DHCP Relay Issues over IPSec VPN

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Paul1977

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#290400 9-Nov-2021 09:42

I'm not sure if this is the best forum for this, as I'm having trouble identifying whether it's a networking issue or a DHCP server issue.

DHCP server is Windows 2019 at main site in subnet A and is configured with scopes for all subnets.
Branch site 1 has two subnets B & C
Branch site 2 has two subnets D & E
Branch site 3 has one subnet F

Subnets aren't VLANs, they are physical subnets with routers between them. Branch sites are connected to main site via site-to-site IPsec VPNs.

Everythings is working perfectly, except DHCP relay from some subnets isn't working...

Subnets B, C, & D DHCP relay works
Subnets E & F DHCP relay doesn't work

When I attempt to obtain an IP address for a client in subnet F (for example) I can see that DHCP traffic (UDP 67/68) is passing through the router at the main site, but the client times out and doesn't get an address.

If I manually assign an address to the client (or set up a local DHCP server to issue addresses) full communication between subnets E/F and A works.

For testing I've allowed unrestricted traffic between Subnet A and E/F, and have ensured that the Windows Firewall on the DHCP server is not restricting incoming traffic to UDP ports 67/68.

I'm at a complete loss as I can't see any difference between the subnets/scopes that are working and those that aren't.

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Dynamic

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#2809769 9-Nov-2021 09:57

We've always had DHCP on each site in case links go down, so I can't comment specifically on troubleshooting this beyond generic thoughts.

Can you turn up logging levels to maximum and see if that reveals anything? Ideally aiming to compare a complete DHCP transaction to an incomplete transaction. That would hopefully reveal whether the requests are actually making it to the server and the response is not getting back to the client.

I'm picking it is something subtly different in the routing rules, perhaps even just the order of the rules influencing how the traffic is handled. Good luck!

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MadEngineer

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#2809770 9-Nov-2021 10:06

Use wire shark. You’ll see all the faults clear as day

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Paul1977

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#2809896 9-Nov-2021 14:27

OK, so turns out the dhcp-relay function on EdgeRouters doesn't work over a VTI interface. Worked around it by using the dhcp-relay option built into dnsmasq.

But interestingly that means I had two different problems, as none of the other sites use EdgeRouters. I mistakenly thought it would be a the same issue, but I'm half way there.