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UDP across VPN spotty with 9.1.11 and Big Sur 11.2.3

New Contributor

UDP across VPN spotty with 9.1.11 and Big Sur 11.2.3

This started when I switched from a MacOS Mojave machine to a MacOS Big Sur (11.2.3) machine.  Pulse Secure version on both is 9.1.11.


I have mosh ( installed via homebrew on both machines.  With the VPN up and established, the mojave-based mosh client successfully connects across the VPN and communicates via UDP (typically port 60001 or thereabouts).


This does not work on the Big Sur machine: mosh successfully connects, but then is unable to communicate via UDP.  However, if I start netcat (/usr/bin/nc) and connect to the same server on the same udp port from the same Big Sur machine, communication works just fine.


At first I thought it was a Big Sur issue with mosh, but I have confirmed that I can use mosh client frrom the Big Sur machine to any machine in my local network (not traversing the VPN).


Doing more investigation with wireshark capturing device utun2 and traffic between my Big Sur machine and the remote/VPN machine, and netcat, I can confirm that wireshark on the mac does not display all UDP datagrams.  Loss happens regardless of origination side, and the loss is not 100%. Important: the lossiness ONLY occurs if the "listening" side of the datagram connection is on the Big Sur machine and the "connecting" side is on the remote machine.  If the roles are reversed, all datagrams travel both directions.



Big Sur:  nc -u 60001

remote:  nc -l -u 60001



Big Sur: nc -l -u 60001

remote: nc -u 60001