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>Chapter 31. <SPAN
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>libpq</SPAN
> - C Library</TD
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="LIBPQ-SSL"
>31.18. SSL Support</A
></H1
><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
HREF="ssl-tcp.html"
>Section 17.9</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 <TT
CLASS="FILENAME"
>openssl.cnf</TT
> and is located in the
directory reported by <TT
CLASS="LITERAL"
>openssl version -d</TT
>. This default
can be overridden by setting environment variable
<TT
CLASS="ENVAR"
>OPENSSL_CONF</TT
> to the name of the desired configuration
file.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="LIBQ-SSL-CERTIFICATES"
>31.18.1. Client Verification of Server Certificates</A
></H2
><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,
<ACRONYM
CLASS="ACRONYM"
>SSL</ACRONYM
> certificate verification must be used.
</P
><P
> If the parameter <TT
CLASS="LITERAL"
>sslmode</TT
> is set to <TT
CLASS="LITERAL"
>verify-ca</TT
>,
libpq will verify that the server is trustworthy by checking the
certificate chain up to a trusted certificate authority
(<ACRONYM
CLASS="ACRONYM"
>CA</ACRONYM
>). If <TT
CLASS="LITERAL"
>sslmode</TT
> is set to <TT
CLASS="LITERAL"
>verify-full</TT
>,
libpq will <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>also</I
></SPAN
> verify that the server host name matches its
certificate. The SSL connection will fail if the server certificate cannot
be verified. <TT
CLASS="LITERAL"
>verify-full</TT
> is recommended in most
security-sensitive environments.
</P
><P
> In <TT
CLASS="LITERAL"
>verify-full</TT
> mode, the <TT
CLASS="LITERAL"
>cn</TT
> (Common Name) attribute
of the certificate is matched against the host name. If the <TT
CLASS="LITERAL"
>cn</TT
>
attribute starts with an asterisk (<TT
CLASS="LITERAL"
>*</TT
>), it will be treated as
a wildcard, and will match all characters <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>except</I
></SPAN
> a dot
(<TT
CLASS="LITERAL"
>.</TT
>). 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, the certificate(s) of one or more
trusted <ACRONYM
CLASS="ACRONYM"
>CA</ACRONYM
>s must be
placed in the file <TT
CLASS="FILENAME"
>~/.postgresql/root.crt</TT
> in the user's home
directory. (On Microsoft Windows the file is named
<TT
CLASS="FILENAME"
>%APPDATA%\postgresql\root.crt</TT
>.)
</P
><P
> Certificate Revocation List (CRL) entries are also checked
if the file <TT
CLASS="FILENAME"
>~/.postgresql/root.crl</TT
> exists
(<TT
CLASS="FILENAME"
>%APPDATA%\postgresql\root.crl</TT
> on Microsoft
Windows).
</P
><P
> The location of the root certificate file and the CRL can be changed by
setting
the connection parameters <TT
CLASS="LITERAL"
>sslrootcert</TT
> and <TT
CLASS="LITERAL"
>sslcrl</TT
>
or the environment variables <TT
CLASS="ENVAR"
>PGSSLROOTCERT</TT
> and <TT
CLASS="ENVAR"
>PGSSLCRL</TT
>.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> For backwards compatibility with earlier versions of PostgreSQL, if a
root CA file exists, the behavior of
<TT
CLASS="LITERAL"
>sslmode</TT
>=<TT
CLASS="LITERAL"
>require</TT
> will be the same
as that of <TT
CLASS="LITERAL"
>verify-ca</TT
>, meaning the server certificate
is validated against the CA. Relying on this behavior is discouraged,
and applications that need certificate validation should always use
<TT
CLASS="LITERAL"
>verify-ca</TT
> or <TT
CLASS="LITERAL"
>verify-full</TT
>.
</P
></BLOCKQUOTE
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="LIBPQ-SSL-CLIENTCERT"
>31.18.2. Client Certificates</A
></H2
><P
> If the server requests a trusted client certificate,
<SPAN
CLASS="APPLICATION"
>libpq</SPAN
> will send the certificate stored in
file <TT
CLASS="FILENAME"
>~/.postgresql/postgresql.crt</TT
> in the user's home
directory. The certificate must be signed by one of the certificate
authorities (<ACRONYM
CLASS="ACRONYM"
>CA</ACRONYM
>) trusted by the server. A matching
private key file <TT
CLASS="FILENAME"
>~/.postgresql/postgresql.key</TT
> must also
be present. The private
key file must not allow any access to world or group; achieve this by the
command <TT
CLASS="COMMAND"
>chmod 0600 ~/.postgresql/postgresql.key</TT
>.
