CIDR-mask</replaceable>  <replaceable>authentication-method</replaceable>  <optional><replaceable>authentication-option</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 using TCP/IP networks.
       Note that TCP/IP connections are disabled unless the server is
       started with the <option>-i</option> option or the
       <varname>tcpip_socket</> configuration parameter is enabled.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>hostssl</literal></term>
     <listitem>
      <para>
       This record matches connection attempts using SSL over TCP/IP.
       <literal>host</literal> records will match either SSL or
       non-SSL connection attempts, but <literal>hostssl</literal>
       records require SSL connections.
      </para>

      <para>
       To make use of this option the server must be built
       with SSL support enabled. Furthermore, SSL must be enabled by
       enabling the <varname>ssl</varname> configuration parameter
       (see <xref linkend="runtime-config"> for more information).
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>hostnossl</literal></term>
     <listitem>
      <para>
       This record is similar to <literal>hostssl</> but with the
	   opposite logic: it matches only regular connection attempts not
	   using SSL.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable>database</replaceable></term>
     <listitem>
      <para>
       Specifies which databases 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>samegroup</> specifies that
       the requested user must a member of the group with the same
       name as the requested database.  Otherwise, this is the name of
       a specific <productname>PostgreSQL</productname> database.
       Multiple database names can be supplied by separating them with
       commas.  A file containing database names can be specified by
       preceding the file name with <literal>@</>. The file must be in
       the same directory as <filename>pg_hba.conf</>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable>user</replaceable></term>
     <listitem>
      <para>
       Specifies which <productname>PostgreSQL</> users this record
       matches. The value <literal>all</literal> specifies that it
       matches all users.  Otherwise, this is the name of a specific
       <productname>PostgreSQL</productname> user. Multiple user names
       can be supplied by separating them with commas. Group names can
       be specified by preceding the group name with <literal>+</>. A
       file containing user names can be specified by preceding the
       file name with <literal>@</>. The file must be in the same
       directory as <filename>pg_hba.conf</>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable>IP-address</replaceable></term>
     <term><replaceable>IP-mask</replaceable></term>
     <listitem>
      <para>
       These two fields contain IP address and mask values in standard
       dotted decimal notation. (IP addresses can only be specified
       numerically, not as domain or host names.)  Taken together they
       specify the client machine IP addresses that this record
       matches.  The precise logic is that
<programlisting>
(<replaceable>actual-IP-address</replaceable> xor <replaceable>IP-address-field</replaceable>) and <replaceable>IP-mask-field</replaceable>
</programlisting>
       must be zero for the record to match.
      </para>

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

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

    <varlistentry>
     <term><replaceable>CIDR-mask</replaceable></term>
     <listitem>
      <para>
        This field may be used as an alternative to the 
        <replaceable>IP-mask</replaceable> notation.  It is an
	integer specifying the number of high-order bits 
        to set in the mask.  The number must
        be between 0 and 32 (in the case of an IPv4 address) or 128
        (in the case of an IPv6 address) inclusive. 0 will match any
        address, while 32/128 will match only the exact host specified.
        The same matching logic is used as for a dotted notation
        <replaceable>IP-Mask</replaceable>.
      </para>

      <para>
        There must be no white space between the <replaceable>IP-address</replaceable>
        and the <literal>/</literal> or the <literal>/</literal> and the
        <replaceable>CIDR-mask</replaceable>, or the file will not be parsed 
        correctly.
      </para>

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

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

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

       <varlistentry>
        <term><literal>reject</></term>
        <listitem>
         <para>
          The connection is rejected unconditionally. This is useful for
          <quote>filtering out</> certain hosts from a group.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>md5</></term>
        <listitem>
         <para>
          Requires the client to supply an MD5 encrypted password for
          authentication. This is the only method that allows encrypted
          passwords to be stored in <structname>pg_shadow</structname>.
          See <xref linkend="auth-password"> for details.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>crypt</></term>
        <listitem>
         <para>
          Like the <literal>md5</literal> method but uses older <function>crypt()</>
          encryption, which is needed for pre-7.2 clients.
          <literal>md5</literal> is preferred for 7.2 and later clients.
          See <xref linkend="auth-password"> for details.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>password</></term>
        <listitem>
         <para>
          Same as <literal>md5</>, but 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>krb4</></term>
        <listitem>
         <para>
          Kerberos V4 is used to authenticate the user. This is only
          available for TCP/IP connections.  See <xref
          linkend="kerberos-auth"> for details.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>krb5</></term>
        <listitem>
         <para>
          Kerberos V5 is used to authenticate the user. This is only
          available for TCP/IP connections.  See <xref
          linkend="kerberos-auth"> for details.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>ident</></term>
        <listitem>
         <para>
          Obtain the operating system user name of the client (for
          TCP/IP connections by contacting the ident server on the
          client, for local connections by getting it from the
          operating system) and check if the user is allowed to
          connect as the requested database user by consulting the map
          specified after the <literal>ident</literal> key word.
         </para>

