Alan Cox wrote: > > You increase it and you shortcut on shared lans. Thats really a seperate > issue to the question of which source is used. If you loopback someone > elses address on your own lo device I'm not suprised weird > shit happens, put the alias on eth0 where it belongs. > Woah! I'm not talking about JUST load-balanced networks. This is a far more generic problem. As an example, I have a router/firewall for the office that has two interface cards, both with perfectly valid internal addresses - these addresses aren't used anywhere else on the network. Two of the interfaces are: 172.20.240.2/24 and 172.24.0.1/16. My default gateway is 172.20.240.1 and there aren't any other non-interface routes. If I do: # arp -d 172.24.0.80 # ping -I 172.20.240.2 172.24.0.80 I see: 16:18:40.856328 arp who-has 172.24.0.80 tell 172.20.240.2 16:18:40.856431 arp reply 172.24.0.80 is-at 0:50:da:44:f:37 Now, since 172.24.0.80 is a Linux box, it's happily adding 172.20.240.2 into its ARP table and reply to it, hence the reply. But if I was to do this in the other direction (arp -d 172.20.240.1; ping -I 172.24.0.1 172.20.240.1) then I'd lose connectivity over my default route because 172.20.240.1 won't accept ARP packets from IP numbers not on the connected subnet. The <incomplete> ARP entry will block any further ARP requests from valid IP numbers. This, in my opinion, isn't on. There must be thousands of Linux installations out there that (1) have more than one interfaces and (2) are connected to routers or firewalls that drop ARP requests in the same way. Actually, it's not that bad in this case as the next hop will probably send an ARP request at some point which will override it - but that's really not the point. If the routeing was asymetric, or the next hop had a static ARP entry for me, all communication would quite simply be lost. It'd just take the first packet after an ARP entry times out to be from the wrong IP address, and the host would be off the net. I personally don't think "shared LANs" should be favoured over best practice. Even in the case of shared LANs, nothing "breaks" as David Miller suggests would happen if the ARPs were fixed. Thanks, Richard - : send the line "unsubscribe linux-net" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
