On Wed, Mar 07, 2007 at 10:41:16AM +0100, Jan F. Schnellbaecher wrote:

> Can anybody explain how I can use the server name extension from the first
> TLS handshake message (Client Hello)?

Download a 0.9.9 dev snapshot and see the CHANGES file:

  *) Add initial support for TLS extensions, specifically for the server_name
     extension so far.  The SSL_SESSION, SSL_CTX, and SSL data structures now
     have new members for a host name.  The SSL data structure has an
     additional member SSL_CTX *initial_ctx so that new sessions can be
     stored in that context to allow for session resumption, even after the
     SSL has been switched to a new SSL_CTX in reaction to a client's
     server_name extension.

     New functions (subject to change):

         SSL_get_servername()
         SSL_get_servername_type()
         SSL_set_SSL_CTX()

     New CTRL codes and macros (subject to change):

         SSL_CTRL_SET_TLSEXT_SERVERNAME_CB
                                 - SSL_CTX_set_tlsext_servername_callback()
         SSL_CTRL_SET_TLSEXT_SERVERNAME_ARG
                                      - SSL_CTX_set_tlsext_servername_arg()
         SSL_CTRL_SET_TLSEXT_HOSTNAME           - SSL_set_tlsext_host_name()

     openssl s_client has a new '-servername ...' option.

     openssl s_server has new options '-servername_host ...', '-cert2 ...',
     '-key2 ...', '-servername_fatal' (subject to change).  This allows
     testing the HostName extension for a specific single host name ('-cert'
     and '-key' remain fallbacks for handshakes without HostName
     negotiation).  If the unrecogninzed_name alert has to be sent, this by
     default is a warning; it becomes fatal with the '-servername_fatal'
     option.

     [Peter Sylvester,  Remy Allais, Christophe Renou]

> I would like to use it to return an appropriate certificate to avoid a
> CN mismatch.

Most clients don't implement this extension, so this is presumably with
a client you control...

-- 
        Viktor.
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