DHCP Giving Out IP For Wrong Subnet

sthaug at nethelp.no sthaug at nethelp.no
Mon Dec 16 18:19:16 UTC 2013

> > We've been running into this problem where we have the DHCP server giving out a wrong IP address for a device which is running on a different subnet.  These subnets are on VLANs.  For example, below are configured two shared networks:  The first, ADMIN-POOL, is on eth3.  It is the physical interface.  Below that is the shared network GENERAL, which is on eth3.50 and is a virtual interface on eth3.
> There’s your problem. Don’t use interface aliases (multiple VLANs on one physical interface) with a DHCP server - it gets confused. The DHCP server somewhat bypasses the operating system IP stack for local clients.

It's possible that this is a Linux-specific problem. I have used ISC
DHCP with multiple VLAN-based interfaces on FreeBSD without problems.

> Instead, you should connect all of these VLANs to a switch or router and then use a DHCP relay, per VLAN, to relay these requests to the single address of the DHCP server.

Fully agreed.

Steinar Haug, Nethelp consulting, sthaug at nethelp.no

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