Find the process listening on a port (not showing in lsof)

Question

Update:

The port was opened by NFS. I figured this out by configuring NFS to listen on a known port.

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Using Ubuntu 18.04

I want to know which process or kernel function listens on UDP port 38637.

I have researched this problem and found similar questions:

• https://superuser.com/questions/42843/finding-the-process-that-is-using-a-certain-port-in-linux
• https://unix.stackexchange.com/questions/106561/finding-the-pid-of-the-process-using-a-specific-port
• https://unix.stackexchange.com/questions/35292/quickly-find-which-files-belongs-to-a-specific-inode-number
• Network port open, but no process attached?
• netstat shows a listening port with no pid but lsof does not
• Permission denied on /proc/ some process

I have tried all remedies I could find...

• Unmounted all network drives
• Rebooted. After reboot the same port listens.
• Ran virus/trojan scans with clamav and sophos-av
• nmap and nc both successfully connect on the port
• Tried all the following inspections:

netstat

Shows no process ID

➜ sudo netstat -ulpen | grep 38637
udp        0      0 0.0.0.0:38637           0.0.0.0:*                           0          36601      -
udp6       0      0 :::38637                :::*                                0          36602      -

lsof

Shows nothing at all

➜ sudo lsof -i udp:38637
➜ echo $?
1

rpcinfo

Doesn't list the port of interest

➜ sudo rpcinfo -p | grep 38637
➜ echo $?
1

fuser

Troubling?

➜ sudo fuser -n udp 38637
Cannot stat file /proc/16147/fd/1023: Permission denied
Cannot stat file /proc/16374/fd/1023: Permission denied
Cannot stat file /proc/16375/fd/1023: Permission denied
Cannot stat file /proc/16380/fd/1023: Permission denied
Cannot stat file /proc/16381/fd/1023: Permission denied
Cannot stat file /proc/16382/fd/1023: Permission denied
Cannot stat file /proc/18061/fd/1023: Permission denied
Cannot stat file /proc/18177/fd/1023: Permission denied
Cannot stat file /proc/18183/fd/1023: Permission denied
Cannot stat file /proc/18188/fd/1023: Permission denied
Cannot stat file /proc/18189/fd/1023: Permission denied
Cannot stat file /proc/18190/fd/1023: Permission denied

inode doesn't give me anything either

➜ sudo fuser --inode 36601
Specified filename 36601 does not exist.

debugfs

Nada.

➜ sudo debugfs -R 'ncheck 36601' /dev/sda1
debugfs 1.44.1 (24-Mar-2018)
Inode   Pathname

I would appreciate any other suggestions on where to look for answers on why this port listens.