I suppose one could abuse SSL_set_msg_callback() to create a filter that rewrites the initial re-handshake message into something innocuous. Though I doubt that would work, once the client starts a handshake, it expects a response from the server, and may well time out and close if it does not get one. The way TLS works, it is always the client starting a renegotiation. The server can send a message asking the client to renegotiate, but the client can ignore that. I don't believe the opposite is true.
From: openssl-users [mailto:[email protected]] On Behalf Of Sashank Mullapudi (samullap) Sent: Monday, November 28, 2016 10:56 PM To: [email protected] Cc: Ram Mohan R (rmohanr) <[email protected]>; Nikhil Mittal (nimittal) <[email protected]>; Anil Kumar (anilkum) <[email protected]> Subject: Re: [openssl-users] Disabling Client-Initiated TLS renegotiation Resending this hoping for a response from someone who has information on disabling TLS renegotiation from the Client side. Thanks, Sashank From: samullap <[email protected]<mailto:[email protected]>> Date: Tuesday, 22 November 2016 at 12:21 PM To: "[email protected]<mailto:[email protected]>" <[email protected]<mailto:[email protected]>> Cc: "Ram Mohan R (rmohanr)" <[email protected]<mailto:[email protected]>>, "Anil Kumar (anilkum)" <[email protected]<mailto:[email protected]>>, "Nikhil Mittal (nimittal)" <[email protected]<mailto:[email protected]>> Subject: Disabling Client-Initiated TLS renegotiation Hi, As part of securing our web interfaces, we wanted to disable client-initiated TLS renegotiation. The reasoning for this requirement is as follows- Generally, renegotiation of TLS sessions is much more resource-intensive for the server than the client, and should therefore not be performed at will to avoid degrading performance. Disabling client from renegotiating secures the server from undergoing a DoS attack due to continuous renegotiation requests. I see that there is an option SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION, but that is to secure the renegotiation, not disable it. I wanted to check if there is a patch or flag available to disable any negotiation initiated from the client side. Thanks and Regards, Sashank
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