arp entries for local interfaces disappearing ?

[Luigi Rizzo]

Can someone explain why, in my arp table, i don't have an
entry for my local machine ? I just noticed this by chance,
on my 4.9 box at the office, and now again at home:

    > ifconfig
    rl0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
	    inet 10.0.1.55 netmask 0xffffff00 broadcast 10.0.1.255
	    inet 10.1.1.236 netmask 0xffffff00 broadcast 10.1.1.255
	    ether 00:40:f4:34:b1:4b
	    media: Ethernet autoselect (100baseTX <full-duplex>)
	    status: active
    lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
	    inet 127.0.0.1 netmask 0xff000000 
    > arp -na
    ? (10.0.1.52) at 00:02:2d:08:a3:3b on rl0 [ethernet]
    ? (10.0.1.64) at 52:54:05:de:99:7c on rl0 [ethernet]
    > 

the interface is actually up and moving traffic -- the box is talking
to 10.0.1.52.
The route magically reappears if i try to open a session to the
local box using the interface's address:

    > ssh 10.0.1.55
      ... bla bla bla ...
    > arp -na
    ? (10.0.1.52) at 00:02:2d:08:a3:3b on rl0 [ethernet]
    ? (10.0.1.55) at 00:40:f4:34:b1:4b on rl0 permanent [ethernet]
    ? (10.0.1.64) at 52:54:05:de:99:7c on rl0 [ethernet]

and it's marked permanent, so it should not expire! Yet it
looks like they do anyways...

cheers
luigi