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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>34.18. SSL Support</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="libpq-ldap.html" title="34.17. LDAP Lookup of Connection Parameters" /><link rel="next" href="libpq-threading.html" title="34.19. Behavior in Threaded Programs" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">34.18. SSL Support</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="libpq-ldap.html" title="34.17. LDAP Lookup of Connection Parameters">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="libpq.html" title="Chapter 34. libpq - C Library">Up</a></td><th width="60%" align="center">Chapter 34. <span xmlns="http://www.w3.org/1999/xhtml" class="application">libpq</span> - C Library</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="libpq-threading.html" title="34.19. Behavior in Threaded Programs">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="LIBPQ-SSL"><div class="titlepage"><div><div><h2 class="title" style="clear: both">34.18. SSL Support</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="libpq-ssl.html#LIBQ-SSL-CERTIFICATES">34.18.1. Client Verification of Server Certificates</a></span></dt><dt><span class="sect2"><a href="libpq-ssl.html#LIBPQ-SSL-CLIENTCERT">34.18.2. Client Certificates</a></span></dt><dt><span class="sect2"><a href="libpq-ssl.html#LIBPQ-SSL-PROTECTION">34.18.3. Protection Provided in Different Modes</a></span></dt><dt><span class="sect2"><a href="libpq-ssl.html#LIBPQ-SSL-FILEUSAGE">34.18.4. SSL Client File Usage</a></span></dt><dt><span class="sect2"><a href="libpq-ssl.html#LIBPQ-SSL-INITIALIZE">34.18.5. SSL Library Initialization</a></span></dt></dl></div><a id="id-1.7.3.25.2" class="indexterm"></a><p>
<span class="productname">PostgreSQL</span> has native support for using <acronym class="acronym">SSL</acronym>
connections to encrypt client/server communications for increased
security. See <a class="xref" href="ssl-tcp.html" title="18.9. Secure TCP/IP Connections with SSL">Section 18.9</a> for details about the server-side
<acronym class="acronym">SSL</acronym> functionality.
</p><p>
<span class="application">libpq</span> reads the system-wide
<span class="productname">OpenSSL</span> configuration file. By default, this
file is named <code class="filename">openssl.cnf</code> and is located in the
directory reported by <code class="literal">openssl version -d</code>. This default
can be overridden by setting environment variable
<code class="envar">OPENSSL_CONF</code> to the name of the desired configuration
file.
</p><div class="sect2" id="LIBQ-SSL-CERTIFICATES"><div class="titlepage"><div><div><h3 class="title">34.18.1. Client Verification of Server Certificates</h3></div></div></div><p>
By default, <span class="productname">PostgreSQL</span> will not perform any verification of
the server certificate. This means that it is possible to spoof the server
identity (for example by modifying a DNS record or by taking over the server
IP address) without the client knowing. In order to prevent spoofing,
the client must be able to verify the server's identity via a chain of
trust. A chain of trust is established by placing a root (self-signed)
certificate authority (<acronym class="acronym">CA</acronym>) certificate on one
computer and a leaf certificate <span class="emphasis"><em>signed</em></span> by the
root certificate on another computer. It is also possible to use an
<span class="quote">“<span class="quote">intermediate</span>”</span> certificate which is signed by the root
certificate and signs leaf certificates.
</p><p>
To allow the client to verify the identity of the server, place a root
certificate on the client and a leaf certificate signed by the root
certificate on the server. To allow the server to verify the identity
of the client, place a root certificate on the server and a leaf
certificate signed by the root certificate on the client. One or more
intermediate certificates (usually stored with the leaf certificate)
can also be used to link the leaf certificate to the root certificate.
</p><p>
Once a chain of trust has been established, there are two ways for
the client to validate the leaf certificate sent by the server.
If the parameter <code class="literal">sslmode</code> is set to <code class="literal">verify-ca</code>,
libpq will verify that the server is trustworthy by checking the
certificate chain up to the root certificate stored on the client.
If <code class="literal">sslmode</code> is set to <code class="literal">verify-full</code>,
libpq will <span class="emphasis"><em>also</em></span> verify that the server host
name matches the name stored in the server certificate. The
SSL connection will fail if the server certificate cannot be
verified. <code class="literal">verify-full</code> is recommended in most
security-sensitive environments.
</p><p>
In <code class="literal">verify-full</code> mode, the host name is matched against the
certificate's Subject Alternative Name attribute(s), or against the
Common Name attribute if no Subject Alternative Name of type <code class="literal">dNSName</code> is
present. If the certificate's name attribute starts with an asterisk
(<code class="literal">*</code>), the asterisk will be treated as
a wildcard, which will match all characters <span class="emphasis"><em>except</em></span> a dot
(<code class="literal">.</code>). This means the certificate will not match subdomains.
