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Testing for SSL renegotiation

December 15, 2009

Edit: Please note that the test described here works only with OpenSSL version that was not patched to deal with insecure renegotiation. I recommend that you download version 0.9.8k directly from the OpenSSL web site and compile a special binary to use for testing.

Someone asked me how to test for SSL connection renegotiation, so I thought I would also write here for the benefit of everyone. Testing is easy provided you have access to an un-patched version of OpenSSL. To test, you will use the s_client tool (you'll type the bits in blue):

$ openssl s_client -connect www.ssllabs.com:443
[snip... a lot of openssl output]
28874:error:1409E0E5:SSL routines:SSL3_WRITE_BYTES:ssl handshake failure:s3_pkt.c:530:

The idea is that you connect to an SSL server and start by typing the first line of a request. You then type a single uppercase letter R on a single line, which tells OpenSSL to ask for renegotiation. I am aware of the following outcomes:

  • Your HTTP request completes, which means that renegotiation is enabled
  • You get an error (one such possible error is shown in the example above), which means that renegotiation did not work
  • The connection blocks and timeouts after a while, which is how OpenSSL 0.9.8l deals with renegotiation.

Of course, a SSL Labs report will tell you whether a particular server supports renegotiation.