Re: Odd result and underlying mistake

On 03/16/2018 02:54 PM, Leroy Tennison wrote:
> Posting for other's benefit in case someone else does this.  I searched 
> the web without finding an answer then discovered the issue.  What I 
> saw in a tcpdump output (because things weren't working) was
> 
> Request who-has <target IP address> tell <target IP address>
> 
> Where <target IP address> was a local interface address, quite odd since 
> the local interface should know its own MAC address.

That sounds like a Gratuitous ARP.

> The problem was that I had accidentally used the local interface IP 
> address in 'ip route add default via <local interface IP address> dev 
> <local interface>' instead of 'ip route add default via <gateway IP 
> address accessible from local interface> dev <interface>'.

I think I just reproduced this in a network namespace.

When I do this, I don't see "Request who-has <target IP address> tell 
<target IP address>".  Instead I see "Request who-has <target IP 
address> tell <NetNS IP address>".

19:26:13.919415 ARP, Request who-has 8.8.8.8 tell 192.0.2.1, length 28
19:26:14.943348 ARP, Request who-has 8.8.8.8 tell 192.0.2.1, length 28
19:26:15.967318 ARP, Request who-has 8.8.8.8 tell 192.0.2.1, length 28
19:26:16.991390 ARP, Request who-has 8.8.8.8 tell 192.0.2.1, length 28
19:26:18.015337 ARP, Request who-has 8.8.8.8 tell 192.0.2.1, length 28

This in and of itself seems odd to me.  Why is Linux ARPing for an 
address that is obviously not local to the subnet?  (I bound 
192.0.2.1/24, Test-Net-1, to the interface in the NetNS.)

Once I bound 8.8.8.8/32 to the vEth in my main NetNS [1] I saw an ARP 
reply.  But pings to 8.8.8.8/32 timed out.

19:28:22.651995 ARP, Request who-has 8.8.8.8 tell 192.0.2.1, length 28
19:28:22.652010 ARP, Reply 8.8.8.8 is-at ca:b0:eb:fa:ef:ab, length 28
19:28:22.652013 IP 192.0.2.1 > 8.8.8.8: ICMP echo request, id 11202, seq 
1, length 64
19:28:23.711384 IP 192.0.2.1 > 8.8.8.8: ICMP echo request, id 11202, seq 
2, length 64
19:28:24.735382 IP 192.0.2.1 > 8.8.8.8: ICMP echo request, id 11202, seq 
3, length 64
19:28:25.759387 IP 192.0.2.1 > 8.8.8.8: ICMP echo request, id 11202, seq 
4, length 64

When I checked routing on my main NetNS, I found that 192.0.2.0/24 was 
going out my default gateway.  [2]

So I added a route for 192.0.2.1/32 to go out the vEth device that had 
8.8.8.8/32 bound to it.  (But now "via <IP>", just "dev <device>".

ip route add 192.0.2.0/24 dev n1

After doing that, I'm actually able to ping 8.8.8.8 from within the 
network namespace.  IMHO this shouldn't be possible as it's only got a 
route to 192.0.2.0/24.

1) I actually don't know what the main / default routing namespace 
equivalent is.  As far as I can tell, there's no term for it.  At least 
not that I've found.
2) What's surprising by this is that I frequently have 192.0.2.0/24 
bound to a dummy interface on my machine.



-- 
Grant. . . .
unix || die