allow to switch off disabled renegotiation proxy to SSL client certificate protected backends

[WolfgangFahl@…]

​https://www.ruby-forum.com/topic/6875137

describes the scenario. A backend SSL Client Certificate secured server needs renegotiation.

Nginx fails 1.8.x fails badly on this with my Ubuntu 14.04.3 LTS
after having successfully solved the riddle in ​http://serverfault.com/questions/622855/nginx-proxy-to-back-end-with-ssl-client-certificate-authentication/746816#746816

The workaround to reconfigure the backend Apache Server is more than ugly. The worst part is the major effort needed to find out this problem in the first place - at lease nginx should mention that it actively forced the problem. curl/wget/openssl all run with no problem just nginx behaves this badly.

[Sergey Kandaurov <pluknet@…>]

In 6982:ac9b1df5b246/nginx:

SSL: disabled renegotiation detection in client mode.

CVE-2009-3555 is no longer relevant and mitigated by the renegotiation
info extension (secure renegotiation). On the other hand, unexpected
renegotiation still introduces potential security risks, and hence we do
not allow renegotiation on the server side, as we never request renegotiation.

On the client side the situation is different though. There are backends
which explicitly request renegotiation, and disabled renegotiation
introduces interoperability problems. This change allows renegotiation
on the client side, and fixes interoperability problems as observed with
such backends (ticket #872).

Additionally, with TLSv1.3 the SSL_CB_HANDSHAKE_START flag is currently set
by OpenSSL when receiving a NewSessionTicket message, and was detected by
nginx as a renegotiation attempt. This looks like a bug in OpenSSL, though
this change also allows better interoperability till the problem is fixed.

[Sergey Kandaurov]

Fixed in the upcoming 1.13.0.