path: root/doc/src/sgml/client-auth.sgml

<!--
$Header: /cvsroot/pgsql/doc/src/sgml/client-auth.sgml,v 1.32 2002/01/20 22:19:55 petere Exp $
-->

<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
  about that. It is therefore obviously essential to restrict which
  database user name(s) a given client can connect as.
 </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 and database; some authentication methods
  allow you to restrict by user name as well.
 </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 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 and lines beginning
   with a hash character (<quote>#</quote>) are ignored. A record is
   made up of a number of fields which are separated by spaces and/or
   tabs.  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 or names,
   and the authentication method to be used for connections matching
   these parameters.
   The first record that matches the type, client address, and requested
   database name of a connection attempt is used to do the
   authentication step.  There is no <quote>fall-through</> or
   <quote>backup</>: if one record is chosen and the authentication
   fails, the following records are not considered. If no record
   matches, the access will be denied.
  </para>

  <para>
   A record may have one of the three formats
   <synopsis>
local   <replaceable>database</replaceable> <replaceable>authentication-method</replaceable> [ <replaceable>authentication-option</replaceable> ]
host    <replaceable>database</replaceable> <replaceable>IP-address</replaceable> <replaceable>IP-mask</replaceable> <replaceable>authentication-method</replaceable> [ <replaceable>authentication-option</replaceable> ]
hostssl <replaceable>database</replaceable> <replaceable>IP-address</replaceable> <replaceable>IP-mask</replaceable> <replaceable>authentication-method</replaceable> [ <replaceable>authentication-option</replaceable> ]
    </synopsis>
   The meaning of the fields is as follows:

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

    <varlistentry>
     <term><literal>host</literal></term>
     <listitem>
      <para>
       This record pertains to connection attempts over TCP/IP
       networks. Note that TCP/IP connections are completely disabled
       unless the server is started with the <option>-i</option> switch or
       the equivalent configuration parameter is set.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>hostssl</literal></term>
     <listitem>
      <para>
       This record pertains to connection attempts with 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 match only SSL connections.)
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable>database</replaceable></term>
     <listitem>
      <para>
       Specifies the database that this record applies to. 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.  Otherwise,
       this is the name of a specific <productname>PostgreSQL</productname>
       database.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable>IP address</replaceable></term>
     <term><replaceable>IP mask</replaceable></term>
     <listitem>
      <para>
       These two fields specify to which client machines a
       <literal>host</literal> or <literal>hostssl</literal>
       record applies, based on their IP
       address. (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 method that users must use to authenticate themselves
       when connecting under the control of this authentication 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 mostly
          useful to <quote>filter out</> certain hosts from a group.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>password</></term>
        <listitem>
         <para>
          The client is required to supply a password which is required to
	  match the database password that was set up for the user.
         </para>

         <para>
          An optional file name may be specified after the
          <literal>password</literal> keyword. This file is expected to
          contain a list of users who may connect using this record,
          and optionally alternative passwords for them.
         </para>

         <para>
          The password is sent over the wire in clear text. For better
          protection, use the <literal>md5</literal> or 
          <literal>crypt</literal> methods.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>md5</></term>
        <listitem>
         <para>
          Like the <literal>password</literal> method, but the password
          is sent over the wire encrypted using a simple
          challenge-response protocol. This protects against incidental
          wire-sniffing.  This is now the recommended choice for
	  password-based authentication.
         </para>

