netstat output - diff between 'link' and 'inet' counters

[Matthew Seaman]

On Thu, May 20, 2004 at 11:23:01AM -0600, Nathan Kinkade wrote:
> I delved into trying to determine the cause of an unreasonably high
> number of Ierrs on a few FreeBSD routers we have setup on campus.  While
> probing through the netstat output on the machines I realized that I
> don't understand the exact difference between the 'inet' and 'link'
> protocol families.  Now, I understand the difference between IP and
> ethernet, but the byte and packet counts for 'inet' and 'link' don't
> seem to match what I would expect for those protocols, respectively.
> This tells me that the numbers being logged must differ from my
> expectations.  Generally I notice that the 'inet' counts for an
> interface are a relatively small fraction of that for the 'link'
> counts for the same interface.  However, on our main FreeBSD router that
> provides NAT and access to the internet the numbers are somewhat
> reversed, with 'inet' counts being much higher than the 'link' counts.
> Is there someone who can explain to me exactly what packet and byte
> counts actually represent for the 'inet' and 'link' families?

I surmise that you're talking about the per-interface statistics as
reported by 'netstat -i' or 'netstat -I ifN' rather than any other set
of flags to netstat.  Let's look at what I get on my system:

    % netstat -I de0
    Name    Mtu Network       Address              Ipkts Ierrs    Opkts Oerrs  Coll
    de0    1500 <Link#1>    00:40:05:a5:8d:b7   149504     0   111734     4     0
    de0    1500 81.2.69.216/2 smtp               70771     -   120940     -     -
    de0    1500 fe80:1::240 fe80:1::240:5ff:f        0     -        3     -     -
    de0    1500 81.2.69.219/3 arbitrary         371042     -   301860     -     -

Now, link#1 corresponds to my local network (from 'netstat -r'):

    81.2.69.216/29     link#1             UC          2        0    de0

So the Ipkts count is for all the packets passing that interface with
a destination address matching the 81.2.69.216/29 network but not
including packets to one of the specific addresses on that
interface. That includes many packets for some unused addesses out of
my netblock[*] and also packets to the broadcast address 81.2.69.219

The other three entries are for the specific addresses assigned to
that interface -- I have the principal IP number on the interface as
81.2.69.218, and a jail using 81.2.69.219, plus the automatically
assigned IPv6 link-local address.  (IPv6 traffic mostly goes via a
gif(4) tunnel which acts like a different interface.

	Cheers,

	Matthew

[*] It's a feature of the way my network is set up that all such
packets will hit the de0 interface of that machine.  Normally a
network switch will prevent irrelevant traffic from hitting that
network interface.

-- 
Dr Matthew J Seaman MA, D.Phil.                       26 The Paddocks
                                                      Savill Way
PGP: http://www.infracaninophile.co.uk/pgpkey         Marlow
Tel: +44 1628 476614                                  Bucks., SL7 1TH UK
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