BT DNS returns NXDOMAIN for our business domain - all BT customers affected, works on every other IS

[briantruongau]

Hi all,

Hoping a moderator can help escalate this one, as it's costing us customers.

Our web app at builder.printerp.ai cannot be reached by any of our customers on BT broadband. On every other UK ISP — and on any public DNS — it loads fine. We've confirmed this across multiple separate BT customers on different BT lines.

BT's DNS returns "Non-existent domain":

C:\> nslookup builder.printerp.ai
Server:  hub.home.arpa
Address: 192.168.1.254

*** hub.home.arpa can't find builder.printerp.ai: Non-existent domain

This isn't a caching issue — customers have flushed DNS and rebooted their hubs repeatedly over several days with no change.

Here's the part that makes me think it's a filtering false positive:

Other hostnames in the exact same DNS zone resolve perfectly on the same BT connection, at the same moment, returning identical IP addresses:

api.printerp.ai            104.21.66.237, 172.67.165.220   -> resolves OK
builder.printerp.ai        104.21.66.237, 172.67.165.220   -> NXDOMAIN

Same nameservers, same IPs, same zone. Only that one hostname fails, and only on BT. If a customer sets their DNS to 8.8.8.8 it works instantly — so this is happening at BT's resolvers, not in our DNS config.

For what it's worth, the domain is clean everywhere I've checked:

• Google Safe Browsing — "No unsafe content found"
• Spamhaus — "no issues"
• Netcraft — no fraud or malware indicators

We're a legitimate B2B business — the site is a tool UK print and apparel companies use to prepare artwork and manage print orders. We're BT Business customers ourselves.

I've emailed [email protected] as well, but posting here in case a moderator can help move it along.

Is there anything else I can provide to get this looked at?

Thanks.

[imjolly · Accepted solution]

this is a residential customer help customer forum and as business user  you need to post here

https://business.forums.bt.com/

[briantruongau]

Many thanks for your information. 

[bobpullen]

Would be useful to understand what DNS server your affected customers' routers are using. If I query one of the BT DNS server IPs it looks good from where I'm sat: -

~$ dig builder.printerp.ai @81.139.56.100

; <<>> DiG 9.18.39-0ubuntu0.22.04.4-Ubuntu <<>> builder.printerp.ai @81.139.56.100
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 15937
;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1232
;; QUESTION SECTION:
;builder.printerp.ai.           IN      A

;; ANSWER SECTION:
builder.printerp.ai.    300     IN      A       104.21.66.237
builder.printerp.ai.    300     IN      A       172.67.165.220

[briantruongau]

@bobpullen — thanks, here's the data you asked for.

An affected customer on a BT line, after a DNS flush:

C:\> nslookup builder.printerp.ai
Server:  hub.home.arpa
Address: 192.168.1.254

*** hub.home.arpa can't find builder.printerp.ai: Non-existent domain

The resolver serving that line:

C:\> nslookup -type=txt o-o.myaddr.l.google.com
o-o.myaddr.l.google.com text = "81.134.101.38"

Querying that same IP from outside the BT network:

$ dig builder.printerp.ai @81.134.101.38
;; ->>HEADER<<- opcode: QUERY, status: NOERROR
builder.printerp.ai. 300 IN A 172.67.165.220
builder.printerp.ai. 300 IN A 104.21.66.237

Both published resolvers also answer correctly for me:

$ dig builder.printerp.ai @81.139.56.100  → NOERROR, 172.67.165.220 / 104.21.66.237
$ dig builder.printerp.ai @81.139.57.100  → NOERROR, 172.67.165.220 / 104.21.66.237

Other hostnames in the same zone resolve fine on the same BT line at the same time, returning identical IPs:

api.printerp.ai            → resolves
builder-stage.printerp.ai  → resolves
app.printerp.ai            → resolves
builder.printerp.ai        → NXDOMAIN

I'll leave the interpretation to BT. What I can say is the domain is clean on Google Safe Browsing, Spamhaus and Netcraft, and it resolves normally on every other UK ISP.

I've emailed [email protected] three times (2, 3 and 4 September) with no reply, no acknowledgement and no case reference. BT Business live chat couldn't raise one either. @BethM pointed me to that address and I've followed it exactly.

Is there another route to someone who can look at this? Happy to provide anything else that helps. My pleasure.

[licquorice]

Unfortunately not, you've raised it with the correct team but BT only move at glacial speed!

Keep hammering them and they might get fed up and reply.

[bobpullen]

@briantruongau - very odd.

My lookups were from an EE line which for all intents and purposes is pretty much the same as a BT line.

If one of your affected customers queries the BT DNS IP directly, rather than going via the router does it answer?

Any commonality with the type of BT hub they’re using? The newer Smart Hub 3 does some IPS and parental control filtering slightly differently I believe.

[Steve_M]

I think the correct email address is [email protected] but from my BT line, it resolves perfect:

Non-authoritative answer:

Name: builder.printerp.ai

Address: 2606:4700:3032::ac43:a5dc

Name: builder.printerp.ai

Address: 2606:4700:3033::6815:42ed

Non-authoritative answer:

Name: builder.printerp.ai

Address: 104.21.66.237

Name: builder.printerp.ai

Address: 172.67.165.220