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runtime.sgml 72.06 KiB
<!-- $PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.427 2009/04/24 20:46:16 momjian Exp $ -->
<chapter Id="runtime">
<title>Server Setup and Operation</title>
<para>
This chapter discusses how to set up and run the database server
and its interactions with the operating system.
</para>
<sect1 id="postgres-user">
<title>The <productname>PostgreSQL</productname> User Account</title>
<indexterm>
<primary>postgres user</primary>
</indexterm>
<para>
As with any other server daemon that is accessible to the outside world,
it is advisable to run <productname>PostgreSQL</productname> under a
separate user account. This user account should only own the data
that is managed by the server, and should not be shared with other
daemons. (For example, using the user <literal>nobody</literal> is a bad
idea.) It is not advisable to install executables owned by this
user because compromised systems could then modify their own
binaries.
</para>
<para>
To add a Unix user account to your system, look for a command
<command>useradd</command> or <command>adduser</command>. The user
name <systemitem>postgres</systemitem> is often used, and is assumed
throughout this book, but you can use another name if you like.
</para>
</sect1>
<sect1 id="creating-cluster">
<title>Creating a Database Cluster</title>
<indexterm>
<primary>database cluster</primary>
</indexterm>
<indexterm>
<primary>data area</primary>
<see>database cluster</see>
</indexterm>
<para>
Before you can do anything, you must initialize a database storage
area on disk. We call this a <firstterm>database cluster</firstterm>.
(<acronym>SQL</acronym> uses the term catalog cluster.) A
database cluster is a collection of databases that is managed by a
single instance of a running database server. After initialization, a
database cluster will contain a database named <literal>postgres</literal>,
which is meant as a default database for use by utilities, users and third
party applications. The database server itself does not require the
<literal>postgres</literal> database to exist, but many external utility
programs assume it exists. Another database created within each cluster
during initialization is called
<literal>template1</literal>. As the name suggests, this will be used
as a template for subsequently created databases; it should not be
used for actual work. (See <xref linkend="managing-databases"> for
information about creating new databases within a cluster.)
</para>
<para>
In file system terms, a database cluster will be a single directory
under which all data will be stored. We call this the <firstterm>data
directory</firstterm> or <firstterm>data area</firstterm>. It is completely up to you where you choose to store your data. There is no
default, although locations such as
<filename>/usr/local/pgsql/data</filename> or
<filename>/var/lib/pgsql/data</filename> are popular. To initialize a
database cluster, use the command <xref
linkend="app-initdb">,<indexterm><primary>initdb</></> which is
installed with <productname>PostgreSQL</productname>. The desired
file system location of your database cluster is indicated by the
<option>-D</option> option, for example
<screen>
<prompt>$</> <userinput>initdb -D /usr/local/pgsql/data</userinput>
</screen>
Note that you must execute this command while logged into the
<productname>PostgreSQL</productname> user account, which is
described in the previous section.
</para>
<tip>
<para>
As an alternative to the <option>-D</option> option, you can set
the environment variable <envar>PGDATA</envar>.
<indexterm><primary><envar>PGDATA</envar></primary></indexterm>
</para>
</tip>
<para>
<command>initdb</command> will attempt to create the directory you
specify if it does not already exist. It is likely that it will not
have the permission to do so (if you followed our advice and created
an unprivileged account). In that case you should create the
directory yourself (as root) and change the owner to be the
<productname>PostgreSQL</productname> user. Here is how this might
be done:
<screen>
root# <userinput>mkdir /usr/local/pgsql/data</userinput>
root# <userinput>chown postgres /usr/local/pgsql/data</userinput>
root# <userinput>su postgres</userinput>
postgres$ <userinput>initdb -D /usr/local/pgsql/data</userinput>
</screen>
</para>
<para>
<command>initdb</command> will refuse to run if the data directory
looks like it has already been initialized.</para>
<para>
Because the data directory contains all the data stored in the
database, it is essential that it be secured from unauthorized
access. <command>initdb</command> therefore revokes access
permissions from everyone but the
<productname>PostgreSQL</productname> user.
</para>
<para>
However, while the directory contents are secure, the default
client authentication setup allows any local user to connect to the
database and even become the database superuser. If you do not
trust other local users, we recommend you use one of
<command>initdb</command>'s <option>-W</option>, <option>--pwprompt</option>
or <option>--pwfile</option> options to assign a password to the
database superuser.<indexterm><primary>password</><secondary>of the
superuser</></indexterm> Also, specify <option>-A md5</> or
<option>-A password</> so that the default <literal>trust</> authentication
mode is not used; or modify the generated <filename>pg_hba.conf</filename>
file after running <command>initdb</command>,
<emphasis>before</> you start the server for the first time. (Other
reasonable approaches include using <literal>ident</literal> authentication
or file system permissions to restrict connections. See <xref
linkend="client-authentication"> for more information.)
</para>
<para>
<command>initdb</command> also initializes the default
locale<indexterm><primary>locale</></> for the database cluster.
Normally, it will just take the locale settings in the environment
and apply them to the initialized database. It is possible to
specify a different locale for the database; more information about
that can be found in <xref linkend="locale">. The default sort order used
within the particular database cluster is set by
<command>initdb</command>, and while you can create new databases using
different sort order, the order used in the template databases that initdb
creates cannot be changed without dropping and recreating them.
There is also a performance impact for using locales
other than <literal>C</> or <literal>POSIX</>. Therefore, it is
important to make this choice correctly the first time.
</para>
<para>
<command>initdb</command> also sets the default character set encoding
for the database cluster. Normally this should be chosen to match the
locale setting. For details see <xref linkend="multibyte">.
</para>
<sect2 id="creating-cluster-nfs">
<title>Network File Systems</title>
<indexterm zone="creating-cluster-nfs">
<primary>Network File Systems</primary>
</indexterm>
<indexterm><primary><acronym>NFS</></><see>Network File Systems</></>
<indexterm><primary>Network Attached Storage (<acronym>NAS</>)</><see>Network File Systems</></>
<para>
Many installations create database clusters on network file systems.
Sometimes this is done directly via <acronym>NFS</>, or by using a
Network Attached Storage (<acronym>NAS</>) device that uses
<acronym>NFS</> internally. <productname>PostgreSQL</> does nothing
special for <acronym>NFS</> file systems, meaning it assumes
<acronym>NFS</> behaves exactly like locally-connected drives
(<acronym>DAS</>, Direct Attached Storage). If client and server
<acronym>NFS</> implementations have non-standard semantics, this can
cause reliability problems (see <ulink
url="http://www.time-travellers.org/shane/papers/NFS_considered_harmful.html"></ulink>).
Specifically, delayed (asynchronous) writes to the <acronym>NFS</>
server can cause reliability problems; if possible, mount
<acronym>NFS</> file systems synchronously (without caching) to avoid
this. Also, soft-mounting <acronym>NFS</> is not recommended.
(Storage Area Networks (<acronym>SAN</>) use a low-level
communication protocol rather than <acronym>NFS</>.)
</para>
</sect2>
</sect1>
<sect1 id="server-start">
<title>Starting the Database Server</title>
<para>
Before anyone can access the database, you must start the database
server. The database server program is called
<command>postgres</command>.<indexterm><primary>postgres</></>
The <command>postgres</command> program must know where to
find the data it is supposed to use. This is done with the
<option>-D</option> option. Thus, the simplest way to start the
server is:
<screen>
$ <userinput>postgres -D /usr/local/pgsql/data</userinput>
</screen>
which will leave the server running in the foreground. This must be done while logged into the <productname>PostgreSQL</productname> user
account. Without <option>-D</option>, the server will try to use
the data directory named by the environment variable <envar>PGDATA</envar>.
