System Administration & Network Administration
ubuntu dhcp dhcp-server isc-dhcp ubuntu-12.10
Updated Fri, 12 Aug 2022 12:27:50 GMT

Computer gets IP from DHCP server but has no internet connection

I've installed an isc-dhcp-server on Ubuntu 12.10 and I'm trying to setup a DHCP server on a head node for six worker computers in the local network. The head node itself is a DHCP client receiving the IP-address (on eth0) from an other computer in the network.

IPv4 IP forwarding is enabled on the head node. In /etc/sysctl.conf the following line was added:

net.ipv4.ip_forward = 1

The following rules are set in /etc/rc.local:

/sbin/iptables -P FORWARD ACCEPT
/sbin/iptables --table nat -A POSTROUTING -o eth0 -j MASQUERADE

Furthermore, INTERFACES="eth0" is set in /etc/default/isc-dhcp-server.

The dhcpd.conf file contains (for the moment only one worker computer is in the file):

ddns-update-style none;
default-lease-time 3600;
max-lease-time 7200;
subnet netmask {
  host hostName {
    hardware ethernet macOfHost;
  option subnet-mask;
  option broadcast-address;
  option routers;

The worker node effectively gets the IP address, but it has no internet connection. A ping to is successful, as well as a ping to the computer that serves as a DHCP server for the head node (a ping to The problem doesn't seem to have something to do with DNS as a ping to an IP-address (such as fails.


The network topology is as follows. There is switch connecting a computer (with internet access and running a DHCP server, IP address and 7 other computers. One of these 7 computers gets an IP from the computer. The IP it gets is and the internet on that computer works fine. Now we have a DHCP server running on in order to provide internet access to the other 6 computers, but that fails. They get IP addresses but have no internet access. We are not allowed to modify anything on the node so it should be feasible to make internet work with this setup.

Does someone know what the problem could be?

Output of /sbin/route -n on the client:

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface         UG    0      0        0 eth1     U     1000   0        0 eth1   U     1      0        0 eth1

Output of sudo iptables -L -v -n: Chain INPUT (policy ACCEPT 2146 packets, 1551K bytes) pkts bytes target prot opt in out source destination

Chain FORWARD (policy ACCEPT 59 packets, 3762 bytes)
 pkts bytes target     prot opt in     out     source               destination         
Chain OUTPUT (policy ACCEPT 1908 packets, 213K bytes)
 pkts bytes target     prot opt in     out     source               destination

Output of ip ro sh default via dev eth0 proto static dev eth0 scope link metric 1000 dev eth0 proto kernel scope link src metric 1


Everything is working now. It turned out to be a DNS problem; changing the option routers to option routers fixed the issue. Pings must have been blocked somewhere in the network as responses never came through, but surfing in the browser to a specific IP-address was possible, which made me realise that DNS was the problem. Thanks everyone for your help.