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I found, that sometimes DHCP server from behind my main router answers DHCP request from clients in the LAN. Below is an example

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Notebook is connected to LAN via access point. There is DHCP server running on LAN at pfSense. There is also DHCP server running on Router3. Sometimes, notebook receives address from Router3.

The question is: how it can be and how to patch this bleach?

In my firewall I have a rule 192.168.100.0/24 to pass so that I could open router web GUI from any clients. But I don't want it's DHCP server serves...

UPDATE 1

Probably I was wrongly blaming Router3 device. I found another device that could provide DHCP services: I have also multimedia player with built-in AP functionality. I have turned it on some time ago, but it didn't worked as I expected, so I forgot about it. Although it has 192.168.100.1 undocumented DHCP server inside. I deduced this by MAC address of that fake DHCP server, which has first bytes the same as of this device. Now I turned AP off and will see how it will behave.

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    Indeed, you'll need to either enable NAT routing on the pfSense system, or use the upstream DHCP server Router3 – Austin Hemmelgarn Jul 25 '17 at 18:09
  • @RuiFRibeiro how does second router hears broadcast requests? Why do they pass trhough pfsense? – Dims Jul 25 '17 at 18:16
  • @RuiFRibeiro I want pfSense do DHCP for LAN and any other servers do their DHCP in their subnets, not around the entire globe! – Dims Jul 25 '17 at 18:18
  • okok, a Internet label would have helped. I think I am understanding it better now. Either you configured a DHCP relay in pfsense, or there are some cables in the wrong sockets, or you configured a bridge. Or you used a single NIC instead of two NICs in pfsense...or ultimately, your notebook has a static IP address. Giving the benefit of doubt, I would go for the DHCP relay possibility. – Rui F Ribeiro Jul 25 '17 at 18:20
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    @RuiFRibeiro nothing applied except DHCP relay, which I don't know how to configure; pfSense has separate DHCP relay service, which I have stopped. Inside DHCP server I can't find relay options. – Dims Jul 25 '17 at 19:07
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You were getting that IP address because you had the DHCP relay service activated.

The "DHCP relay" functionality that is present in many brands of firewalls and routers, is a functionality to propagate in a router the DHCP requests for a DHCP server that is on another network e.g. to be able to use only a DHCP server for several networks - but then the DHCP server has to be configured and aware that are several networks - while the one at router3 only gives IP addresses of one and only network.

Has you find out, disabling the DHCP relay functionality in pfSense, you won't get more on that position of the network an address of 192.168.100.0/24

  • So, how to disable DHCP relay in pfSense? – Dims Jul 25 '17 at 20:29
  • @Dims I though you said you stopped it; if that is the case, I can tell you later on. In my mobile now. – Rui F Ribeiro Jul 25 '17 at 20:30
  • I stopped it, yes, but DHCP in another subnet is still reacheable. – Dims Jul 25 '17 at 20:34
  • for that to happen without DHCP relay, AP1 has to be connected to the 192.168.100.5 interface and not to 192.168.10.1. The AP could not care less about it´s own IP, it is just for administration (normally it is a bridge). Unless it doing routing, and you redefined in the wifi interface 192.168.100.0/24 again by mistake. – Rui F Ribeiro Jul 25 '17 at 20:42
  • So, you just don't beleive that things happen as I describe. Okay. – Dims Jul 25 '17 at 20:53

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