If that variable is not provided either, it will fail.
</para>
<para>
Normally it is better to start <command>postgres</command> in the
background. For this, use the usual shell syntax:
<screen>
$ <userinput>postgres -D /usr/local/pgsql/data >logfile 2>&1 &</userinput>
</screen>
It is important to store the server's <systemitem>stdout</> and
<systemitem>stderr</> output somewhere, as shown above. It will help
for auditing purposes and to diagnose problems. (See <xref
linkend="logfile-maintenance"> for a more thorough discussion of log
file handling.)
</para>
<para>
The <command>postgres</command> program also takes a number of other
command-line options. For more information, see the
<xref linkend="app-postgres"> reference page
and <xref linkend="runtime-config"> below.
</para>
<para>
This shell syntax can get tedious quickly. Therefore the wrapper
program
<xref linkend="app-pg-ctl"><indexterm><primary>pg_ctl</primary></indexterm>
is provided to simplify some tasks. For example:
<programlisting>
pg_ctl start -l logfile
</programlisting>
will start the server in the background and put the output into the
named log file. The <option>-D</option> option has the same meaning
here as for <command>postgres</command>. <command>pg_ctl</command>
is also capable of stopping the server.
</para>
<para>
Normally, you will want to start the database server when the
computer boots.<indexterm><primary>booting</><secondary>starting
the server during</></> Autostart scripts are operating-system-specific.
There are a few distributed with
<productname>PostgreSQL</productname> in the
<filename>contrib/start-scripts</> directory. Installing one will require
root privileges.
</para>
<para>
Different systems have different conventions for starting up daemons
at boot time. Many systems have a file
<filename>/etc/rc.local</filename> or
<filename>/etc/rc.d/rc.local</filename>. Others use
<filename>rc.d</> directories. Whatever you do, the server must be
run by the <productname>PostgreSQL</productname> user account
<emphasis>and not by root</emphasis> or any other user. Therefore you
probably should form your commands using <literal>su -c '...'
postgres</literal>. For example:
<programlisting>
su -c 'pg_ctl start -D /usr/local/pgsql/data -l serverlog' postgres
</programlisting>
</para>
<para>
Here are a few more operating-system-specific suggestions. (In each
case be sure to use the proper installation directory and user
name where we show generic values.)
<itemizedlist>
<listitem>
<para>
For <productname>FreeBSD</productname>, look at the file
<filename>contrib/start-scripts/freebsd</filename> in the
<productname>PostgreSQL</productname> source distribution.
<indexterm><primary>FreeBSD</><secondary>start script</secondary></>
</para>
</listitem>
<listitem>
<para>
On <productname>OpenBSD</productname>, add the following lines
to the file <filename>/etc/rc.local</filename>:
<indexterm><primary>OpenBSD</><secondary>start script</secondary></>
<programlisting>
if [ -x /usr/local/pgsql/bin/pg_ctl -a -x /usr/local/pgsql/bin/postgres ]; then
su - -c '/usr/local/pgsql/bin/pg_ctl start -l /var/postgresql/log -s' postgres
echo -n ' postgresql'
fi
</programlisting>
</para>
</listitem>
<listitem>
<para>
On <productname>Linux</productname> systems either add
<indexterm><primary>Linux</><secondary>start script</secondary></>
<programlisting>
/usr/local/pgsql/bin/pg_ctl start -l logfile -D /usr/local/pgsql/data
</programlisting>
to <filename>/etc/rc.d/rc.local</filename> or look at the file
<filename>contrib/start-scripts/linux</filename> in the
<productname>PostgreSQL</productname> source distribution.
</para>
</listitem>
<listitem>
<para>
On <productname>NetBSD</productname>, either use the
<productname>FreeBSD</productname> or
<productname>Linux</productname> start scripts, depending on
preference. <indexterm><primary>NetBSD</><secondary>start script</secondary></>
</para>
</listitem>
<listitem>
<para>
On <productname>Solaris</productname>, create a file called
<filename>/etc/init.d/postgresql</filename> that contains
the following line:
<indexterm><primary>Solaris</><secondary>start script</secondary></>
<programlisting>
su - postgres -c "/usr/local/pgsql/bin/pg_ctl start -l logfile -D /usr/local/pgsql/data"
</programlisting>
Then, create a symbolic link to it in <filename>/etc/rc3.d</> as
<filename>S99postgresql</>.
</para>
</listitem>
</itemizedlist>
</para>
<para>
While the server is running, its
<acronym>PID</acronym> is stored in the file
<filename>postmaster.pid</filename> in the data directory. This is
used to prevent multiple server instances from
running in the same data directory and can also be used for
shutting down the server. </para>
<sect2 id="server-start-failures">
<title>Server Start-up Failures</title>
<para>
There are several common reasons the server might fail to
start. Check the server's log file, or start it by hand (without
redirecting standard output or standard error) and see what error
messages appear. Below we explain some of the most common error
messages in more detail.
</para>
<para>
<screen>
LOG: could not bind IPv4 socket: Address already in use
HINT: Is another postmaster already running on port 5432? If not, wait a few seconds and retry.
FATAL: could not create TCP/IP listen socket
</screen>
This usually means just what it suggests: you tried to start
another server on the same port where one is already running.
However, if the kernel error message is not <computeroutput>Address
already in use</computeroutput> or some variant of that, there might
be a different problem. For example, trying to start a server
on a reserved port number might draw something like:
<screen>
$ <userinput>postgres -p 666</userinput>
LOG: could not bind IPv4 socket: Permission denied
HINT: Is another postmaster already running on port 666? If not, wait a few seconds and retry.
FATAL: could not create TCP/IP listen socket
</screen>
</para>
<para>
A message like
<screen>
FATAL: could not create shared memory segment: Invalid argument
DETAIL: Failed system call was shmget(key=5440001, size=4011376640, 03600).
</screen>
probably means your kernel's limit on the size of shared memory is
smaller than the work area <productname>PostgreSQL</productname>
is trying to create (4011376640 bytes in this example). Or it could
mean that you do not have System-V-style shared memory support
configured into your kernel at all. As a temporary workaround, you
can try starting the server with a smaller-than-normal number of
buffers (<xref linkend="guc-shared-buffers">). You will eventually want
to reconfigure your kernel to increase the allowed shared memory
size. You might also see this message when trying to start multiple
servers on the same machine, if their total space requested
exceeds the kernel limit.
</para>
<para>
An error like
<screen>
FATAL: could not create semaphores: No space left on device
DETAIL: Failed system call was semget(5440126, 17, 03600).
</screen>
does <emphasis>not</emphasis> mean you've run out of disk
space. It means your kernel's limit on the number of <systemitem
class="osname">System V</> semaphores is smaller than the number
<productname>PostgreSQL</productname> wants to create. As above,
you might be able to work around the problem by starting the
server with a reduced number of allowed connections
(<xref linkend="guc-max-connections">), but you'll eventually want to
increase the kernel limit.
</para>
<para>
If you get an <quote>illegal system call</> error, it is likely that shared memory or semaphores are not supported in your kernel at
all. In that case your only option is to reconfigure the kernel to
enable these features.
</para>
<para>
Details about configuring <systemitem class="osname">System V</>
<acronym>IPC</> facilities are given in <xref linkend="sysvipc">.
</para>
</sect2>
<sect2 id="client-connection-problems">
<title>Client Connection Problems</title>
<para>
Although the error conditions possible on the client side are quite
varied and application-dependent, a few of them might be directly
related to how the server was started up. Conditions other than
those shown below should be documented with the respective client
application.
