Date: Fri, 12 Jan 2018 11:56:47 +0100 From: "O. Hartmann" <o.hartmann@walstatt.org> 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: <20180112115639.3b31073f@freyja.zeit4.iv.bundesimmobilien.de> In-Reply-To: <5e6811ff-70c6-ee74-bf04-1319e9002b29@yandex.ru> References: <20180109102813.63c32899@freyja.zeit4.iv.bundesimmobilien.de> <5e6811ff-70c6-ee74-bf04-1319e9002b29@yandex.ru>
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On Tue, 9 Jan 2018 21:23:54 +0300 "Andrey V. Elsukov" <bu7cher@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.
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