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If my server has only the cipher suites enabled which are defined by TLSv1.2 does that force a client to also use TLSv1.2 or else not be able to connect or can the connection fallback to TLSv1.1 or 1.0 and still negotiate the TLSv1.2 cipher? I do understand that the GCM block modes are TLSv1.2 only but unsure about the others.

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It forces clients who report that they support TLSv1.2 to use it, and causes all other client connections, those who only support a maxium of TLSv1.0 or TLSv1.1, to fail.

At this point, unless you know exactly who your clients are and that they do, in fact, support TLSv1.2, (if this is in any enterprise Intranet environment, for instance) I would not recommend this configuration. There is a large portion of the Internet that does not yet have the level of support required for this to not turn out to be a disaster.

  • Can you explain the answer here then? security.stackexchange.com/questions/65159/… - "TLS is backward compatible, in the following sense: when the client connects, it sends a ClientHello message that specifies the highest version that it supports; the server then responds with a ServerHello that defines the protocol version which will be used. That way, a TLS server can perfectly support TLS 1.2 and yet interoperate properly with clients who know only SSL 3.0, TLS 1.0 or TLS 1.1." – user53029 Oct 15 '14 at 23:44
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    @user53029. The backwards compatibility described in that answer presumes that the server also supports the older versions of SSL/TLS, and can then choose to use them if that is all a client supports. If you disable the older versions on the server, it will not be able to choose them, and the connection will fail. – Xander Oct 15 '14 at 23:49
  • Ok thanks - so what I am getting at now is that simply disabling all but the TLSv1.2 ciphers on the server does not effectively disable earlier versions of SSL/TLS, correct? This has to be done elsewhere in the config? – user53029 Oct 15 '14 at 23:53
  • @user53029 Ah! I understand now. No, I'm not aware of any TLSv1.2 cipher suites that are backwards compatible. As far as I know, this will leave you with TLSv1.2, and older versions of TLS with no cipher suites. – Xander Oct 16 '14 at 12:53
  • Ok, for example you are saying that if I have a server that has only suites enabled that were added in TLSv1.2, such as TLS_RSA_WITH_AES_256_CBC_SHA256, BUT also still supports TLS1.0/1.1. If a client that only supports the 1.0/1.1 protocol tries to connect it will fail to do so because it can't negotiate the cipher listed above? – user53029 Oct 16 '14 at 14:38

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