         <para>
          If you use the map <literal>sameuser</literal>, the user
          names are required to be identical. If not, the map name is
          looked up in the file <filename>pg_ident.conf</filename>
          in the same directory as <filename>pg_hba.conf</filename>.
          The connection is accepted if that file contains an
          entry for this map name with the operating-system user name
          and the requested <productname>PostgreSQL</productname> user
          name.
         </para>

         <para>
          For local connections, this only works on machines that
          support Unix-domain socket credentials (currently
          <systemitem class=osname>Linux</>, <systemitem
          class=osname>FreeBSD</>, <systemitem class=osname>NetBSD</>,
          <systemitem class=osname>OpenBSD</>, and 
          <systemitem class=osname>BSD/OS</>).
         </para>

         <para>
          See <xref linkend="auth-ident"> below 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>
      </variablelist>

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

    <varlistentry>
     <term><replaceable>authentication-option</replaceable></term>
     <listitem>
      <para>
       The meaning of this optional field depends on the chosen
       authentication method and is described in the next section.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </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>

  <important>
   <para>
    Do not prevent the superuser from accessing the <literal>template1</literal>
    database.  Various utility commands need access to <literal>template1</literal>.
   </para>
  </important>

  <para>
   The <filename>pg_hba.conf</filename> file is read on start-up and when
   the main server process (<command>postmaster</>) 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 <command>postmaster</>
   (using <literal>pg_ctl reload</> or <literal>kill -HUP</>) to make it
   re-read the file.
  </para>

  <para>
   An example of a <filename>pg_hba.conf</filename> file is 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>An example <filename>pg_hba.conf</filename> file</title>
<programlisting>
# Allow any user on the local system to connect to any database under
# any user name using Unix-domain sockets (the default for local
# connections).
#
# TYPE  DATABASE    USER        IP-ADDRESS        IP-MASK           METHOD
local   all         all                                             trust

# The same using local loopback TCP/IP connections.
#
# TYPE  DATABASE    USER        IP-ADDRESS        IP-MASK           METHOD
host    all         all         127.0.0.1         255.255.255.255   trust     

# The same as the last line but using a CIDR mask
#
# TYPE  DATABASE    USER        IP-ADDRESS/CIDR-mask  METHOD
host    all         all         127.0.0.1/32          trust     

# Allow any user from any host with IP address 192.168.93.x to connect
# to database "template1" as the same user name that ident reports for
# the connection (typically the Unix user name).
# 
# TYPE  DATABASE    USER        IP-ADDRESS        IP-MASK           METHOD
host    template1   all         192.168.93.0      255.255.255.0     ident sameuser

# The same as the last line but using a CIDR mask
#
# TYPE  DATABASE    USER        IP-ADDRESS/CIDR-mask  METHOD
host    template1   all         192.168.93.0/24       ident sameuser

# Allow a user from host 192.168.12.10 to connect to database
# "template1" if the user's password is correctly supplied.
# 
# TYPE  DATABASE    USER        IP-ADDRESS        IP-MASK           METHOD
host    template1   all         192.168.12.10     255.255.255.255   md5

# In the absence of preceding "host" lines, these two lines will
# reject all connection from 192.168.54.1 (since that entry will be
# matched first), but allow Kerberos V connections from anywhere else
# on the Internet.  The zero mask means that no bits of the host IP
# address are considered so it matches any host.
# 
# TYPE  DATABASE    USER        IP-ADDRESS        IP-MASK           METHOD
host    all         all         192.168.54.1      255.255.255.255   reject
host    all         all         0.0.0.0           0.0.0.0           krb5

# Allow users from 192.168.x.x hosts to connect to any database, if
# they pass the ident check.  If, for example, ident says the user is
# "bryanh" and he requests to connect as PostgreSQL user "guest1", the
# connection is allowed if there is an entry in pg_ident.conf for map
# "omicron" that says "bryanh" is allowed to connect as "guest1".
#
# TYPE  DATABASE    USER        IP-ADDRESS        IP-MASK           METHOD
host    all         all         192.168.0.0       255.255.0.0       ident omicron

# If these are the only three lines for local connections, they will
# allow local users to connect only to their own databases (databases
# with the same name as their user name) except for administrators and
# members of group "support" who may connect to all databases.  The file
# $PGDATA/admins contains a list of user names.  Passwords are required in
# all cases.
#
# TYPE  DATABASE    USER        IP-ADDRESS        IP-MASK           METHOD
local   sameuser    all                                             md5
local   all         @admins                                         md5
local   all         +support                                        md5

# The last two lines above can be combined into a single line:
local   all         @admins,+support                                md5

