[openssl-users] Disabling Client-Initiated TLS renegotiation

Wall, Stephen swall at redcom.com
Tue Nov 29 13:35:45 UTC 2016

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:openssl-users-bounces at openssl.org] On Behalf Of Sashank Mullapudi (samullap)
Sent: Monday, November 28, 2016 10:56 PM
To: openssl-users at openssl.org
Cc: Ram Mohan R (rmohanr) <rmohanr at cisco.com>; Nikhil Mittal (nimittal) <nimittal at cisco.com>; Anil Kumar (anilkum) <anilkum at cisco.com>
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.


From: samullap <samullap at cisco.com<mailto:samullap at cisco.com>>
Date: Tuesday, 22 November 2016 at 12:21 PM
To: "openssl-users at openssl.org<mailto:openssl-users at openssl.org>" <openssl-users at openssl.org<mailto:openssl-users at openssl.org>>
Cc: "Ram Mohan R (rmohanr)" <rmohanr at cisco.com<mailto:rmohanr at cisco.com>>, "Anil Kumar (anilkum)" <anilkum at cisco.com<mailto:anilkum at cisco.com>>, "Nikhil Mittal (nimittal)" <nimittal at cisco.com<mailto:nimittal at cisco.com>>
Subject: Disabling Client-Initiated TLS renegotiation


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,
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