Date: Wed, 29 Aug 2001 00:26:15 +0100 (BST) From: Joshua Goodall <joshua@roughtrade.net> To: <freebsd-net@FreeBSD.ORG> Subject: Re: Gratuitous ARP (summary) Message-ID: <Pine.LNX.4.33.0108282339230.23691-100000@elm.phenome.org> In-Reply-To: <Pine.LNX.4.10.10108281117590.1887-100000@ruby.ccmr.cornell.edu>
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On Tue, 28 Aug 2001, Mitch Collinsworth wrote: > It's not clear what Jushua is asking for, but my guess is proxy arp. > See arp(8), in particular the -s flag. Then I will clarify and say that what I want is precisely described in section 4.7 of TCP/IP Illustrated Vol. 1 (Stevens) and looks like this on a tcpdump: bloo:~# tcpdump -teli fxp0 arp net 10.1.1 & tcpdump: listening on fxp0 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". However a synthesis of the suggestions from Terry & Ruslan yields what I wanted: 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. b) in the case where the address is the primary address of the interface, an ifconfig will cycle the route *and* delete the ARP cache entries for the subnet. However, using arping to the interface primary address (not broadcast) gives a correctly formatted, non-destructive gratuitous ARP. So I'm happy now. Curiously, an ifconfig for an alias will generate a gratuitous ARP for that alias even if the -arp option is supplied. I'm not sure this is correct behaviour. Joshua To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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