# The database column can also use lists and file names, but not groups:
local   db1,db2,@demodbs  all                                       md5
</programlisting>
   </example>
 </sect1>

 <sect1 id="auth-methods">
  <title>Authentication methods</title>
  <para>
   The following describes 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 as
    whatever database user he specifies (including the database superuser).
    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 may 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_directory</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 it; 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>crypt</>
   </indexterm>
   <indexterm>
    <primary>password</primary>
    <secondary>authentication</secondary>
   </indexterm>

   <para>
    The password-based authentication methods are <literal>md5</>,
    <literal>crypt</>, and <literal>password</>. These methods operate
    similarly except for the way that the password is sent across the
    connection. If you are at all concerned about password
    <quote>sniffing</> attacks then <literal>md5</> is preferred, with
    <literal>crypt</> a second choice if you must support pre-7.2
    clients. Plain <literal>password</> should especially be avoided for
    connections over the open Internet (unless you use SSL, SSH, or
    other communications security wrappers around the connection).
   </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_shadow</> system
    catalog table. Passwords can be managed with the SQL
    commands <command>CREATE USER</command> and <command>ALTER
    USER</command>, e.g., <userinput>CREATE USER foo WITH PASSWORD
    'secret';</userinput>. By default, that is, if no password has
    been set up, the stored password is null and
    password authentication will always fail for that user.
   </para>

   <para>
    To restrict the set of users that are allowed to connect to
    certain databases, list the users in the <replaceable>user</>
    column of <filename>pg_hba.conf</filename>, as explained in the
    previous section.
   </para>

  </sect2>

  <sect2 id="kerberos-auth">
   <title>Kerberos authentication</title>

   <indexterm zone="kerberos-auth">
    <primary>Kerberos</primary>
   </indexterm>

   <para>
    <productname>Kerberos</productname> is an industry-standard secure
    authentication system suitable for distributed computing over a public
    network. A description of the <productname>Kerberos</productname> system
    is far beyond the scope of this document; in all generality it can be
    quite complex (yet powerful). The <ulink
    url="http://www.nrl.navy.mil/CCS/people/kenh/kerberos-faq.html">Kerberos
    <acronym>FAQ</></ulink> or <ulink url="ftp://athena-dist.mit.edu">MIT
    Project Athena</ulink> can be a good starting point for exploration.
    Several sources for <productname>Kerberos</> distributions exist.
   </para>

   <para>
    While <productname>PostgreSQL</> supports both Kerberos 4 and 
    Kerberos 5, only Kerberos 5 is recommended.  Kerberos 4 is
    considered insecure and no longer recommended for general
    use.
   </para>

   <para>
    In order to use <productname>Kerberos</>, support for it must be
    enabled at build time.  See <xref linkend="installation"> for more
    information.  Both Kerberos 4 and 5 are supported, but only one
    version can be supported in any one build.
   </para>

   <para>
    <productname>PostgreSQL</> operates like a normal Kerberos service.
    The name of the service principal is
    <literal><replaceable>servicename</>/<replaceable>hostname</>@<replaceable>realm</></literal>, where
    <replaceable>servicename</> is <literal>postgres</literal> (unless a
    different service name was selected at configure time with
    <literal>./configure --with-krb-srvnam=whatever</>).
    <replaceable>hostname</> 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 must have their <productname>PostgreSQL</> user
    name as their first component, for example
    <literal>pgusername/otherstuff@realm</>. At present the realm of
    the client is not checked by <productname>PostgreSQL</>; so if you
    have cross-realm authentication enabled, then any principal in any
    realm that can communicate with yours will be accepted.
   </para>

   <para>
    Make sure that your server key file is readable (and preferably only
    readable) by the <productname>PostgreSQL</productname> server
    account.  (See also <xref linkend="postgres-user">). The location of the
    key file is specified with the <varname>krb_server_keyfile</> run-time
    configuration parameter. (See also <xref
    linkend="runtime-config">.) The default is <filename>/etc/srvtab</>
    if you are using Kerberos 4 and
    <filename>FILE:/usr/local/pgsql/etc/krb5.keytab</> (or whichever
    directory was specified as <varname>sysconfdir</> at build time)
    with Kerberos 5.
   </para>

   <para>
    To generate the keytab file, use for example (with version 5)
<screen>
<prompt>kadmin% </><userinput>ank -randkey postgres/server.my.domain.org</>
<prompt>kadmin% </><userinput>ktadd -k krb5.keytab postgres/server.my.domain.org</>
</screen>
    Read the <productname>Kerberos</> documentation for details.
   </para>

   <para>
    When connecting to the database make sure you have a ticket for a
    principal matching the requested database user name. An example: For
    database user name <literal>fred</>, both principal
    <literal>fred@EXAMPLE.COM</> and
    <literal>fred/users.example.com@EXAMPLE.COM</> can be used to
    authenticate to the database server.
   </para>

