Date: Thu, 7 Dec 2006 22:05:02 +0800 From: "Paul Hamilton" <paulh@bdug.org.au> To: <a@zeos.net>, <freebsd-questions@freebsd.org> Subject: RE: How does my computer work with an empty arp table? Message-ID: <0a0301c71a08$b09761c0$6600a8c0@w2k2> In-Reply-To: <20061204061444.GA653@host.my.domain>
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> -----Original Message----- > From: owner-freebsd-questions@freebsd.org=20 > [mailto:owner-freebsd-questions@freebsd.org] On Behalf Of a@zeos.net > Sent: Monday, 4 December 2006 2:15 PM > To: freebsd-questions@freebsd.org > Subject: Re: How does my computer work with an empty arp table? >=20 >=20 > On Sun, Dec 03, 2006 at 02:53:44PM -0800, Atom Powers wrote: > > On 12/3/06, a@zeos.net <a@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=20 > > >host one second ago. > >=20 > > The ARP table is a cache of known ARP<->IP addresses. If=20 > there are no=20 > > addresses in the ARP table then the system will send out an ARP=20 > > broadcast to discover the ARP address that belongs to the=20 > IP address.=20 > > Of course only the Ethernet hosts on your local network will be in=20 > > your ARP table. > >=20 > > -- > > -- > > Perfection is just a word I use occasionally with mustard. > > --Atom Ray Powers-- >=20 > Thank you for response. >=20 > But why there is no MAC address of my ADSL modem connected=20 > via Ethernet? Does my host send broadcast frames to=20 > communicate with modem everytime? >=20 > Furthermore, when I ping the modem, a proper entry appears in table: >=20 > -------------------------------------------------------------- > ---------- > $arp -a >=20 > $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=3D0 ttl=3D254 time=3D1.298 ms >=20 > --- rt.my.domain ping statistics --- > 1 packets transmitted, 1 packets received, 0% packet loss=20 > round-trip min/avg/max/stddev =3D 1.298/1.298/1.298/0.000 ms >=20 > $arp -a > rt.my.domain (192.168.1.1) at 00:13:49:61:f9:b2 on rl0 [ethernet] > -------------------------------------------------------------- > ---------- >=20 > But no entry appears when I communicate trough the modem. >=20 > How can I watch what is going on? >=20 You can see what's going on by su'ing to root, and typeing: =20 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 =20
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