Re: ipfw: manpage: semantics of "receive" and "xmit" interfaces

On Tue, 9 Jan 2018 21:23:54 +0300
"Andrey V. Elsukov" <bu7cher_at_yandex.ru> wrote:

> On 09.01.2018 12:28, O. Hartmann wrote:
> > In section RULE OPTIONS, there is recv|xmit|via explained (a bit). There is
> > also an example:
> > 
> > ipfw add deny ip from any to any out recv ed0 xmit ed1
> > 
> > Can someone explain a bit more what the semantics of these is? I get
> > especially confused by the subsequent blocks of text following the line I
> > mentioned above. Since not everybody using FreeBSD is capable of studying
> > the kernel sources, I have difficulties to put those statements in line
> > with a visualization of the packet flow. A local host receiving a packets
> > destined for the local host can not have xmit interface? If I imagine, that
> > the recv interface might be the interface adjacent directly to the in/out
> > port depicted in section PACKET FLOW it doesn't give me any idea why there
> > is no xmit interface.   
> 
> When your system has two interfaces ed0 and ed1, and it acts as router,
> a forwarded packet can be checked by firewall two times:
> 
> 1. When a packet is received on ed0 interface, mbuf associated with this
> packet gets a property "receiving interface". This packet is checked for
> inbound direction and can be matched by "in" and "recv ed0" opcodes.
> If it was not dropped by rules, it will go through IP stack and can be
> forwarded according to routing table via interface ed1.
> 
> 2. When the routing decision was made (i.e. outbound interface is
> determined) a packet checked by firewall again, now for outbound
> direction. And it can be matched by "out" and "xmit ed1" opcodes. The
> opcode "recv ed0" still can be matched too, but "in" opcode will not
> matched.
> 
> A packet destined for local host is consumed by local IP stack and will
> not forwarded. It is checked by firewall only one time (usually). Thus
> it can not have xmit interface.
> 

Thanks very much for the explanation.