If the connection is made using an IP address instead of a host name, the
IP address will be matched (without doing any DNS lookups).
</p><p>
To allow server certificate verification, one or more root certificates
must be placed in the file <code class="filename">~/.postgresql/root.crt</code>
in the user's home directory. (On Microsoft Windows the file is named
<code class="filename">%APPDATA%\postgresql\root.crt</code>.) Intermediate
certificates should also be added to the file if they are needed to link
the certificate chain sent by the server to the root certificates
stored on the client.
</p><p>
Certificate Revocation List (CRL) entries are also checked
if the file <code class="filename">~/.postgresql/root.crl</code> exists
(<code class="filename">%APPDATA%\postgresql\root.crl</code> on Microsoft
Windows).
</p><p>
The location of the root certificate file and the CRL can be changed by
setting
the connection parameters <code class="literal">sslrootcert</code> and <code class="literal">sslcrl</code>
or the environment variables <code class="envar">PGSSLROOTCERT</code> and <code class="envar">PGSSLCRL</code>.
</p><div class="note"><h3 class="title">Note</h3><p>
For backwards compatibility with earlier versions of PostgreSQL, if a
root CA file exists, the behavior of
<code class="literal">sslmode</code>=<code class="literal">require</code> will be the same
as that of <code class="literal">verify-ca</code>, meaning the server certificate
is validated against the CA. Relying on this behavior is discouraged,
and applications that need certificate validation should always use
<code class="literal">verify-ca</code> or <code class="literal">verify-full</code>.
</p></div></div><div class="sect2" id="LIBPQ-SSL-CLIENTCERT"><div class="titlepage"><div><div><h3 class="title">34.18.2. Client Certificates</h3></div></div></div><p>
If the server attempts to verify the identity of the
client by requesting the client's leaf certificate,
<span class="application">libpq</span> will send the certificates stored in
file <code class="filename">~/.postgresql/postgresql.crt</code> in the user's home
directory. The certificates must chain to the root certificate trusted
by the server. A matching
private key file <code class="filename">~/.postgresql/postgresql.key</code> must also
be present. The private
key file must not allow any access to world or group; achieve this by the
command <code class="command">chmod 0600 ~/.postgresql/postgresql.key</code>.
On Microsoft Windows these files are named
<code class="filename">%APPDATA%\postgresql\postgresql.crt</code> and
<code class="filename">%APPDATA%\postgresql\postgresql.key</code>, and there
is no special permissions check since the directory is presumed secure.
The location of the certificate and key files can be overridden by the
connection parameters <code class="literal">sslcert</code> and <code class="literal">sslkey</code> or the
environment variables <code class="envar">PGSSLCERT</code> and <code class="envar">PGSSLKEY</code>.
</p><p>
The first certificate in <code class="filename">postgresql.crt</code> must be the
client's certificate because it must match the client's private key.
<span class="quote">“<span class="quote">Intermediate</span>”</span> certificates can be optionally appended
to the file — doing so avoids requiring storage of intermediate
certificates on the server (<a class="xref" href="runtime-config-connection.html#GUC-SSL-CA-FILE">ssl_ca_file</a>).
</p><p>
For instructions on creating certificates, see <a class="xref" href="ssl-tcp.html#SSL-CERTIFICATE-CREATION" title="18.9.5. Creating Certificates">Section 18.9.5</a>.
</p></div><div class="sect2" id="LIBPQ-SSL-PROTECTION"><div class="titlepage"><div><div><h3 class="title">34.18.3. Protection Provided in Different Modes</h3></div></div></div><p>
The different values for the <code class="literal">sslmode</code> parameter provide different
levels of protection. SSL can provide
protection against three types of attacks:
</p><div class="variablelist"><dl class="variablelist"><dt><span class="term">Eavesdropping</span></dt><dd><p>If a third party can examine the network traffic between the
client and the server, it can read both connection information (including
the user name and password) and the data that is passed. <acronym class="acronym">SSL</acronym>
uses encryption to prevent this.
</p></dd><dt><span class="term">Man in the middle (<acronym class="acronym">MITM</acronym>)</span></dt><dd><p>If a third party can modify the data while passing between the
client and server, it can pretend to be the server and therefore see and
modify data <span class="emphasis"><em>even if it is encrypted</em></span>. The third party can then
forward the connection information and data to the original server,
making it impossible to detect this attack. Common vectors to do this
include DNS poisoning and address hijacking, whereby the client is directed
to a different server than intended. There are also several other
attack methods that can accomplish this. <acronym class="acronym">SSL</acronym> uses certificate
verification to prevent this, by authenticating the server to the client.