</para>
<para>
<screen>
psql: could not connect to server: Connection refused
Is the server running on host "server.joe.com" and accepting
TCP/IP connections on port 5432?
</screen>
This is the generic <quote>I couldn't find a server to talk
to</quote> failure. It looks like the above when TCP/IP
communication is attempted. A common mistake is to forget to
configure the server to allow TCP/IP connections.
</para>
<para>
Alternatively, you'll get this when attempting Unix-domain socket
communication to a local server:
<screen>
psql: could not connect to server: No such file or directory
Is the server running locally and accepting
connections on Unix domain socket "/tmp/.s.PGSQL.5432"?
</screen>
</para>
<para>
The last line is useful in verifying that the client is trying to
connect to the right place. If there is in fact no server
running there, the kernel error message will typically be either
<computeroutput>Connection refused</computeroutput> or
<computeroutput>No such file or directory</computeroutput>, as
illustrated. (It is important to realize that
<computeroutput>Connection refused</computeroutput> in this context
does <emphasis>not</emphasis> mean that the server got your
connection request and rejected it. That case will produce a
different message, as shown in <xref
linkend="client-authentication-problems">.) Other error messages
such as <computeroutput>Connection timed out</computeroutput> might
indicate more fundamental problems, like lack of network
connectivity.
</para>
</sect2>
</sect1>
<sect1 id="kernel-resources">
<title>Managing Kernel Resources</title>
<para>
A large <productname>PostgreSQL</> installation can quickly exhaust
various operating system resource limits. (On some systems, the
factory defaults are so low that you don't even need a really <quote>large</> installation.) If you have encountered this kind of
problem, keep reading.
</para>
<sect2 id="sysvipc">
<title>Shared Memory and Semaphores</title>
<indexterm zone="sysvipc">
<primary>shared memory</primary>
</indexterm>
<indexterm zone="sysvipc">
<primary>semaphores</primary>
</indexterm>
<para>
Shared memory and semaphores are collectively referred to as
<quote><systemitem class="osname">System V</>
<acronym>IPC</></quote> (together with message queues, which are not
relevant for <productname>PostgreSQL</>). Almost all modern
operating systems provide these features, but not all of them have
them turned on or sufficiently sized by default, especially systems
with BSD heritage. (On <systemitem class="osname">Windows</>,
<productname>PostgreSQL</> provides its own replacement
implementation of these facilities, and so most of this section
can be disregarded.)
</para>
<para>
The complete lack of these facilities is usually manifested by an
<errorname>Illegal system call</> error upon server start. In
that case there's nothing left to do but to reconfigure your
kernel. <productname>PostgreSQL</> won't work without them.
</para>
<para>
When <productname>PostgreSQL</> exceeds one of the various hard
<acronym>IPC</> limits, the server will refuse to start and
should leave an instructive error message describing the problem
encountered and what to do about it. (See also <xref
linkend="server-start-failures">.) The relevant kernel
parameters are named consistently across different systems; <xref
linkend="sysvipc-parameters"> gives an overview. The methods to set
them, however, vary. Suggestions for some platforms are given below.
Be warned that it is often necessary to reboot your machine, and
possibly even recompile the kernel, to change these settings.
</para>
<table id="sysvipc-parameters">
<title><systemitem class="osname">System V</> <acronym>IPC</> parameters</>
<tgroup cols="3">
<thead>
<row>
<entry>Name</>
<entry>Description</>
<entry>Reasonable values</>
</row>
</thead>
<tbody>
<row>
<entry><varname>SHMMAX</></>
<entry>Maximum size of shared memory segment (bytes)</>
<entry>at least several megabytes (see text)</entry>
</row>
<row>
<entry><varname>SHMMIN</></> <entry>Minimum size of shared memory segment (bytes)</>
<entry>1</>
</row>
<row>
<entry><varname>SHMALL</></>
<entry>Total amount of shared memory available (bytes or pages)</>
<entry>if bytes, same as <varname>SHMMAX</varname>; if pages, <literal>ceil(SHMMAX/PAGE_SIZE)</literal></>
</row>
<row>
<entry><varname>SHMSEG</></>
<entry>Maximum number of shared memory segments per process</>
<entry>only 1 segment is needed, but the default is much higher</>
</row>
<row>
<entry><varname>SHMMNI</></>
<entry>Maximum number of shared memory segments system-wide</>
<entry>like <varname>SHMSEG</> plus room for other applications</>
</row>
<row>
<entry><varname>SEMMNI</></>
<entry>Maximum number of semaphore identifiers (i.e., sets)</>
<entry>at least <literal>ceil((max_connections + autovacuum_max_workers) / 16)</literal></>
</row>
<row>
<entry><varname>SEMMNS</></>
<entry>Maximum number of semaphores system-wide</>
<entry><literal>ceil((max_connections + autovacuum_max_workers) / 16) * 17</literal> plus room for other applications</>
</row>
<row>
<entry><varname>SEMMSL</></>
<entry>Maximum number of semaphores per set</>
<entry>at least 17</>
</row>
<row>
<entry><varname>SEMMAP</></>
<entry>Number of entries in semaphore map</>
<entry>see text</>
</row>
<row>
<entry><varname>SEMVMX</></>
<entry>Maximum value of semaphore</>
<entry>at least 1000 (The default is often 32767, don't change unless forced to)</>
</row>
</tbody>
</tgroup>
</table>
<para>
<indexterm><primary>SHMMAX</primary></indexterm> The most important
shared memory parameter is <varname>SHMMAX</>, the maximum size, in
bytes, of a shared memory segment. If you get an error message from
<function>shmget</> like <errorname>Invalid argument</>, it is
likely that this limit has been exceeded. The size of the required
shared memory segment varies depending on several
<productname>PostgreSQL</> configuration parameters, as shown in
<xref linkend="shared-memory-parameters">. (Any error message you might
get will include the exact size of the failed allocation request.)
You can, as a temporary solution, lower some of those settings to
avoid the failure. While it is possible to get
<productname>PostgreSQL</> to run with <varname>SHMMAX</> as small as 2 MB, you need considerably more for acceptable performance. Desirable
settings are in the tens to hundreds of megabytes.
</para>
<para>
Some systems also have a limit on the total amount of shared memory in
the system (<varname>SHMALL</>). Make sure this is large enough
for <productname>PostgreSQL</> plus any other applications that
are using shared memory segments. (Caution: <varname>SHMALL</>
is measured in pages rather than bytes on many systems.)
</para>
<para>
Less likely to cause problems is the minimum size for shared
memory segments (<varname>SHMMIN</>), which should be at most
approximately 500 kB for <productname>PostgreSQL</> (it is
usually just 1). The maximum number of segments system-wide
(<varname>SHMMNI</>) or per-process (<varname>SHMSEG</>) are unlikely
to cause a problem unless your system has them set to zero.
</para>
<para>
<productname>PostgreSQL</> uses one semaphore per allowed connection
(<xref linkend="guc-max-connections">) and allowed autovacuum worker
process (<xref linkend="guc-autovacuum-max-workers">), in sets of 16.
Each such set will
also contain a 17th semaphore which contains a <quote>magic
number</quote>, to detect collision with semaphore sets used by
other applications. The maximum number of semaphores in the system
is set by <varname>SEMMNS</>, which consequently must be at least
as high as <varname>max_connections</> plus
<varname>autovacuum_max_workers</>, plus one extra for each 16
allowed connections plus workers (see the formula in <xref
linkend="sysvipc-parameters">). The parameter <varname>SEMMNI</>
determines the limit on the number of semaphore sets that can
exist on the system at one time. Hence this parameter must be at
least <literal>ceil((max_connections + autovacuum_max_workers) / 16)</>.
