netdev - Re: No cmsg for SOCK

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Message-Id: <200707141938.12028@auguste.remlab.net>
Date:	Sat, 14 Jul 2007 19:38:11 +0300
From:	Rémi Denis-Courmont <rdenis@...phalempin.com>
To:	Daniel Schaffrath <daniel.schaffrath@....com>
Cc:	linux-net@...r.kernel.org, Linux NetDev <netdev@...r.kernel.org>
Subject: Re: No cmsg for SOCK_STREAM?

Le samedi 14 juillet 2007, Daniel Schaffrath a écrit :
> 'man 7 ip' sais that IP_RECVOPTS is not supported for SOCK_STREAM
> sockets.
>
> But what if for debugging/measuring purposes I wanted to record the
> route of the datagrams containing the stream segments? Are there any
> other ways to get the IP options? Like a second raw socket to deliver
> cmsgs to user space and later resubmit for kernel TCP processing and
> normal delivery to application -- if that is possible at all.

The STREAM does not return data on a per-packet basis, so it would be 
total non-sense to have IP_RECVOPTS on STREAM sockets. What would the 
kernel do when there is re-assembly before delivery to userland, or if 
a retransmitted segment has different options?!

FYI, the "advanced" socket API (for IPv6) consistently notes that 
sending/receiving IP(v6) options per-packet is only defined for DGRAM 
and RAW sockets.

You have to raw sockets, or better yet, BPF to do this.

-- 
Rémi Denis-Courmont
http://www.remlab.net/
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