Smart Hub - Can I change DHCP/DNS settings for LAN?

[Hi_ImTemmy]

Hello,

I have a EE Smart HUB.

I want to set DNS settings for devices on my LAN (Not for the WAN / Internet connection - that's a different thing entirely) Other routers would offer options like this:

However in the Smart HUB I can only find:

Is there any way of setting static DNS for DHCP?

Thanks.

[XRaySpeX]

There are no DNS settings just for the LAN. DNS works on the basis of "I may not know the answer but I know a man who does". The LAN points to the router, 192.168.1.254, as its DNS & the router in turn points to actual external DNS.

DNS really has nowt to do with DHCP other than it is passed on to DHCP-connected devices.

What are you actually trying to do?

[mikeliuk]

Hi @Hi_ImTemmy ,

You've made an interesting observation as my original belief was that consumer routers make no distinction between the DNS set for the router itself ("for the WAN") versus the DNS setting for the LAN.

I have three routers in production: Netgear MR110, Cisco/Linksys DD-WRT, and a Linksys OpenWRT router all of which have custom DNS settings. What I find, as shown below, is that the DHCP response returns the router as the DNS and forwards requests to the chosen DNS set in the router. The OpenWRT router documents this the most clearly with the label "DNS forwardings".

I suspect that the behaviour with your router in the DD-WRT screenshot will be the same that the DHCP request will specify the DNS as the router and will forward to the DNS set in the management page. I think it's likely you have a Linux server around so can run the below nmap command/script to see the DNS specified in the DHCP response. Obviously if you want to change the DNS on a per-server basis, you can do this in /etc/resolv.conf but I suspect you want to do it for many other non-server devices if you want it going out by DHCP.

[x@x ~]# for i in br{1..3}; do nmap --script broadcast-dhcp-discover -e $i; done
Starting Nmap 7.70 ( https://nmap.org ) at 2021-09-20 20:19 BST
Pre-scan script results:
| broadcast-dhcp-discover:
| Response 1 of 1:
| IP Offered: 192.168.X.149
| DHCP Message Type: DHCPOFFER
| Server Identifier: 192.168.X.254
| IP Address Lease Time: 2m00s
| Renewal Time Value: 1m00s
| Rebinding Time Value: 1m45s
| Subnet Mask: 255.255.255.0
| Broadcast Address: 192.168.X.255
| Domain Name Server: 192.168.X.254
|_ Router: 192.168.X.254
WARNING: No targets were specified, so 0 hosts scanned.
Nmap done: 0 IP addresses (0 hosts up) scanned in 2.78 seconds
Starting Nmap 7.70 ( https://nmap.org ) at 2021-09-20 20:19 BST
Pre-scan script results:
| broadcast-dhcp-discover:
| Response 1 of 1:
| IP Offered: 192.168.Y.249
| DHCP Message Type: DHCPOFFER
| Server Identifier: 192.168.Y.1
| IP Address Lease Time: 2m00s
| Renewal Time Value: 1m00s
| Rebinding Time Value: 1m45s
| Subnet Mask: 255.255.255.0
| Broadcast Address: 192.168.Y.255
| Router: 192.168.Y.1
| Domain Name Server: 192.168.Y.1
|_ Domain Name: lan
WARNING: No targets were specified, so 0 hosts scanned.
Nmap done: 0 IP addresses (0 hosts up) scanned in 3.43 seconds
Starting Nmap 7.70 ( https://nmap.org ) at 2021-09-20 20:20 BST
Pre-scan script results:
| broadcast-dhcp-discover:
| Response 1 of 1:
| IP Offered: 192.168.Z.69
| DHCP Message Type: DHCPOFFER
| Server Identifier: 192.168.Z.254
| IP Address Lease Time: 2m00s
| Renewal Time Value: 1m00s
| Rebinding Time Value: 1m45s
| Subnet Mask: 255.255.255.0
| Broadcast Address: 192.168.Z.255
| Router: 192.168.Z.254
| Domain Name: net
| Interface MTU: 1500
|_ Domain Name Server: 192.168.Z.254
WARNING: No targets were specified, so 0 hosts scanned.
Nmap done: 0 IP addresses (0 hosts up) scanned in 3.57 seconds