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Subject: RE: boot problems & related on hkbaja47
From: [email protected] (Jeff Hill)
To: "'Ned Arnold'" <[email protected]>
Cc: "EPICS-tech-talk (E-mail)" <[email protected]>
Date: Thu, 18 Jun 1998 10:05:01 -0600
Ned,

I read my TCP/IP book this morning and discovered some additional information
on ARP. ARP cache entries typically expire, and are therefore deleted, 20 minutes 
after the last time that they were used. When a host boots it issues a "gratuitous
ARP request" which is only an ARP request for the Ethernet address of the host that
is booting. If any hosts on the network have the booting host's IP address then they 
reply, and the booting host logs a duplicate IP address message on its console.
ARP requests are broadcasts, and therefore all hosts on the LAN receive
this "gratuitous ARP request". It is therefore possible for correctly implemented 
(apparently implementations vary) ARP caches to update themselves when they
see a "gratuitous ARP request" which specifies a new Ethernet address/IP address
binding for the host that is booting.

> 
> This seems to explain my recent problem, but I replace CPU's in the field
> and give them the same IP number without needing to reboot all the 
> workstations. It only seemed necessary if the ARCH changed. 
> 
> Any explanations ?
> 

Only unreliable speculations:

o Apparently BSD derived systems update the ARP cache in response to
"gratuitous ARP requests", and SVR4 derived systems do not. Perhaps 
Solaris doesn't?

o The Ethernet addresses are assigned to manufactures in blocks. Perhaps 
there is something built into the Solaris ARP where it does not update the cache in
response to the "gratuitous ARP request" if the Ethernet address changes
to a different allocated block of Ethernet addresses??? This might be
a security measure which attempts to prevent "spoofing" (i.e. assumption)
of some other hosts IP addresses.

Jeff



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