FS#50852 - IPv6 Path MTU Discovery not working

Attached to Project: Arch Linux
Opened by Stefan Huhn (stefguy) - Wednesday, 21 September 2016, 09:06 GMT
Last edited by freswa (frederik) - Thursday, 20 February 2020, 21:16 GMT
Task Type Bug Report
Category Kernel
Status Closed
Assigned To No-one
Architecture All
Severity Medium
Priority Normal
Reported Version
Due in Version Undecided
Due Date Undecided
Percent Complete 100%
Votes 0
Private No

Details

I am running several Arch Linux and Ubuntu 16.04 machines behind a FritzBox 7430 DSL router. My provider provides me with native IPv4 and IPv6. IPv4 works fine, however, I have issues with IPv6 on the Arch boxes.
I get a valid IPv6 address configured via IPv6 auto configuration and I am able to ping -6 to the outsite world. However, I am not able to initiate TCP connections via IPv6. One idea was to poke around with the MTU settings for the interface connected to the router and that made it finally work: manually setting the MTU size to 1492 allows me to make TCP connections to the outside world.
The default MTU of the interface is 1500, same on the Ubuntu boxes. My understanding of IPv6 is that when a machine creates a packet too large it gets notified to automatically adjust its MTU via ICMP. It seems to me like this is working on Ubuntu, but not on Arch.


Steps to reproduce:
- attach Arch Linux machine to a network where IPv6 auto configuration is enabled
- check that your interface configured a valid IPv6 address (ip addr show)
- try to ping a machine via IPv6 (eg. ping -6 www.google.com)
- try to download something via wget (wget --no-check-certificate https://[some::ip::v6:address]
-- not working
- manually decrease MTU (ip link set dev enp0s20u4 mtu 1492)
- try to download something via wget again (wget --no-check-certificate https://[some::ip::v6:address]
-- works like a charm

Please provide with information what kind of logs/trace would be helpful to solve this issue, I'm quite lost with this ..
This task depends upon

Closed by  freswa (frederik)
Thursday, 20 February 2020, 21:16 GMT
Reason for closing:  Works for me
Comment by Jan de Groot (JGC) - Wednesday, 21 September 2016, 09:09 GMT
Do you block ICMP in your firewall?
Comment by Stefan Huhn (stefguy) - Wednesday, 21 September 2016, 09:22 GMT
No. ICMP is not blocked. On the FritzBox I have double-checked this via the UI, on the Arch boxes there are absolutely no iptables/ip6tables rules in place. Same for Ubuntu, where it seems to work.
Comment by Stefan Huhn (stefguy) - Wednesday, 21 September 2016, 09:24 GMT
Additionally, it's the fritzbox uplink which requires an MTU of 1492 (DSL Link). So the source of the ICMP packet should be the fritzbox, not someplace 'outside'. The fritzbox firewall might not be involved at all.
Comment by Andreas Radke (AndyRTR) - Tuesday, 10 December 2019, 09:40 GMT
Is this still an issue?

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