6brd: bridge NDP between equal-prefix IPv6 network segments (v0.0.1)
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"The original motivation of Aboriginal Linux was that back around 2002 Knoppix
 was the only Linux distro paying attention to the desktop, and Knoppix's 700
 megabyte live CD included the same 100 megabytes of packages Linux From Scratch
 built, which provided a roughly equivalent command line experience as the 1.7
 megabytes tomsrtbt which was based on busybox and uClibc."
                                                                 --- Rob Landley


About
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ndppd [1] is often used to relay NDP [2] between network segments; however, for
segments with the same prefix (typically a /64), ndppd had a routing problem:
usually the upstream segment has a higher routing priority, so when the relay
receives an upstream packet, it will attempt to forward the packet to the
upstream segment instead of the downstream segment(s).

[1] <https://github.com/DanielAdolfsson/ndppd>.
[2] <https://en.wikipedia.org/wiki/Neighbor_Discovery_Protocol>.

odhcpd [3] (based on 6relayd [4]) solves this by not relaying NSs and NAs by
itself, but pinging the destination address on network segment(s) other than the
source segment; it then watches the neighbour table for changes, and sets up
entries for said address in the NDP proxy table as well as the routing table
accordingly.

[3] <https://git.openwrt.org/?p=project/odhcpd.git>.
[4] <https://github.com/sbyx/6relayd>.

Unfortunately, odhcpd seems to be designed for OpenWRT in a nearly exclusive
way: the code also implements DHCPv6 (server or relay), router discovery (server
or relay), which is largely undesirable since these are usually handled better
by other dedicated packages; it also uses OpenWRT's libubox and UCI system,
which complicates the build procedure and the configuration.

6brd [5] is a solution to this: it focuses on bridging NDP between equal-prefix
IPv6 network segments using the mechanism mentioned above, and leaves the
unrelated functionalities to other software like radvd [6]; additionally, 6brd
is extremely simple to build and use (see below).

[5] <https://gitea.com/CasperVector/6brd>.
[6] <http://www.litech.org/radvd/>.


Usage
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6brd current builds and runs on Linux; see `Makefile' for how to build it.
It is invoked as described below:
  # 6brd [-f] [-v ...] [~][!]iface1 [~][!]iface2 [[~][!]iface3 ...]
      -f:   foreground and send log messages to stderr
      -v:   increase verbosity by 1, at most 2 increments
       ~:   only proxy DAD messages for the specified interface
       !:   do not learn routes for neighbours on the interface

For instance, if eth0 (y:y:y:y:z:z:z:z/64) is connected to the IPv6-enabled
upstream subnet, and you want to enable IPv6 on the wlan0 subnet, you can
typically use
  # 6brd '!eth0' wlan0
Of course, before doing this, you need to enable IPv6 packet forwarding:
  # sysctl net.ipv6.conf.all.forwarding=1
I also find the following desirable:
  # sysctl net.ipv6.conf.all.accept_dad=0
  # sysctl net.ipv6.conf.all.accept_ra=0
  # sysctl net.ipv6.conf.all.autoconf=0
Appropriate configuration for addresses and routing is also necessary:
  # ip addr add y:y:y:y:z:z:z:z/64 dev eth0 noprefixroute
  # ip addr add y:y:y:y:z:z:z:z/64 dev wlan0 noprefixroute
  # ip route add y:y:y:y::/64 dev eth0 metric 256
  # ip route add y:y:y:y::/64 dev wlan0 metric 512
Also note that eth0 is qualified with `!' due to the difference in routing
priority.  Finally, if you want to provide SLAAC on the downstream network
segment(s), radvd will be necessary.


Copying
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Based on OpenWRT's odhcpd, a successor to Steven Barth's 6relayd.
Licence: GPL v2 <https://www.gnu.org/licenses/gpl-2.0.html>.