DNS issue with caldav.gmx.com

Byron Tompkins

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Apr 5, 2022, 9:00:34 AM4/5/22

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Firstly, I just want to say that I'm an end user in this situation and am not a networking DNS specialist. Also, I'm located in Australia.

Just over a month ago (28.02.2022) I started encountering an issue where I could not synchronise my calendar entries using the caldav protocol whilst connected to my mobile phone providers (Vodafone TPG Australia) network.

The issue is experienced across a range of devices (iPhones, iPads, MacBooks, Samsung Galaxy A50) but I managed to get to a point where I've determined the issue is something to do with DNS but as I'n not a DNS networking specialist, I don't know what the issue is I just know that it exists.

I've had multiple phone calls to Vodafone over the past month and on one of those calls the analyst made comment which kind of alluded to the fact that Vodafone AU may use Google DNS so that's why I'm here.

Essentially, if I use Quad9 DNS (9.9.9.9 & 149.112.112.112) my calendar synchronisation works. If I use the Vodafone AU default DNS or Google DNS, calendar synchronisation fails.

Now, on Vodafone AU and Google DNS, if I traceroute to caldav.gmx.com, the end result is either caldav.gmx.com or calendar.gmx.com.

If I traceroute caldav.gmx.com with the Quad9 dns settings, the end result is caldav-live.gmx.com.

Here's the interesting part. If I then try and traceroute to caldav-live.gmx.com on Vodafone AU or Google DNS, it can't resolve the server yet, if I try and traceroute to the IP address of the server (217.72.199.80) it resolves it with the name caldav-live.gmx.com.

My question is, to get this fixed so I can synchronise my calendar successfully, who to I need to notify of this issue and what do I tell them is broken and what they need to fix?

Oh, did I mention that I've been in e-mail contact with GMX multiple times and it's always the same story, they can't replicate my problem.

I've attached traceroute logs below.

Traceroute - Vodafone Mobile (Quad9 Public DNS).pdf

Traceroute - Vodafone Network - caldav-live.gmx.com.pdf

Traceroute - Vodafone Mobile (Standard DNS Settings).pdf

Traceroute - Vodafone Network - IP address of caldav-live.gmx.com.pdf

Claus Mattsson

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Apr 7, 2022, 9:10:48 AM4/7/22

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The caldav.gmx.com leads to those two IPs, that you described in your traceroutes. You also demonstrated accesscapabilities for those two ips, again from your traceroutes.

I can't reproduce any situation, where I am unable to get an IP for any of the ressources. You may try this link: Query: caldav.gmx.com - Google Public DNS

If one of the IPs arent hosting your caldav service, then it should be a case for GMX to resolve.

Byron Tompkins

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Apr 7, 2022, 9:10:55 AM4/7/22

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Thought it might be interesting to perform some NSLOOKUP tests for this situation. Interesting results. It appears that in the situation where synchronisation does work, NSLOOKUP of caldav.gmx.com and caldav-live.gmx.com returns the same IP address where as in the situation where synchronisation doesn't work, caldav.gmx.com returns a different IP address to Quad9 and caldav-live.gmx.com returns an NXDOMAIN can't find server error.

Not sure what they mean though, if someone can advise? I really just want to be able to tell someone who is the right person what they need to do to fix this. I'm am just the end user after all.

NSLookups via Google DNS (8.8.8.8)

mixedbeanz@Byrons-Air ~ % nslookup caldav.gmx.com

Server: 8.8.8.8

Address: 8.8.8.8#53

Non-authoritative answer:

caldav.gmx.com canonical name = calendar.gmx.com.

Name: calendar.gmx.com

Address: 217.72.206.50

mixedbeanz@Byrons-Air ~ % nslookup caldav-live.gmx.com

Server: 8.8.8.8

Address: 8.8.8.8#53

** server can't find caldav-live.gmx.com: NXDOMAIN

NSLookups via Cloudflare DNS (1.1.1.1)

mixedbeanz@Byrons-Air ~ % nslookup caldav.gmx.com