How does my computer work with an empty arp table?

[Paul Hamilton]

> -----Original Message-----
> From: owner-freebsd-questions at freebsd.org 
> [mailto:owner-freebsd-questions at freebsd.org] On Behalf Of a at zeos.net
> Sent: Monday, 4 December 2006 2:15 PM
> To: freebsd-questions at freebsd.org
> Subject: Re: How does my computer work with an empty arp table?
> 
> 
> On Sun, Dec 03, 2006 at 02:53:44PM -0800, Atom Powers wrote:
> > On 12/3/06, a at zeos.net <a at zeos.net> wrote:
> > >My computer is connected to ISP via ADSL and works properly.
> > >
> > >I typed
> > >
> > >arp -a
> > >
> > >and saw an empty table, although I pinged successfully an Internet 
> > >host one second ago.
> > 
> > The ARP table is a cache of known ARP<->IP addresses. If 
> there are no 
> > addresses in the ARP table then the system will send out an ARP 
> > broadcast to discover the ARP address that belongs to the 
> IP address. 
> > Of course only the Ethernet hosts on your local network will be in 
> > your ARP table.
> > 
> > --
> > --
> > Perfection is just a word I use occasionally with mustard.
> > --Atom Ray Powers--
> 
> Thank you for response.
> 
> But why there is no MAC address of my ADSL modem connected 
> via Ethernet? Does my host send broadcast frames to 
> communicate with modem everytime?
> 
> Furthermore, when I ping the modem, a proper entry appears in table:
> 
> --------------------------------------------------------------
> ----------
> $arp -a
> 
> $ping -c 1 rt # It is my modem
> PING rt.my.domain (192.168.1.1): 56 data bytes
> 64 bytes from 192.168.1.1: icmp_seq=0 ttl=254 time=1.298 ms
> 
> --- rt.my.domain ping statistics ---
> 1 packets transmitted, 1 packets received, 0% packet loss 
> round-trip min/avg/max/stddev = 1.298/1.298/1.298/0.000 ms
> 
> $arp -a
> rt.my.domain (192.168.1.1) at 00:13:49:61:f9:b2 on rl0 [ethernet]
> --------------------------------------------------------------
> ----------
> 
> But no entry appears when I communicate trough the modem.
> 
> How can I watch what is going on?
> 

You can see what's going on by su'ing to root, and typeing:  

  tcpdump -ni rl0

then try pinging, or sending your traffic.  You will then see the ADSL modem
reply (or the PPoE traffic etc).

Cheers,

Paul