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31.18. SSL Support
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31.18. SSL Support

Postgres Pro has native support for usingSSL connections to encrypt client/server communications for increased security. SeeSection 17.9 for details about the server-sideSSL functionality.

libpq reads the system-wideOpenSSL configuration file. By default, this file is namedopenssl.cnf and is located in the directory reported byopenssl version -d. This default can be overridden by setting environment variableOPENSSL_CONF to the name of the desired configuration file.

By default,Postgres Pro 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 certificatesigned 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 parametersslmode is set toverify-ca, libpq will verify that the server is trustworthy by checking the certificate chain up to the root certificate stored on the client. Ifsslmode is set toverify-full, libpq willalso 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.verify-full is recommended in most security-sensitive environments.

Inverify-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 dNSName is present. If the certificate's name attribute starts with an asterisk (*), the asterisk will be treated as a wildcard, which will match all charactersexcept 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~/.postgresql/root.crt in the user's home directory. (On Microsoft Windows the file is named%APPDATA%\postgresql\root.crt.) 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.

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 file and the CRL can be changed by setting the connection parameterssslrootcert andsslcrl or the environment variablesPGSSLROOTCERT andPGSSLCRL.

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 file~/.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 commandchmod 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 parameterssslcert andsslkey or the environment variablesPGSSLCERT andPGSSLKEY.

The first certificate inpostgresql.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'sroot.crt file.

For instructions on creating certificates, seeSection 17.9.3.

31.18.3. Protection Provided in Different Modes

The different values for thesslmode parameter provide different levels of protection. SSL can provide protection against three types of attacks:

Eavesdropping

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 dataeven 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, SSL usage must be configured onboth 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 thesslmode parameter toverify-full orverify-ca, and providing the system with a root certificate to verify against. This is analogous to using anhttpsURL 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.

AllSSL 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.Table 31.1 illustrates the risks the differentsslmode values protect against, and what statement they make about security and overhead.

Table 31.1. SSL Mode Descriptions

sslmodeEavesdropping protectionMITM protectionStatement
disableNoNoI don't care about security, and I don't want to pay the overhead of encryption.
allowMaybeNoI don't care about security, but I will pay the overhead of encryption if the server insists on it.
preferMaybeNoI don't care about encryption, but I wish to pay the overhead of encryption if the server supports it.
requireYesNoI 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-caYesDepends on CA-policyI want my data encrypted, and I accept the overhead. I want to be sure that I connect to a server that I trust.
verify-fullYesYesI 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 betweenverify-ca andverify-full depends on the policy of the rootCA. If a publicCA is used,verify-ca allows connections to a server thatsomebody else may have registered with theCA. In this case,verify-full should always be used. If a localCA is used, or even a self-signed certificate, usingverify-ca often provides enough protection.

The default value forsslmode isprefer. 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.

Table 31.2 summarizes the files that are relevant to the SSL setup on the client.

Table 31.2. Libpq/Client SSL File Usage

FileContentsEffect
~/.postgresql/postgresql.crtclient certificaterequested by server
~/.postgresql/postgresql.keyclient private keyproves client certificate sent by owner; does not indicate certificate owner is trustworthy
~/.postgresql/root.crttrusted certificate authoritieschecks that server certificate is signed by a trusted certificate authority
~/.postgresql/root.crlcertificates revoked by certificate authoritiesserver certificate must not be on this list

31.18.5. SSL Library Initialization

If your application initializeslibssl and/orlibcrypto libraries andlibpq is built withSSL support, you should callPQinitOpenSSL to telllibpq that thelibssl and/orlibcrypto libraries have been initialized by your application, so thatlibpq will not also initialize those libraries. Seehttp://h41379.www4.hpe.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, int do_crypto);

Whendo_ssl is non-zero,libpq will initialize theOpenSSL library before first opening a database connection. Whendo_crypto is non-zero, thelibcrypto library will be initialized. By default (ifPQinitOpenSSL 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 eitherOpenSSL or its underlyinglibcrypto library, youmust 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 toPQinitOpenSSL(do_ssl, do_ssl). It is sufficient for applications that initialize both or neither ofOpenSSL andlibcrypto.

PQinitSSL has been present sincePostgreSQL 8.0, whilePQinitOpenSSL was added inPostgreSQL 8.4, soPQinitSSL might be preferable for applications that need to work with older versions oflibpq.

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