Lowering the number
of allowed connections is a temporary workaround for failures,
which are usually confusingly worded <errorname>No space
left on device</>, from the function <function>semget</>.
</para>
<para>
In some cases it might also be necessary to increase
<varname>SEMMAP</> to be at least on the order of
<varname>SEMMNS</>. This parameter defines the size of the semaphore
resource map, in which each contiguous block of available semaphores
needs an entry. When a semaphore set is freed it is either added to
an existing entry that is adjacent to the freed block or it is
registered under a new map entry. If the map is full, the freed
semaphores get lost (until reboot). Fragmentation of the semaphore
space could over time lead to fewer available semaphores than there
should be.
</para>
<para>
The <varname>SEMMSL</> parameter, which determines how many
semaphores can be in a set, must be at least 17 for
<productname>PostgreSQL</>.
</para>
<para>
Various other settings related to <quote>semaphore undo</>, such as
<varname>SEMMNU</> and <varname>SEMUME</>, are not of concern
for <productname>PostgreSQL</>.
</para>
<variablelist> <varlistentry>
<term><systemitem class="osname">AIX</></term>
<indexterm><primary>AIX</><secondary>IPC configuration</></>
<listitem>
<para>
At least as of version 5.1, it should not be necessary to do
any special configuration for such parameters as
<varname>SHMMAX</varname>, as it appears this is configured to
allow all memory to be used as shared memory. That is the
sort of configuration commonly used for other databases such
as <application>DB/2</application>.</para>
<para> It might, however, be necessary to modify the global
<command>ulimit</command> information in
<filename>/etc/security/limits</filename>, as the default hard
limits for file sizes (<varname>fsize</varname>) and numbers of
files (<varname>nofiles</varname>) might be too low.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><systemitem class="osname">BSD/OS</></term>
<indexterm><primary>BSD/OS</><secondary>IPC configuration</></>
<listitem>
<formalpara>
<title>Shared Memory</>
<para>
By default, only 4 MB of shared memory is supported. Keep in
mind that shared memory is not pageable; it is locked in RAM.
To increase the amount of shared memory supported by your
system, add something like the following to your kernel configuration
file:
<programlisting>
options "SHMALL=8192"
options "SHMMAX=\(SHMALL*PAGE_SIZE\)"
</programlisting>
<varname>SHMALL</> is measured in 4 kB pages, so a value of
1024 represents 4 MB of shared memory. Therefore the above increases
the maximum shared memory area to 32 MB.
For those running 4.3 or later, you will probably also need to increase
<varname>KERNEL_VIRTUAL_MB</> above the default <literal>248</>.
Once all changes have been made, recompile the kernel, and reboot.
</para>
</formalpara>
<para>
For those running 4.0 and earlier releases, use <command>bpatch</>
to find the <varname>sysptsize</> value in the current
kernel. This is computed dynamically at boot time.
<screen>
$ <userinput>bpatch -r sysptsize</>
<computeroutput>0x9 = 9</>
</screen>
Next, add <varname>SYSPTSIZE</> as a hard-coded value in the
kernel configuration file. Increase the value you found using
<command>bpatch</>. Add 1 for every additional 4 MB of
shared memory you desire.
<programlisting>
options "SYSPTSIZE=16"
</programlisting>
<varname>sysptsize</> cannot be changed by <command>sysctl</command>.
</para>
<formalpara>
<title>Semaphores</>
<para>
You will probably want to increase the number of semaphores
as well; the default system total of 60 will only allow about
50 <productname>PostgreSQL</productname> connections. Set the values you want in your kernel configuration file, e.g.:
<programlisting>
options "SEMMNI=40"
options "SEMMNS=240"
</programlisting>
</para>
</formalpara>
</listitem>
</varlistentry>
<varlistentry>
<term><systemitem class="osname">FreeBSD</></term>
<indexterm><primary>FreeBSD</><secondary>IPC configuration</></>
<listitem>
<para>
The default settings are only suitable for small installations
(for example, default <varname>SHMMAX</varname> is 32
MB). Changes can be made via the <command>sysctl</command> or
<command>loader</command> interfaces. The following
parameters can be set using <command>sysctl</command>:
<screen>
<prompt>$</prompt> <userinput>sysctl -w kern.ipc.shmall=32768</userinput>
<prompt>$</prompt> <userinput>sysctl -w kern.ipc.shmmax=134217728</userinput>
<prompt>$</prompt> <userinput>sysctl -w kern.ipc.semmap=256</userinput>
</screen>
To have these settings persist over reboots, modify
<filename>/etc/sysctl.conf</filename>.
</para>
<para>
The remaining semaphore settings are read-only as far as
<command>sysctl</command> is concerned, but can be changed
before boot using the <command>loader</command> prompt:
<screen>
<prompt>(loader)</prompt> <userinput>set kern.ipc.semmni=256</userinput>
<prompt>(loader)</prompt> <userinput>set kern.ipc.semmns=512</userinput>
<prompt>(loader)</prompt> <userinput>set kern.ipc.semmnu=256</userinput>
</screen>
Similarly these can be saved between reboots in
<filename>/boot/loader.conf</filename>.
</para>
<para>
You might also want to configure your kernel to lock shared
memory into RAM and prevent it from being paged out to swap.
This can be accomplished using the <command>sysctl</command>
setting <literal>kern.ipc.shm_use_phys</literal>.
</para>
<para>
If running in FreeBSD jails by enabling <application>sysctl</>'s
<literal>security.jail.sysvipc_allowed</>, <application>postmaster</>s
running in different jails should be run by different operating system
users. This improves security because it prevents non-root users
from interfering with shared memory or semaphores in a different jail,
and it allows the PostgreSQL IPC cleanup code to function properly.
(In FreeBSD 6.0 and later the IPC cleanup code doesn't properly detect
processes in other jails, preventing the running of postmasters on the
same port in different jails.)
</para>
<para>
<systemitem class="osname">FreeBSD</> versions before 4.0 work like
<systemitem class="osname">NetBSD</> and <systemitem class="osname">
OpenBSD</> (see below).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><systemitem class="osname">NetBSD</></term>
<term><systemitem class="osname">OpenBSD</></term>
<indexterm><primary>NetBSD</><secondary>IPC configuration</></>
<indexterm><primary>OpenBSD</><secondary>IPC configuration</></>
<listitem>
<para>
The options <varname>SYSVSHM</> and <varname>SYSVSEM</> need
to be enabled when the kernel is compiled. (They are by
default.) The maximum size of shared memory is determined by
the option <varname>SHMMAXPGS</> (in pages). The following
shows an example of how to set the various parameters
(<systemitem class="osname">OpenBSD</> uses <literal>option</> instead):
<programlisting>
options SYSVSHM
options SHMMAXPGS=4096
options SHMSEG=256
options SYSVSEM
options SEMMNI=256
options SEMMNS=512
options SEMMNU=256
options SEMMAP=256
</programlisting>
</para>
<para>
You might also want to configure your kernel to lock shared
memory into RAM and prevent it from being paged out to swap.
This can be accomplished using the <command>sysctl</command>
setting <literal>kern.ipc.shm_use_phys</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><systemitem class="osname">HP-UX</></term>
<indexterm><primary>HP-UX</><secondary>IPC configuration</></>
<listitem>
<para>
The default settings tend to suffice for normal installations.
On <productname>HP-UX</> 10, the factory default for
<varname>SEMMNS</> is 128, which might be too low for larger
database sites.
