"500 Command unrecognized" loop with -sV -sC -p2121 -dd #1478
Description
Initially was using -O -sV -sC on a number of hosts, then narrowed it down to the above command as minimal to reproduce a seemingly frozen nmap run.
Running
sudo nmap [ip] -sV -sC -p2121 -dd
eventually results in this looping over and over, many times per second:
NSOCK INFO [30.0650s] nsock_trace_handler_callback(): Callback: READ SUCCESS for EID 5122 [[ip]:2121] (27 bytes): 500 Command unrecognized...
NSOCK INFO [30.0650s] nsock_write(): Write request for 6 bytes to IOD #4 EID 5131 [[ip]:2121]
NSOCK INFO [30.0650s] nsock_trace_handler_callback(): Callback: WRITE SUCCESS for EID 5131 [[ip]:2121]
NSOCK INFO [30.0650s] nsock_read(): Read request from IOD #4 [[ip]:2121] (timeout: 7000ms) EID 5138
NSOCK INFO [30.0670s] nsock_trace_handler_callback(): Callback: READ SUCCESS for EID 5138 [[ip]:2121] (27 bytes): 500 Command unrecognized...
NSOCK INFO [30.0670s] nsock_write(): Write request for 6 bytes to IOD #4 EID 5147 [[ip]:2121]
NSOCK INFO [30.0670s] nsock_trace_handler_callback(): Callback: WRITE SUCCESS for EID 5147 [[ip]:2121]
NSOCK INFO [30.0670s] nsock_read(): Read request from IOD #4 [[ip]:2121] (timeout: 7000ms) EID 5154
NSOCK INFO [30.0690s] nsock_trace_handler_callback(): Callback: READ SUCCESS for EID 5154 [[ip]:2121] (27 bytes): 500 Command unrecognized...
NSOCK INFO [30.0690s] nsock_write(): Write request for 6 bytes to IOD #4 EID 5163 [[ip]:2121]
NSOCK INFO [30.0690s] nsock_trace_handler_callback(): Callback: WRITE SUCCESS for EID 5163 [[ip]:2121]
NSOCK INFO [30.0690s] nsock_read(): Read request from IOD #4 [[ip]:2121] (timeout: 7000ms) EID 5170
Happy to submit the IP privately, if that'll help.