On Microsoft Windows these files are named
<TT
CLASS="FILENAME"
>%APPDATA%\postgresql\postgresql.crt</TT
> and
<TT
CLASS="FILENAME"
>%APPDATA%\postgresql\postgresql.key</TT
>, 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 <TT
CLASS="LITERAL"
>sslcert</TT
> and <TT
CLASS="LITERAL"
>sslkey</TT
> or the
environment variables <TT
CLASS="ENVAR"
>PGSSLCERT</TT
> and <TT
CLASS="ENVAR"
>PGSSLKEY</TT
>.
</P
><P
> In some cases, the client certificate might be signed by an
<SPAN
CLASS="QUOTE"
>"intermediate"</SPAN
> certificate authority, rather than one that is
directly trusted by the server. To use such a certificate, append the
certificate of the signing authority to the <TT
CLASS="FILENAME"
>postgresql.crt</TT
>
file, then its parent authority's certificate, and so on up to a
<SPAN
CLASS="QUOTE"
>"root"</SPAN
> authority that is trusted by the server. The root
certificate should be included in every case where
<TT
CLASS="FILENAME"
>postgresql.crt</TT
> contains more than one certificate.
</P
><P
> Note that <TT
CLASS="FILENAME"
>root.crt</TT
> lists the top-level CAs that are
considered trusted for signing server certificates. In principle it need
not list the CA that signed the client's certificate, though in most cases
that CA would also be trusted for server certificates.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="LIBPQ-SSL-PROTECTION"
>31.18.3. Protection Provided in Different Modes</A
></H2
><P
> The different values for the <TT
CLASS="LITERAL"
>sslmode</TT
> parameter provide different
levels of protection. SSL can provide
protection against three types of attacks:
<P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
>Eavesdropping</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
>Man in the middle (<ACRONYM
CLASS="ACRONYM"
>MITM</ACRONYM
>)</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"
><I
CLASS="EMPHASIS"
>even if it is encrypted</I
></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
>Impersonation</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"
><I
CLASS="EMPHASIS"
>both the client and the server</I
></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 <TT
CLASS="LITERAL"
>sslmode</TT
> parameter to <TT
CLASS="LITERAL"
>verify-full</TT
> or
<TT
CLASS="LITERAL"
>verify-ca</TT
>, and providing the system with a root certificate to
verify against. This is analogous to using an <TT
CLASS="LITERAL"
>https</TT
>
<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 tradeoff that has to be made between performance
and security. <A
HREF="libpq-ssl.html#LIBPQ-SSL-SSLMODE-STATEMENTS"
>Table 31-1</A
>
illustrates the risks the different <TT
CLASS="LITERAL"
>sslmode</TT
> values
protect against, and what statement they make about security and overhead.
</P
><DIV
CLASS="TABLE"
><A
NAME="LIBPQ-SSL-SSLMODE-STATEMENTS"
></A
><P
><B
>Table 31-1. SSL Mode Descriptions</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><COL><THEAD
><TR
><TH
><TT
CLASS="LITERAL"
>sslmode</TT
></TH
><TH
>Eavesdropping protection</TH
><TH
><ACRONYM
CLASS="ACRONYM"
>MITM</ACRONYM
> protection</TH
><TH
>Statement</TH
></TR
></THEAD
><TBODY
><TR
><TD
><TT
CLASS="LITERAL"
>disable</TT
></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
><TT
CLASS="LITERAL"
>allow</TT
></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
><TT
CLASS="LITERAL"
>prefer</TT
></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
><TT
CLASS="LITERAL"
>require</TT
></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
><TT
CLASS="LITERAL"
>verify-ca</TT
></TD
><TD
>Yes</TD
><TD
><TT
CLASS="LITERAL"
>Depends on CA</TT
>-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
><TT
CLASS="LITERAL"
>verify-full</TT
></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
><P
> The difference between <TT
CLASS="LITERAL"
>verify-ca</TT
> and <TT
CLASS="LITERAL"
>verify-full</TT
>
depends on the policy of the root <ACRONYM
CLASS="ACRONYM"
>CA</ACRONYM
>. If a public
<ACRONYM
CLASS="ACRONYM"
>CA</ACRONYM
> is used, <TT
CLASS="LITERAL"
>verify-ca</TT
> allows connections to a server
that <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>somebody else</I
></SPAN
> may have registered with the <ACRONYM
CLASS="ACRONYM"
>CA</ACRONYM
>.
