Created attachment 183594 [details]
LibAlias: allocate NAT ports using exhaustive search
Currently libalias allocates NAT ports from a pool of 32768 ports (from ALIAS_PORT_BASE to ALIAS_PORT_BASE + ALIAS_PORT_MASK), by randomly trying GET_NEW_PORT_MAX_ATTEMPTS (20) ports and returning the first unused one, otherwise failing.
On a heavily loaded setup, as the number of ports in use approaches 32768, requests for ports will increasingly fail even though there are free ports. With 30000 ports in use, 17.12% of requests fail, with 31000 ports in use, 32.98% of requests fail, with 32000 ports in use, 62.23% of requests fail.
This behavior is erratic, and both PF and IPFILTER allocate ports by exhaustively searching the entire port range from a random starting point (eg. pf_get_sport() in sys/netpfil/pf/pf_lb.c), which is guaranteed to find a port if any are available. The attached patch changes libalias to do the same.