</para>
<para>
<acronym>IPC</> parameters can be set in the <application>System
Administration Manager</> (<acronym>SAM</>) under
<menuchoice><guimenu>Kernel
Configuration</><guimenuitem>Configurable Parameters</></>. Hit
<guibutton>Create A New Kernel</> when you're done.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><systemitem class="osname">Linux</></term>
<indexterm><primary>Linux</><secondary>IPC configuration</></>
<listitem>
<para>
The default maximum segment size is 32 MB, which is only adequate
for small <productname>PostgreSQL</productname> installations.
However, the remaining
defaults are quite generously sized, and usually do not require
changes. The maximum shared memory segment size can be changed via the
<command>sysctl</command> interface. For example, to allow 128 MB,
and explicitly set the maximum total shared memory size to 2097152
pages (the default):<screen>
<prompt>$</prompt> <userinput>sysctl -w kernel.shmmax=134217728</userinput>
<prompt>$</prompt> <userinput>sysctl -w kernel.shmall=2097152</userinput>
</screen>
In addition these settings can be saved between reboots in
<filename>/etc/sysctl.conf</filename>.
</para>
<para>
Older distributions might not have the <command>sysctl</command> program,
but equivalent changes can be made by manipulating the
<filename>/proc</filename> file system:
<screen>
<prompt>$</prompt> <userinput>echo 134217728 >/proc/sys/kernel/shmmax</userinput>
<prompt>$</prompt> <userinput>echo 2097152 >/proc/sys/kernel/shmall</userinput>
</screen>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><systemitem class="osname">MacOS X</></term>
<indexterm><primary>MacOS X</><secondary>IPC configuration</></>
<listitem>
<para>
In OS X 10.2 and earlier, edit the file
<filename>/System/Library/StartupItems/SystemTuning/SystemTuning</>
and change the values in the following commands:
<programlisting>
sysctl -w kern.sysv.shmmax
sysctl -w kern.sysv.shmmin
sysctl -w kern.sysv.shmmni
sysctl -w kern.sysv.shmseg
sysctl -w kern.sysv.shmall
</programlisting>
</para>
<para>
In OS X 10.3 and later, these commands have been moved to
<filename>/etc/rc</> and must be edited there. Note that
<filename>/etc/rc</> is usually overwritten by OS X updates (such as
10.3.6 to 10.3.7) so you should expect to have to redo your editing
after each update.
</para>
<para>
In OS X 10.3.9 and later, instead of editing <filename>/etc/rc</>
you can create a file named <filename>/etc/sysctl.conf</>,
containing variable assignments such as:
<programlisting>
kern.sysv.shmmax=4194304
kern.sysv.shmmin=1
kern.sysv.shmmni=32
kern.sysv.shmseg=8
kern.sysv.shmall=1024
</programlisting>
This method is better than editing <filename>/etc/rc</> because
your changes will be preserved across system updates. Note that
<emphasis>all five</> shared-memory parameters must be set in
<filename>/etc/sysctl.conf</>, else the values will be ignored.
</para>
<para>
Beware that recent releases of OS X ignore attempts to set
<varname>SHMMAX</> to a value that isn't an exact multiple of 4096.
</para>
<para>
<varname>SHMALL</> is measured in 4 kB pages on this platform. </para>
<para>
In all OS X versions, you'll need to reboot to make changes in the
shared memory parameters take effect.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><systemitem class="osname">SCO OpenServer</></term>
<indexterm><primary>SCO OpenServer</><secondary>IPC configuration</></>
<listitem>
<para>
In the default configuration, only 512 kB of shared memory per
segment is allowed. To increase the setting, first change to the
directory <filename>/etc/conf/cf.d</>. To display the current value of
<varname>SHMMAX</>, run:
<programlisting>
./configure -y SHMMAX
</programlisting>
To set a new value for <varname>SHMMAX</>, run:
<programlisting>
./configure SHMMAX=<replaceable>value</>
</programlisting>
where <replaceable>value</> is the new value you want to use
(in bytes). After setting <varname>SHMMAX</>, rebuild the kernel:
<programlisting>
./link_unix
</programlisting>
and reboot.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><systemitem class="osname">Solaris</></term>
<indexterm><primary>Solaris</><secondary>IPC configuration</></>
<listitem>
<para>
At least in version 2.6, the default maximum size of a shared
memory segment is too low for <productname>PostgreSQL</>. The
relevant settings can be changed in <filename>/etc/system</>,
for example:
<programlisting>
set shmsys:shminfo_shmmax=0x2000000
set shmsys:shminfo_shmmin=1
set shmsys:shminfo_shmmni=256
set shmsys:shminfo_shmseg=256
set semsys:seminfo_semmap=256
set semsys:seminfo_semmni=512
set semsys:seminfo_semmns=512
set semsys:seminfo_semmsl=32
</programlisting>
You need to reboot for the changes to take effect.
</para>
<para>
See also <ulink
url="http://sunsite.uakom.sk/sunworldonline/swol-09-1997/swol-09-insidesolaris.html"></>
for information on shared memory under
<productname>Solaris</>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><systemitem class="osname">UnixWare</></term>
<indexterm><primary>UnixWare</><secondary>IPC configuration</></>
<listitem>
<para>
On <productname>UnixWare</> 7, the maximum size for shared
memory segments is only 512 kB in the default configuration.
To display the current value of <varname>SHMMAX</>, run:
<programlisting>
/etc/conf/bin/idtune -g SHMMAX
</programlisting>
which displays the current, default, minimum, and maximum
values. To set a new value for <varname>SHMMAX</>,
run:
<programlisting>
/etc/conf/bin/idtune SHMMAX <replaceable>value</>
</programlisting>
where <replaceable>value</> is the new value you want to use
(in bytes). After setting <varname>SHMMAX</>, rebuild the
kernel:
<programlisting>
/etc/conf/bin/idbuild -B
</programlisting>
and reboot.
</para>
</listitem>
</varlistentry>
</variablelist>
<table id="shared-memory-parameters">
<title><productname>PostgreSQL</productname> shared memory usage</>
<tgroup cols="2">
<thead>
<row>
<entry>Usage</>
<entry>Approximate shared memory bytes required (as of 8.3)</>
</row>
</thead>
<tbody>
<row>
<entry>Connections</>
<entry>(1800 + 270 * <xref
linkend="guc-max-locks-per-transaction">) * <xref
linkend="guc-max-connections"></entry>
</row>
<row>
<entry>Autovacuum workers</>
<entry>(1800 + 270 * <xref
linkend="guc-max-locks-per-transaction">) * <xref
linkend="guc-autovacuum-max-workers"></entry>
</row>
<row>
<entry>Prepared transactions</>
<entry>(770 + 270 * <xref
linkend="guc-max-locks-per-transaction">) * <xref linkend="guc-max-prepared-transactions"></entry>
</row>
<row>
<entry>Shared disk buffers</>
<entry>(<xref linkend="guc-block-size"> + 208) * <xref linkend="guc-shared-buffers"></entry>
</row>
<row>
<entry>WAL buffers</> <entry>(<xref linkend="guc-wal-block-size"> + 8) * <xref linkend="guc-wal-buffers"></entry>
</row>
<row>
<entry>Fixed space requirements</>
<entry>770 kB</entry>
</row>
</tbody>
</tgroup>
</table>
</sect2>
<sect2>
<title>Resource Limits</title>
<para>
Unix-like operating systems enforce various kinds of resource limits
that might interfere with the operation of your
<productname>PostgreSQL</productname> server. Of particular
importance are limits on the number of processes per user, the
number of open files per process, and the amount of memory available
to each process. Each of these have a <quote>hard</quote> and a
<quote>soft</quote> limit. The soft limit is what actually counts
but it can be changed by the user up to the hard limit. The hard
limit can only be changed by the root user. The system call
<function>setrlimit</function> is responsible for setting these
parameters. The shell's built-in command <command>ulimit</command>
(Bourne shells) or <command>limit</command> (<application>csh</>) is
used to control the resource limits from the command line. On
BSD-derived systems the file <filename>/etc/login.conf</filename>
controls the various resource limits set during login. See the
operating system documentation for details. The relevant
parameters are <varname>maxproc</varname>,
<varname>openfiles</varname>, and <varname>datasize</varname>. For
example:
<programlisting>
default:\
...