In this case, <TT
CLASS="LITERAL"
>verify-full</TT
> should always be used. If
a local <ACRONYM
CLASS="ACRONYM"
>CA</ACRONYM
> is used, or even a self-signed certificate, using
<TT
CLASS="LITERAL"
>verify-ca</TT
> often provides enough protection.
</P
><P
> The default value for <TT
CLASS="LITERAL"
>sslmode</TT
> is <TT
CLASS="LITERAL"
>prefer</TT
>. 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"
><H2
CLASS="SECT2"
><A
NAME="LIBPQ-SSL-FILEUSAGE"
>31.18.4. SSL Client File Usage</A
></H2
><P
> <A
HREF="libpq-ssl.html#LIBPQ-SSL-FILE-USAGE"
>Table 31-2</A
> summarizes the files that are
relevant to the SSL setup on the client.
</P
><DIV
CLASS="TABLE"
><A
NAME="LIBPQ-SSL-FILE-USAGE"
></A
><P
><B
>Table 31-2. Libpq/Client SSL File Usage</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><THEAD
><TR
><TH
>File</TH
><TH
>Contents</TH
><TH
>Effect</TH
></TR
></THEAD
><TBODY
><TR
><TD
><TT
CLASS="FILENAME"
>~/.postgresql/postgresql.crt</TT
></TD
><TD
>client certificate</TD
><TD
>requested by server</TD
></TR
><TR
><TD
><TT
CLASS="FILENAME"
>~/.postgresql/postgresql.key</TT
></TD
><TD
>client private key</TD
><TD
>proves client certificate sent by owner; does not indicate
certificate owner is trustworthy</TD
></TR
><TR
><TD
><TT
CLASS="FILENAME"
>~/.postgresql/root.crt</TT
></TD
><TD
>trusted certificate authorities</TD
><TD
>checks that server certificate is signed by a trusted certificate
authority</TD
></TR
><TR
><TD
><TT
CLASS="FILENAME"
>~/.postgresql/root.crl</TT
></TD
><TD
>certificates revoked by certificate authorities</TD
><TD
>server certificate must not be on this list</TD
></TR
></TBODY
></TABLE
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="LIBPQ-SSL-INITIALIZE"
>31.18.5. SSL Library Initialization</A
></H2
><P
> If your application initializes <TT
CLASS="LITERAL"
>libssl</TT
> and/or
<TT
CLASS="LITERAL"
>libcrypto</TT
> 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 <TT
CLASS="LITERAL"
>libssl</TT
> and/or <TT
CLASS="LITERAL"
>libcrypto</TT
> libraries
have been initialized by your application, so that
<SPAN
CLASS="APPLICATION"
>libpq</SPAN
> will not also initialize those libraries.
See <A
HREF="http://h71000.www7.hp.com/doc/83final/ba554_90007/ch04.html"
TARGET="_top"
>http://h71000.www7.hp.com/doc/83final/ba554_90007/ch04.html</A
>
for details on the SSL API.
</P
><P
> <P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="LIBPQ-PQINITOPENSSL"
></A
><CODE
CLASS="FUNCTION"
>PQinitOpenSSL</CODE
>
</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 <TT
CLASS="PARAMETER"
>do_ssl</TT
> is non-zero, <SPAN
CLASS="APPLICATION"
>libpq</SPAN
>
will initialize the <SPAN
CLASS="APPLICATION"
>OpenSSL</SPAN
> library before first
opening a database connection. When <TT
CLASS="PARAMETER"
>do_crypto</TT
> is
non-zero, the <TT
CLASS="LITERAL"
>libcrypto</TT
> 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 <TT
CLASS="LITERAL"
>libcrypto</TT
> library, you <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>must</I
></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
><A
NAME="LIBPQ-PQINITSSL"
></A
><CODE
CLASS="FUNCTION"
>PQinitSSL</CODE
>
</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
<TT
CLASS="LITERAL"
>PQinitOpenSSL(do_ssl, do_ssl)</TT
>.
It is sufficient for applications that initialize both or neither
of <SPAN
CLASS="APPLICATION"
>OpenSSL</SPAN
> and <TT
CLASS="LITERAL"
>libcrypto</TT
>.
</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
>.
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