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Old 01-20-2005, 03:27 PM   #1
Registered: May 2001
Posts: 74

Rep: Reputation: 15
arp giving me incomplete mac addresses

Hi Guys,
In recent times I'm getting this annoying problem with linux more and more. It seems I get an <incomplete> for an Ip address in my arp table. I noticed it for the first time a few years ago and now it seems to be happening quite frequently. It doesn't seem to matter what version of RedHat I use. Most of the boxes have custom kernels due to specific needs not met in the canned kernel. The most frequent occurence happend like this:

Got an email from the ISP stating they are changeing networks. The email included a new IP address mask and gateway. I changed the box to use the new info and cannot ping the gateway address. Ran arp and found the <incomplete> entry for the new address. Changed it back to the old one and everything worked fine. In this instince I do not know what the gateway device manufacturer is (no MAC, no block Id, = idunno who made the card) but have seen this problem with many gateway devices including CISCO.

I figure if I replace the nic in the firewall everything will work, but as stated before it is getting more bothersome as the problem persists.

Thanks for your time in reading this.
Old 02-14-2005, 10:23 PM   #2
LQ Newbie
Registered: Feb 2005
Location: Burnaby
Posts: 4

Rep: Reputation: 0
When your computer attempts to contact another PC, it will send an ARP Request out. The PC with that IP will reply with its MAC address. If there is no PC with that IP address, your PC will not get a reply, and (in Linux) your ARP table will show that IP as incomplete. So there is probably no problem on your end. Your ISP has probably not yet changed their router's IP address.

You can test your PC by running a sniffer. If you have a GUI, I recommend ethereal (easy and neat). If you do not have a GUI, I suggest using tcpdump. Run man tcpdump for help on using tcpdump. Once you run the sniffer, try pinging you gateway. You will notice that you send out at least 1 ARP Request. You will probably not see an ARP Reply though.

If you do not get an ARP reply, contact your ISP and tell them exactly what you did and that you did not get an ARP Reply. They will tell you what IP your gateway is using and when the switch will take effect.

Good Luck!!
Old 01-02-2014, 06:36 AM   #3
Registered: Apr 2005
Location: France
Distribution: Debian
Posts: 127

Rep: Reputation: 19
I get this for my Vista PC ... really need to ditch that OS.

Before you ask, the firewall is turned off on my Vista box, the kids use it for gaming and I have a DSL router - in router mode.


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