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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 to be used can be selected
  on the basis of (client) host, 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 users who have no local
  account, and in such cases there need be no connection between
  database user names and OS user names.
 </para>

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

  <indexterm zone="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 area
   is initialized by <command>initdb</command>.
  </para>

  <para>
   The general format of the <filename>pg_hba.conf</filename> file is of
   a set of records, one per line. Blank lines are ignored, as is any
   text after the <quote>#</quote> 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 three formats
   <synopsis>
local   <replaceable>database</replaceable> <replaceable>user</replaceable> <replaceable>authentication-method</replaceable> [ <replaceable>authentication-option</replaceable> ]
host    <replaceable>database</replaceable> <replaceable>user</replaceable> <replaceable>IP-address</replaceable> <replaceable>IP-mask</replaceable> <replaceable>authentication-method</replaceable>
hostssl <replaceable>database</replaceable> <replaceable>user</replaceable> <replaceable>IP-address</replaceable> <replaceable>IP-mask</replaceable> <replaceable>authentication-method</replaceable>
    </synopsis>
   The meaning of the fields is as follows:

   <variablelist>
    <varlistentry>
     <term><literal>local</literal></term>
     <listitem>
      <para>
       This record applies to connection attempts using Unix domain
       sockets.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>host</literal></term>
     <listitem>
      <para>
       This record applied to 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
       <literal>tcpip_socket</> <filename>postgresql.conf</>
       configuration parameter is enabled.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>hostssl</literal></term>
     <listitem>
      <para>
       This record applies to connection attempts using SSL over
       TCP/IP. To make use of this option the server must be
       built with SSL support enabled. Furthermore, SSL must be
       enabled with the <option>-l</> option or equivalent configuration
       setting when the server is started.  (Note: <literal>host</literal>
       records will match either SSL or non-SSL connection attempts, but
       <literal>hostssl</literal> records requires SSL connections.)
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable>database</replaceable></term>
     <listitem>
      <para>
       Specifies the database for this record. The value
       <literal>all</literal> specifies that it applies to all
       databases, while the value <literal>sameuser</> identifies the
       database with the same name as the connecting user. The value
       <literal>samegroup</> identifies a group with the same name as
       the database name. Only members of this group can connect to the
       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 the user for this record. The value
       <literal>all</literal> specifies that it applies to 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 specify the client machine IP addresses
       (<literal>host</literal> or <literal>hostssl</literal>) for this
       record. (Of course IP addresses can be spoofed but this
       consideration is beyond the scope of
       <productname>PostgreSQL</productname>.) The precise logic is that
       <blockquote>
        <informalfigure>
         <programlisting>(<replaceable>actual-IP-address</replaceable> xor <replaceable>IP-address-field</replaceable>) and <replaceable>IP-mask-field</replaceable></programlisting>
        </informalfigure>
       </blockquote>
       must be zero for the record to match.
      </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
          any user that has login access to the client host to connect as
          any <productname>PostgreSQL</productname> user whatsoever.
         </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 pg_shadow.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>crypt</></term>
        <listitem>
         <para>
          Like <literal>md5</literal> method but uses older crypt
          encryption, which is needed for pre-7.2 clients.
          <literal>md5</literal> is preferred for 7.2 and later clients.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>password</></term>
        <listitem>
         <para>
          Same as "md5", but the password is sent in cleartext over the
          network. This should not be used on untrusted networks.
         </para>
         </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.
         </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.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>ident</></term>
        <listitem>
	 <para>
          For TCP/IP connections, authentication is done by contacting
          the <firstterm>ident</firstterm> server on the client host.
          This is only as secure as the client machine. You must specify
          the map name after the 'ident' keyword. It determines how to
          map remote user names to PostgreSQL user names. If you use
          "sameuser", the user names are assumed to be identical. If
          not, the map name is looked up in the $PGDATA/pg_ident.conf
          file. The connection is accepted if that file contains an
          entry for this map name with the ident-supplied user name and
          the requested PostgreSQL user name.
         </para>
         <para>
          On machines that support unix-domain socket credentials
          (currently Linux, FreeBSD, NetBSD, and BSD/OS), ident allows
          reliable authentication of 'local' connections without ident
          running on the local machine.
         </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>
         <para>
          Following the <literal>ident</> keyword, an <firstterm>ident
          map</firstterm> name should be supplied which specifies which
          operating system users equate with which database users. See
          below for details.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>pam</></term>
        <listitem>
         <para>
          This authentication type operates similarly to
          <firstterm>password</firstterm> 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</> keyword in the
          file. For more information about PAM, please read the <ulink
          url="http://www.kernel.org/pub/linux/libs/pam/"><productname>L
          inux-PAM</productname> Page</ulink> and the <ulink
          url="http://www.sun.com/software/solaris/pam/"><systemitem
          class="osname">Solaris</> PAM Page</ulink>.
         </para>
        </listitem>
       </varlistentry>

      </variablelist>

      </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 connections but require a password for
   remote TCP connections. In this case a record specifying
   <literal>trust</> authentication for connections from 127.0.0.1 would
   appear before a record specifying password authentication for a wider
   range of allowed client IP addresses.
  </para>

  <para>
    <indexterm>
     <primary>SIGHUP</primary>
    </indexterm>
   The <filename>pg_hba.conf</filename> file is read on start-up and when
   the <application>postmaster</> receives a
   <systemitem>SIGHUP</systemitem> signal. If you edit the file on an
   active system, you will need to signal the <application>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 below for details on the
   different authentication methods.

