how to force an unresponsive host to renew its dhcp lease


dhcp server has stopped for some reason. during the outage, the lease expired for one of the clients. i noticed the host (client) is unreachable, and need to do some work on it- right now.

more info:

found the client was unreachable via ssh or ping, and tcpdump on the gateway showed no traffic to its ip address except failed pings. looked up its mac in /etc/dhcpd.conf and started tcpdump on the gateway to listen for packets with that mac address as its source (perhaps it picked up a rogue ip address, etc.?). this eventually showed the host periodically tried to renew its dhcp lease:

tcpdump -i em0 -nt ether src 40:16:7e:28:b6:15 > xid:0xdc16986e [|bootp] [tos 0x10] > xid:0xdc16986e [|bootp] [tos 0x10]

that discovery was followed by (on the gateway/dhcp server):

pgrep -lf dhcpd

oops, no output, so:

Listening on athn0 (
Listening on em3 (
Listening on em2 (
Listening on em1 (
Listening on em0 (

but I wanna talk to the host, NOW. so, pulled the ethernet cable, waited 5 seconds, and plugged it back in. by the time I got back from behind the rack, tcpdump was already showing normal traffic from the host.

that’s all folks!
Wed Aug 8 23:42:07 PDT 2018