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I run arp --delete and it leaves the entry but it's simply marked as incomplete. I expected --delete to entirely remove the entry.
Does anyone understand what is occurring here?

$ sudo arp -avn
? ( at 00:cc:cc:bb:dd:86 [ether] on eth0
Entries: 1  Skipped: 0  Found: 9

$ sudo arp -d

$ sudo arp -avn
? ( at <incomplete> [ether] on eth0

This is on Ubuntu 10.04 .

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1 Answer 1

up vote 6 down vote accepted

The entry will be removed, just be patient.

(If you want the shortest possible answer: incomplete == deleted)

Let's say "delete" is the wrong word for the action. What's really happening here is that the entry is manually set to the state "request sent, no answer" (thus "incomplete" ARP process) as if the machine would be really unreachable.

Now, the entry will be completely removed soon unless it gets a new valid ARP response in the meantime. In that case the entry would be re-added anyways even if it was removed instead of being marked as incomplete. So there's no actual pro or con to this behaviour.

But keep in mind that we're talking of a cache. Deleting things from caches is hard and expensive. It's way more efficient to invalidate an entry and wait if it gets replaced before it is finally removed. But for the system it's totally no difference if the entry is gone from the list or just marked incomplete.

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And there is no way to really delete it so that a subsequent need to know the destination would trigger a "new" ARP query? –  Skaperen Feb 7 '13 at 2:09
But exactly that's the case here. If anything looks up an entry that is marked "incomplete" a new ARP request is send. If it is answered, the entry is updated and no longer "incomplete". If no answer returns, the entry is removed from the list after some time. (That's why I think it's pretty useless to "--delete" an entry manually. If the machine lives, the entry will be refreshed. If it is really gone, then why "--delete" the entry manually anyways?) –  Karma Fusebox Feb 7 '13 at 2:29
... it comes to my mind that you might want to "change" the ARP address for an IP address. In this special case you can "--delete" manually and have the next ARP request return the new address. But for this scenario, it doesn't matter at all if the entry is visibly removed from the list or marked "incomplete". For the tech involved, it's the same. –  Karma Fusebox Feb 7 '13 at 2:45

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