Website DNS works but only in some situations

Question

This is really confusing; There is a website which we host that I have been told has gone down, but checking that website from my own computer it loads fine. However checking the website from the mobile data on my phone it says it can't connect (unrecognised domain name).

Checking the website on other browsers using VPN on my computer connects ok on some regions (Europe). But on others ( Asia or America regions) it comes back with the same issue; "Unrecognised domain name"

MX toolbox says (as one example):

Sorry, we couldn't find any name servers for <site>

failed VPN states:

Server IP address could not be found.

But the DNS on the hosting server has not changed, the hosting server has not changed, and the website appears correctly on some 3rd party (isitmeorjustdown type places).

• How can I confirm this issue?

• What can be the cause of this issue?

• How can this be fixed ASAP?

thanks.

Answer 1

NS DNS records

Start by examining the NS records for the domain. I use dig on the command line, but you can also use versions of it online.

$ dig NS example.com
;; ANSWER SECTION:
example.com.        0   IN  NS  a.iana-servers.net.
example.com.        0   IN  NS  b.iana-servers.net.

You should see between 2 and 4 NS records for your domain. They are usually subdomains of your DNS hosting company. If you see old NS records or records other than what your DNS host has given you, then you have a problem that you need to fix. You can edit your NS records at your domain registrar.

A and CNAME records

Next examine the A and CNAME records for your base domain and the www subdomain at each of your name servers.

$ dig @a.iana-servers.net example.com
;; ANSWER SECTION:
example.com.        86400   IN  A   93.184.216.34

$ dig @a.iana-servers.net www.example.com
;; ANSWER SECTION:
www.example.com.    86400   IN  A   93.184.216.34

$ dig @b.iana-servers.net example.com
;; ANSWER SECTION:
example.com.        86400   IN  A   93.184.216.34

$ dig @b.iana-servers.net www.example.com
;; ANSWER SECTION:
www.example.com.    86400   IN  A   93.184.216.34

For most sites, each query should have exactly one record. Having multiple IP addresses for a website is possible if you have have multiple servers that all serve the exact same content. However, if you don't have multiple servers and don't use a CDN, having multiple IP addresses is a mistake that can lead to your problems.

Similarly, each DNS name server should return the same records for your site. If one of them is returning something different, that can cause the the problems you describe.

Test the site

For each of your IP addresses, connect to your site on that IP address for all combinations of http/https and www/no-www. I use curl for this type of debugging.

$ curl --resolve example.com:80:93.184.216.34 --head http://example.com/
HTTP/1.1 200 OK
Content-Length: 648

$ curl --resolve www.example.com:80:93.184.216.34 --head http://www.example.com/
HTTP/1.1 200 OK
Content-Length: 648

$ curl --resolve example.com:443:93.184.216.34 --head https://example.com/
HTTP/1.1 200 OK
Content-Length: 648

$ curl --resolve www.example.com:443:93.184.216.34 --head https://www.example.com/
HTTP/1.1 200 OK
Content-Length: 648

Make sure that each of your URLs returns the correct status codes. For most sites, I would expect three of those four URLs to redirect. Take note of any connection errors or security certificate problems that you encounter.

If your site has multiple IP addresses, each IP address should return the same content using this method.