ipv6 disabled crash fritz box 8.03 bug

[katsuro]

As the owner of two AVM Fritz!Box DSL routers (7520 rev. B and 5690 Pro International), I'm experiencing a recurring problem when IPv6 support is disabled – particularly when using Telekom DSL in Germany.

Problem Description:

• On the 7520, disabling IPv6 causes the router to crash regularly – usually every 22–26 hours, sometimes even more frequently (every 10–12 hours).
• I initially thought this was due to the older hardware or a potential memory leak, so I upgraded to a 5690 Pro.
• The 5690 Pro worked fine during the first week, with IPv6 enabled.
• However, as soon as I disabled IPv6, the router crashed overnight. After reboot, I saw the generic message "A report has been sent" – but with no clear explanation of the cause.

Additional Notes:

• The crash only happens when IPv6 is disabled.
• This appears to be a firmware-related bug, likely in Fritz!OS version 8.03.
• Both routers are used with Telekom DSL in Germany.
• The issue is reproducible and affects both old and new hardware models.

 

[Loxley]

Hy @katsuro,
Welcome to the forum. Regarding your question:
It is normally possible to switch off IPV6. I am also with Telekom, but I use a Speedport router. I can switch off IPV6 there without any problems.
To be on the safe side, Telekom interrupts the IPV4 connection once a day and assigns a new address. Could this be the reason?

 

[katsuro]

Hy @katsuro,
Welcome to the forum. Regarding your question:
It is normally possible to switch off IPV6. I am also with Telekom, but I use a Speedport router. I can switch off IPV6 there without any problems.
To be on the safe side, Telekom interrupts the IPV4 connection once a day and assigns a new address. Could this be the reason?

No, I don't think that's the cause.
If I keep IPv6 enabled, my connection remains stable — no interruptions at all.
The IP address stays the same until I manually reboot the router or the DSL link drops for some other reason.
So the daily IPv4 reconnection by Telekom doesn’t seem to affect the stability in my case — only disabling IPv6 causes regular crashes.

 

[Loxley]

Ok, it was just a guess on my part. But one question please: Why do you want to switch off IPV6?
Does it interfere or what is the reason?

 

[katsuro]

Ok, it was just a guess on my part. But one question please: Why do you want to switch off IPV6?
Does it interfere or what is the reason?

After checking my DNS setup and how my services are exposed, I realized that I don’t actually need IPv6 in my current setup.

• My public DNS only resolves to IPv4.
• I run a few services, but they are only configured for IPv4, including port forwarding.
• In some rare cases, DNS returned IPv6 addresses, which I hadn’t configured firewall rules, so I disabled IPv6 just to be safe and avoid unexpected external access.

 

[Loxley]

If you do not configure firewall rules for IPV6 on your router, everything is normally blocked.
So you can leave ipv6 enabled without any harm.

 

[katsuro]

If you do not configure firewall rules for IPV6 on your router, everything is normally blocked.
So you can leave ipv6 enabled without any harm.

Yes, I understand that, but I still prefer to have IPv6 disabled.
The problem is: whenever I disable IPv6, my router eventually crashes and restarts.
This is pretty annoying, especially because reconnecting over DSL takes 3–4 minutes every time.
So even though IPv6 may be safe by default, disabling it shouldn't cause instability — and that's the core issue I'm trying to highlight.

 

[Loxley]

I agree with you. The router should not crash if IPV6 is deactivated. You could perhaps contact AVM support. Maybe they know something.

 

[Stationary]

Telekom interrupts the IPV4 connection once a day and assigns a new address

No. Telekom hasn‘t done this widely anymore since years:

This 7590 is connected since more than three months.
If your FB reconnects every night, then it is more likely, that you use the FB settings to request a new connection every night:

The FB only reconnects daily if „Zwangstrennung“ is checked. Otherwise it will run for months without a reconnect, unless the DSLAM forces one - which it sometimes does when the controller believes that the connection could or should be run at a different speed. In fact, with my 5690 Pro I had such DSLAM induced reconnect a few days ago and afterwards the link was faster.

But back to the IPv6 issue. At the moment I am having the 5690 run on vDSL250 with IPv6 switched off. Thus far, no problem.

 

[Loxley]

Okay, yes. You have to set “Zwangstrennung” and you can also switch it off. What I wanted to say was: There is an option to disconnect once a day. But that doesn't seem to be the cause.
Sorry I didn't express myself exactly.

 

[katsuro]

At the moment I am having the 5690 run on vDSL250 with IPv6 switched off.

Same VDSL250 for me. Also could you please check how much SNR you have?
Could I ask you to check your SNR (Signal-to-Noise Ratio) values, especially at night?
In my case, I usually get about 10–11 dB during the day, but it drops to 8–9 dB at night.
Just wondering if you’re seeing something similar.

 

[Stationary]

An SNR of about 7-9 dB is typical for my connection. It never reaches double digit values.

 

[katsuro]

Anhang anzeigen 10971
An SNR of about 7-9 dB is typical for my connection. It never reaches double digit values.

Hmm, I have more stable on download and lower at upload with 80m link. Interesting...
Thanks for info!