Published: 03-02-2014 | Author: Remy van Elst | Text only version of this article

When connecting to a server, clients should verify the validity of the server certificate using either a Certificate Revocation List (CRL), or an Online Certificate Status Protocol (OCSP) record. The problem with CRL is that the lists have grown huge and takes forever to download.

OCSP is much more lightweight, as only one record is retrieved at a time. But the side effect is that OCSP requests must be made to a 3rd party OCSP responder when connecting to a server, which adds latency and potential failures. In fact, the OCSP responders operated by CAs are often so unreliable that browser will fail silently if no response is received in a timely manner. This reduces security, by allowing an attacker to DoS an OCSP responder to disable the validation.

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The solution is to allow the server to send its cached OCSP record during the TLS handshake, therefore bypassing the OCSP responder. This mechanism saves a roundtrip between the client and the OCSP responder, and is called OCSP Stapling.

The server will send a cached OCSP response only if the client requests it, by announcing support for the status_request TLS extension in its CLIENT HELLO.

Most servers will cache OCSP response for up to 48 hours. At regular intervals, the server will connect to the OCSP responder of the CA to retrieve a fresh OCSP record. The location of the OCSP responder is taken from the Authority Information Access field of the signed certificate.

This tutorial is also available for nginx

What is OCSP Stapling

OCSP stapling is defined in the IETF RFC 6066. The term "stapling" is a popular term used to describe how the OCSP response is obtained by the web server. The web server caches the response from the CA that issued the certificate. When an SSL/TLS handshake is initiated, the response is returned by the web server to the client by attaching the cached OCSP response to the CertificateStatus message. To make use of OCSP stapling, a client must include the "status_request" extension with its SSL/TSL Client "Hello" message.

OCSP stapling presents several advantages including the following:

The relying party receives the status of the web servers certificate when it is needed (during the SSL/TLS handshake).

No additional HTTP connection needs to be set up with the issuing CA.

OCSP stapling provides added security by reducing the number of attack vectors.

Read one of the following links for more information on OCSP and OCSP stapling.

Requirements

You need at least Apache 2.3.3 and later plus OpenSSL 0.9.8h or later for this to work. This is not available in the current Ubuntu LTS releases (12.04), it has 2.2.22 and CentOS 6 has 2.2.15. Either search for PPA's/unofficial repositories or compile them yourself.

You also need create a firewall exception to allow your server to make outbound connections to the upstream OCSP's. You can view all OCSP URI's from a website using this one liner:

OLDIFS=$IFS; IFS=':' certificates=$(openssl s_client -connect google.com:443 -showcerts -tlsextdebug -tls1 2>&1 </dev/null | sed -n '/-----BEGIN/,/-----END/ {/-----BEGIN/ s/^/:/; p}'); for certificate in ${certificates#:}; do echo $certificate | openssl x509 -noout -ocsp_uri; done; IFS=$OLDIFS

It results for google.com in:

http://clients1.google.com/ocsp http://gtglobal-ocsp.geotrust.com

Replace google.com with your domain. Also note that you need the GNU version of sed and bash. It does not work on OS X or BSD.

Apache Configuration

Add the below configuration to your virtualhost:

SSLUseStapling on SSLStaplingCache "shmcb:logs/stapling-cache(150000)"

Here's the explanation for the two lines:

SSLUseStapling

OCSP stapling relieves the client of querying the OCSP responder on its own, but it should be noted that with the RFC 6066 specification, the server's CertificateStatus reply may only include an OCSP response for a single cert. For server certificates with intermediate CA certificates in their chain (the typical case nowadays), stapling in its current implementation therefore only partially achieves the stated goal of "saving roundtrips and resources" - see also RFC 6961 (TLS Multiple Certificate Status Extension).

SSLStaplingCache

Configures the cache used to store OCSP responses which get included in the TLS handshake if SSLUseStapling is enabled. Configuration of a cache is mandatory for OCSP stapling. With the exception of none and nonenotnull, the same storage types are supported as with SSLSessionCache

The shmbc part:

This makes use of a high-performance cyclic buffer (approx. size bytes in size) inside a shared memory segment in RAM (established via /path/to/datafile) to synchronize the local OpenSSL memory caches of the server processes. This is the recommended session cache. To use this, ensure that mod_socache_shmcb is loaded.

You can also give a few more options. For example, a freshness timeout, how old the OCSP response can be:

SSLStaplingResponseMaxAge 900

This lets the response only be max 15 minutes old (900 seconds).

If your apache server is behind a HTTP proxy and you need to do your OCSP queries through a proxy you can use SSLStaplingForceURL . This overrides the URL provided by the certificate:

SSLStaplingForceURL http://internal-proxy.example.org

Restart your apache to load the new configuration:

service apache2 restart

And it should work. Let's test it.

Testing it

Fire up a terminal and use the following OpenSSL command to connect to your website:

openssl s_client -connect example.org:443 -tls1 -tlsextdebug -status

In the response, look for the following:

OCSP response: ====================================== OCSP Response Data: OCSP Response Status: successful (0x0) Response Type: Basic OCSP Response Version: 1 (0x0) Responder Id: 99E4405F6B145E3E05D9DDD36354FC62B8F700AC Produced At: Feb 3 04:25:39 2014 GMT Responses: Certificate ID: Hash Algorithm: sha1 Issuer Name Hash: 0226EE2F5FA2810834DACC3380E680ACE827F604 Issuer Key Hash: 99E4405F6B145E3E05D9DDD36354FC62B8F700AC Serial Number: C1A3D8D00D72FCE483CD84759E9EC0BC Cert Status: good This Update: Feb 3 04:25:39 2014 GMT Next Update: Feb 7 04:25:39 2014 GMT

That means it is working. If you get a response like below, it is not working:

OCSP response: no response sent

You can also use the SSL Labs test to see if OCSP stapling works.

Sources

Tags: apache