It's nicer to hide the check away in the various
xyz_add_default_dependencies() calls, rather than making it explicit in
the caller, and thus require deeper nesing.
With this change services by default will no longer require
basic.target, but instead only after it it via After=basic.target.
However, they will still Require= on sysinit.target.
This has the benefit that when booting into emergency mode it is
relatively safe to actviate individual services, as this will not pull
the entirety of basic.target anymore, thus avoid everything listed in
sockets.target and suchlike. However, during the usual boot no change
should be noticed.
This is managed by the kernel, but we should track whether or not we have
a configured IPv6LL address. This fixes two issues:
- we now wait for IPv6LL before considering the link ready
- we now wait for IPv6LL before attempting to do NDisc or DHCPv6
these protocols relies on an LL address being available.
Router Discovery is a core part of IPv6, which by default is handled by the kernel.
However, the kernel implementation is meant as a fall-back, and to fully support
the protocol a userspace implementation is desired.
The protocol essentially listens for Router Advertisement packets from routers
on the local link and use these to configure the client automatically. The four
main pieces of information are: what kind (if any) of DHCPv6 configuration should
be performed; a default gateway; the prefixes that should be considered to be on
the local link; and the prefixes with which we can preform SLAAC in order to pick
a global IPv6 address.
A lot of additional information is also available, which we do not yet fully
support, but which will eventually allow us to avoid the need for DHCPv6 in the
common case.
Short-term, the reason for wanting this is in userspace was the desire to fully
track all the addresses on links we manage, and that is not possible for addresses
managed by the kernel (as the kernel does not expose to us the fact that it
manages these addresses). Moreover, we would like to support stable privacy
addresses, which will soon be mandated and the legacy MAC-based global addresses
deprecated, to do this well we need to handle the generation in userspace. Lastly,
more long-term we wish to support more RA options than what the kernel exposes.
The previous behavior:
When DHCPv6 was enabled, router discover was performed first, and then DHCPv6 was
enabled only if the relevant flags were passed in the Router Advertisement message.
Moreover, router discovery was performed even if AcceptRouterAdvertisements=false,
moreover, even if router advertisements were accepted (by the kernel) the flags
indicating that DHCPv6 should be performed were ignored.
New behavior:
If RouterAdvertisements are accepted, and either no routers are found, or an
advertisement is received indicating DHCPv6 should be performed, the DHCPv6
client is started. Moreover, the DHCP option now truly enables the DHCPv6
client regardless of router discovery (though it will probably not be
very useful to get a lease withotu any routes, this seems the more consistent
approach).
The recommended default setting should be to set DHCP=ipv4 and to leave
IPv6AcceptRouterAdvertisements unset.
Enabling address acquisition, configuring the client and starting the client are now
split out. This to better handle the client being repeatedly enabled due to router
advertisements.
As the data passed is very different, we introduce four different callbacks:
- Generic - router discovery timed out or state machine stopped
- Router - router and link configuration received
- Prefix onlink - configuration for an onlink prefix received
- Prefix autonomous - configuration for to configure a SLAAC address for a prefix received
This way we do not rely on the size MAX* constants from the kernel headers, as these will
be out-of-sync in case we have old headers and new defines in missing.h.
Of course, ideally we'd just use normal synchronous bus calls, but this
is out of the question as long as we rely on dbus-daemon (which logs to
journald, and thus cannot use to avoid cyclic sync loops). Hence,
instead, reuse the wait logic already implemented for --sync, and use a
signal in one direction, and a mtime watch file for the reply.