</p></dd><dt><span class="term">Impersonation</span></dt><dd><p>If a third party can pretend to be an authorized client, it can
simply access data it should not have access to. Typically this can
happen through insecure password management. <acronym class="acronym">SSL</acronym> uses
client certificates to prevent this, by making sure that only holders
of valid certificates can access the server.
</p></dd></dl></div><p>
</p><p>
For a connection to be known secure, SSL usage must be configured
on <span class="emphasis"><em>both the client and the server</em></span> before the connection
is made. If it is only configured on the server, the client may end up
sending sensitive information (e.g. passwords) before
it knows that the server requires high security. In libpq, secure
connections can be ensured
by setting the <code class="literal">sslmode</code> parameter to <code class="literal">verify-full</code> or
<code class="literal">verify-ca</code>, and providing the system with a root certificate to
verify against. This is analogous to using an <code class="literal">https</code>
<acronym class="acronym">URL</acronym> for encrypted web browsing.
</p><p>
Once the server has been authenticated, the client can pass sensitive data.
This means that up until this point, the client does not need to know if
certificates will be used for authentication, making it safe to specify that
only in the server configuration.
</p><p>
All <acronym class="acronym">SSL</acronym> options carry overhead in the form of encryption and
key-exchange, so there is a trade-off that has to be made between performance
and security. <a class="xref" href="libpq-ssl.html#LIBPQ-SSL-SSLMODE-STATEMENTS" title="Table 34.1. SSL Mode Descriptions">Table 34.1</a>
illustrates the risks the different <code class="literal">sslmode</code> values
protect against, and what statement they make about security and overhead.
</p><div class="table" id="LIBPQ-SSL-SSLMODE-STATEMENTS"><p class="title"><strong>Table 34.1. SSL Mode Descriptions</strong></p><div class="table-contents"><table class="table" summary="SSL Mode Descriptions" border="1"><colgroup><col /><col /><col /><col /></colgroup><thead><tr><th><code class="literal">sslmode</code></th><th>Eavesdropping protection</th><th><acronym class="acronym">MITM</acronym> protection</th><th>Statement</th></tr></thead><tbody><tr><td><code class="literal">disable</code></td><td>No</td><td>No</td><td>I don't care about security, and I don't want to pay the overhead
of encryption.
</td></tr><tr><td><code class="literal">allow</code></td><td>Maybe</td><td>No</td><td>I don't care about security, but I will pay the overhead of
encryption if the server insists on it.
</td></tr><tr><td><code class="literal">prefer</code></td><td>Maybe</td><td>No</td><td>I don't care about encryption, but I wish to pay the overhead of
encryption if the server supports it.
</td></tr><tr><td><code class="literal">require</code></td><td>Yes</td><td>No</td><td>I want my data to be encrypted, and I accept the overhead. I trust
that the network will make sure I always connect to the server I want.
</td></tr><tr><td><code class="literal">verify-ca</code></td><td>Yes</td><td><code class="literal">Depends on CA</code>-policy</td><td>I want my data encrypted, and I accept the overhead. I want to be
sure that I connect to a server that I trust.
</td></tr><tr><td><code class="literal">verify-full</code></td><td>Yes</td><td>Yes</td><td>I want my data encrypted, and I accept the overhead. I want to be
sure that I connect to a server I trust, and that it's the one I
specify.
</td></tr></tbody></table></div></div><br class="table-break" /><p>
The difference between <code class="literal">verify-ca</code> and <code class="literal">verify-full</code>
depends on the policy of the root <acronym class="acronym">CA</acronym>. If a public
<acronym class="acronym">CA</acronym> is used, <code class="literal">verify-ca</code> allows connections to a server
that <span class="emphasis"><em>somebody else</em></span> may have registered with the <acronym class="acronym">CA</acronym>.
In this case, <code class="literal">verify-full</code> should always be used. If
a local <acronym class="acronym">CA</acronym> is used, or even a self-signed certificate, using
<code class="literal">verify-ca</code> often provides enough protection.
</p><p>
The default value for <code class="literal">sslmode</code> is <code class="literal">prefer</code>. As is shown
in the table, this makes no sense from a security point of view, and it only
promises performance overhead if possible. It is only provided as the default
for backward compatibility, and is not recommended in secure deployments.