   <para>
    If you use <application>mod_auth_kerb</application> from
    <ulink url="http://modauthkerb.sf.net">http://modauthkerb.sf.net</ulink>
    and <application>mod_perl</application> on your
    <productname>Apache</productname> web server, you can use
    <literal>AuthType KerberosV5SaveCredentials</literal> with a
    <application>mod_perl</application> script. This gives secure
    database access over the web, no extra passwords required.
   </para>

  </sect2>

  <sect2 id="auth-ident">
   <title>Ident-based authentication</title>

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

   <para>
    The ident authentication method works by inspecting the client's
    operating system user name and determining the allowed database
    user names by using a map file that lists the permitted
    corresponding user name pairs.  The determination of the client's
    user name is the security-critical point, and it works differently
    depending on the connection type.
   </para>

   <sect3>
    <title>Ident Authentication over TCP/IP</title>

   <para>
    The <quote>Identification Protocol</quote> is described in
    <citetitle>RFC 1413</citetitle>. 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</> knows both <replaceable>X</> and
    <replaceable>Y</> when a physical connection is established, it
    can interrogate the ident server on the host of the connecting
    client and could theoretically determine the operating system user
    for any given connection this way.
   </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 he chooses. This authentication method is
    therefore only appropriate for closed networks where each client
    machine is under tight control and where the database and system
    administrators operate in close contact. 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>
   </sect3>

   <sect3>
    <title>Ident Authentication over Local Sockets</title>

   <para>
    On systems supporting <symbol>SO_PEERCRED</symbol> requests for
    Unix-domain sockets (currently <systemitem
    class="osname">Linux</>, <systemitem class="osname">FreeBSD</>,
    <systemitem class="osname">NetBSD</>, <systemitem class=osname>OpenBSD</>, 
    and <systemitem class="osname">BSD/OS</>), ident authentication can also 
    be applied to local connections. In this case, no security risk is added by
    using ident authentication; indeed it is a preferable choice for
    local connections on such systems.
   </para>

    <para>
     On systems without <symbol>SO_PEERCRED</> requests, ident
     authentication is only available for TCP/IP connections. As a
     work around, it is possible to specify the <systemitem
     class="systemname">localhost</> address <systemitem
     class="systemname">127.0.0.1</> and make connections to this
     address.
    </para>
    </sect3>

   <sect3>
    <title>Ident Maps</title>

   <para>
    When using ident-based authentication, after having determined the
    name of the operating system user that initiated the connection,
    <productname>PostgreSQL</productname> checks whether that user is
    allowed to connect as the database user he is requesting to connect
    as. This is controlled by the ident map argument that follows the
    <literal>ident</> key word in the <filename>pg_hba.conf</filename>
    file. There is a predefined ident map <literal>sameuser</literal>,
    which allows any operating system user to connect as the database
    user of the same name (if the latter exists). Other maps must be
    created manually.
   </para>

   <para>
    Ident maps
    other than <literal>sameuser</literal> are defined in the file
    <filename>pg_ident.conf</filename><indexterm><primary>pg_ident.conf</primary></indexterm>
    in the data directory, which contains lines of the general form:
<synopsis>
<replaceable>map-name</> <replaceable>ident-username</> <replaceable>database-username</>
</synopsis>
    Comments and whitespace are handled in the usual way. 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 which operating system user is allowed to connect
    as which database user. The same <replaceable>map-name</> can be
    used repeatedly to specify more user-mappings within a single map.
    There is no restriction regarding how many database users a given
    operating system user may correspond to and vice versa.
   </para>

  <para>
   The <filename>pg_ident.conf</filename> file is read on start-up and
   when the main server process (<command>postmaster</>) 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 <command>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 setup, anyone
    logged in to a machine on the 192.168 network that does not have the
    Unix 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</> himself or as <literal>guest1</>.
   </para>

   <example id="example-pg-ident.conf">
    <title>An example <filename>pg_ident.conf</> file</title>
<programlisting>
# MAPNAME     IDENT-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>
   </sect3>
  </sect2>

  <sect2 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>. You can
    optionally supply you own service name after the <literal>pam</>
    key word in the file <filename>pg_hba.conf</filename>. For more information about PAM, please read
    the <ulink
    url="http://www.kernel.org/pub/linux/libs/pam/"><productname>Linux-PAM</>
    Page</ulink> and the <ulink
    url="http://www.sun.com/software/solaris/pam/"><systemitem
    class="osname">Solaris</> PAM Page</ulink>.
   </para>
  </sect2>
 </sect1>

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

   <para>
    Genuine 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 authorizing 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 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 may or may not be the right thing.
   </para>

   <tip>
   <para>
    The server log may 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 log.
   </para>
   </tip>
  </sect1>

 </chapter>
