LinuxSA Mailing list archives

  From: Glen Turner <glen.turner@adelaide.edu.au>
  To  : linuxsa@linuxsa.org.au
  Date: Wed, 07 Apr 1999 03:48:22 +0930

Re: Duplicate MAC addresses

Hi Richard,

Media access control addresses do not need to be unique
in IP.  IP works fine with multidrop serial lines for
example.

However, IEEE 802-series MAC addresses are *defined*
as being globally unique.  And I can actually
dream up two scenarios where this difference counts
[one of the joys of working in a uni environment
is that you see a *lot* of weird stuff :-) ].



Option (2) fails in a proxy ARP environment.

       A-------1-------| |
                       |C|-----3---------
       B-------2-------| |

Let's say C proxy ARPs onto subnet 3 for subnets 1 and 2.  If
A and B have the same ethernet address, the proxy ARP responses
generated by C will be indeterminate, leading to packet loss
for either A or B.

Proxy ARP makes no sense for MAC address schemes that are not
globally unique.  A popular application of proxy ARP is to
do link-layer protocol conversion without building a full
IP router.  So you typically see it in IP-over-token-ring/
IP-over-ethernet and IP-over-ethernet/IP-over-LocalTalk
protocol converters.


Option (3) may fail if both machines do DHCP and share the
same DHCP server.  The correct behaviour for a DHCP server
on detecting that one MAC address has obtained two IP addresses
is to reject the lease renewal request from the machine that
booted first and to log an attempted denial of service attack
[implied by "Security considerations" section of RFC1531].
Continued rejection of the IP address lease renewal will
eventually leave the non-last booting machine without an IP
address.  Ouch.

Cheers,
Glen

-- 
 Glen Turner                               Network Specialist
 Tel: (08) 8303 3936          Information Technology Services
 Fax: (08) 8303 4400         The University of Adelaide  5005
 Email: glen.turner@adelaide.edu.au           South Australia

-- 
Check out the LinuxSA web pages at http://www.linuxsa.org.au/
To unsubscribe from the LinuxSA list:
  mail linuxsa-request@linuxsa.org.au with "unsubscribe" as the subject