   <example id="example-pg-hba.conf">
    <title>An example <filename>pg_hba.conf</filename> file</title>
<programlisting>
# TYPE       DATABASE    USER       IP_ADDRESS    MASK               AUTHTYPE

# Allow any user on the local system to connect to any
# database under any user name, but only via an IP connection:

host         all         all        127.0.0.1     255.255.255.255    trust     

# The same, over Unix-socket connections:

local        all         all                                         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 on that
# host identifies him as (typically his Unix user name):

host         template1   all        192.168.93.0  255.255.255.0      ident sameuser

# Allow a user from host 192.168.12.10 to connect to database "template1"
# if the user's password is correctly supplied:

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 attempts from 192.168.54.1 (since that entry will be
# matched first), but allow Kerberos V5-validated 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:

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 <productname>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":

host         all        all         192.168.0.0    255.255.0.0        ident omicron

# If these are the only two lines for local connections, they will allow
# local users to connect only to their own databases (database named the
# same as the user name), except for administrators who may connect to
# all databases.  The file $PGDATA/admins lists the user names who are
# permitted to connect to all databases.  Passwords are required in all
# cases.  (If you prefer to use ident authorization, an ident map can
# serve a parallel purpose to the password list file used here.)

local        sameuser   all                                            md5
local        all        @admins                                        md5
</programlisting>
   </example>
  </para>
 </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 postmaster 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 system-level
    protection on connections to the postmaster port.
   </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 postmaster's socket file using
    file-system permissions.  To do this, set the parameter
    <varname>unix_socket_permissions</varname> (and possibly
    <varname>unix_socket_group</varname>) in <filename>postgresql.conf</>,
    as described in <xref linkend="runtime-config-general">.  Or you could
    set <varname>unix_socket_directory</varname> to place the socket file
    in a suitably restricted directory.
   </para>

   <para>
    Setting file-system permissions only helps for Unix-socket connections.
    Local TCP connections are not restricted by it; therefore, if you want
    to use 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 connections
    if you trust every user on every machine that is allowed to connect
    to the postmaster by the <filename>pg_hba.conf</> lines that specify
    <literal>trust</>.  It is seldom reasonable to use <literal>trust</>
    for any TCP connections other than those from 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>
   </indexterm>

   <para>
    Password-based authentication methods include <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. Ordinarily, the
    password for each database user is stored in the pg_shadow system
    catalog table. Passwords can be managed with the query language
    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 <literal>NULL</literal> 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 separated by commas, or in a separate
    file. The file should contain user names separated by commas or one
    user name per line, and be in the same directory as
    <filename>pg_hba.conf</>. Mention the (base) name of the file
    preceded with <literal>@</>in the <literal>USER</> column. The
    <literal>DATABASE</> column can similarly accept a list of values or
    a file name. You can also specify group names by preceding the group
    name with <literal>+</>.
   </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">Kerb
    eros <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>
    In order to use <productname>Kerberos</>, support for it must be
    enabled at build time. Both Kerberos 4 and 5 are supported
    (<literal>./configure --with-krb4</> or <literal>./configure
    --with-krb5</> respectively), although 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
    <replaceable>servicename/hostname@realm</>, 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 domain 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
    <replaceable>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 <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_krb</application> 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>
   <title>Ident-based authentication</title>

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

   <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>

   <para>
    On systems supporting <symbol>SO_PEERCRED</symbol> requests for
    Unix-domain sockets, 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>
    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</> keyword 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>
    <indexterm><primary>pg_ident.conf</primary></indexterm> Ident maps
    other than <literal>sameuser</literal> are defined in the file
    <filename>pg_ident.conf</filename> 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>
    <indexterm>
     <primary>SIGHUP</primary>
    </indexterm>
   The <filename>pg_ident.conf</filename> file is read on start-up and
   when the <application>postmaster</> receives a
   <systemitem>SIGHUP</systemitem> signal. If you edit the file on an
   active system, you will need to signal the <application>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 <systemitem>bryanh</>, <systemitem>ann</>, or
    <systemitem>robert</> would not be granted access. Unix user
    <systemitem>robert</> would only be allowed access when he tries to
    connect as <productname>PostgreSQL</> user <systemitem>bob</>, not
    as <systemitem>robert</> or anyone else. <systemitem>ann</> would
    only be allowed to connect as <systemitem>ann</>. User
    <systemitem>bryanh</> would be allowed to connect as either
    <systemitem>bryanh</> himself or as <systemitem>guest1</>.
   </para>

   <example id="example-pg-ident.conf">
    <title>An example <filename>pg_ident.conf</> file</title>
<programlisting>
#MAP           IDENT-NAME   POSTGRESQL-NAME

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>
  </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>
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>
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 1:  user "andym" does not exist
</ProgramListing>
    The indicated user name was not found.
   </para>

   <para>
<ProgramListing>
FATAL 1:  Database "testdb" does not exist in the system catalog.
</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>

   <para>
    Note that 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>
  </sect1>

 </chapter>
