34.18. SSL Support
PostgreSQL has native support for using SSL connections to encrypt client/server communications for increased security. See Section 18.9 for details about the server-side SSL functionality.
libpq reads the system-wide
OpenSSL configuration file. By default, this
file is named
openssl.cnf and is located in the
directory reported by
openssl version -d. This default
can be overridden by setting environment variable
OPENSSL_CONF to the name of the desired configuration
34.18.1. Client Verification of Server Certificates
By default, PostgreSQL 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 (CA) certificate on one computer and a leaf certificate signed by the root certificate on another computer. It is also possible to use an “intermediate” certificate which is signed by the root certificate and signs leaf certificates.
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.
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
sslmode is set to
libpq will verify that the server is trustworthy by checking the
certificate chain up to the root certificate stored on the client.
sslmode is set to
libpq will also 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
verify-full is recommended in most
verify-full 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
present. If the certificate's name attribute starts with an asterisk
*), the asterisk will be treated as
a wildcard, which will match all characters except a dot
.). 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).
To allow server certificate verification, one or more root certificates
must be placed in the file
in the user's home directory. (On Microsoft Windows the file is named
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.
Certificate Revocation List (CRL) entries are also checked
if the file
%APPDATA%\postgresql\root.crl on Microsoft
The location of the root certificate file and the CRL can be changed by
the connection parameters
or the environment variables
For backwards compatibility with earlier versions of PostgreSQL, if a
root CA file exists, the behavior of
require will be the same
as that of
verify-ca, meaning the server certificate
is validated against the CA. Relying on this behavior is discouraged,
and applications that need certificate validation should always use
34.18.2. Client Certificates
If the server attempts to verify the identity of the
client by requesting the client's leaf certificate,
libpq will send the certificates stored in
~/.postgresql/postgresql.crt in the user's home
directory. The certificates must chain to the root certificate trusted
by the server. A matching
private key file
~/.postgresql/postgresql.key must also
be present. The private
key file must not allow any access to world or group; achieve this by the
chmod 0600 ~/.postgresql/postgresql.key.
On Microsoft Windows these files are named
%APPDATA%\postgresql\postgresql.key, 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
sslkey or the
The first certificate in
postgresql.crt must be the
client's certificate because it must match the client's private key.
“Intermediate” certificates can be optionally appended
to the file — doing so avoids requiring storage of intermediate
certificates on the server (ssl_ca_file).
For instructions on creating certificates, see Section 18.9.5.
34.18.3. Protection Provided in Different Modes
The different values for the
sslmode parameter provide different
levels of protection. SSL can provide
protection against three types of attacks:
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. SSL uses encryption to prevent this.
- Man in the middle (MITM)
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 even if it is encrypted. 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. SSL uses certificate verification to prevent this, by authenticating the server to the client.
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. SSL uses client certificates to prevent this, by making sure that only holders of valid certificates can access the server.
For a connection to be known secure, SSL usage must be configured
on both the client and the server 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
sslmode parameter to
verify-ca, and providing the system with a root certificate to
verify against. This is analogous to using an
URL for encrypted web browsing.
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.
All SSL 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. Table 34.1
illustrates the risks the different
protect against, and what statement they make about security and overhead.
Table 34.1. SSL Mode Descriptions
|Eavesdropping protection||MITM protection||Statement|
|No||No||I don't care about security, and I don't want to pay the overhead of encryption.|
|Maybe||No||I don't care about security, but I will pay the overhead of encryption if the server insists on it.|
|Maybe||No||I don't care about encryption, but I wish to pay the overhead of encryption if the server supports it.|
|Yes||No||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.|
|Yes||Depends on CA policy||I want my data encrypted, and I accept the overhead. I want to be sure that I connect to a server that I trust.|
|Yes||Yes||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.|
The difference between
depends on the policy of the root CA. If a public
CA is used,
verify-ca allows connections to a server
that somebody else may have registered with the CA.
In this case,
verify-full should always be used. If
a local CA is used, or even a self-signed certificate, using
verify-ca often provides enough protection.
The default value for
prefer. 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.
34.18.4. SSL Client File Usage
Table 34.2 summarizes the files that are relevant to the SSL setup on the client.
Table 34.2. Libpq/Client SSL File Usage
|client certificate||requested by server|
|client private key||proves client certificate sent by owner; does not indicate certificate owner is trustworthy|
|trusted certificate authorities||checks that server certificate is signed by a trusted certificate authority|
|certificates revoked by certificate authorities||server certificate must not be on this list|
34.18.5. SSL Library Initialization
If your application initializes
libcrypto libraries and libpq
is built with SSL support, you should call
PQinitOpenSSL to tell libpq
have been initialized by your application, so that
libpq will not also initialize those libraries.
Allows applications to select which security libraries to initialize.
void PQinitOpenSSL(int do_ssl, int do_crypto);
do_sslis non-zero, libpq will initialize the OpenSSL library before first opening a database connection. When
do_cryptois non-zero, the
libcryptolibrary will be initialized. By default (if
PQinitOpenSSLis not called), both libraries are initialized. When SSL support is not compiled in, this function is present but does nothing.
If your application uses and initializes either OpenSSL or its underlying
libcryptolibrary, you must 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.
Allows applications to select which security libraries to initialize.
void PQinitSSL(int do_ssl);
This function is equivalent to
PQinitOpenSSL(do_ssl, do_ssl). It is sufficient for applications that initialize both or neither of OpenSSL and
PQinitSSLhas been present since PostgreSQL 8.0, while
PQinitOpenSSLwas added in PostgreSQL 8.4, so
PQinitSSLmight be preferable for applications that need to work with older versions of libpq.