ARP configuration parameters?

[jcallen at narsil.com (Jerry Callen)]

Does anyone know where I can find a good description of the configuration
parameters for ARP? I'll explain why in a moment, but the parameters I'm 
talking about are those described in /proc/sys/net/ipv4:

$ head /proc/sys/net/ipv4/*
==> /proc/sys/net/ipv4/arp_check_interval <==
6000

==> /proc/sys/net/ipv4/arp_confirm_interval <==
30000

==> /proc/sys/net/ipv4/arp_confirm_timeout <==
500

==> /proc/sys/net/ipv4/arp_dead_res_time <==
6000

==> /proc/sys/net/ipv4/arp_max_tries <==
3

==> /proc/sys/net/ipv4/arp_res_time <==
500

==> /proc/sys/net/ipv4/arp_timeout <==
6000

==> /proc/sys/net/ipv4/ip_dynaddr <==
0

==> /proc/sys/net/ipv4/ip_forward <==
0

The reason I'm wondering about these is that there seems to be an
awful lot of ARP traffic on my network. The network looks, at the
moment, like this:

MediaOne     <---> SonicWall  <---->  cheap  <--->  Linux box A
Cable modem        (firewall)          hub            ("salt")
                    ("sonic")           ^
                                        |
                                        |
                                        v
                                NetGear FS508 10/100
                                   enet switch
                                 ^      ^      ^
                                 |      |      |
                  Linux box B <--+      |      +--> W98 (ugh) laptop
                  ("ginger")            v             ("cayenne")
                                     Macintosh
                                    ("cinnamon")

I'm running tcpdump on "salt" (which is on the hub with the firewall
machine "sonic"). I see several bits of odd behavior:

1) Some machines appear to be sending non-broadcast ARP requests. For 
   instance, Here's a case where "salt" sent an ARP request specifically to
   "sonic" TO GET SONIC's ADDRESS!

   13:52:28.835598 0:40:5:50:99:13 0:e0:4f:23:78:0 arp 42: 
                   arp who-has sonic.narsil.com tell salt.narsil.com
   13:52:28.835598 0:e0:4f:23:78:0 0:40:5:50:99:13 arp 60: 
                   arp reply sonic.narsil.com is-at 0:e0:4f:23:78:0

2) Generally the SonicWall replied twice to each ARP request. I will
   send mail to Sonic to ask them about this; RFC826 (which describes 
   ARP) doesn't seem to suggest this behavior. Has anyone seen other
   systems do this?

3) I see a lot of ARP traffic (in particular these non-broadcast ARPs)
   about every 6 minutes. Unless the units are very strange (hundreths
   of a minute?) this doesn't seem to correspond to any of the tunable
   parameters in /proc.

Anyway, before I start grovelling source code, I figured I'd ask around
to see if there's a write-up on this stuff somewhere.

[I'm also curious as to whether any of this traffic might be generated
by the ethernet switch, which must maintain its own cache of which
MAC addresses are on which ports. But this is a pretty low-end switch,
so I doubt it's got much in the way of brains...]

-- Jerry Callen                      Mobile: 617-388-3990
   Narsil                            FAX:    617-876-5331
   63 Orchard Street                 email:  jcallen at narsil.com
   Cambridge, MA 02140-1328

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