Handshake with IKEA Trådfri gateway fails due to resume state

[eriklupander]

Hi!

I tried to use your splendid DTLS library when trying to talk to my IKEA Trådfri Gateway. However, I couldn't get the handshake to complete. After a bit of debugging, I discovered that the session resume check on session_handshake.go#361 was returning true during the handshake, which resulted in ClientKeyExchange never being sent to the Gateway.

After commenting out the !s.resumed check, the handshake completes just fine and subsequent data transfer works as expected. Regrettably, I'm not knowledgable enough of DTLS and handshakes to try to find the issue and create a proper Pull Request. Sorry.

Note that I 've been able to connect to at least one other CoAP-server running DTLS without my little fix so I honestly have no idea whether it's an issue with how the IKEA Gateway works or if there's something quirky with the handshake.

Anyway - just wanted to let you know.

Thanks!

[bocajim]

Erik, I can definitely add a flag as a workaround to disable session resumption. The session resumption kicks in when the client sends a specific sessionId during the handshake. If you grab me a tcpdump/wireshark of a handshake, I'll be happy to take a look at exactly what their client is sending. It is possible that the session resumption is incorrectly resuming an old session instead of creating a new one. Thanks, Jim

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On Mon, Mar 4, 2019 at 4:28 PM ErikL ***@***.***> wrote: Hi! I tried to use your splendid DTLS library when trying to talk to my IKEA Trådfri Gateway. However, I couldn't get the handshake to complete. After a bit of debugging, I discovered that the session resume check on session_handshake.go#361 was returning true during the handshake, which resulted in ClientKeyExchange never being sent to the Gateway. After commenting out the !s.resumed check, the handshake completes just fine and subsequent data transfer works as expected. Regrettably, I'm not knowledgable enough of DTLS and handshakes to try to find the issue and create a proper Pull Request. Sorry. Note that I 've been able to connect to at least one other CoAP-server running DTLS without my little fix so I honestly have no idea whether it's an issue with how the IKEA Gateway works or if there's something quirky with the handshake. Anyway - just wanted to let you know. Thanks! — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#2>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABNc6MCbiYtmr_rUOtaZIb-xR0kfrCDhks5vTZAKgaJpZM4bdPnk> .

[eriklupander]

Thanks!
I'm not familiar with tcpdump, but I recorded three successful handshakes (running my fork that has the session resume commented out).

I've attached three files:

Log output (debug level) from the library during a completed handshake.
handshake-log.txt

Tcpdump to the console using -vv and -X flags:
handshake-dump-console.txt

Tcpdump to disk using -vv and -X flags
handshake-dump.txt

Thanks!