[openssl-dev] [openssl.org #4025] Bug? DTLS server does not respond if HelloVerifyRequest message lost

[Michael Tuexen]

> On 28 Aug 2015, at 22:52, Ken Ballou via RT <rt at openssl.org> wrote:
> 
> I originally found this in version 1.0.1e, but this also appears to be
> present in the latest master branch of the git repository.
> 
> If a DTLS server has been configured to require a cookie exchange, it
> appears the server never retransmits the HelloVerifyRequest message if the
> client sends another ClientHello with sequence number zero and no cookie. 
> But, this means that if the HelloVerifyRequest message from the server is
> lost (it's UDP, so anything can happen), the client will never be able to
> connect.
> 
> Is this intentional behavior?  I can imagine this may be an attempt to
> thwart a DoS attack, but it seems the attacker has to do as much work as
> the system under attack by resending the ClientHello again.
The server isn't retransmitting the HelloVerifyRequest message, the client
should retransmit the ClientHello. See
https://tools.ietf.org/html/rfc6347#section-3.2.1

Best regards
Michael
> 
> I am attaching source code (in C) for a small (single source file) test
> program to reproduce this.  The test program uses separate read and write
> datagram BIOs to simulate a lost UDP datagram.  After the program sends
> the initial ClientHello and fails to read the HelloVerifyRequest, the user
> is prompted to press "enter."  When the user does so, the program replaces
> the read BIO in the SSL object with the correct BIO and retries
> SSL_connect.  In a wireshark trace, one can see that the server never
> resends the HelloVerifyRequest in reply.  (Pass the server IP address and
> port as command line parameters.)
> 
> I have tested this with "openssl s_server" running on localhost:
> 
> % ./apps/openssl version
> OpenSSL 1.1.0-dev xx XXX xxxx
> 
> % ./apps/openssl s_server -cert keycert.pem -accept 5555 -dtls1
> 
> Then:
> 
> % test-lost-helloverifyrequest 127.0.0.1 5555
> 
> - Ken
> <test-lost-helloverifyrequest.c>_______________________________________________
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