<!-- $Header: /cvsroot/pgsql/doc/src/sgml/client-auth.sgml,v 1.9 2000/11/21 20:44:31 tgl Exp $ -->
<chapter id="client-authentication">
<title>Client Authentication</title>
<para>
When a client application connects to the database server, it specifies which
<productname>Postgres</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>Postgres</productname> offers client authentication by
(client) host and by database, with a number of different
authentication methods available.
</para>
<para>
<productname>Postgres</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 Unix user ids. 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 usernames and Unix
usernames.
</para>
<sect1 id="pg-hba.conf">
<title>The <filename>pg_hba.conf</filename> file</title>
<para>
Client authentication is controlled by the file
<filename>pg_hba.conf</filename> in the $PGDATA directory, e.g.,
<filename>/usr/local/pgsql/data/pg_hba.conf</filename>. (HBA stands
for host-based authentication.) A default <filename>pg_hba.conf</filename>
file is installed when the
data area is initialized by <application>initdb</application>.
</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 and cannot be continued across several lines.
</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> 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.
</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>Postgres</productname>
database.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable>IP address</replaceable></term>
<term><replaceable>IP mask</replaceable></term>
<listitem>
<para>
These two fields control to which hosts a
<literal>host</literal> record applies, based on their IP
address. (Of course IP addresses can be spoofed but this
consideration is beyond the scope of
<productname>Postgres</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 to that database. The possible choices follow,
details are in <xref linkend="auth-methods">.
<variablelist>
<varlistentry>
<term>trust</>
<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>Postgres</productname> user whatsoever.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>reject</>
<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>password</>
<listitem>
<para>
The client is required to supply a password with the connection
attempt which is required to match the 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 that this record pertains to, and
optionally alternative passwords.
</para>
<para>
The password is sent over the wire in clear text. For better
protection, use the <literal>crypt</literal> method.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>crypt</>
<listitem>
<para>
Like the <literal>password</literal> method, but the password
is sent over the wire encrypted using a simple
challenge-response protocol. This is still not
cryptographically secure but it protects against incidental
wire-sniffing. The name of a file may follow the
<literal>crypt</literal> keyword that contains a list of users
that this record pertains to.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>krb4</>
<listitem>
<para>
Kerberos V4 is used to authenticate the user. This is only
available for TCP/IP connections.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>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>ident</term>
<listitem>
<para>
The ident server on the client host is asked for the identity
of the connecting user. <productname>Postgres</productname>
then verifies whether the so identified operating system user
is allowed to connect as the database user that is requested.
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>
</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 there.
</para>
</listitem>
</varlistentry>
</variablelist>
The first record that matches a connection attempt's client IP address
and requested database name 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>
The <filename>pg_hba.conf</filename> file is re-read during each
connection attempt. It is therefore trivial to modify access
permissions while the server is running: just edit 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 crypt
# 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 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
</programlisting>
</example>
</para>
</sect1>
<sect1 id="auth-methods">
<title>Authentication methods</title>
<para>
The following describes the authentication methods in detail.
</para>
<sect2>
<title>Password authentication</title>
<para>
<productname>Postgres</> database passwords are separate from any
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</> 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 passes password
authentication, 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>Postgres</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>crypt</> method. The file will still be evaluated as
usual but the password field will simply be ignored and the
<literal>pg_shadow</> password will 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 still appear to work but the password one is
actually changing is not the password that the system will end up
using.
</para>
</sect2>
<sect2 id="kerberos-auth">
<title>Kerberos authentication</title>
<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).
</para>
<para>
<productname>Postgres</> should operate like a normal Kerberos
service. The name of the service principal is normally
<literal>postgres</literal>, unless it was changed during the
build. Make sure that your server key file is readable (and
preferably only readable) by the Postgres 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 defails.
</para>
<para>
In the <productname>Kerberos</> 5 hooks, the following assumptions
are made about user and service naming:
<itemizedlist>
<listitem>
<para>
User principal names (anames) are assumed to contain the actual
Unix/<productname>Postgres</> user name in the first component.
</para>
</listitem>
<listitem>
<para>
The <productname>Postgres</> service is assumed to be have two
components, the service name and a hostname, canonicalized as
in Version 4 (i.e., with all domain suffixes removed).
</para>
</listitem>
</itemizedlist>
<informaltable>
<tgroup cols="2">
<thead>
<row>
<entry>Parameter</>
<entry>Example</>
</row>
</thead>
<tbody>
<row>
<entry>user</>
<entry>frew@S2K.ORG</>
</row>
<row>
<entry>user</>
<entry>aoki/HOST=miyu.S2K.Berkeley.EDU@S2K.ORG</>
</row>
<row>
<entry>host</>
<entry>postgres_dbms/ucbvax@S2K.ORG</>
</row>
</tbody>
</tgroup>
</informaltable>
</para>
<para>
If you use mod_auth_krb and mod_perl on your Apache web server,
you can use AuthType KerberosV5SaveCredentials with a mod_perl
script. This gives secure database access over the web, no extra
passwords required.
</para>
</sect2>
<sect2>
<title>Ident-based authentication</title>
<para>
The <quote>Identification Protocol</quote> is described in
<citetitle>RFC 1413</citetitle>. Virtually every Unix-like
operating systems 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>Postgres</> 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. 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>
When using ident-based authentication, after having determined the
operating system user that initiated the connection,
<productname>Postgres</productname> determines as what database
system user he may connect. This is controlled by the ident map
argument that follows the <literal>ident</> keyword in the
<filename>pg_hba.conf</filename> file. The simplest ident map is
<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 are held 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. There is also no restriction regarding how many
database users a given operating system may correspond to and vice
versa.
</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 bryanh, ann, or robert would not be granted access.
Unix user robert would only be allowed access when he tries to
connect as Postgres user <quote>bob</quote>, not as <quote>robert</quote>
or anyone else. <quote>ann</quote> would only be allowed to connect as
<quote>ann</>. User bryanh would be allowed to connect as either
<quote>bryanh</> himself or as <quote>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 doesn't 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's 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're
providing, or check your Kerberos or IDENT software if the
complaint mentions one of those authentication types.
</para>
<para>
<ProgramListing>
FATAL 1: SetUserId: user 'joeblow' is not in 'pg_shadow'
</ProgramListing>
This is the fancy way of saying that the user doesn't exist at all.
</para>
<para>
<ProgramListing>
FATAL 1: Database testdb does not exist in pg_database
</ProgramListing>
The database you're trying to connect to doesn't exist. Note that
if you don't specify a database name, it defaults to the database
user name, which may or may not be the right thing.
</para>
</sect1>
</chapter>