Unable to connect to screwfix.com (dns issue)

[#1]

@chagoi5 

Unless there’s been a recent change of which I am unaware you can’t change the DNS resolvers on a Hub One. You can bypass these by changing them on most devices.

[#2]

On an Android go
Settings->Connections->More connection settings
and set a Private DNS. 8.8.8.8 or 8.8.4.4 are the Google ones that work so bypassing the Plusnet ones

[#3]

Hi All,

Thanks for all the input.

So rather than risk bricking my Plusnet One Hub I've swapped it out for a VDSL2 router I already had here.

That enabled me to:

• Setup it up and test the Broadband username/password was as I found is the xxxxx@plusdsl.net and your account password.
• Setup the DNS setting to google public ones in the router and not every device I have!
• Also it solved an issue I had a while ago where the Wi-Fi from the Plusnet One Hub had become unreliable. Had disabled it and used a separate Wi-Fi Access point. 

If anyone interested I used a TP-Link TD-W9970,   Yes I know a very low spec router but it works.   Will probably treat myself to something much better!

One again thanks all.

Guess screwfix and Plusnet need a chat!

[#4]

Hi confusedasasnak

Thanks,  Most of my android devices worked exactly like that!  but my SONY XA2 phone nope!

I found I had to 'forget' the Wi-Fi and then add it manually and select manual/Static IP and enter all the details including a public DNS

[#5]

I have a potential workaround that I can try and implement remotely that should work at router level but I would need a willing guinea pig with a Hub One who is experiencing this problem. There's also no guarantee that it will work so it could result in you having to factory reset should anything go awry.

[#6]

@bobpullen what does that involve doing? - have you figured out a way to enable DNS over TCP on the router?

[#7]

 I also have the very same problem in that I am unable to open the Screwfix website from my Android 12 tablet. I would add that I am not very computer literate!!!  I have tried my Brave browser as well as Chrome and Firefox and Opera browers but all have the same failure message. However by pure chance I happened to turn on my Surfshark VPN and low and behold I can now connect to Screwfix with no problem at all. I assume that the fact that Surfshark changes the address resolves the DNS/TCP issue.

[#8]

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@alitster wrote:

@bobpullen what does that involve doing? - have you figured out a way to enable DNS over TCP on the router?

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No, it isn't as simple as that.

The Hub One's core DNS implementation does not support eDNS nor TCP fallback, and implementing it would be far from trivial. This isn't isolated to the Hub One; there are other (admittedly older) routers on the market that are exactly the same.

What I would be looking to do is override the DNS addresses assigned to the WAN inteface of the hub so it uses either an alternative set of Plusnet addresses, or a third party DNS provider's like Cloudflare/Google etc. This doesn't resolve the crux of the issue but it should result in smaller DNS responses which I'm hopeful won't be prone to the problem.

The default Plusnet DNS resolvers return an extra section of records when compared to other DNS hosts and that's contributing to the issue.

[#9]

So it is the Plusnet DNS service and not our routers or a Screwfix problem?

[#10]

I would have thought that the Screwfix site must have something to do with the problem as in my case it appears to be the only website I am unable to connect to without using the VPN.

[#11]

@confusedasasnak the issue is due to the size of the DNS response for www.screwfix.com and the fact that the PlusNet HubOne doesn't support DNS queries over TCP.

In it's most basic form, when a DNS lookup is performed it'll be done over UDP, the maximum size for this is 512 bytes, the response exceeds that so the Truncated flag is set. This indicates to your local resolver to attempt it again over TCP where it doesn't have that limit. The HubOne doesn't listen for DNS queries over TCP and so it fails. 

As @OHIODAN mentioned, it will work fine if you use a VPN service as that will override your DNS server setting to use it's own.

