odhcp6c: "safe" parameter causes keeping invalid prefixes valid

[Jonas Lochmann]

I am operating OpenWrt routers with mwan3 and also know other people
that do this. A longstanding issue is that IPv6 allows deprecated
rules and mwan3 uses them and then has connectivity issues.

However, deprecated routes should still work. So I suspected an issue
in the upstream routers (issue was always with IPv6 RA with
Fritz! routers, but never when OpenWrt itself did PPPoE). A packet
capture showed that what OpenWrt considerd valid but deprecated was
sent as invalid (valid lifetime = zero) from the uplink.

odhcp6c handles RA prefixes here [1] by calling odhcp6c_update_entry;
the third parameter (called "safe") is hardcoded as 7200 (2 hours).
In its implementation, something strange is done using it [2]:

803         if (x && x->valid > new->valid && new->valid < safe)
804                 new->valid = safe;

Note: x is the previous data for that prefix.

If new->valid is zero, then it is very likely smaller than x->valid and
at the same time smaller than safe (2 hours). In this case, valid = 0
is replaced with valid = 2 hours. As the next RA is received within
two hours, it is again extended to the full two hours.

RFC 4861 recommends another behavior:

> If the prefix is already present in the host's Prefix List as
> the result of a previously received advertisement, reset its
> invalidation timer to the Valid Lifetime value in the Prefix
> Information option.  If the new Lifetime value is zero, time-out
> the prefix immediately (see Section 6.3.5).

Now, one could add valid = zero as special case. However, I see an
bigger issue. I suspect that the whole if statement has a logic error
and something else was intended. Maybe clamping valid to up to two
hours?

[1] https://git.openwrt.org/?p=project/odhcp6c.git;a=blob;f=src/ra.c;h=01a8b721d719f0e0e545a3ca7471c47490597969;hb=77e1ae21e67f81840024ffe5bb7cf69a8fb0d2f0#l511
[2] https://git.openwrt.org/?p=project/odhcp6c.git;a=blob;f=src/odhcp6c.c;h=8354820746131b8d0c9d01f7141af635edc0b8fe;hb=77e1ae21e67f81840024ffe5bb7cf69a8fb0d2f0#l803