         <para>
	  The name of a file may follow the
          <literal>md5</literal> keyword.  It contains a list of users
          who may connect using this record.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>crypt</></term>
        <listitem>
         <para>
          Like the <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. The <literal>crypt</>
          method is not compatible with encrypting passwords in
          <filename>pg_shadow</>, and may fail if client and server
          machines have different implementations of the crypt() library
          routine.
         </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>
	  The identity of the user as determined on login to the
	  operating system is used by <productname>PostgreSQL</productname>
	  to determine whether the user
          is allowed to connect as the requested database user.
	  For TCP/IP connections the user's identity is determined by
	  contacting the <firstterm>ident</firstterm> server on the client
	  host.  (Note that this is only as reliable as the remote ident
	  server; ident authentication should never be used for remote hosts
	  whose administrators are not trustworthy.)
	  On operating systems
	  supporting <symbol>SO_PEERCRED</> requests for Unix domain sockets,
	  ident authentication is possible for local connections;
	  the system is then asked for the connecting user's identity.
	 </para>
         <para>
	  On systems without <symbol>SO_PEERCRED</> requests, ident authentication
	  is only available for TCP/IP connections.  As a workaround,
	  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>
          The <replaceable>authentication option</replaceable> following
          the <literal>ident</> keyword specifies the name of an
          <firstterm>ident map</firstterm> that 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>, with the main difference that
          it will use PAM (Pluggable Authentication Modules) as the
          authentication mechanism. The <replaceable>authentication
          option</replaceable> following the <literal>pam</> keyword
          specifies the service name that will be passed to PAM. The
          default service name is <literal>postgresql</literal>.
          For more information about PAM, please read the <ulink
          url="http://www.kernel.org/pub/linux/libs/pam/"><productname>Linux-PAM</productname>
          Page</ulink> and/or the <ulink 
          url="http://www.sun.com/software/solaris/pam/"><systemitem class="osname">Solaris</> PAM
          Page</ulink>.
         </para>
        </listitem>
       </varlistentry>

      </variablelist>

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

    <varlistentry>
     <term><replaceable>authentication option</replaceable></term>
     <listitem>
      <para>
       This field is interpreted differently depending on the
       authentication method, as described above.
      </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
   very 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 startup
   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    IP_ADDRESS    MASK               AUTHTYPE  MAP

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

host         all         127.0.0.1     255.255.255.255    trust     

# The same, over Unix-socket connections:

local        all                                          trust

# Allow any user from any host with IP address 192.168.93.x to
# connect to database "template1" as the same username that ident on that
# host identifies him as (typically his Unix username):

host         template1   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 in pg_shadow is correctly supplied:

host         template1   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        192.168.54.1   255.255.255.255    reject
host         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        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                                     md5
local        all                                          md5  admins
</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>password</primary>
   </indexterm>
   <indexterm>
    <primary>MD5</>
   </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 obsolete 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 set of users in a separate file (one user name
    per line) in the same directory that <filename>pg_hba.conf</> is in,
    and mention the (base) name of the file after the
    <literal>password</>, <literal>md5</>, or <literal>crypt</> keyword,
    respectively, in <filename>pg_hba.conf</>. If you do not use this
    feature, then any user that is known to the database system can
    connect to any database (so long as he supplies the correct password,
    of course).
   </para>

   <para>
    These files can also be used to apply a different set of passwords
    to a particular database or set thereof. In that case, the files
    have a format similar to the standard Unix password file
    <filename>/etc/passwd</filename>, that is,
<synopsis>
<replaceable>username</replaceable>:<replaceable>password</replaceable>
</synopsis>
    Any extra colon-separated fields following the password are
    ignored. The password is expected to be encrypted using the
    system's <function>crypt()</function> function. The utility
    program <application>pg_passwd</application> that is installed
    with <productname>PostgreSQL</productname> can be used to manage
    these password files.
   </para>

   <para>
    Lines with and without passwords can be mixed in secondary
    password files. Lines without password indicate use of the main
    password in <literal>pg_shadow</> that is managed by
    <command>CREATE USER</> and <command>ALTER USER</>. Lines with
    passwords will cause that password to be used. A password entry of
    <quote>+</quote> also means using the pg_shadow password.
   </para>

   <para>
    Alternative passwords cannot be used when using the <literal>md5</>
    or <literal>crypt</> methods. The file will be read as
    usual, but the password field will simply be ignored and the
    <literal>pg_shadow</> password will always be used.
   </para>

   <para>
    Note that using alternative passwords like this means that one can
    no longer use <command>ALTER USER</command> to change one's
    password. It will appear to work but the password one is
    changing is not the password that the system will end up
    using.
   </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>
    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 a system.
   </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 startup
   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 username 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 joeblow, 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 'joeblow'
</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 "joeblow" 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>