Re: BOOTP server sends ARP requests

[Ken Yap <ken@nlc.net.au>]

>Hello everybody!
>
>I've got a problem with my BOOTP-Server.
>It gets a BOOTP-request from a BOOTP-Client.
>Then it looks up the name of the client in der BOOTPTAB-File
>and asks the NAMESERVER for its IP.
>Then the Servers sends the following ARP request for six times:
>who knows 192.168.100.20 tell 192.168.100.10
>(who knows "client" tell "server")
>The nothing happens. After 5 to 7 minutes the clients sends a new
>request and the whole procedure restarts.
>
>This it what tcpdump told me.
>
>Of course no one could answer these requests,
>but why does the server send them.
>It should know the clients MAC from the BOOTPTAB-File.
>It used it already to look up the hostname of the client.

Which bootpd are you running?

You can find a full story from W. Richard Stevens' (vale WRS) book in
the chapter on bootp, but the problem is that bootpd typically runs
as a userland application and has no direct access to the kernel ARP
tables. Therefore bootpd has two options: 1. either broadcast the bootp
reply to the client or 2. inject the MAC address into the ARP cache and
then do a unicast.  The latter can be done either through a special
system call e.g. ioctl(SIOCSRARP,...) to update the ARP cache, or by
broadcasting an ARP update which is then picked up by the server itself.
So it looks like your bootpd is failing to do one of the last two options,
you have to figure out which one.  For example in ISC dhcpd, it was
recommended for some OSes to set a special route for 255.255.255.255 so
that the broadcast update could be received.
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