:datasize-cur=256M:\
:maxproc-cur=256:\
:openfiles-cur=256:\
...
</programlisting>
(<literal>-cur</literal> is the soft limit. Append
<literal>-max</literal> to set the hard limit.)
</para>
<para>
Kernels can also have system-wide limits on some resources.
<itemizedlist>
<listitem>
<para>
On <productname>Linux</productname>
<filename>/proc/sys/fs/file-max</filename> determines the
maximum number of open files that the kernel will support. It can
be changed by writing a different number into the file or by
adding an assignment in <filename>/etc/sysctl.conf</filename>.
The maximum limit of files per process is fixed at the time the
kernel is compiled; see
<filename>/usr/src/linux/Documentation/proc.txt</filename> for
more information.
</para>
</listitem>
</itemizedlist>
</para>
<para>
The <productname>PostgreSQL</productname> server uses one process
per connection so you should provide for at least as many processes as allowed connections, in addition to what you need for the rest
of your system. This is usually not a problem but if you run
several servers on one machine things might get tight.
</para>
<para>
The factory default limit on open files is often set to
<quote>socially friendly</quote> values that allow many users to
coexist on a machine without using an inappropriate fraction of
the system resources. If you run many servers on a machine this
is perhaps what you want, but on dedicated servers you might want to
raise this limit.
</para>
<para>
On the other side of the coin, some systems allow individual
processes to open large numbers of files; if more than a few
processes do so then the system-wide limit can easily be exceeded.
If you find this happening, and you do not want to alter the
system-wide limit, you can set <productname>PostgreSQL</>'s <xref
linkend="guc-max-files-per-process"> configuration parameter to
limit the consumption of open files.
</para>
</sect2>
<sect2>
<title>Linux Memory Overcommit</title>
<para>
In Linux 2.4 and later, the default virtual memory behavior is not
optimal for <productname>PostgreSQL</productname>. Because of the
way that the kernel implements memory overcommit, the kernel might
terminate the <productname>PostgreSQL</productname> server (the
master server process) if the memory demands of
another process cause the system to run out of virtual memory.
</para>
<para>
If this happens, you will see a kernel message that looks like
this (consult your system documentation and configuration on where
to look for such a message):
<programlisting>
Out of Memory: Killed process 12345 (postgres).
</programlisting>
This indicates that the <filename>postgres</filename> process
has been terminated due to memory pressure.
Although existing database connections will continue to function
normally, no new connections will be accepted. To recover,
<productname>PostgreSQL</productname> will need to be restarted.
</para>
<para>
One way to avoid this problem is to run
<productname>PostgreSQL</productname> on a machine where you can
be sure that other processes will not run the machine out of
memory. If memory is tight, increasing the swap space of the
operating system can help avoiding the problem, because the
out-of-memory (OOM) killer is invoked whenever physical memory and
swap space are exhausted.
</para>
<para>
On Linux 2.6 and later, an additional measure is to modify the
kernel's behavior so that it will not <quote>overcommit</> memory.
Although this setting will not prevent the <ulink
url="http://lwn.net/Articles/104179/">OOM killer</> from being invoked
altogether, it will lower the chances significantly and will therefore
lead to more robust system behavior. This is done by selecting strict
overcommit mode via <command>sysctl</command>:
<programlisting>sysctl -w vm.overcommit_memory=2
</programlisting>
or placing an equivalent entry in <filename>/etc/sysctl.conf</>.
You might also wish to modify the related setting
<literal>vm.overcommit_ratio</>. For details see the kernel documentation
file <filename>Documentation/vm/overcommit-accounting</>.
</para>
<para>
Some vendors' Linux 2.4 kernels are reported to have early versions
of the 2.6 overcommit <command>sysctl</command> parameter. However, setting
<literal>vm.overcommit_memory</> to 2
on a kernel that does not have the relevant code will make
things worse not better. It is recommended that you inspect
the actual kernel source code (see the function
<function>vm_enough_memory</> in the file <filename>mm/mmap.c</>)
to verify what is supported in your copy before you try this in a 2.4
installation. The presence of the <filename>overcommit-accounting</>
documentation file should <emphasis>not</> be taken as evidence that the
feature is there. If in any doubt, consult a kernel expert or your
kernel vendor.
</para>
</sect2>
</sect1>
<sect1 id="server-shutdown">
<title>Shutting Down the Server</title>
<indexterm zone="server-shutdown">
<primary>shutdown</>
</indexterm>
<para>
There are several ways to shut down the database server. You control
the type of shutdown by sending different signals to the master
<command>postgres</command> process.
<variablelist>
<varlistentry>
<term><systemitem>SIGTERM</systemitem><indexterm><primary>SIGTERM</></></term>
<listitem>
<para>
This is the <firstterm>Smart Shutdown</firstterm> mode.
After receiving <systemitem>SIGTERM</systemitem>, the server
disallows new connections, but lets existing sessions end their
work normally. It shuts down only after all of the sessions terminate.
If the server is in online backup mode, it additionally waits
until online backup mode is no longer active. While backup mode is
active, new connections will still be allowed, but only to superusers
(this exception allows a superuser to connect to terminate
online backup mode).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><systemitem>SIGINT</systemitem><indexterm><primary>SIGINT</></></term>
<listitem>
<para>
This is the <firstterm>Fast Shutdown</firstterm> mode.
The server disallows new connections and sends all existing
server processes <systemitem>SIGTERM</systemitem>, which will cause them
to abort their current transactions and exit promptly. It then
waits for the server processes to exit and finally shuts down.
If the server is in online backup mode, backup mode will be
terminated, rendering the backup useless.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><systemitem>SIGQUIT</systemitem><indexterm><primary>SIGQUIT</></></term>
<listitem>
<para>
This is the <firstterm>Immediate Shutdown</firstterm> mode.
The master <command>postgres</command> process will send a
<systemitem>SIGQUIT</systemitem> to all child processes and exit
immediately, without properly shutting itself down. The child processes
likewise exit immediately upon receiving
<systemitem>SIGQUIT</systemitem>. This will lead to recovery (by
replaying the WAL log) upon next start-up. This is recommended
only in emergencies.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
The <xref linkend="app-pg-ctl"> program provides a convenient
interface for sending these signals to shut down the server.
Alternatively, you can send the signal directly using <command>kill</>
on non-Windows systems.
The <acronym>PID</> of the <command>postgres</command> process can be
found using the <command>ps</command> program, or from the file
<filename>postmaster.pid</filename> in the data directory. For
example, to do a fast shutdown:
<screen>
$ <userinput>kill -INT `head -1 /usr/local/pgsql/data/postmaster.pid`</userinput>
</screen>
</para>
<important>
<para>
It is best not to use <systemitem>SIGKILL</systemitem> to shut down
the server. Doing so will prevent the server from releasing
shared memory and semaphores, which might then have to be done
manually before a new server can be started. Furthermore,
<systemitem>SIGKILL</systemitem> kills the <command>postgres</command>
process without letting it relay the signal to its subprocesses,
so it will be necessary to kill the individual subprocesses by hand as
well.
