Longest matching IPv6 address selection biting me / 2020-01-24

Trying to get devuan updates, I see:

Err:5 http://nl.mirror.devuan.org/merged ascii Release
  404  Not Found [IP: 2001:878:346::116 80]
Err:6 http://nl.mirror.devuan.org/merged ascii-security Release
  404  Not Found [IP: 2001:878:346::116 80]
Err:7 http://nl.mirror.devuan.org/merged ascii-updates Release
  404  Not Found [IP: 2001:878:346::116 80]

While nl.mirror.devuan.org has no shortage of IPv6 and IPv4 addresses:

;; ANSWER SECTION:
nl.mirror.devuan.org.   78083   IN      CNAME   deb.devuan.org.
deb.devuan.org.         78083   IN      CNAME   deb.roundr.devuan.org.
deb.roundr.devuan.org.  845     IN      AAAA    2001:638:a000:1021:21::1
deb.roundr.devuan.org.  845     IN      AAAA    2a01:4f8:140:1102:2b76:955d:b48f:bdf3
deb.roundr.devuan.org.  845     IN      AAAA    2001:878:346::116
deb.roundr.devuan.org.  845     IN      AAAA    2a01:4f8:162:7293::14
deb.roundr.devuan.org.  845     IN      AAAA    2800:a8:c001::a
deb.roundr.devuan.org.  845     IN      AAAA    2a01:4f9:2a:fa9::2
deb.roundr.devuan.org.  845     IN      AAAA    2001:590:3803::31:151
deb.roundr.devuan.org.  845     IN      AAAA    2001:4ca0:4300::1:19
deb.roundr.devuan.org.  845     IN      AAAA    2a02:2a38:1:400:422a:422a:422a:422a
deb.roundr.devuan.org.  845     IN      AAAA    2a0a:e5c0:2:2:400:c8ff:fe68:bef3

;; ANSWER SECTION:
nl.mirror.devuan.org.   78063   IN      CNAME   deb.devuan.org.
deb.devuan.org.         78063   IN      CNAME   deb.roundr.devuan.org.
deb.roundr.devuan.org.  824     IN      A       46.4.50.2
deb.roundr.devuan.org.  824     IN      A       130.225.254.116
deb.roundr.devuan.org.  824     IN      A       190.64.49.124
deb.roundr.devuan.org.  824     IN      A       31.220.0.151
deb.roundr.devuan.org.  824     IN      A       200.236.31.1
deb.roundr.devuan.org.  824     IN      A       131.188.12.211
deb.roundr.devuan.org.  824     IN      A       141.84.43.19
deb.roundr.devuan.org.  824     IN      A       37.187.111.86
deb.roundr.devuan.org.  824     IN      A       5.196.38.18
deb.roundr.devuan.org.  824     IN      A       95.216.15.86
deb.roundr.devuan.org.  824     IN      A       185.38.15.81

I always get the error for 2001:878:346::116 when connecting. This site seems to have a problem with the devuan mirror at the moment, so I'd like to use another one, but apt keeps going back to the same source. This has to do with IPv6 address destination selection (RFC 3484 / RFC 6724).

A good explanation at IPv6 Destination Address Selection – what, why, how - Karl Auer with:

Rule 9, “use longest matching prefix“, will prefer the candidate destination address that shares the greatest number of contiguous leading bits with the source address that would be chosen for it. Such an address is likely to be topologically closer to the source address.

Indeed that address is close to my home network addresses:

2001:0878:0346:0000:0000:0000:0000:0116
2001:0980:14ca:0001::/64

So the "roundr" round robin isn't very round for IPv6 users.

Workaround: reject the address that is giving me problems:

# ip -6 route add unreachable 2001:878:346::116
# apt update
Get:1 http://nl.mirror.devuan.org/merged ascii InRelease [25.6 kB]
Get:2 http://nl.mirror.devuan.org/merged ascii-security InRelease [25.6 kB]
Get:3 http://nl.mirror.devuan.org/merged ascii-updates InRelease [25.6 kB]
Get:5 http://nl.mirror.devuan.org/merged ascii-security/main Sources [185 kB]
Hit:4 http://packages.roundr.devuan.org/merged ascii InRelease
Get:6 http://nl.mirror.devuan.org/merged ascii-security/main amd64 Packages [480 kB]