[Bug 1853669] Re: systemd resolves own hostname to link local ipv6 address

[Andreas L]

The premis is that you have a network device with a "link local" (aka:
"scopeid 0x20<link>") inet6 address.

In my case, "ifconfig enp4s0" shows a line like this:
   inet6 fe80::4687:fcff:fe9e:4ac7  prefixlen 64  scopeid 0x20<link>

If your machine does NOT have such a line, then I must admit, I don't
know how to produce one. Mine had it from default setup.

If your machine DOES have an inet6 link local address, then (in a shell)
run "host $(hostname)" and it is likely to return an inet (ipv4) address
(that's just fine) AND the link local inet6 address.

If your "host $(hostname)" doesn't give you any inet6 address, or gives
you one with %network-name appended, then you might have a different
version of "systemd" than 237-3ubuntu10.33 (as included in 18.04)

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https://bugs.launchpad.net/bugs/1853669

Title:
  systemd resolves own hostname to link local ipv6 address

Status in systemd package in Ubuntu:
  Incomplete

Bug description:
  I've got an ethernet-device that only has a configured ipv4 address,
  and some auto-generated link-local (aka "scope link") ipv6 address.

  Any tool doing a DNS query (and /lib/systemd/systemd-resolved is the
  DNS-server listening on 127.0.0.53) for this host's hostname gets back
  two addresses: the correct ipv4 address, and a broken ipv6 address.

  Unlike on ipv4,  it is possible for the same ipv6-address to be
  assigned to multiple devices, and therefore the address is only valid
  in the context of the eth-device.

  Now, if "ifconfig" shows "inet6 fe80::4687:fcff:fe9e:4ac7  prefixlen 64  scopeid 0x20<link>"
  then "fe80::4687:fcff:fe9e:4ac7" is NOT a connectable address, and syscall connect() typically fails with EINVAL.

  To make it a valid address, it needs to be suffixed with a "%" and the device name, like:
  fe80::4687:fcff:fe9e:4ac7%enp4s0

  Either the resolver can return the link name attached to the address
  separated with a "%" char, or it needs to ignore link-local inet6
  addresses.

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