MTU mismatch with 7.08 and "Unknown DTLS packet"

Chaskiel Grundman cgrundman at gmail.com
Fri Jan 5 08:31:04 PST 2018


It turns out I was mistaken. The gateway does *not* split the packet
into 2 DTLS packets. It sends one large DTLS packet and openconnect
reads the first 1290 bytes of plaintext as one packet and the rest as
another.

I do not use official anyconnect clients myself except on windows,
which I have gotten a capture from. The ciphersuites are identical.
the captures include the dtls handshake, and the largest data packet
sent during setup. The linux capture also includes the packets
corresponding to a >1290 byte ping (I used 1391 instead of 1291, so
there's extra stuff coming back from the gateway corresponding to the
IP fragmentation, but that does not obscure the fact that the gateway
sends a packet with a 1344 byte DTLS payload, but the max that
openconnect sends is 1328)

windows anyconnect:
https://drive.google.com/file/d/1wlbp8GUXYO8SR2l1TQX6avj8OgL7j5xi/view?usp=drive_web
linux openconnect:
https://drive.google.com/file/d/1ZdFMsCo9WUa7bCw0EVY5DWxd8dFD7MRD/view?usp=drive_web



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