Accept Filtering on FreeBSD:

SO_ACCEPTFILTER is not available on Linux. There is a socket option called TCP_DEFER_ACCEPT, which is roughly equivalent to the “dataready” accept filter on FreeBSD. It’s not quite as good as “httpready”, since with TCP_DEFER_ACCEPT, accept() will return as soon as the socket becomes readable (i.e. after at least one byte of the request is received).