The introduction of address lifetimes also means that lwIP correctly
supports transitions between PREFERRED and DEPRECATED address states,
and that means that the source address selection must be changed to
take this into account. Adding this feature to the previous algorithm
would have resulted in a mess, so this patch rewrites the algorithm to
stay close to the rules described in RFC 6724 (formerly 3484) Sec. 5.
This yields the following changes:
- Rule 2 ("prefer appropriate scope") is now fully implemented, most
importantly allowing larger-scope addresses to be picked if no
smaller-scope addresses are available (e.g., a global address may
now be used to connect to a unique-local address);
- Rule 3 ("avoid deprecated addresses") is now also fully implemented;
- unknown-scope addresses are also supported, with lowest priority;
- the link between the prescribed rules and the actual algorithm is
made much more explicit, hopefully allowing future improvements to
be made more easily.
For reasons explained in comments, one previous deviation from the RFC
on Rule 2 is retained for now.
As laid out in RFC 5942, the assumption that a dynamically assigned
(SLAAC/DHCPv6) address implies an on-link subnet, is wrong. lwIP does
currently make that assumption, routing packets according to local
address subnets rather than the on-link prefix list. The result is
that packets may not make it to their destination due to incorrect
routing decisions.
This patch changes the routing algorithms to be (more) compliant with
RFC 5942, by implementing the following new routing policies:
- all routing decisions check the on-link prefix list first, and
select a default router for off-link routing only if there is no
matching entry in the on-link prefix list;
- dynamically assigned addresses (from address autoconfiguration) are
considered /128 assignments, and thus, no routing decisions are taken
based on matches against their (/64) subnet anymore;
- more generally, all addresses that have a lifetime are considered
dynamically assigned and thus of size /128, which is the required
behavior for externally implemented SLAAC clients and DHCPv6;
- statically assigned (i.e., manually configured) addresses are still
considered /64 assignments, and thus, their associated subnet is
considered for routing decisions, in order to behave as generally
expected by end users and to retain backward compatibility;
- the link-local address in IPv6 address slot #0 is considered static
and thus has no lifetime and an implied /64 subnet, although link-
local routing is currently always handled separately anyway.
IPv6 source address selection is kept as is, as the subnet tests in
the algorithm serve as poor man's longest-common-prefix equivalent
there (RFC 6724 Sec. 5, Rule 8).
Previously, IPv6 routing could select a next-hop router on a netif
that was down or disconnected, potentially resulting in packets being
dropped unnecessarily. This patch changes router selection to take
into account the state of the router's associated netif, eliminating
such unnecessary packet loss.
Also, this patch fixes the test for router validity, which was
erroneously based on the router's invalidation timer rather than its
neighbor cache entry state. Given that an expired router has no
associated neighbor cache entry, no invalid routers would previously
ever be returned.
Finally, this patch also adds round-robin selection of routers that
are not known to be reachable or probably reachable, as per RFC 4861
Sec. 6.3.6 point (2). Support for this feature was partially present
but not actually functional.
This patch rearranges the code division between nd6.c and ip6.c such
that the latter does not need to access ND6-internal data structures
(specifically, "default_router_list") directly anymore.
Generally speaking, packets with a loopback destination address -
127.0.0.1 for IPv4 and ::1 for IPv6 - should not be accepted on
non-loopback interfaces. For IPv4, this is implied by RFC 1122
Sec. 3.2.1.3. For IPv6, it is mandated by RFC 4291 Sec. 2.5.3.
Failure to perform this filtering may have security implications, as
applications that bind sockets to loopback addresses may not expect
that nodes on the local external network be able to produce traffic
that will arrive at such sockets.
With this patch, lwIP drops packets that are sent to a loopback
address but do not originate from the interface that has the loopback
address assigned to it. This approach works regardless of whether it
is lwIP or the system using it that implements a loopback netif. The
only exception that must be made is for configurations that enable
netif packet loopback but disable the lwIP loopback netif: in that
case, loopback packets are routed across non-loopback netifs and would
thus be lost by the new filter as well.
For IPv6, loopback-destined packets are also no longer forwarded; the
IPv4 forwarding code already had a check for that.
As a small performance improvement, the IPv6 link-local/loopback
address check is now performed only once per packet rather than
repeatedly for every candidate netif.
In general, netif_default may be NULL, and various places in the code
already check for this case before attempting to dereference the
netif_default pointer. Some places do not perform this check though,
and may cause null pointer dereferences if netif_default is not set.
This patch adds NULL checks to those places as well.
In the BSD socket API world, IP_HDRINCL is a socket option for "raw"
sockets that indicates whether sent packets already include an IP
header. Within lwIP, "IP_HDRINCL" is redefined as a special value
that indicates to lwIP-internal functions that an IP header is already
included. While somewhat related, the two meanings are different and,
on platforms that define the IP_HDRINCL socket option, this results in
a conflict. This patch renames the lwIP one to "LWIP_IP_HDRINCL",
thus resolving the conflict.
ip_addr_t is used for all generic IP addresses for the API, ip(4/6)_addr_t are only used internally or when initializing netifs or when calling version-related functions
