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bug#23311: TLS handshake error

From: Ludovic Courtès
Subject: bug#23311: TLS handshake error
Date: Wed, 20 Apr 2016 13:18:38 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)

address@hidden (Ludovic Courtès) skribis:

> $ while ./pre-inst-env guix download https://mirror.hydra.gnu.org/index.html 
> ; do : ; done

Interestingly, the same loop with wget (which uses the very same GnuTLS)
goes on forever.

It turns out that instead of the default TLS cipher suite priority
string (“NORMAL”), wget does:

--8<---------------cut here---------------start------------->8---
    case secure_protocol_auto:
      err = gnutls_priority_set_direct (session, "NORMAL:%COMPAT:-VERS-SSL3.0", 
--8<---------------cut here---------------end--------------->8---

The code doesn’t explain why, but GnuTLS’s documentation has this bit
(info "(gnutls) Priority Strings"):

--8<---------------cut here---------------start------------->8---
%COMPAT                          will enable compatibility
                                 mode.  It might mean that
                                 violations of the protocols
                                 are allowed as long as maximum
                                 compatibility with problematic
                                 clients and servers is
                                 achieved.  More specifically
                                 this string would disable TLS
                                 record random padding,
                                 tolerate packets over the
                                 maximum allowed TLS record,
                                 and add a padding to TLS
                                 Client Hello packet to prevent
                                 it being in the 256-512 range
                                 which is known to be causing
                                 issues with a commonly used
--8<---------------cut here---------------end--------------->8---

Indeed, as soon as we add %COMPAT, ‘gnutls-cli’ et al. send a 253-byte
client hello (instead of 261) and the problem vanishes.

Commit 967ee481e893fd77ff8ca896188e20e425331bf2 does that.


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