topic Re: Disabling DHCP causes DNS issues in Archived Posts

Disabling DHCP causes DNS issues

leenowell — Thu, 03 Jun 2021 17:38:11 GMT

Hi All,

I am having issues with sporadic massive ping times so because I had an old Brightbox router, EE sent me a new Brightbox 1 hoping it will fix the problem.

On the new router, if I disable DHCP I can ping an external IP address fine (e.g. 8.8.8.8) but get name resolution errors when I ping by name (e.g. www.bbc.co.uk ). Doing nothing else other than switching DHCP back on resolves the issue. DNS is set to "get from ISP" but have also tried manually putting in Google's DNS servers and EE's DNS servers and it still doesn;t work.

The interface of the old and new router is the same but the old one works as expected and the new one has this DNS issue.

Only other thing to add is that when I got the router the wireless light was off (and I couldn't see the SSID) however the EE call centre person remotely connected to the router and it said it was on. A factory reset solved the problem. Also, when I change settings, sometimes it seems to kill the webserver on the router as I can no longer connect to the admin site. Rebooting the router seems to fix it. I wonder if somehow the firmware is messed up on the router they shipped me?

Any help appreciated.

Thanks

Lee.

Re: Disabling DHCP causes DNS issues

XRaySpeX — Thu, 03 Jun 2021 19:34:00 GMT

Yes, it will do that. Why are you disabling the DHCP? Unless you are fixing IP addys in the devices themselves it can't work as no device has an IP addy.. In this case the device you are pinging from cannot be seen by the router to return a DNS result. So the device fails the lookup. In the same way if you run the command nslookup bbc.co.uk it will return "(DN)Server: UnKnown".
When you couldn't see your SSID & Wireless was OFF you had the Broadcast SSID setting Disabled. All CS did was Enable it remotely.
If you are using WiFi & you change some router settings it will sometimes get lost & you will need to reconnect, often after a router reboot.

Re: Disabling DHCP causes DNS issues

leenowell — Thu, 03 Jun 2021 19:58:45 GMT

thanks @XRaySpeX for your reply. In answer to your questions/ comments.

1. I am disabling DHCP because I have a another DHCP server on the network instead. I have the internal network on a different subnet to the external one and is bridged using ClearOS which provides all the internal network capabilities e.g. DHCP, internal DNS, This all works fine with the old router but not the new one. Also, pinging external ip address (8.8.8.8) comes back ok to the device so does that.

2. When the agent logged in remotely to the router she could see it was turned on however the lights and lack of SSID would indicate it was off. It was the factory reset I did which seemed to activate the WiFi.

3. Sorry should have mentioned, When I couldn't connect to the admin page, I could ping the IP address of the router. Also, the laptop was connected to the router with an Ethernet cable rather than WiFi.

thanks again for your help

Lee.

Re: Disabling DHCP causes DNS issues

mikeliuk — Thu, 03 Jun 2021 21:08:43 GMT

Hi @leenowell ,

Would it be ok to disconnect all your personal network devices from the service provider's router and test the service provider's router in isolation to see if the issues disappear?

Your description of the network topology is unclear and it is not clear to me how many other routers and switches are involved.

Assuming it is not too difficult to do, I would recommend using a double-NAT configuration to test for basic functionality.

If it is not feasible to reconfigure the network topology, it would be useful to have a high-level description of your network topology (including physical or wireless links, and subnets involved). I would be particularly interested to hear how you expect routing to work and how you tell your connected devices what the default gateway is in order to reach the service provider's network and the internet.

Re: Disabling DHCP causes DNS issues

leenowell — Fri, 04 Jun 2021 09:01:25 GMT

Hi @mikeliuk Thanks very much for your reply.

Yes I have been able to reproduce the problem with just my laptop (manually configured with static IP address) connected to the router. If I have DHCP switched on it works fine. Switching DHCP off (laptop unchanged and still on same IP settings) causes the DNS issue (i.e. can't resolve www.bbc.co.uk). Turning DHCP back on and the problem is solved. My old EE Brightbox seems to work fine when I turn DHCP off.

Thanks

Lee.

Re: Disabling DHCP causes DNS issues

mikeliuk — Fri, 04 Jun 2021 09:51:27 GMT

Hi @leenowell ,

Is the static IP address in the same subnet as the service provider's router? Are you able to ping your router from the laptop? Are you able to do a nslookup of www.bbc.co.uk when explicitly giving the DNS server? (I.e. "nslookup www.bbc.co.uk 8.8.8.8"). Does this fail if you omit the explicit DNS server?

You have described that manual configuration fails but automatic configuration (using DHCP) works. This is pretty good evidence that your manual configuration is wrong and a misconfiguration of your laptop. DHCP provides an IP address, DNS servers, and default gateway. I would recommend to check the output of "ipconfig" if you use Windows 10, and "ip a" and "ip route" if you use Linux.

