Re: pcap_t not writeable on OpenBSD

[Fernando Gont <fernando () gont com ar>]

On 01/10/2012 07:57 PM, Guy Harris wrote:

> > I'm doing I/O multiplexing with the pcap descriptor, and it turns
> > out that on OpenBSD the underlying descriptor for a pcap_t is never
> > writeable.
>
> I presume from "I'm doing I/O multiplexing" that by "writeable"
> you're referring to "writeable" as in "a select() or poll() 

Yes.


> > For injection, on linux anyhow, a PF_PACKET device is always 
> > writeable, but I'm pretty sure it discards packets if you write
> > too fast.
>
> the OpenBSD bpfwrite() routine does not appear to block for any "no
> room to buffer the packet" reason (it just calls the interface's
> if_output routine to send the packet, and I think that routine is
> supposed to fail, not block, if there's no buffering space available
> in the driver), and the FreeBSD one behaves the same.

So... what would be the portable way to make sure that pcap_inject() or
the like do not block, but also do not discard packets because of my app
sending "too fast"?


> So:
>
> it appears that on all of the BSD-flavored platforms there's nothing
> to multiplex on the output side - the service offered when sending
> raw packets is a "best effort" service with no guarantee that the
> packet will even make it onto the wire (which is pretty much what you
> get with, for example, Ethernet in any case);

I would expect that if there are no buffers available, pcap_inject() or
blocks, rather than silently fail. (i.e., how do I control the rate at
which I'm sending? how do I know if I should be retransmitting? etc.)



> on most platforms select() will always say "you can write", with
> OpenBSD as an exception and DragonFly BSD as another possible
> exception - poll() might be the same except that it might also fail
> on Mac OS X, and kqueues will probably fail with an error on all
> platforms if you try to create a EVFILT_WRITE kevent for a BPF
> device.

Thanks *so* *much* for your help and analysis. I really appreciate it.

I will post to the relevant OpenBSD list, to hear what they have to say...

Thanks!

Best regards,
-- 
Fernando Gont
e-mail: fernando () gont com ar || fgont () si6networks com
PGP Fingerprint: 7809 84F5 322E 45C7 F1C9 3945 96EE A9EF D076 FFF1



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