Bug#653623: apt: DNS resolution fails on IPv6-only systems
I'm experiencing this with a multi-stack system for quite some time now.
Often the lookups for about half of the repositories fail.
I just had a look at it with Wireshark. What happens is a query is sent
to the DNS server, which immediately (i. e. without any unusual delay)
replies with a positive result set. BUT: This response is rejected by
the client with an ICMP "Port unreachable" message, indicating that apt
under some circumstances closes the socket after sending the request
without waiting for a response.
About half of my 10 repositories could not be resolved when testing.
There seems to be a 50% chance for each often resulting in a number of 4
or 5 failed repositories, but I've also encountered 3 and 7.
After I found this bug report, I tried deleting the link-local IPv6
address of the external interface (leaving only IPv4 addresses), which
reduced the number of failing repositories to just 1 or 2 (sometimes 3).
Adding the address raised the number to the initial value again.
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