Re: ALPN support for GnuTLS connections

[Eric Marsden]

On 10/10/2024 15:54, Robert Pluim wrote:
> Patch below. Works in my limited testing.

Excellent, I can confirm that this works with the PostgreSQL 17.0 use 
case that I mentioned upthread, as well as with test servers from 
OpenSSL and Rustls (see the attached test file).

Remaining questions in my mind:

(1) It would be useful for elisp code to be able to determine whether 
Emacs has ALPN support. The elisp code will generally know that the 
service it's connecting to requires ALPN, and it would be useful to be 
able to inform the user that they should upgrade Emacs, instead of 
getting a generic "connection failed" error. The C preprocessor test 
HAVE_GNUTLS_ALPN_SET_PROTOCOLS  isn't visible from elisp, nor is (I 
think?) the binding to gnutls_alpn_set_protocols. This might also be 
useful for other features such as the AEAD support. Perhaps a function 
such as gnutls-feature-available-p(:alpn) ?

(2) The current behaviour of connection failing only depending on the 
server's ALPN setting is I think less than ideal. If the server is not 
configured to request ALPN, sending ALPN does not lead to failure. If 
server is looking for ALPN and wants another protocol, the connection 
fails. I think the default behaviour should be for the connection to 
fail if one of the ALPN protocols requested by the client is not 
selected: this seems to be more consistent and avoids a security attack 
called ALPACA, https://alpaca-attack.com/. This just requires a change 
to use GNUTLS_ALPN_MANDATORY (as well as updates to the documentation)

        ret = gnutls_alpn_set_protocols (state, protocols, count, 
GNUTLS_ALPN_MANDATORY);

In fact I see reading the ALPACA web page that TLS clients are 
recommended to use the SNI extension to indicate the server name that 
they wish to connect to, which gnutls.c is not currently doing. One 
thing at a time!

(3) Perhaps you could add the attached tiny patch to the logging support 
for gnutls.c (which is very verbose), so that an EAGAIN doesn't pollute 
logs at level 1.


Thanks,

Eric

alpn.el
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log-retry.diff
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