[mythtv-users] Questions on PXE booting a frontend

Michael Cook waxrat at comcast.net
Mon Aug 3 23:25:08 UTC 2009


Brian Wood <beww at beww.org> writes:

> On Monday 03 August 2009 13:04:18 Ronald Frazier wrote:
>> I too have had no problem letting my myth backend run dhcp for the
>> diskless frontend systems and letting my router handle dhcp for
>> everything else. My dhcpd.conf file is similar to Jelte's, but there
>> are a few differences. Here is what I have. Don't ask my why I did
>> things any specific way. I just copied setting from some tutorials and
>> adjusted accordingly.
>
> If it's working I wonder if it is because of luck. Perhaps one of
> the machines takes a lot longer to respond to a DHCP request or
> some other mechanism is preventing both dhcp servers from trying
> to work at the same time, or they are sufficiently stubborn to try
> again when they get interfered with, and the retry intervals are
> sufficiently different to prevent collision (not "collision" in
> the ethernet sense, but in the "two dhcp servers trying to work at
> the same time" sense).
>
> The conventional wisdom is that more than one dhcp server on a
> network is asking for trouble.

Well, the protocol was designed to allow multiple DHCP servers on
the same broadcast medium.  But it's possible that doing so would
uncover bugs in one or another DHCP server.  (Years ago I
encountered a DHCP server that would leak a little memory each time
it saw traffic from another DHCP server, and the leaky server wouuld
eventually crash when it ran out of memory.)

FWIW, the protocol is:

1. client: DISCOVER ("Is there anybody out there?")
2. server: OFFER ("You could ask for this here lease, if you wish")
3. client: REQUEST ("Okay, give me this here lease")
4. server: ACK ("You got it")

When the client sends a DISCOVER and there are multiple DHCP
servers, the client might get multiple OFFERs, but the client would
send only one REQUEST (and would let the other OFFERs will expire).

FWIW, I've been running two DHCP servers on my home network for
years.  One is my Vonage Linksys router, and the other is my mythtv
backend.  (I've configured them to use non-overlapping ranges of IP
addresses within the subnet, but I think that's not strictly
necessary.)

Michael


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