Nmap simultaneous scanning

Question

Quick Nmap question. If there are X amount of ports per machine to scan and I want to scan 2 machines at once (no more no less), is there any way of forcing nmap to always be scanning 2 different machines? Further question: If there is an odd number of machines will both threads converge on one IP or is there a way of stopping that.

Answer 1

I believe the flag that you're looking for would be max-parellelism

TIMING AND PERFORMANCE:
 Options which take <time> are in seconds, or append 'ms' (milliseconds),
 's' (seconds), 'm' (minutes), or 'h' (hours) to the value (e.g. 30m).
 -T<0-5>: Set timing template (higher is faster)
 --min-hostgroup/max-hostgroup <size>: Parallel host scan group sizes
 --min-parallelism/max-parallelism <numprobes>: Probe parallelization
 --min-rtt-timeout/max-rtt-timeout/initial-rtt-timeout <time>: Specifies
     probe round trip time.
 --max-retries <tries>: Caps number of port scan probe retransmissions.
 --host-timeout <time>: Give up on target after this long
 --scan-delay/--max-scan-delay <time>: Adjust delay between probes
 --min-rate <number>: Send packets no slower than <number> per second
 --max-rate <number>: Send packets no faster than <number> per second

Answer 2

Nmap scans in parallel by default, but not by using threads. Instead, it uses non-blocking I/O functions to continue sending new probes while it waits for responses to old ones. For most scan types, this involves running a packet capture during the scan and alternating between sending new probes and processing captured response packets. There are lots of options for tuning performance.