Host unavailable only when phone in WIFI

I am running a nextcloud 18.0.11 on an own server behind a fritzbox. Since the last client of the android nextcloud app all users suffer a connection issue, when the phone is logged into a WIFI-network. The connection issue seems not to be there, when accessing from mobile network or even VPN to the university network. There is no connection attempt appearing in the nextcloud log, the app shows the message “Server nicht verfügbar”, when being opened while connected to a WIFI.

Setup Information:

  • Nextcloud 18.0.11 (and previously 0.10), manual installation with apache, PHP 7.4.3, MySQL
  • SSH encryption provided by letsencrypt/certbot
  • External domain is
  • Android app version for which the error occurred: 3.14.1

The rather unusual port was chosen for security reasons and up to now never made any problems. The fritzbox is forwarding the port to the server. The WIFI issue seems not to depend on the respective internet provider. Also even in WIFI the nextcloud can be reached by browser from the phone.

This is an expected behavior because a Fritzbox blocks connection attempts on the LAN to a local service which is being addressed by the external DNS name by default. You should search the internet and/or the forum for “DNS rebind”, to find a solution. AVM provides a detailed describtion of this function on their website too,

most likely the problem is related to IPv6 which becomes preferred by more clients over time: Nextcloud at home with IPv4 and IPv6 German and CALDAV calendar: certificate issue in local network

I solved the problem by setting up a local DNS (pihole) and configuring the router to use this DNS server (I had to go the hard way as there is no way to disable Fritzbox internal IPv6 DNS server which always uses DNS server configured in the internet settings)

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Ok, that’s also a way to go, but that’s strange, because it wasn’t that difficult to disable IPv6 on my Fritzbox?!

(Heimnetz > Netzwerk > Netzwerkeinstellungen: IP-Adressen > IPv6-Konfiguration > IPv6-Adressen: DNSv6-Server im Heimnetz etc.)

I didn’t test if completely disabling DHCPv6 would do… but then the question is how the hosts should acquire an IPv6 address?

But is DHCPv6 is enabled I have 3 options and each of them includes DNSv6 server. For me it looks like using IPv6 requires DNSv6 as well. For this reason I choose the way to change “internet” DNS of the Fritzbox to my internal DNS where I just block the myfritz DNS record and provide my reverse proxy IP.

The config is still “work in progress” on my side as sometimes fritzbox DNS responds with public IPs but most of the time it works good… so at the moment it is good enough for me.

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is this even the case, if actually accessing from the outside? The szenario i was refering to was trying to access a nextclound instance from my wifi, while the actual server is located in a different network (meaning another house)

I would not expect this to happen if you access your server from internet… There is some some difference in accessing/port forwarding of request initiated from IPv4 and IPv6 in Fritzbox… but till now I didn’t see any issues from outside local network. I didn’t often access my NC from private network… but I my feeling is public networks work fine in general…

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