Date: Sat, 1 Sep 2001 12:49:38 -0400 From: Paul Chvostek <paul@it.ca> To: Joshua Goodall <joshua@roughtrade.net> Cc: net@freebsd.org Subject: Re: Gratuitous ARP (summary) Message-ID: <20010901124938.A41615@gahch.it.ca> In-Reply-To: <Pine.LNX.4.33.0108282339230.23691-100000@elm.phenome.org>; from joshua@roughtrade.net on Wed, Aug 29, 2001 at 12:26:15AM %2B0100 References: <Pine.LNX.4.10.10108281117590.1887-100000@ruby.ccmr.cornell.edu> <Pine.LNX.4.33.0108282339230.23691-100000@elm.phenome.org>
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On Wed, Aug 29, 2001 at 12:26:15AM +0100, Joshua Goodall wrote: > 0:a0:c9:ca:73:5f Broadcast arp 42: arp who-has 10.1.1.1 tell 10.1.1.1 > > and is primarily used for collision detection and network-local arp-cache > priming, but also has applications in IP address migration (a common > high-availability technique). I should not have described it as an "arp > reply" since it is, of course, an "arp who-has". Yeah, that got me too -- I thought you were trying to manually implement proxy-arp by sending out is-at's. I saw this behaviour with a Cisco router that was managing a dialup pool and sending gratuitous is-at's every time a new IP address was assigned out of the pool to a new PPP session. It became a problem when the pool got into the 1000's of lines, because every machine on the network was maintaining a huge honkin' local arp table rather than just relying on routing. > a) in the case where the address is an alias, re-issuing the > ifconfig ... alias results in a gratuitous ARP for the alias address > without losing the subnet route & ARP cache entries. However I use a > netmask of 255.255.255.255 for all aliases in the same subnet as the > primary, in line with the ifconfig(8) manual. FWIW, on aliased IPs, I seem to be unable to generate the who-has arps unless I specify the netmask. Just doing "ifconfig if0 a.b.c.d alias" does not seem to be sufficient. But the actual value of the netmask should not affect ARP, since ARP doesn't know about CIDR. > So I'm happy now. Woo hoo! :) I'd still like to know how to generate a gratuitous is-at. I mean, besides forging it in dsniff's arpspoof. -- Paul Chvostek <paul@it.ca> Operations / Development / Abuse / Whatever vox: +1 416 598-0000 IT Canada http://www.it.ca/ To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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