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Date:      Tue, 9 Jan 2018 15:00:10 -0800 (PST)
From:      "Rodney W. Grimes" <freebsd-rwg@pdx.rh.CN85.dnsmgr.net>
To:        "Andrey V. Elsukov" <bu7cher@yandex.ru>
Cc:        "O. Hartmann" <ohartmann@walstatt.org>, freebsd-current <freebsd-current@freebsd.org>, freebsd-ipfw@freebsd.org
Subject:   Re: ipfw: manpage: semantics of "receive" and "xmit" interfaces
Message-ID:  <201801092300.w09N0AAo028968@pdx.rh.CN85.dnsmgr.net>
In-Reply-To: <5e6811ff-70c6-ee74-bf04-1319e9002b29@yandex.ru>

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> 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.

in, recv and via options 


> 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

in, recv and via options

> 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.

And a packet generated localy would not have a recv interface.
 
> -- 
> WBR, Andrey V. Elsukov
> 

-- 
Rod Grimes                                                 rgrimes@freebsd.org



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