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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>33.18. 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 V1.79.1" /><link rel="prev" href="libpq-ldap.html" title="33.17. LDAP Lookup of Connection Parameters" /><link rel="next" href="libpq-threading.html" title="33.19. 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">33.18. SSL Support</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="libpq-ldap.html" title="33.17. LDAP Lookup of Connection Parameters">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="libpq.html" title="Chapter 33. libpq - C Library">Up</a></td><th width="60%" align="center">Chapter 33. <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 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="libpq-threading.html" title="33.19. 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">33.18. SSL Support</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="libpq-ssl.html#LIBQ-SSL-CERTIFICATES">33.18.1. Client Verification of Server Certificates</a></span></dt><dt><span class="sect2"><a href="libpq-ssl.html#LIBPQ-SSL-CLIENTCERT">33.18.2. Client Certificates</a></span></dt><dt><span class="sect2"><a href="libpq-ssl.html#LIBPQ-SSL-PROTECTION">33.18.3. Protection Provided in Different Modes</a></span></dt><dt><span class="sect2"><a href="libpq-ssl.html#LIBPQ-SSL-FILEUSAGE">33.18.4. SSL Client File Usage</a></span></dt><dt><span class="sect2"><a href="libpq-ssl.html#LIBPQ-SSL-INITIALIZE">33.18.5. 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">33.18.1. 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">33.18.2. 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">33.18.3. 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 SSL-secured, 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 33.1. SSL Mode Descriptions">Table 33.1</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 33.1. 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>Depends on CA 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">33.18.4. SSL Client File Usage</h3></div></div></div><p>
   <a class="xref" href="libpq-ssl.html#LIBPQ-SSL-FILE-USAGE" title="Table 33.2. Libpq/Client SSL File Usage">Table 33.2</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 33.2. 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">33.18.5. 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.
  </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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