PostgreSQL 8.4.1 Documentation
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# 30.17. SSL Support

PostgreSQL has native support for using SSL connections to encrypt client/server communications for increased security. See Section 17.8 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 file.

## 30.17.1. Certificate verification

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 this, SSL certificate verification must be used.

If the parameter `sslmode` is set to `verify-ca` libpq will verify that the server is trustworthy by checking the certificate chain up to a trusted CA. If `sslmode` is set to `verify-full`, libpq will also verify that the server hostname matches that of the certificate. The SSL connection will fail if the server certificate cannot be verified. `verify-full` is recommended in most security sensitive environments.

In `verify-full` mode, the `cn` attribute of the certificate is matched against the hostname. If the `cn` attribute starts with an asterisk (`*`), it will be treated as a wildcard, and 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 hostname, the IP address will be matched (without doing any DNS lookups).

To allow verification, the certificate of a trusted CA must be placed in the file `~/.postgresql/root.crt` in the user's home directory. (On Microsoft Windows the file is named `%APPDATA%\postgresql\root.crt`.)

Certificate Revocation List (CRL) entries are also checked if the file `~/.postgresql/root.crl` exists (`%APPDATA%\postgresql\root.crl` on Microsoft Windows).

The location of the root certificate store and the CRL can be overridden by the connection parameters `sslrootcert` and `sslcrl` or the environment variables `PGSSLROOTCERT` and `PGSSLCRL`.

## 30.17.2. Client certificates

If the server requests a trusted client certificate, libpq will send the certificate stored in file `~/.postgresql/postgresql.crt` in the user's home directory. The certificate must be signed by one of the certificate authorities (CA) 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 command `chmod 0600 ~/.postgresql/postgresql.key`. On Microsoft Windows these files are named `%APPDATA%\postgresql\postgresql.crt` and `%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 connection parameters `sslcert` and `sslkey` or the environment variables `PGSSLCERT` and `PGSSLKEY`.

## 30.17.3. Protection provided in different modes

The different values for the `sslmode` parameter provide different levels of protection, in different environments. SSL itself provides protection against three different types of attacks:

Table 30-2. SSL attacks

Type | Description
--- | ---
Eavesdropping | If a third party can listen to the network traffic between the client and the server, it can read both connection information (including the username 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.
Impersonation | 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, the two first of these have to be set up 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, this is controlled by setting the `sslmode` parameter to `verify-full` or `verify-ca`, and providing the system with a root certificate to verify against. This is analogous to using a `https` 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 this only in the server configuration.

All SSL options carry overhead in the form of encryption and key-exchange, and it is a tradeoff that has to be made between performance and security. The following table illustrates the risks the different `sslmode` values protect against, and what statement they make about security and overhead:

Table 30-3. SSL mode descriptions

`sslmode` | Eavesdropping protection | MITM protection | Statement
--- | --- | --- | ---
`disabled` | No | No | I don't care about security, and I don't want to pay the overhead of encryption.
`allow` | Maybe | No | I don't care about security, but I will pay the overhead of encryption if the server insists on it.
`prefer` | Maybe | No | I don't care about encryption, but I wish to pay the overhead of encryption if the server supports it.
`require` | 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.
`verify-ca` | 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.
`verify-full` | 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 `verify-ca` and `verify-full` 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 to succeed. 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 `sslmode` is `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 backwards compatiblity, and not recommended in secure deployments.

## 30.17.4. SSL File Usage

Table 30-4. Libpq/Client SSL File Usage

File | Contents | Effect
--- | --- | ---
`~/.postgresql/postgresql.crt` | client certificate | requested by server
`~/.postgresql/postgresql.key` | client private key | proves client certificate sent by owner; does not indicate certificate owner is trustworthy
`~/.postgresql/root.crt` | trusted certificate authorities | checks server certificate is signed by a trusted certificate authority
`~/.postgresql/root.crl` | certificates revoked by certificate authorities | server certificate must not be on this list

## 30.17.5. SSL library initialization

If your application initializes `libssl` and/or `libcrypto` libraries and libpq is built with SSL support, you should call `PQinitOpenSSL` to tell libpq that the `libssl` and/or `libcrypto` libraries have been initialized by your application, so that libpq will not also initialize those libraries. See http://h71000.www7.hp.com/doc/83final/BA554_90007/ch04.html for details on the SSL API.

`PQinitOpenSSL`

  Allows applications to select which security libraries to initialize.

              void PQinitOpenSSL(int do_ssl, init do_crypto);

  When `do_ssl` is non-zero, libpq will initialize the OpenSSL library before first opening a database connection. When `do_crypto` is non-zero, the `libcrypto` library will be initialized. By default (if `PQinitOpenSSL` is 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 `libcrypto` library, 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.

`PQinitSSL`

  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 `libcrypto`.

  `PQinitSSL` has been present since PostgreSQL 8.0, while `PQinitOpenSSL` was added in PostgreSQL 8.4, so `PQinitSSL` might be preferable for applications that need to work with older versions of libpq.

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