arp command returning weird info for Apple routers

1

When I use arp -a command, my two Apple routers (Extreme and Express) return weird results, unlike my other results. The IP address isn't a LAN IP address like the other results and the MAC address says "(incomplete)". Below is the output:

MBP:~ User$ arp -a
airport-express.local (169.xxx.xx.xxx) at (incomplete) on en1 [ethernet]
airport-extreme.local (169.xxx.xxx.xx) at (incomplete) on en1 [ethernet]
? (189.xx.xx.xx) at MAC_ADDR on en1 ifscope [ethernet]
? (189.xx.xx.xx) at MAC_ADDR on en1 ifscope [ethernet]
? (189.xx.xx.xx) at MAC_ADDR on en1 ifscope [ethernet]
? (189.xx.xx.xx) at MAC_ADDR on en1 ifscope [ethernet]
? (189.xx.xx.xx) at MAC_ADDR on en1 ifscope [ethernet]
? (189.xx.xx.xx) at MAC_ADDR on en1 ifscope [ethernet]
? (189.xx.xx.xx) at MAC_ADDR on en1 ifscope [ethernet]

How are the routers returning different IP addresses? Is there a way to specify the IP you want in an arp command?

(Note: The actual modem is listed in the normal LAN IP addresses. And a ping, traceroute, or netstat returned nothing but "host is down" when looking into the Apple router IP addresses.)

masterninja01

Posted 2013-09-15T21:43:12.710

Reputation: 198

It might help to have some insight into your network topology. What's acting as the gateway/router? What's acting as the DHCP server? Is there really no NAT involved? Are the Apple routers acting as routers or just bridges? – Spiff – 2013-09-16T02:55:27.960

The DHCP and NAT are given by the modem (gateway) which is located somewhere else. So, the apple routers are acting as bridges right now. – masterninja01 – 2013-09-16T16:30:46.470

Perhaps you're using the routers only as bridges and so they're not participating in the IP networking. – David Schwartz – 2013-09-16T21:48:54.053

Answers

1

Those 169 addresses are probably actually 169.254.x.x addresses, which makes them IPv4 link-local addresses, which devices assign to themselves when they haven't yet gotten, or can't get, an IP address lease via DHCP. It could be that your computer saw your APs while they were rebooting, and tried to contact them at their link-local addresses, but didn't receive a response, perhaps because those units had gotten IP address leases via DHCP by then, and had thus ditched their link-local addresses by then.

It might be interesting to see what your Apple devices are advertising as their addresses via Bonjour:

dns-sd -G v4v6 airport-express.local
^C
dns-sd -G v4v6 airport-extreme.local
^C

(You have to Control-C out of the dns-sd command after it gives you what you need, otherwise it will sit there forever watching for, and reporting, any changes in that information.)

It could be that both of those devices are also in the list of 189.x.x.x hosts for which you were successfully able to resolve a MAC address.

If the Apple devices are still reporting via Bonjour that they have those 169.[254].x.x addresses, then something else might be going wrong with ARP. Perhaps the networking code in those devices forgot to tell Bonjour that they no longer have the 169.254.x.x addresses.

Spiff

Posted 2013-09-15T21:43:12.710

Reputation: 84 656

When I do the dns-sd -G v4v6 command it gives me 3 things: 1) the local ip address, 2) what seems to be the MAC address, 3) the same 169.254.xx.xx ip. – masterninja01 – 2013-09-18T08:01:55.537

The thing that looks like a MAC address is most likely an IPv6 link-local address. So if by (1) you're saying that it has a 189.x.x.x address on your network, can you see if that 189.x.x.x address shows up properly in your ARP table? – Spiff – 2013-09-18T19:38:11.537

At first the ARP table doesn't show the routers with their hostname or the IPv4 link-local. After a while of checking back it finally gives it to me. So, it has the hostname and IPv4 link-local btu below it also has the LAN ip address with "?" for the hostname. It shows both after awhile. – masterninja01 – 2013-09-19T09:38:37.510

Asked: 2013-09-15T21:43:12.710

Viewed: 1 893 times

Active: 2013-09-16T21:41:18.823