Once you have determined the correct manual/static configuration for the laptop, you may go on to configure further network devices such as other routers, otherwise adding other network devices (e.g. in bridge mode) would simply cause more confusion.

Edit: please may you explain what you mean by this statement: "The interface of the old and new router is the same"?

Edit2: please may you follow this guide to manually configure your laptop with a static IP address (inside the relevant range), and be sure to select appropriate DNS servers (e.g. 1.1.1.1 and 1.0.0.1), and the gateway as the IP address of your service provider's router. https://support.microsoft.com/en-us/windows/change-tcp-ip-settings-bd0a07af-15f5-cd6a-363f-ca2b6f391ace

Edit3: the confusing thing is why turning dhcp on and off should have any impact on a statically configured device. Highly recommend there should be no other routers or switches connected to the service provider's router when carrying out this debugging (I assume already the case).

Edit 4: I guess one fool-proof approach is to set the laptop to obtain its configuration by dhcp, connect to router to obtain configuration, verify it works, record the known-good configuration (IP, subnet, gateway, DNS servers), disable dhcp on the router, apply the recorded configuration statically to the laptop, reboot laptop, verify the static configuration has stuck, check whether the known-good functionality is still present.

Re: Disabling DHCP causes DNS issues

leenowell — Fri, 04 Jun 2021 10:13:33 GMT

Hi @mikeliuk thanks for your response. In answer to your questions etc. (sorry can;t see how to do these inline in this interface)

Yes laptop (Linux) is in same subnet as router and can ping it and also ping 8.8.8.8. The DNS setting on the static IP config is set to the IP address of the router.

I tested your nslookup by specifying Google DNS and that works fine so does setting the laptop DNS manually to be 8.8.8.8. I tried manually setting the DNS on the router to be 8.8.8.8 and 8.8.4.4 and that doesn't work either (with the laptop DNS setting as IP address of router).

When I talk about DHCP enabled working, the laptop settings don't change and are set to the same manual configuration all the time. The only thing I am doing is enabling and disabling the DHCP server on the router - I am not actually using it.

By "The interface of the old and new router is the same" I meant that the configuration UI is the same for both routers so therefore I am sure that that the settings I am changing are identical even though they have different results.

re:Edit3 - yes this is very odd. Only thing I can think of is that there is a firmware bug/ issue where turning DHCP off corrupts the routers DNS settings and even manually changing them doesn't sort it. Is there I way to tell what the router thinks the DNS settings are (i.e. not what the UI says it is)? Yes for all these tests only thing connected to the router is the laptop.

I have temporarily changed the laptop settings to write this response so will run the following as suggested in a minute and post the results

- ip a; ip route

- edit 4. Just tried them on this setup and it doesn't give DNS information. How do I get that?

If it helps any, switching back to the old router (same config as the new one as far as I can tell) resolves the problem without changing anything else. This is in test mode (i.e. only laptop) and when connected to full network. This would imply it is something in the new router? Either a bug/ fault or the same setting behaves differently somehow

Thanks once again

Lee.

Re: Disabling DHCP causes DNS issues

leenowell — Fri, 04 Jun 2021 10:25:01 GMT

Hi @mikeliuk

I have run the 2 x ip commands with the laptop DNS set to

a. 8.8.8.8 (works fine)

b. the router IP address (has the issue)

For both tests router DNS manually set to 8.8.8; 8.8.4.4. In summary the output from "ip a" is identical for both (did a diff). There is a slight difference in "ip route". For test a the output is

$ ip route
default via 192.168.10.1 dev enp0s25 proto static metric 100
169.254.0.0/16 dev enp0s25 scope link metric 1000
192.168.10.0/24 dev enp0s25 proto kernel scope link src 192.168.10.56 metric 100

for test b the "100" at the end of the first line changes to 20100

Not sure if this helps? No DNS info as per earlier post so not sure how to get that from command line?

Re: Disabling DHCP causes DNS issues

mikeliuk — Fri, 04 Jun 2021 10:53:56 GMT

Hi @leenowell ,

The key clue seems to be that your service provider's router sometimes does not function as a DNS server which should forward requests to other DNS servers when it cannot resolve a name itself (perhaps from a cache). It's possible that when dhcp is enabled, your service provider's router does function properly as a DNS server and this is something that can be checked.