</p></div><div class="sect2" id="LIBPQ-SSL-FILEUSAGE"><div class="titlepage"><div><div><h3 class="title">34.18.4. SSL Client File Usage</h3></div></div></div><p>
<a class="xref" href="libpq-ssl.html#LIBPQ-SSL-FILE-USAGE" title="Table 34.2. Libpq/Client SSL File Usage">Table 34.2</a> summarizes the files that are
relevant to the SSL setup on the client.
</p><div class="table" id="LIBPQ-SSL-FILE-USAGE"><p class="title"><strong>Table 34.2. Libpq/Client SSL File Usage</strong></p><div class="table-contents"><table class="table" summary="Libpq/Client SSL File Usage" border="1"><colgroup><col /><col /><col /></colgroup><thead><tr><th>File</th><th>Contents</th><th>Effect</th></tr></thead><tbody><tr><td><code class="filename">~/.postgresql/postgresql.crt</code></td><td>client certificate</td><td>requested by server</td></tr><tr><td><code class="filename">~/.postgresql/postgresql.key</code></td><td>client private key</td><td>proves client certificate sent by owner; does not indicate
certificate owner is trustworthy</td></tr><tr><td><code class="filename">~/.postgresql/root.crt</code></td><td>trusted certificate authorities</td><td>checks that server certificate is signed by a trusted certificate
authority</td></tr><tr><td><code class="filename">~/.postgresql/root.crl</code></td><td>certificates revoked by certificate authorities</td><td>server certificate must not be on this list</td></tr></tbody></table></div></div><br class="table-break" /></div><div class="sect2" id="LIBPQ-SSL-INITIALIZE"><div class="titlepage"><div><div><h3 class="title">34.18.5. SSL Library Initialization</h3></div></div></div><p>
If your application initializes <code class="literal">libssl</code> and/or
<code class="literal">libcrypto</code> libraries and <span class="application">libpq</span>
is built with <acronym class="acronym">SSL</acronym> support, you should call
<code class="function">PQinitOpenSSL</code> to tell <span class="application">libpq</span>
that the <code class="literal">libssl</code> and/or <code class="literal">libcrypto</code> libraries
have been initialized by your application, so that
<span class="application">libpq</span> will not also initialize those libraries.
See <a class="ulink" href="http://h41379.www4.hpe.com/doc/83final/ba554_90007/ch04.html" target="_top">http://h41379.www4.hpe.com/doc/83final/ba554_90007/ch04.html</a>
for details on the SSL API.
</p><p>
</p><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PQINITOPENSSL"><span class="term">
<code class="function">PQinitOpenSSL</code>
<a id="id-1.7.3.25.9.3.1.1.1.2" class="indexterm"></a>
</span></dt><dd><p>
Allows applications to select which security libraries to initialize.
</p><pre class="synopsis">
void PQinitOpenSSL(int do_ssl, int do_crypto);
</pre><p>
</p><p>
When <em class="parameter"><code>do_ssl</code></em> is non-zero, <span class="application">libpq</span>
will initialize the <span class="application">OpenSSL</span> library before first
opening a database connection. When <em class="parameter"><code>do_crypto</code></em> is
non-zero, the <code class="literal">libcrypto</code> library will be initialized. By
default (if <code class="function">PQinitOpenSSL</code> is not called), both libraries
are initialized. When SSL support is not compiled in, this function is
present but does nothing.
</p><p>
If your application uses and initializes either <span class="application">OpenSSL</span>
or its underlying <code class="literal">libcrypto</code> library, you <span class="emphasis"><em>must</em></span>
call this function with zeroes for the appropriate parameter(s)
before first opening a database connection. Also be sure that you
have done that initialization before opening a database connection.
</p></dd><dt id="LIBPQ-PQINITSSL"><span class="term">
<code class="function">PQinitSSL</code>
<a id="id-1.7.3.25.9.3.1.2.1.2" class="indexterm"></a>
</span></dt><dd><p>
Allows applications to select which security libraries to initialize.
</p><pre class="synopsis">
void PQinitSSL(int do_ssl);
</pre><p>
</p><p>
This function is equivalent to
<code class="literal">PQinitOpenSSL(do_ssl, do_ssl)</code>.
It is sufficient for applications that initialize both or neither
of <span class="application">OpenSSL</span> and <code class="literal">libcrypto</code>.
</p><p>
<code class="function">PQinitSSL</code> has been present since
<span class="productname">PostgreSQL</span> 8.0, while <code class="function">PQinitOpenSSL</code>
was added in <span class="productname">PostgreSQL</span> 8.4, so <code class="function">PQinitSSL</code>
might be preferable for applications that need to work with older
versions of <span class="application">libpq</span>.
</p></dd></dl></div><p>
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