Beginning a few months ago, two of the systems on my LAN, a desktop and a laptop, have periodically become mutually uncommunicative. I believe it started occurring when I did a massive -Syu on the laptop, installing six months worth of updates for the laptop at once and effectively upgrading 1/2 of the installed packages... but I'm not positive. The issue still occurs regularly with both systems running 3.0.x kernels.
The issue typically occurs after the desktop has been up for a few days, but on rare occasion it's present when the desktop boots. When the issue does occur, other systems on the LAN are able to reach both systems as per normal. It's a simple switched network, and both systems have manually-assigned static IPs.
I've done a fair bit of testing with tshark and have determined that the issue presents itself as such: The laptop is able to reach the desktop, with ICMP echo requests and nmap scans going through just fine. However, a response is never received by the laptop.
When running tshark on the laptop and sending traffic from the desktop, an oddity occurs. Although the laptop's static IP is 192.168.0.3 (and this is explicitly present as an argument to ping, nmap, etc.) the desktop is actually sending out an ARP broadcast, attempting to get the MAC of an IP in the Philippines - far outside the LAN's 192.168.0.0/24 range.
What makes this doubly odd is that I've manually added a correct entry into the desktop's ARP cache for the laptop's IP (via arp -s), yet it's apparently ignored and the bogus ARP broadcasts continue. Interestingly, the arping utility is able to broadcast for the correct IP's MAC, and does receive a reply from the laptop - but the issue persists for everything except arping.
If I put an unused IP (e.g. 192.168.0.32) along with the laptop's MAC into the desktop's ARP cache and ping that new address, predictably the ICMP echo requests do get to the laptop, but of course aren't acknowledged as the laptop isn't using that IP. Actually moving the laptop to another IP results in the systems being able to reach each other normally, but the bizarre ARP broadcasts persist when attempting to reach the laptop's old IP, 192.168.0.3.
A further observation I made (which simply added to the confusion): An Arch VM, running with bridged networking on the desktop, is perfectly capable of reaching the laptop - I can SSH in without issue. Yet nothing (aside from arping) is even able to get a reply from the laptop when run from a shell on the desktop. Because the VM works, I've essentially ruled out any sort of hardware or driver issue.
The part where I'm stuck is... what's below the ARP cache? I have no idea why "ping 192.168.0.3" doesn't work when there is an explicitly-placed entry for the IP in the ARP cache, and how an explicitly-specified IP can turn into an ARP broadcast for a completely different IP (in the Philippines, no less) just baffles me.
Any help is appreciated.
On Sat, Sep 3, 2011 at 4:39 PM, swh root@pwnly.com wrote:
Any help is appreciated.
Just to get prerequisites out of the way, have you done the
for f in $(find /etc -type f -name '*pacnew'); do orig = ${f%.pacnew} diff -u $orig $f > /tmp/$orig.diff done
then review the config changes and make updates accordingly dance?
On Sat, Sep 3, 2011 at 5:44 PM, Jeremiah Dodds jeremiah.dodds@gmail.com wrote:
Just to get prerequisites out of the way, have you done the
for f in $(find /etc -type f -name '*pacnew'); do orig = ${f%.pacnew} diff -u $orig $f > /tmp/$orig.diff done
then review the config changes and make updates accordingly dance?
The desktop is my primary system, so I do tend to keep its config files fully in order.
The laptop is something of a test bench system, so it's got a number of *.pacnew files, but I'm convinced they're all unrelated to networking. Most of them are actually user/group related and are there just because they're necessarily modified after initial install - /etc/{group,passwd,shadow,sudoers} and such.
It really seems like an extreme long-shot, that the laptop would be able to consistently cause strange low-level networking issues in the desktop, but I'm grasping at straws here.
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