A. With dhcp enabled:

a. "nmap -p 53 <router_internal_ip_address>" to see that the DNS port is open

b. "nslookup www.bbc.co.uk <router_internal_ip_address>" # I expect this to work

B. With dhcp disabled:

a. "nmap -p 53 <router_internal_ip_address>" to see that the DNS port is open, closed, or filtered

b. "nslookup www.bbc.co.uk <router_internal_ip_address>" # I expect this to fail

In Linux your DNS servers will show in /etc/resolv.conf

NetworkManager and perhaps other things may edit this file (refer to documentation for your distribution to see how to set DNS servers). On distributions where the DNS servers are set statically, an edit to /etc/resolv.conf may be sufficient if nothing changes it, otherwise would need to use distribution-specific methods to make a choice stick.

Edit: apologies, DHCP does not update DNS but the above debugging still applies to figure out what is wrong. https://www.ietf.org/proceedings/45/I-D/draft-ietf-dhc-dhcp-dns-10.txt

Edit2: I guess the last check is that with /etc/resolv.conf should be check with router dhcp enabled and disabled to see if the file is changed at the same time as the good and bad behaviour. Most likely it will stay the same and point to the service provider's router's internal IP address. Presumably you should be able to work around the issue by always setting your device DNS servers to be known-good public ones but you shouldn't have to do this and the default automatic behaviour should work fine in a sane world.

Re: Disabling DHCP causes DNS issues

leenowell — Fri, 04 Jun 2021 12:03:30 GMT

Hi @mikeliuk Thanks for the reply again.. Looks like we are getting somewhere - thanks 😉

So... I run your tests with the following results

With DHCP Enabled I get

PORT STATE SERVICE
53/tcp open domain

and the nslookup works (i.e. brings back some BBC host names and IP addresses)

With DHCP disabled I get

PORT STATE SERVICE
53/tcp closed domain

and the nslookup times out.

I checked /etc/resolv.conf and the DNS config doesn't appear to be there (I am running Ubuntu 20.04) all it has is

nameserver 127.0.0.53
options edns0 trust-ad

Thanks

Lee.

Re: Disabling DHCP causes DNS issues

mikeliuk — Fri, 04 Jun 2021 12:51:40 GMT

Hi @leenowell ,

I think that's pretty conclusive. When you disable the DHCP functionality on the service provider's router, you also lose DNS functionality (i.e. the DNS service or daemon). It's possible that two distinct things are being turned off, or it's one service or daemon providing both the DHCP and DNS services (much like dnsmasq).

It's your choice what to do about this. The IP address 127.0.0.53 refers to your localhost/laptop. The entire subnet 127.0.0.0/8 refers to your localhost/laptop. (So you need to configure DNS on your devices another way.)

Personally I keep DHCP enabled on all my routers and daisy chain them for a triple-NAT configuration as this is simple and robust.

If you choose to go for a more advanced configuration. Have fun with that. I'm sure myself and others would be ok to hear of your network topology and intended configuration if you encounter any hiccups. You should have sufficient information to overcome your immediate DNS issue.

Re: Disabling DHCP causes DNS issues

leenowell — Fri, 04 Jun 2021 13:03:03 GMT

Thanks very much @mikeliuk for your help on this. So at least we have got to the root cause, the ultimate solution is a bit more tricky 🙂 So it looks like the old router must leave DNS running even though DHCP is disabled. Is there any logical reason why they would deliberately disable DNS if you didn't want the router to act as the DHCP server?

So my topology is..... I have the router on one subnet and all the internal network (routers, switch etc.) on a different subnet. I then have a PC running ClearOS which acts as the gateway (dual NIC) between the 2 and it provides all the network services e.g. DHCP, internal DNS, Virus / Malware scanning, Firewall etc etc.

So if I understand this correctly, the edge devices will need to have the ClearOS box as the DNS server in order to resolve internal names. Somehow I would then need to configure ClearOS to then forward to Google's DNS (e.g. or the EE ones) rather than forwarding on to the router. Is that correct?

Thanks

Lee.

Re: Disabling DHCP causes DNS issues

leenowell — Fri, 04 Jun 2021 13:33:12 GMT

Hi @mikeliuk

Just wanted to say a huge thank you for your help. I have changed the DNS entries in ClearOS to the Google ones for now and it seems to be working fine again. Still can't see the logic of disabling DHCP also killing DNS as the DNS servers should be given by the ISP but hey we got to the bottom of it. Can now go back to the original problem of solving my intermittent broadband "outage" issue.

Thanks once again

Lee.

Re: Disabling DHCP causes DNS issues

mikeliuk — Fri, 04 Jun 2021 14:50:39 GMT

Hi @leenowell ,

I can't immediately think of any reason why anyone would deliberately disable DNS when the option given is to disable DHCP. It seems to fail the least astonishment principle so I would guess they are inter-related behind the scenes (e.g. one service provides both and they are either tightly coupled or the developer was too lazy to decouple these functions). The GUI developer probably did not account for the laziness of the backend developer so didn't mark the option as disabling both DNS and DHCP (this seems to be the current behaviour but a future version of the firmware could change this if a developer becomes embarrassed by this thread, perhaps).

