Zephyr IPv6 default router / prefix networking options

Michael Scott

Hello Devs,

I'd like to discuss some different approaches to a simple setup for BLE and iwpan 802.15.4 nodes connected to a gateway via 6lowpan (in a star pattern).  No mesh involved.

On the gateway side, I'm running a NAT64 container which picks up traffic destined for the 64:ff9b::/96 network and translates it to IPv4 before sending it out.  On the return path it gets re-translated back from IPv4 to the 64:ff9b::/96 IP and sent back to the original node that sent the data.   Paired with that I'm also running a DNS64 container on the gateway which accepts DNS queries from the connected nodes (BLE and 802.15.4 6lowpan) and if an IPv4 address would normally be returned it translates it to the 64:ff9b::/96 prefix (DNS64).

Using this setup, each node *should* have basic access to the internet even on home networks where there is no IPv6 available.

On each Zephyr node,  we currently only have a link-local address configured.

Enter the complications:

If I ask Zephyr to send a packet to a 64:ff9b::/96-based address (via the gateway IP) it gets denied because link-local addresses are not "proper" according to RFCs and the function here:

Here are some of my proposed changes to solve this:

1. First solution was to setup RADVD on the gateway to present a "proper" IPv6 network to the connected nodes.  Here we encounter initial connection failures due to the RA packets not being received prior to the application choosing to communicate via the network.  Looks like @jukkar's L4 notifications would come in handy here if we can craft the right requirements for the "UP" and "DOWN" notifications.  Also, I think that RADVD is overkill in this scenario.  We are talking about a hand full of nodes on 1 subnet w/ 1 statically configured router IP.  It will never change and if the gateway goes down, the nodes would never be connected in the first place.

2. Second solution was to add a Kconfig option to subsys/net/ip/net_if.c which would change the behavior of is_proper_ipv6_address().  In effect saying "go ahead and use the link-local address" we know what we're doing.  I'm not sure how the networking maintainers feel about that as it does contradict RFC.  Seems like a one-off fix.

3. A third solution was to add Kconfig options to subsys/net/lib/config for NET_APP_MY_IPV6_ROUTER, NET_APP_MY_IPV6_PREFIX and NET_APP_MY_IPV6_PREFIX_LEN.  These would effectively configure the nodes at boot-time as if the related RA packets had been received (no lifetime expiration). These changes might allow us to modify all of our IPv6 samples so that multiple nodes can be running at the same time.

Question to @jukkar or other network maintainers: What's the preferred setup?  I know there is movement on this front but I'm not sure of the specifics.

- Mike

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