Debugging an ARP issue (no resp to ARP requests)

From: krinkin.m.u@gmail.com (Mike Krinkin)
To: kernelnewbies@lists.kernelnewbies.org
Subject: Debugging an ARP issue (no resp to ARP requests)
Date: Tue, 20 Jan 2015 03:11:06 +0300	[thread overview]
Message-ID: <20150120001106.GA12045@kmu-tp-x230> (raw)
In-Reply-To: <CAC+QLdSjV6i6Hs5em0Xm3W6hZq6GuqF=ZGfrMGTdmjoxKgTsXA@mail.gmail.com>

Hi,

On Mon, Jan 19, 2015 at 03:42:58PM -0800, Mandeep Sandhu wrote:
> Hi All,
> 
> Please let me know if this is not the correct ML for such a question
> (or if there's a more appropriate list for it).
> 
> I'm currently debugging an issue where Linux is not responding to ARP
> requests (testing with custom network interface h/w).
> 
> I have 2 network interfaces which are basically interfaces on a custom
> network device sitting on the PCI bus.
> 
> My setup looks like follows:
> 
> xeth0 - 192.168.2.1
> xeth2 - 192.168.2.2
> 
> xeth0/2 are the interfaces created for the custom device (via a lkm)
> 
> Both these interfaces are on the local machine and are connected
> back-to-back on the custom device, so packets sent from one arrive on
> the other.
> 
> I'm testing this setup by sending a single ARP pack, forced out of one
> interface, for the other interface's IP.
> 
> $ sudo arping -c 1 -i xeth0 192.168.2.2
> ARPING 192.168.2.2
> 
> --- 192.168.2.2 statistics ---
> 1 packets transmitted, 0 packets received, 100% unanswered (0 extra)
> 
> If I start tcpdump on the receive interface (xeth2), I see the ARP
> packet arrive, but I don't see any response go out (I've attached the
> pcap file for anyone interested in looking at the received packet).
> The ARP packet's contents seem to be proper under wireshark.
> 
> Is there any statistics I can look at to see if the packet was dropped
> by linux during ARP processing?
> 
> I've put debug stmts in the LKM, but I don't see the kernel IP layer
> calling the xmit function for an ARP reply (however the xmit function
> for the ARP request, from xeth0, is being called).

Have you tried to put debug prints in arp_process function?

> 
> I don't see any drops in "netstat -s" output as well.
> 
> Any hints appreciated.

Just a random guess, but try to disable reverse path filtering (rp_filter).

> 
> Thanks,
> -mandeep

> ????\x02\0\x04\0\0\0\0\0\0\0\0\0\0\0\x04\0\x01\0\0\0w??T?g\b\0<\0\0\0<\0\0\0??????\x02\x10\x18?~e\b\x06\0\x01\b\0\x06\x04\0\x01\x02\x10\x18?~e??\x02\x01\0\0\0\0\0\0??\x02\x02\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0
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