Dropped ICMP "Datagram too big" Packets

[GrayShade]

I tried a few times to enable IPv6 on my home router, but some pages didn't load. After researching a bit, I blamed it to a PMTU issue and assumed there's a problem with either my router firmware or my ISP.

Hoever, today I ran a Netalyzr test over IPv4 and it reported:

The path between your network and our system supports an MTU of at least 1500 bytes, and the path between our system and your network has an MTU of 1492 bytes. The path MTU bottleneck that fails to properly report the ICMP "too big" is between [router ip] and your host. The path between our system and your network does not appear to report properly when the sender needs to fragment traffic.​

I have an ASUS RT-N66U router running MerlinWRT 378.56_2. The MTU and MRU settings are configured as 1492, but on I'm on PPPoE and I don't know if those should be adjusted for the encapsulation overhead. I also don't know if this is a MTU or firewall issue.

Do you have any hints?

 

[charlie2alpha]

Do you have native IPv6 or do you use a tunneling service?

 

[GrayShade]

Do you have native IPv6 or do you use a tunneling service?

Native, but I performed the Netalyzr test over IPv4 (IPv6 disabled).

 

[GrayShade]

I tried lowering MTU and MRU to 1464 and I got:

The path between your network and our system supports an MTU of at least 1500 bytes, and the path between our system and your network has an MTU of 1464 bytes. The path MTU bottleneck that fails to properly report the ICMP "too big" is between 213.248.83.170 and *. The path between our system and your network does not appear to report properly when the sender needs to fragment traffic.​

 

[charlie2alpha]

In that case, something appears to block the necessary ICMP packets. Do you have any custom firewall rules or scripts?

 

[charlie2alpha]

Also, try to disable temporarily the firewall on your pc and run the test again.

 

[GrayShade]

Also, try to disable temporarily the firewall on your pc and run the test again.

No firewall on the PC, I'll try disabling the one on the router.

 

[GrayShade]

No firewall on the PC, I'll try disabling the one on the router.

No difference.

On the other hand, I enabled IPv6 again and Netalyzr says that the IPv6 MTU test succeeded. test-ipv6.com does the same, but then why am I having random issues from time to time?