From a quick look at the ClearOS server, that is indeed the logical place to be running an alternative DNS service, forwarding to public DNS servers (Quad9 might be considered for security, your mileage may vary). You'll need to check the documentation to ensure the forwarding works ok and that devices can access DNS ok (likely via the ClearOS server). In the optimal configuration, nothing should need to be done on the device side (e.g. mobiles and laptops) as the devices default to DHCP (clients, obviously).

Edit: just noting a corner case that it's possible some other option could decouple the DNS function from the DHCP function, but looking at the webpage for ClearOS, that seems to be the correct location for the on-site DNS server anyway.

Edit2: for the avoidance of doubt, we've really not discussed the service provider's own DNS servers here much. If the DNS service is running on the service provider's router on your site, it is likely to forward to the service provider's own DNS servers by default and in the first instance (these DNS server IP addresses should be discoverable on this forum somewhere). In theory, the service provider's DNS servers should be the fastest to reach within their own infrastructure but for debugging purposes, users are often pointed to public DNS servers which are often very robust.

Re: Disabling DHCP causes DNS issues

leenowell — Fri, 04 Jun 2021 15:17:58 GMT

Hi @mikeliuk

Yes good points particularly the use of the EE DNS servers. I would much rather the external DNS done by the router especially since the EE ones could (in theory) change over time and as you say should be the fastest.

I will give EE a call to see if there are any other router options from them and take if from there. Is there somewhere I can raise router "bugs"?

Thanks

Lee.

Re: Disabling DHCP causes DNS issues

mikeliuk — Fri, 04 Jun 2021 16:12:43 GMT

In practice, I think you will be better off with public DNS servers such as Quad9, cloudflare (1.1.1.1, 1.0.0.1), or Google DNS at least in the short term and especially if you want to disable DHCP on your service provider's router.

Although the service provider's DNS is good in theory, I've never heard anyone raving about how good they are. More commonly you will hear of people complaining of a DNS issue that is usually traced to the service provider's DNS and the solution is usually to switch to a public DNS server of high repute.

I'm not aware of the correct route to raise a router issue, that firmware updates do get released does suggest there is some way of recognizing issues. It's not even clear the problem is exactly as diagnosed as at least one other person would need to confirm they see the same thing to have confidence. If the service provider's router is based on a commercially available router, it would be good to simultaneously report to both the service provider and manufacturer (doubtless one or both parties would reject the issue as not their problem).

You also need to consider how many of the service provider's customers would choose to disable DHCP and then accidentally lose DNS (if this happens to any second person or second device at all). If you are the only person impacted, the issue will be placed at the appropriate point in the TODO list and this position could be below the "never" line.

DNS lookups are also cached for a time which could make it largely irrelevant which DNS server you forward to. Sometimes the frequency of updates is more important where a user is running a website and changing DNS entries and will often find those entries would be updated/available more quickly via public DNS servers than a service provider's DNS servers.

Many technical questions and choices have no best answer. I generally recommend going for the choice which saves the most time and reduces the amount of thinking required. The exception is if a person is actively trying to learn something, in which case go for the most difficult, fully-featured, production-ready option available.

I'm aware this is a lot of text and really no answer at all! 😂 I do believe the above is true though. 😎

Re: Disabling DHCP causes DNS issues

XRaySpeX — Sat, 05 Jun 2021 00:57:18 GMT

@mikeliuk wrote:

It's not even clear the problem is exactly as diagnosed as at least one other person would need to confirm they see the same thing to have confidence.

Here! I do so confirm as I implied in post 2.

I suspect that:

The OP's original router which didn't have the issue was the old flat Brightbox 1.
The OP's new router which does have this issue is the newer upright Brightbox 1R or 2.

Re: Disabling DHCP causes DNS issues

mikeliuk — Sat, 05 Jun 2021 07:50:50 GMT

Hi @XRaySpeX ,

Many thanks. Just so that I am clear, please may I check that you also have the same service provider's device and when DHCP is enabled you see port 53/tcp open on that device, but that when you disable DHCP, you find that port 53/tcp is closed?

Once you have confirmed this, we will have reproduced the observation of the OP and would have reason to believe this is the current typical behaviour of that version of the firmware for perhaps a proportion of such devices, or all such devices with that firmware version.

Re: Disabling DHCP causes DNS issues

XRaySpeX — Sat, 05 Jun 2021 09:47:52 GMT

@mikeliuk : All my ports are Stealth.

Re: Disabling DHCP causes DNS issues

mikeliuk — Sat, 05 Jun 2021 14:28:11 GMT

Wrong network.

This thread pertains to the internal private subnet of the service provider's router, not the WAN. 🤓