netdev - Re: Is it possible to get device information via CMSG?

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Message-ID: <20180528065701.dqdjbrqb34zfsfvo@unicorn.suse.cz>
Date:   Mon, 28 May 2018 08:57:01 +0200
From:   Michal Kubecek <mkubecek@...e.cz>
To:     "Eric S. Raymond" <esr@...rsus.com>
Cc:     netdev@...r.kernel.org
Subject: Re: Is it possible to get device information via CMSG?

On Sat, May 26, 2018 at 05:39:12AM -0400, Eric S. Raymond wrote:
> I'm trying to untangle some nasty code in the Mills implementation of
> NTP.  I could simplify it a lot if there were a way to query a packet
> to find out the name of the network interface it arrived on.  (At the
> moment the code has to iterate over all interfaces checking for
> traffic on each one just so it doesn't lose that information.)
> 
> This seems like the kind of thing the CMSG macros are intended to
> support, but I can't find anywhere a specification of what cmsg_level
> and cmsg_type values are valid and what their semantics are.
> 
> So I have two questions:
> 
> 1. Is there a cmsg_level/cmsg_type combination that will return the
> name of the device the packet arrived through?

Not name directly, AFAIK, but you can set SOL_IP / IP_PKTINFO (or
SOL_IPV6 / IPV6_RECVPKTINFO) socket option and get IP_PKTINFO
(IPV6_PKTINFO) message with recvmsg(). This will tell you incoming
interface index so that you can look the name up. See ip(7) or ipv6(7)
for format of the message (struct ip_pktinfo, struct in6_pktinfo).

However, I suspect that userspace application is not really interested
in incoming interface name but rather in destination address of the
incoming packet which is also provided in IP_PKTINFO / IPV6_PKTINFO
message. 

Michal Kubecek