ARP(4) spoofing?

Ian Smith smithi at
Mon Mar 17 15:00:38 UTC 2008

In freebsd-questions Digest, Vol 207, Issue 2
On Mon, 17 Mar 2008 03:29:04 -0600 Modulok <modulok at> wrote:
 > > > Would this be ARP(4) spoofing, or is it just me? How would I
 > > > confirm it?
 > > >
 > > > arp: is on lo0 but got reply from xx:xx:xx:xx:xx:xx on em1
 > > > This is on a FreeBSD router, em1 is Internet-facing. (em0)
 > > > is LAN facing and permanent entry in the arp cache. This happens
 > > > constantly and is slowly filling my log files.
 > > What does an "ifconfig -a" on your machine show? It looks like you've
 > > configured your loopback interface to also have
 > [-]Modulok> ifconfig -au inet
 >         options=b<RXCSUM,TXCSUM,VLAN_MTU>
 >         inet netmask 0xffffff00 broadcast
 >         options=b<RXCSUM,TXCSUM,VLAN_MTU>
 >         inet 66.x.x.x netmask 0xffffff80 broadcast 66.x.x.255
 > lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
 >         inet netmask 0xff000000
 > Just for fun, the entry in the arp cache:
 > [-]Modulok> arp -an | grep
 > ? ( at (myEthernetAddress) on em0 permanent [ethernet]

You've omitted even obfuscated ether addresses, and haven't said if
xx:xx:xx:xx:xx:xx is one of yours or one of your LAN's or an unknown,
so I'm assuming the latter, and that address isn't shown by arp -an?

Does 'netstat -finet -rn' show anything useful re MACs connected?

 > Concerning the arp(4) DIAGNOSTICS section (Just thinking aloud here:)
 > "Physical connections exist to the same logical IP network on both if0 and
 > if1."
 > Doubtful: LAN---em0[FreeBSD]em1---modem---Internet

What sort of modem?  cable/DSL?  using PPPoE?  Router or bridge?  I'm
wondering if the modem might sometimes? think it was too? Or
the ISP could be misconfigured, depending on how you're connected.  From
the above, looks like em1's on a /25 public subnet?

Best way to find out might be watching something like:
  # tcpdump -pen -i em1
adding such as 'not tcp and not udp' and/or 'and not port blah' and/or
'not host blah and ..' until it's not too busy.

If this is happening every 10 minutes, maybe there's a ping or udp
packet or such associated too? 

 > "an entry already exists in the ARP cache ... and the cable has been
 > disconnected from if0, then reconnected to if1."
 > Nope.
 > "This message can only be issued if the sysctl
 > is set to 1"
 > While I could set the relevant sysctl variable to prevent it from
 > being logged, (which I'll probably end up doing) when strange things
 > happen, I usually like to know about them.

Yeah, usually best not swept under the carpet.

 > Disable the dynamic ARP cache on the external interface and make
 > permanent entries to the ISP's gateway and DNS servers? Perhaps.

What arp entries have you for these now?  (obscure at will, though the
first 3 octet manuf/product codes might be interesting/useful).  Still
don't get what your 'modem' is, if both/all these servers are visible.

 > However, in the event they ever change hardware (and fail to spoof
 > their previous ethernet address), I'd have to manually edit the ARP
 > 3:00am...on a Sunday. Plus these ARP replies, while
 > annoying, are not really harming anything as FreeBSD's ARP appears to
 > prevent address takeover via gratuitous, un-solicited, impersonating
 > ARP replies.

Sure, but it's (at least) misconfiguration, somewhere, by someone ..

 > Come to think of it, that might be it. I haven't looked into whether
 > or not these are replies triggered by requests from the local host (If
 > only I knew a way to do such a thing.) Logic initially rejects the

Again, tcpdump, running in as many terms as needed (here, two)

 > notion. As why would this box be sending out a gratuitous ARP request
 > every 10 minutes through the wrong interface for the given address?

Smells more like incoming so far, on em1 .. did I mention tcpdump? :)

cheers, Ian

More information about the freebsd-questions mailing list