High CPU usage ncat on Windows

[thomasfaingnaert]

The following scenario results in high CPU usage in Windows:

1. Start a server that listens on localhost (IPv4 only):

ncat -lk4 127.0.0.1 6000

2a. Connect to that server, but let ncat try IPv6 connection first:

ncat -v localhost 6000

CPU usage of ncat is extremely high. Oddly enough, this doesn't happen in this alternate scenario:
2b. Connect to that server, but force IPv4:

ncat -v4 localhost 6000

Ncat version (downloaded from https://nmap.org/download.html):

Ncat: Version 7.80 ( https://nmap.org/ncat )

[dmiller-nmap]

Confirmed. Here's the output with a bit more verbosity:

Without -4, 15% CPU usage:

>ncat -vvvvvv localhost 6000
Ncat: Version 7.80 ( https://nmap.org/ncat )
libnsock nsock_set_loglevel(): Set log level to DEBUG
NCAT DEBUG: Using trusted CA certificates from C:\Program Files (x86)\Nmap\ca-bundle.crt.
libnsock nsock_iod_new2(): nsock_iod_new (IOD #1)
libnsock event_new(): event_new (IOD #1) (EID #8)
libnsock nsock_connect_tcp(): TCP connection requested to ::1:6000 (IOD #1) EID 8
libnsock nsock_pool_add_event(): NSE #8: Adding event (timeout in 10000ms)
libnsock nsock_loop(): nsock_loop() started (no timeout). 1 events pending
libnsock nsock_trace_handler_callback(): Callback: CONNECT ERROR [No connection could be made because the target machine actively refused it.  (10061)] for EID 8 [::1:6000]
Ncat: Connection to ::1 failed: No connection could be made because the target machine actively refused it. .
Ncat: Trying next address...
libnsock event_new(): event_new (IOD #1) (EID #16)
libnsock nsock_connect_tcp(): TCP connection requested to 127.0.0.1:6000 (IOD #1) EID 16
libnsock nsock_pool_add_event(): NSE #16: Adding event (timeout in 10000ms)
libnsock event_delete(): event_delete (IOD #1) (EID #8)
libnsock nsock_trace_handler_callback(): Callback: CONNECT SUCCESS for EID 16 [127.0.0.1:6000]
Ncat: Connected to 127.0.0.1:6000.
libnsock nsock_iod_new2(): nsock_iod_new (IOD #2)
libnsock event_new(): event_new (IOD #1) (EID #26)
libnsock nsock_read(): Read request from IOD #1 [127.0.0.1:6000] (timeout: -1ms) EID 26
libnsock nsock_pool_add_event(): NSE #26: Adding event (timeout in 2024526038ms)
libnsock event_new(): event_new (IOD #2) (EID #34)
libnsock nsock_readbytes(): Read request for 0 bytes from IOD #2 [peer unspecified] EID 34
libnsock nsock_pool_add_event(): NSE #34: Adding event (timeout in 2024526038ms)
libnsock event_delete(): event_delete (IOD #1) (EID #16)
libnsock nsock_trace_handler_callback(): Callback: READ EOF for EID 34 [peer unspecified]

With -4, negligible CPU usage:

C:\Program Files (x86)\Windows Kits\10\Debuggers\x64>ncat -vvvvvv -4 localhost 6000
Ncat: Version 7.80 ( https://nmap.org/ncat )
libnsock nsock_set_loglevel(): Set log level to DEBUG
NCAT DEBUG: Using trusted CA certificates from C:\Program Files (x86)\Nmap\ca-bundle.crt.
libnsock nsock_iod_new2(): nsock_iod_new (IOD #1)
libnsock event_new(): event_new (IOD #1) (EID #8)
libnsock nsock_connect_tcp(): TCP connection requested to 127.0.0.1:6000 (IOD #1) EID 8
libnsock nsock_pool_add_event(): NSE #8: Adding event (timeout in 10000ms)
libnsock nsock_loop(): nsock_loop() started (no timeout). 1 events pending
libnsock nsock_trace_handler_callback(): Callback: CONNECT SUCCESS for EID 8 [127.0.0.1:6000]
Ncat: Connected to 127.0.0.1:6000.
libnsock nsock_iod_new2(): nsock_iod_new (IOD #2)
libnsock event_new(): event_new (IOD #1) (EID #18)
libnsock nsock_read(): Read request from IOD #1 [127.0.0.1:6000] (timeout: -1ms) EID 18
libnsock nsock_pool_add_event(): NSE #18: Adding event (timeout in 2024478101ms)
libnsock event_new(): event_new (IOD #2) (EID #26)
libnsock nsock_readbytes(): Read request for 0 bytes from IOD #2 [peer unspecified] EID 26
libnsock nsock_pool_add_event(): NSE #26: Adding event (timeout in 2024478101ms)
libnsock event_delete(): event_delete (IOD #1) (EID #8)

Tried with --nsock-engine poll and --nsock-engine iocp and the connection did not succeed. That should probably be a separate issue; I forget why only the select engine works in Ncat.