dhcp-helper (cannot bind DHCP server socket: Address already in use)

smeezekitty · 02-16-2015, 04:11 PM

I am trying to connect a device that must use a DHCP
network to the ethernet port on my laptop running Ubuntu which connects
to the network through the built in wireless.

I have had no problems doing this before using static IP addresses
but am having a problem doing it with DHCP.

I read the man pages and it seems like dhcp-helper would do what I want.
So I run

Code:

dhcp-helper -b wlan0

and it returns

Code:

dhcp-helper: cannot bind DHcp server socket: Address already in use

Google returns lots of people with the same problem but no workable solution.

Ser Olmy · 02-16-2015, 07:04 PM

The -b parameter probably fails because the DHCP client is already bound to UDP port 68 on the wireless interface. netstat -4 -nlu should be able to confirm that, but I'm not sure it really matters, as I suspect dhcp-helper might not be what you're looking for.

dhcp-helper is a DHCP Relay Agent. It forwards DHCP Discover/Request packets from one network to a DHCP server in another IP network. That DHCP server must have a DHCP scope for the IP network for which the Relay Agent is forwarding requests. It's highly unlikely that this is the case with your setup.

It sounds like you simply want the wired device to get an IP address from the DHCP server servicing the wireless network. For that to happen, the wired and wireless networks must actually be parts of the same Ethernet ("layer 2" in network terms) network. Currently, they are entirely separate networks, and cannot both use IP addresses belonging to the same IP network.

You could create a bridge interface with the both wireless and the wired interface as members. That way, your laptop would act as a switch with one wired and one wireless interface.

smeezekitty · 02-16-2015, 07:57 PM

Quote:

Originally Posted by Ser Olmy

The -b parameter probably fails because the DHCP client is already bound to UDP port 68 on the wireless interface. netstat -4 -nlu should be able to confirm that, but I'm not sure it really matters, as I suspect dhcp-helper might not be what you're looking for.

dhcp-helper is a DHCP Relay Agent. It forwards DHCP Discover/Request packets from one network to a DHCP server in another IP network. That DHCP server must have a DHCP scope for the IP network for which the Relay Agent is forwarding requests. It's highly unlikely that this is the case with your setup.

netstat does indeed show a server on port 68

Quote:

It sounds like you simply want the wired device to get an IP address from the DHCP server servicing the wireless network. For that to happen, the wired and wireless networks must actually be parts of the same Ethernet ("layer 2" in network terms) network. Currently, they are entirely separate networks, and cannot both use IP addresses belonging to the same IP network.

You could create a bridge interface with the both wireless and the wired interface as members. That way, your laptop would act as a switch with one wired and one wireless interface.

How do I do that? Sorry but networking really just isn't my "thing"