dhcp-server subnet configuration for interfaces not being listened to

[Simon Hobson]

Amir Sagie wrote:
>I've recently been using isc-dhcp-server v4.2.4 on a box with multiple
>network interfaces and noticed that it will happily accept subnet
>configurations which are bound to interfaces on which dhcpd is not
>configured to run. Example:
>
># dhcpd -t -cf /etc/dhcpd.conf eth1
>
>where /etc/dhcpd.conf includes:
>
>subnet 192.168.1.0 netmask 255.255.255.0 { ... }
>subnet 192.168.2.0 netmask 255.255.255.0 { ... }
>
>Assuming eth1, eth2 are bound to 192.168.1.0/24, 192.168.1.0/24
>respectively, dhcpd issues no warning when eth2 is omitted from the
>daemon launch command, nor does is log "dropped" DHCP discover messages.
>I am aware that interfaces are expected to come & go and their addresses
>to change, but in more static environments I think a log warning (upon
>daemon launch) is called for, eg:

How does the DHCP server know that those subnets aren't actually being used ?
While it's somewhat contrived, it's perfectly possible that there is a relay agent involved, and the server is actually servicing those subnets - just not directly.

In general, it's expected that the administrator knows what he's doing. The ISC server supports some incredibly flexible setups, and it's impossible for the parser to test all combinations.