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<chapter id="security">
<Title>Security</Title>
<Para>
<Sect1>
<Title>User Authentication</Title>
<Para>
<firstterm>Authentication</firstterm>
is the process by which the backend server and
<application>postmaster</application>
ensure that the user requesting access to data is in fact who he/she
claims to be.
All users who invoke <Productname>Postgres</Productname> are checked against the
contents of the <literal>pg_user</literal> class to ensure that they are
authorized to do so. However, verification of the user's actual
identity is performed in a variety of ways:
<variablelist>
<varlistentry>
<term>
From the user shell
</term>
<listitem>
<para>
A backend server started from a user shell notes the user's (effective)
user-id before performing a
<function>setuid</function>
to the user-id of user <replaceable>postgres</replaceable>.
The effective user-id is used
as the basis for access control checks. No other authentication is
conducted.
<varlistentry>
<term>
From the network
</term>
<listitem>
<para>
If the <Productname>Postgres</Productname> system is built as distributed,
access to the Internet TCP port of the
<application>postmaster</application>
process is available to anyone. The DBA configures the pg_hba.conf file
in the PGDATA directory to specify what authentication system is to be used
according to the host making the connection and which database it is
connecting to. See <citetitle>pg_hba.conf(5)</citetitle>
for a description of the authentication
systems available. Of course, host-based authentication is not fool-proof in
Unix, either. It is possible for determined intruders to also
masquerade the origination host. Those security issues are beyond the
scope of <Productname>Postgres</Productname>.
</variablelist>
<Sect1>
<Title>Access Control</Title>
<Para>
<Productname>Postgres</Productname> provides mechanisms to allow users
to limit the access to their data that is provided to other users.
<variablelist>
<varlistentry>
<term>
Database superusers
</term>
<listitem>
<para>
Database super-users (i.e., users who have <literal>pg_user.usesuper</literal>
set) silently bypass all of the access controls described below with
two exceptions: manual system catalog updates are not permitted if the
user does not have <literal>pg_user.usecatupd</literal> set, and destruction of
system catalogs (or modification of their schemas) is never allowed.
<varlistentry>
<term>
Access Privilege
</term>
<listitem>
<para>
The use of access privilege to limit reading, writing and setting
of rules on classes is covered in
<citetitle>grant/revoke(l)</citetitle>.
<varlistentry>
<term>
Class removal and schema modification
</term>
<listitem>
<para>
Commands that destroy or modify the structure of an existing class,
such as <command>alter</command>,
<command>drop table</command>,
and
<command>drop index</command>,
only operate for the owner of the class. As mentioned above, these
operations are <emphasis>never</emphasis>
permitted on system catalogs.
</variablelist>
<Sect1>
<Title>Functions and Rules</Title>
<Para>
Functions and rules allow users to insert code into the backend server
that other users may execute without knowing it. Hence, both
mechanisms permit users to <firstterm>trojan horse</firstterm>
others with relative impunity. The only real protection is tight
control over who can define functions (e.g., write to relations with
SQL fields) and rules. Audit trails and alerters on
<literal>pg_class</literal>, <literal>pg_user</literal>
and <literal>pg_group</literal> are also recommended.
<Sect2>
<Title>Functions</Title>
<Para>
Functions written in any language except SQL
run inside the backend server
process with the permissions of the user <replaceable>postgres</replaceable> (the
backend server runs with its real and effective user-id set to
<replaceable>postgres</replaceable>. It is possible for users to change the server's
internal data structures from inside of trusted functions. Hence,
among many other things, such functions can circumvent any system
access controls. This is an inherent problem with user-defined C functions.
<Sect2>
<Title>Rules</Title>
<Para>
Like SQL functions, rules always run with the identity and
permissions of the user who invoked the backend server.
<sect2>
<title>
Caveats
</title>
<para>
There are no plans to explicitly support encrypted data inside of
<Productname>Postgres</Productname>
(though there is nothing to prevent users from encrypting
data within user-defined functions). There are no plans to explicitly
support encrypted network connections, either, pending a total rewrite
of the frontend/backend protocol.
<para>
User names, group names and associated system identifiers (e.g., the
contents of <literal>pg_user.usesysid</literal>) are assumed to be unique
throughout a database. Unpredictable results may occur if they are
not.
</chapter>
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