The getent output shows that addresses are being sorted properly for a machine without v6. apt-get and other properly-written software will try the addresses in the order listed there, i.e. all v4 addresses first and only then the v6 addresses.

So... I don't think `Acquire::ForceIPv4 "true";` could fix the problem, because apt-get wouldn't have even tried the v6 addresses if any of the v4 ones were working. If you run `wget http://us.archive.ubuntu.com/ubuntu` while the problem is happening it should give you clearer log messages.

Another possibility is a DNS failure that causes your DNS queries to go missing sometimes. If this is the issue then it probably affects both your A and AAAA queries, but you wouldn't notice it on the AAAA queries if you don't have v6. You would only notice when the A queries go missing and the lookup only returns AAAAs; programs will try v6 "first" if this happens, since v6 is all there is to try.

> And how "::/0" > "::ffff:0:0/96"

It has higher precedence, but DNS results are sorted by "do the labels match?" first and by precedence second (rules 5 and 6 in section 6). The idea is to prefer connecting to addresses where the kernel would select the same type of address (as identified by the labels) for the source address. In your case, the algorithm is looking at something like this:

     src addr               →    dest addr
  <your v4 address>         → 91.189.91.81        (label  4 → 4) (precedence 35)
  <your v4 address>         → 91.189.91.82        (label  4 → 4) (precedence 35)
  <your v4 address>         → 91.189.91.83        (label  4 → 4) (precedence 35)
  fe80::786a:e338:3957:b331 → 2620:2d:4002:1::101 (label 13 → 1) (precedence 40)
  fe80::786a:e338:3957:b331 → 2620:2d:4002:1::102 (label 13 → 1) (precedence 40)
  fe80::786a:e338:3957:b331 → 2620:2d:4002:1::103 (label 13 → 1) (precedence 40)

The first three go first because of the matching label, then the last 3 go last because of the differing label. The two groups of three would then each be sorted by precedence, which you can't see here because both groups are homogeneous.

Note the label sort is the only one that considers your source addresses. If that step wasn't there, the sort order would be the same on machines with and without v6, which would be bad.