</para>
</important>
<para>
To terminate an individual session while allowing other sessions to
continue, use <function>pg_terminate_backend()</> (see <xref
linkend="functions-admin-signal-table">) or send a
<systemitem>SIGTERM</> signal to the child process associated with
the session.
</para>
</sect1>
<sect1 id="preventing-server-spoofing">
<title>Preventing Server Spoofing</title>
<indexterm zone="preventing-server-spoofing">
<primary>server spoofing</primary>
</indexterm>
<para>
While the server is running, it is not possible for a malicious user
to take the place of the normal database server. However, when the
server is down it is possible for a local user to spoof the normal
server by starting their own server. The spoof server could read
passwords and queries sent by clients, but could not return any data
because the <varname>PGDATA</> directory would still be secure because
of directory permissions. Spoofing is possible because any user can start a database server; a client cannot identify an invalid server
unless it is specially configured.
</para>
<para>
The simplest way to prevent invalid servers for <literal>local</>
connections is to use a Unix domain socket directory (<xref
linkend="guc-unix-socket-directory">) that has write permission only
for a trusted local user. This prevents a malicious user from creating
their own socket file in that directory. If you are concerned that
some applications might still reference <filename>/tmp</> for the
socket file and hence be vulnerable to spoofing, during operating system
startup create symbolic link <filename>/tmp/.s.PGSQL.5432</> that points
to the relocated socket file. You also might need to modify your
<filename>/tmp</> cleanup script to preserve the symbolic link.
</para>
<para>
For TCP connections the server
must accept only <literal>hostssl</> connections (<xref
linkend="auth-pg-hba-conf">) and have SSL
<filename>server.key</filename> (key) and
<filename>server.crt</filename> (certificate) files (<xref
linkend="ssl-tcp">). The TCP client must connect using
<literal>sslmode='verify-ca'</> or
<literal>'verify-full'</> and have the required certificate
files present (<xref linkend="libpq-connect">).
</para>
</sect1>
<sect1 id="encryption-options">
<title>Encryption Options</title>
<indexterm zone="encryption-options">
<primary>encryption</primary>
</indexterm>
<para>
<productname>PostgreSQL</productname> offers encryption at several
levels, and provides flexibility in protecting data from disclosure
due to database server theft, unscrupulous administrators, and
insecure networks. Encryption might also be required to secure
sensitive data such as medical records or financial transactions.
</para>
<variablelist>
<varlistentry>
<term>Password Storage Encryption</term>
<listitem>
<para>
By default, database user passwords are stored as MD5 hashes, so
the administrator cannot determine the actual password assigned
to the user. If MD5 encryption is used for client authentication,
the unencrypted password is never even temporarily present on the
server because the client MD5 encrypts it before being sent
across the network.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Encryption For Specific Columns</term>
<listitem>
<para>
The <filename>contrib</> function library
<function>pgcrypto</function> allows certain fields to be stored
encrypted. This is useful if only some of the data is sensitive. The client supplies the decryption key and the data is decrypted
on the server and then sent to the client.
</para>
<para>
The decrypted data and the decryption key are present on the
server for a brief time while it is being decrypted and
communicated between the client and server. This presents a brief
moment where the data and keys can be intercepted by someone with
complete access to the database server, such as the system
administrator.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Data Partition Encryption</term>
<listitem>
<para>
On Linux, encryption can be layered on top of a file system mount
using a <quote>loopback device</quote>. This allows an entire
file system partition be encrypted on disk, and decrypted by the
operating system. On FreeBSD, the equivalent facility is called
GEOM Based Disk Encryption, or <acronym>gbde</acronym>.
</para>
<para>
This mechanism prevents unencrypted data from being read from the
drives if the drives or the entire computer is stolen. This does
not protect against attacks while the file system is mounted,
because when mounted, the operating system provides an unencrypted
view of the data. However, to mount the file system, you need some
way for the encryption key to be passed to the operating system,
and sometimes the key is stored somewhere on the host that mounts
the disk.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Encrypting Passwords Across A Network</term>
<listitem>
<para>
The <literal>MD5</> authentication method double-encrypts the
password on the client before sending it to the server. It first
MD5 encrypts it based on the user name, and then encrypts it
based on a random salt sent by the server when the database
connection was made. It is this double-encrypted value that is
sent over the network to the server. Double-encryption not only
prevents the password from being discovered, it also prevents
another connection from using the same encrypted password to
connect to the database server at a later time.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Encrypting Data Across A Network</term>
<listitem>
<para>
SSL connections encrypt all data sent across the network: the
password, the queries, and the data returned. The
<filename>pg_hba.conf</> file allows administrators to specify
which hosts can use non-encrypted connections (<literal>host</>)
and which require SSL-encrypted connections
(<literal>hostssl</>). Also, clients can specify that they
connect to servers only via SSL. <application>Stunnel</> or <application>SSH</> can also be used to encrypt transmissions.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>SSL Host Authentication</term>
<listitem>
<para>
It is possible for both the client and server to provide SSL
certificates to each other. It takes some extra configuration
on each side, but this provides stronger verification of identity
than the mere use of passwords. It prevents a computer from
pretending to be the server just long enough to read the password
send by the client. It also helps prevent "man in the middle"
attacks where a computer between the client and server pretends to
be the server and reads and passes all data between the client and
server.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Client-Side Encryption</term>
<listitem>
<para>
If the system administrator cannot be trusted, it is necessary
for the client to encrypt the data; this way, unencrypted data
never appears on the database server. Data is encrypted on the
client before being sent to the server, and database results have
to be decrypted on the client before being used.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect1>
<sect1 id="ssl-tcp">
<title>Secure TCP/IP Connections with SSL</title>
<indexterm zone="ssl-tcp">
<primary>SSL</primary>
</indexterm>
<para>
<productname>PostgreSQL</> has native support for using
<acronym>SSL</> connections to encrypt client/server communications
for increased security. This requires that
<productname>OpenSSL</productname> is installed on both client and
server systems and that support in <productname>PostgreSQL</> is
enabled at build time (see <xref linkend="installation">).
</para>
<para>
With <acronym>SSL</> support compiled in, the
<productname>PostgreSQL</> server can be started with
<acronym>SSL</> enabled by setting the parameter
<xref linkend="guc-ssl"> to <literal>on</> in
<filename>postgresql.conf</>. The server will listen for both standard
and <acronym>SSL</> connections on the same TCP port, and will negotiate
with any connecting client on whether to use <acronym>SSL</>. By
default, this is at the client's option; see <xref
linkend="auth-pg-hba-conf"> about how to set up the server to require
use of <acronym>SSL</> for some or all connections.
</para>
<para>
<productname>PostgreSQL</productname> reads the system-wide
<productname>OpenSSL</productname> configuration file. By default, this
file is named <filename>openssl.cnf</filename> and is located in the
directory reported by <literal>openssl version -d</>.
This default can be overridden by setting environment variable
<envar>OPENSSL_CONF</envar> to the name of the desired configuration file.
</para>
<para>
<productname>OpenSSL</productname> supports a wide range of ciphers
and authentication algorithms, of varying strength. While a list of
ciphers can be specified in the <productname>OpenSSL</productname>
configuration file, you can specify ciphers specifically for use by
the database server by modifying <xref linkend="guc-ssl-ciphers"> in
<filename>postgresql.conf</>.
</para>
<note>
<para>
It is possible to have authentication without encryption overhead by
using <literal>NULL-SHA</> or <literal>NULL-MD5</> ciphers. However,
a man-in-the-middle could read and pass communications between client
and server. Also, encryption overhead is minimal compared to the
overhead of authentication. For these reasons NULL ciphers are not
recommended.
