tcpdump - tun/tap virtual interfaces

[Dambielle Yannick]

Hello,
Thank you very much for the details. I think I'm starting to understand the
subject better.
Well, I've did all again from the beggining and now I have the same results
has you had. It works.
10.0.10.1 is routed via lo0, and 10.0.10.2 is routed via tap0.
Icmp requests and replies could be seen with tcpdump on lo0 for 10.0.10.1,
but for 10.0.10.2, the request were passing trough tap0 while the replies
took lo0 (??).
I don't know, if what I'm going to say is the right method for having both
replies and requests passing through the same interface, so please, give me
a better one.

I did the following trick :

# route add 10.0.10.1 10.0.10.2

and the result is :

# route get 10.0.10.1
    route to: 10.0.10.1
destination: 10.0.0.1
   gateway: 10.0.0.1
   interface: tap0

# route get 10.0.10.2
    route to: 10.0.10.2
destination: 10.0.0.2
   interface: tap0

# ping 10.0.0.1
# tcpdump -i lo0 <-- nothing
# tcpdump -i tap0
blablabla.... icmp: echo request
blablabla.... icmp: echo reply

# ping 10.0.0.2
# tcpdump -i lo0 <-- nothing
# tcpdump -i tap0
blablabla.... icmp: echo request
blablabla.... icmp: echo reply

Now I can go back to my jails and my little project :)

Yannick

----- Original Message ----- 
From: "Robert Watson" <rwatson at freebsd.org>
To: "Giovanni P. Tirloni" <gpt at tirloni.org>
Cc: <freebsd-net at freebsd.org>
Sent: Sunday, September 28, 2003 8:05 PM
Subject: Re: tcpdump - tun/tap virtual interfaces


>
> On Sun, 28 Sep 2003, Giovanni P. Tirloni wrote:
>
> > * Robert Watson (rwatson at freebsd.org) wrote:
> >
> > > Do you see anything when you ping the broadcast address or other
foreign
> > > address of the tap interface?  Packets delivered to local IP addresses
> > > generally don't go out an interface.
> >
> >  About Ethernet frames not going out to the wire and being sent to the
> >  loopback..
> >
> >  The check seems to happen at line 291 in if_ethersubr.c and then it
> >  uses the if_simloop() function to copy the packet to the loopback
> >  interface. Is that right?
> >
> >  The rcvif interface is set to the hardware device, how is this used in
> >  this case? What kind of checks are done to the rcvif usually?
> >
> >  I haven't received my copy of Steven's Volume 2 yet so if it's
> >  explained there (as I hope) I will sit in my corner and wait to for it
> >  patiently :)
>
> Ethernet loopback does occur, and BPF will pick those up.  However, the
> loopback you're seeing is actually happening at the IP layer, as a result
> of routing rather than link layer behavior:
>
> 10                 link#6             UC          1        0   tap0
> 10.0.10.1          00:bd:18:a1:11:00  UHLW        0       26    lo0
>
> Local IP addresses have their packets routed to them over lo0, so the
> packets being looked for can be found by doing tcpdump on lo0:
>
> test1# tcpdump -eni lo0 &
> [2] 511
> tcpdump: listening on lo0
> test1# Sep 28 14:03:07 test1 kernel: lo0: promiscuous mode enabled
>
> test1# ping -c 1 10.0.10.1
> PING 10.0.10.1 (10.0.10.1): 56 data bytes
> 64 bytes from 10.0.10.1: icmp_seq=0 ttl=64 time=0.073 ms
>
> --- 10.0.10.1 ping statistics ---
> 1 packets transmitted, 1 packets received, 0% packet loss
> round-trip min/avg/max/stddev = 0.073/0.073/0.073/0.000 ms
> test1# 14:03:12.713690 AF 2 84: 10.0.10.1 > 10.0.10.1: icmp: echo request
> 14:03:12.713724 AF 2 84: 10.0.10.1 > 10.0.10.1: icmp: echo reply
>
> Route command output appended below.
>
> Robert N M Watson             FreeBSD Core Team, TrustedBSD Projects
> robert at fledge.watson.org      Network Associates Laboratories
>
> route get 10.0.10.1
>    route to: 10.0.10.1
> destination: 10.0.10.1
>   interface: lo0
>       flags: <UP,HOST,DONE,LLINFO,WASCLONED,LOCAL>
>  recvpipe  sendpipe  ssthresh  rtt,msec    rttvar  hopcount      mtu
> expire
>        0         0         0         0         0         0      1500
> 0
> test1# route get 10.0.10.2
>    route to: 10.0.10.2
> destination: 10.0.0.0
>        mask: 255.0.0.0
>   interface: tap0
>       flags: <UP,DONE,CLONING>
>  recvpipe  sendpipe  ssthresh  rtt,msec    rttvar  hopcount      mtu
> expire
>        0         0         0         0         0         0      1500
> -100
>
>
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