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I have a remote client machine that is sending out DHCPDISCOVER's. The server is responding with a DHCPOFFER, but there is no DHCPACK.

This repeats about every 30 seconds from the same host. Is there something I can do remotely or do I need to get someone to reboot it? It's in a data centre so I may have to travel there to do it!


Thanks for the suggestions. I've had all the machines rebooted, but I still have issues. I think there is an issue with my configuration. Does this look correct?

#
# /etc/dhcpd.conf for primary DHCP server
#

authoritative;
ddns-update-style none;
deny duplicates;
default-lease-time 600;
max-lease-time 3600;

# Our fixed hosts
host host2  { hardware ethernet xx:xx:xx:xx:xx:xx; fixed-address x.x.x.202; }
host host3  { hardware ethernet xx:xx:xx:xx:xx:xx; fixed-address x.x.x.203; }
host host4  { hardware ethernet xx:xx:xx:xx:xx:xx; fixed-address x.x.x.204; }
host host5  { hardware ethernet xx:xx:xx:xx:xx:xx; fixed-address x.x.x.205; }

subnet x.x.x.128 netmask 255.255.255.128 {
  option subnet-mask 255.255.255.128;
  option broadcast-address x.x.x.255;
  option routers x.x.x.129;
  option domain-name-servers 8.8.8.8, 8.8.4.4;

  # Testing pool.
  pool {
    max-lease-time 300; # 5 minutes
    range x.x.x.250 x.x.x.254;
    deny known-clients;
  }

  # Our hosts - I didn't have this pool declaration before, do I need it if I want
  # the hosts to be running dhcp but always get the same address?
  pool {
    max-lease-time 1800;
    range x.x.x.200 x.x.x.220;
    deny unknown-clients;
  }
}
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DHCPRequest should come before the DHCPAck. Are you seeing that? Try running a packet capture on the server and look for the DHCPDiscover, DHCPOffer, DHCPRequest and DHCPAck to and from the server. Is the client on the same LAN segment as the server? If not, is the router separating the two configured as a DHCP relay? –  joeqwerty Jan 26 '13 at 18:27
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2 Answers

I've seen this a few times and so far I've seen only two reasons:

  • The IP address the DHCP server gave is already in use by another device. Usually you'd see a DHCPNAK though.
  • Your firewall is accepting the traffic to the dhcp server, but not the traffic back

Fortunately both should be easy to test. Ping the IP address and check relevant firewalls.

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Thanks. I pinged the address offered but no response. I then set up a host entry for it forcing it to offer a different address but this didn't seem to help. Will check the firewall. –  Matt Jan 26 '13 at 19:21
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It goes:

CLIENT -> DHCPDISCOVER
SERVER -> DHCPOFFER
CLIENT -> DHCPREQUEST
SERVER -> DHCPACK

You you are missing the DHCPREQUEST before the DHCPACK in your description.

If the client is on a different subnet than the DHCP server the DHCPOFFER is sent unicast to the DHCP-relay on port 67 UDP. The DHCP-relay agent broadcasts the DHCPOFFER to the subnet on UDP port 68.

I'd investigate connectivity issues related to the DHCPOFFER. Track it and see if it finds its way back to the client and if it does, why is the client not DHCPREQUEST:ing the address.

A common dhcp relay agent is the "ip helper-address" option in cisco routers under a specific interface.

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