</para>
</note>
<para>
To start in <acronym>SSL</> mode, the files <filename>server.crt</>
and <filename>server.key</> must exist in the server's data directory.
These files should contain the server certificate and private key,
respectively.
On Unix systems, the permissions on <filename>server.key</filename> must
disallow any access to world or group; achieve this by the command
<command>chmod 0600 server.key</command>.
If the private key is protected with a passphrase, the
server will prompt for the passphrase and will not start until it has
been entered.
</para>
<sect2 id="ssl-client-certificates">
<title>Using client certificates</title>
<para>
To require the client to supply a trusted certificate, place
certificates of the certificate authorities (<acronym>CA</acronym>)
you trust in the file <filename>root.crt</filename> in the data
directory, and set the <literal>clientcert</literal> parameter
to <literal>1</literal> on the appropriate line(s) in pg_hba.conf.
A certificate will then be requested from the client during
SSL connection startup. (See <xref linkend="libpq-ssl"> for a
description of how to set up certificates on the client.) The server will
verify that the client's certificate is signed by one of the trusted
certificate authorities. Certificate Revocation List (CRL) entries
are also checked if the file <filename>root.crl</filename> exists.
<!-- If this URL changes replace it with a URL to www.archive.org. -->
(See <ulink
url="http://h71000.www7.hp.com/DOC/83final/BA554_90007/ch04s02.html"></>
for diagrams showing SSL certificate usage.)
</para>
<para>
The <literal>clientcert</literal> option in <filename>pg_hba.conf</>
is available for all authentication methods, but only for rows
specified as <literal>hostssl</>. Unless specified, the default is
not to verify the client certificate.
</para>
<para> You can use the authentication method <literal>cert</> to use the
client certificate for authenticating users. See
<xref linkend="auth-cert"> for details.
</para>
</sect2>
<sect2 id="ssl-server-files">
<title>SSL Server File Usage</title>
<para>
The files <filename>server.key</>, <filename>server.crt</>,
<filename>root.crt</filename>, and <filename>root.crl</filename>
are only examined during server start; so you must restart
the server for changes in them to take effect.
</para>
<table id="ssl-file-usage">
<title>SSL Server File Usage</title>
<tgroup cols="3">
<thead>
<row>
<entry>File</entry>
<entry>Contents</entry>
<entry>Effect</entry>
</row>
</thead>
<tbody>
<row>
<entry><filename>server.crt</></entry>
<entry>server certificate</entry>
<entry>requested by client</entry>
</row>
<row>
<entry><filename>server.key</></entry>
<entry>server private key</entry>
<entry>proves server certificate sent by owner; does not indicate
certificate owner is trustworthy</entry>
</row>
<row>
<entry><filename>root.crt</></entry>
<entry>trusted certificate authorities</entry>
<entry>checks that client certificate is
signed by a trusted certificate authority</entry>
</row>
<row>
<entry><filename>root.crl</></entry>
<entry>certificates revoked by certificate authorities</entry>
<entry>client certificate must not be on this list</entry>
</row>
</tbody>
</tgroup>
</table>
</sect2>
<sect2 id="ssl-certificate-creation">
<title>Creating a Self-Signed Certificate</title>
<para>
To create a quick self-signed certificate for the server, use the
following <productname>OpenSSL</productname> command:
<programlisting>
openssl req -new -text -out server.req
</programlisting>
Fill out the information that <application>openssl</> asks for. Make sure
you enter the local host name as <quote>Common Name</>; the challenge password can be left blank. The program will generate a key that is
passphrase protected; it will not accept a passphrase that is less
than four characters long. To remove the passphrase (as you must if
you want automatic start-up of the server), run the commands:
<programlisting>
openssl rsa -in privkey.pem -out server.key
rm privkey.pem
</programlisting>
Enter the old passphrase to unlock the existing key. Now do:
<programlisting>
openssl req -x509 -in server.req -text -key server.key -out server.crt
</programlisting>
to turn the certificate into a self-signed certificate and to copy
the key and certificate to where the server will look for them.
Finally do:
<programlisting>
chmod og-rwx server.key
</programlisting>
because the server will reject the file if its permissions are more
liberal than this.
For more details on how to create your server private key and
certificate, refer to the <productname>OpenSSL</> documentation.
</para>
<para>
A self-signed certificate can be used for testing, but a certificate
signed by a certificate authority (<acronym>CA</>) (either one of the
global <acronym>CAs</> or a local one) should be used in production
so the client can verify the server's identity. If all the clients
are local to the organization, using a local <acronym>CA</> is
recommended.
</para>
</sect2>
</sect1>
<sect1 id="ssh-tunnels">
<title>Secure TCP/IP Connections with <application>SSH</application> Tunnels</title>
<indexterm zone="ssh-tunnels">
<primary>ssh</primary>
</indexterm>
<para>
One can use <application>SSH</application> to encrypt the network
connection between clients and a
<productname>PostgreSQL</productname> server. Done properly, this
provides an adequately secure network connection, even for non-SSL-capable
clients.
</para>
<para>
First make sure that an <application>SSH</application> server is
running properly on the same machine as the
<productname>PostgreSQL</productname> server and that you can log in using
<command>ssh</command> as some user. Then you can establish a secure
tunnel with a command like this from the client machine:
<programlisting>
ssh -L 63333:localhost:5432 joe@foo.com
</programlisting>
The first number in the <option>-L</option> argument, 63333, is the
port number of your end of the tunnel; it can be chosen freely.
(IANA reserves ports 49152 through 65535 for private use.) The
second number, 5432, is the remote end of the tunnel: the port
number your server is using. The name or IP address between the
port numbers is the host with the database server you are going to
connect to, as seen from the host you are logging in to, which
is <literal>foo.com</literal> in this example. In order to connect
to the database server using this tunnel, you connect to port 63333 on the local machine:
<programlisting>
psql -h localhost -p 63333 postgres
</programlisting>
To the database server it will then look as though you are really
user <literal>joe</literal> on host <literal>foo.com</literal>
connecting to <literal>localhost</literal> in that context, and it
will use whatever authentication procedure was configured for
connections from this user and host. Note that the server will not
think the connection is SSL-encrypted, since in fact it is not
encrypted between the
<application>SSH</application> server and the
<productname>PostgreSQL</productname> server. This should not pose any
extra security risk as long as they are on the same machine.
</para>
<para>
In order for the
tunnel setup to succeed you must be allowed to connect via
<command>ssh</command> as <literal>joe@foo.com</literal>, just
as if you had attempted to use <command>ssh</command> to set up a
terminal session.
</para>
<para>
You could also have set up the port forwarding as
<programlisting>
ssh -L 63333:foo.com:5432 joe@foo.com
</programlisting>
but then the database server will see the connection as coming in
on its <literal>foo.com</literal> interface, which is not opened by
the default setting <literal>listen_addresses =
'localhost'</literal>. This is usually not what you want.
</para>
<para>
If you have to <quote>hop</quote> to the database server via some
login host, one possible setup could look like this:
<programlisting>
ssh -L 63333:db.foo.com:5432 joe@shell.foo.com
</programlisting>
Note that this way the connection
from <literal>shell.foo.com</literal>
to <literal>db.foo.com</literal> will not be encrypted by the SSH
tunnel.
SSH offers quite a few configuration possibilities when the network
is restricted in various ways. Please refer to the SSH
documentation for details.
</para>
<tip>
<para>
Several other applications exist that can provide secure tunnels using
a procedure similar in concept to the one just described.
</para>
</tip>
</sect1>
</chapter>