[gnutls-devel] GnuTLS | Listening DTLS server responds with HELLO_VERIFY_REQUEST to most messages (#632)

Development of GNU's TLS library gnutls-devel at lists.gnutls.org
Thu Nov 29 15:55:11 CET 2018

New Issue was created.

Issue 632: https://gitlab.com/gnutls/gnutls/issues/632
Author:    Paul F.B.

I am not sure if this is a bug or something intended. Analyzing models of the GnuTLS DTLS server implementation by [state learning](http://www.cs.ru.nl/~joeri/StateMachineInference.html), I noticed that the DTLS server will initially respond with HELLO_VERIFY_REQUEST to not only to CLIENT_HELLO messages, but also to most other messages. Attached is a Wireshark capture of GnuTLS v. 3.5.19 doing this for a FINISHED message, and actually completing the handshake, plus the model obtained (viewable via xdot, BTW I couldn't find much else wrong in it). This is reproducible also on the latest GnuTLS version.

The command for running the server is:
`gnutls-serv --udp --disable-client-cert --pskpasswd some_path_to_keys --port 20000 --priority NORMAL:+PSK:+SRP`

I was wondering if this was intended behavior. It seems weird, as the DTLS specification is quite clear about the flow of a handshake (CLIENT_HELLO/HELLO_VERIFY_REQUEST CLIENT_HELLO/SERVER_HELLO...). Weirder still, HELLO_VERIFY_REQUEST is sent also in response to ALERT messages. 


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