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Date: Wed, 19 Oct 2016 03:03:58 +1100 (EST)
From: Ian Smith <smithi@nimnet.asn.au>
To: Shawn Bakhtiar <shashaness@hotmail.com>
cc: Samira Nazari <nazari.s11@gmail.com>,
 "freebsd-ipfw@freebsd.org" <freebsd-ipfw@freebsd.org>
Subject: Re: change packets with IPFW divert
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On Tue, 18 Oct 2016 14:21:50 +0000, Shawn Bakhtiar wrote:
 > On Oct 18, 2016, at 6:49 AM, Samira Nazari <nazari.s11@gmail.com<mailto:nazari.s11@gmail.com>> wrote:
 > > Hello every one,
 > > When we diverte packets to the specified port with "IPFW divert" ,
 > > we can change it and re-sent to the kernel?

 > Not sure what you mean by change it but:
 > 
 > "Divert sockets are similar to raw IP sockets, except that they can 
 > be bound to a specific divert port via the bind(2) system call.  The 
 > IP address in the bind is ignored; only the port number is 
 > significant.  A divert socket bound to a divert port will receive all 
 > packets diverted to that port by some (here unspecified) kernel 
 > mechanism(s).  Packets may also be written to a divert port, in which 
 > case they re-enter kernel IP packet processing."
 > 
 > -- SRC: https://www.freebsd.org/cgi/man.cgi?query=divert&sektion=4&apropos=0&manpath=FreeBSD+10.3-RELEASE+and+Ports

Apart from divert(4), most likely the best example is the natd(8) code, 
which modifies packet source or destination addresses and (maybe) ports.

Ignoring the NAT processing - or not, as appropriate - the way natd uses 
divert sockets both to receive packets from ipfw and later (perhaps) to 
reinject them for further processing should show clearly how it's done.

cheers, Ian