Web interface for managing an ISC DHCP server

For monitoring a failover setup, I would use OMAPI commands (eg. via omshell) to check the status of the failover-state object.

Considering you have properly configured your dhcpd server to enable OMAPI access, you could use something like:

$ cat check-failover.cmd <<EOF
server localhost
port 7911
key omapi_key <KEY>
connect
new failover-state
set name = "<FAILOVER NAME>"
open
EOF

Then use it like:

$ omshell < check-failover.cmd | grep state
partner-state = 00:00:00:02
local-state = 00:00:00:02

With state being an integer which value is described in dhcpd.8 man page (1 is "startup", 2 is "normal", etc.).

It should be quite easy to write a Nagios/Shinken probe from this.

You can have several DHCP servers on your network, but you cannot set a priority. Addressless clients will broadcast to find one, and will then choose one however they like (usually whichever replies first) and accept its offer.

I suggest that if your existing DHCP server is using its lease data to support anything (port forwarding as you mentioned and maybe dynamic rDNS), that may not work reliably if you add a second DHCP server.

As long as they don't share the same range of IPs, it is possible for them to be authoritative for their own scope (but not the subnet, or globally). Otherwise, or if this configuration can't be specified with your software, neither may be authoritative; this might cause problems for roaming clients connecting to your wireless network. In particular, if they have an RFC1918 address in your subnet assigned while they were elsewhere, for example, they would not get a DHCPNAK when attempting to renew the lease.