Another hack is to edit your hosts file (for windows users see https://www.howtogeek.com/howto/27350/beginner-geek-how-to-edit-your-hosts-file/ ) and add a record for www.screwfix.com to it. This will prevent your computer from doing a DNS lookup and so work. On windows you can get a valid IP by running

nslookup www.screwfix.com 212.159.6.9

that sends the query directly to the PlusNet upstream DNS server bypassing the one on your router.

Just remember to remove it if/when the issue gets resolved!

[#12]

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@bobpullen wrote:

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No, it isn't as simple as that.

The Hub One's core DNS implementation does not support eDNS nor TCP fallback, and implementing it would be far from trivial. This isn't isolated to the Hub One; there are other (admittedly older) routers on the market that are exactly the same.

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You can't simply update / enable TCP for the DNS resolver it's using and push out an update? It seems pretty ludicrous that it's using ancient and obsolete software.

I heard from Screwfix earlier today that some Vodafone customers were having the same issue.

If the below isn't possible it might be worth contacting Screwfix and asking them nicely to remove an IP address from the CNAME record which would also fix it.

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The default Plusnet DNS resolvers return an extra section of records when compared to other DNS hosts and that's contributing to the issue.

---

If you use BIND, there's a simple solution to that. Just set

minimal-responses yes;

in the options section and it'll disable the Authority section in the response.

Ref: https://www.zytrax.com/books/dns/ch7/queries.html#minimal-responses

The response will then fit into a single UDP packet without being truncated. And there's no need to apply any 'fixes' to customers routers.

[#13]

Hi

Not sure if this helps

I am using Kubuntu - Linux

1. If I 1st try to go to Screwfix site using Brave browser ( most probably the same in Chrome browser ), it gives the dns error.
2. I then keep this browser open and then I open Firefox and I can access Screwfix as normal using Firefox.
3. But if I go back to the still open Brave browser, Screwfix site has opened without me doing anything ?

[#14]

Thanks but that does not work on my Samsung A8 tablet. Also tried leaving the unable to connect message open on Brave and then logging in with Opera but no joy there either. I am able to connect with any browser providing Surfshark is enabled.

I have been in touch with Screwfix and they are aware of what is being posted on this forum and have escalated the problem up to their IT Dept.

[#15]

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@alitster wrote:

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@bobpullen wrote:

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No, it isn't as simple as that.

The Hub One's core DNS implementation does not support eDNS nor TCP fallback, and implementing it would be far from trivial. This isn't isolated to the Hub One; there are other (admittedly older) routers on the market that are exactly the same.

---

You can't simply update / enable TCP for the DNS resolver it's using and push out an update? It seems pretty ludicrous that it's using ancient and obsolete software.

Things are rarely as simple behind the scenes

I heard from Screwfix earlier today that some Vodafone customers were having the same issue.

If the below isn't possible it might be worth contacting Screwfix and asking them nicely to remove an IP address from the CNAME record which would also fix it.

They're not technically doing anything wrong, so I'm not sure this approach is going to work. Neither would I say we've much more authority to request such a change than a user of the site does. that said, it would be in Screwfix's best interests, because the issue will not be isolated to the Hub One as I've alluded to previously (and as some Voda customers seem to be witness to).

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The default Plusnet DNS resolvers return an extra section of records when compared to other DNS hosts and that's contributing to the issue.

---

If you use BIND, there's a simple solution to that. Just set

minimal-responses yes;

in the options section and it'll disable the Authority section in the response.

Ref: https://www.zytrax.com/books/dns/ch7/queries.html#minimal-responses

The response will then fit into a single UDP packet without being truncated. And there's no need to apply any 'fixes' to customers routers.

---

No, we are not using BIND, although the same option exists in the software we are using. Seems how it defaults is somewhat down to the interpretation of RFCs. Making changes to the DNS platform carries a fair element of risk given the impact should anything go awry. Because of this, change goverance will be rife - irrespective of how trivial the change is.

We are considering options and in the meantime my offer to make individual changes at the router level remains should anybody want to give it a go. That's the quickest route to resolution for anybody not in a position to work around things themselves.