[OpenWrt-Devel] DHCPv6 flash renumbering patches with HE tunnel breaks

Arjen de Korte arjen+openwrt at de-korte.org
Mon Mar 30 12:45:54 EDT 2015

Citeren Steven Barth <cyrus at openwrt.org>:

> After thinking it through a bit more, I changed the default behavior  
> to the following:
> The preferred lifetime is now as given by the ISP, however the  
> DHCPv6 server will only hand out the address with the highest  
> preferred lifetime (= the ULA unless disabled).

Since I avoid ULA like the plague, this probably won't be a problem  
for me and global IPv6 addresses will be served. But I'm not convinced  
that favoring a ULA prefix (if available) over an ip6prefix is best at  
all times.

> Thus flash renumbering should still work (since only the ULA-address  
> cannot be changed immediately).
> Is that okay with you or do you see any other issues?

Well, one thing that still worries me, is that the behavior of the  
DHCPv6 server is different depending on the state of the O flag. From  
a DHCPv6 client perspective, it shouldn't. When both M- and O-flag are  
set, it is up to the client to decide if it wants to use DHCPv6 or  
SLAAC. You've made it clear that there may be reasons not to use an  
adress provided by DHCPv6 and use SLAAC instead. But this should not  
be the only (or default) way of operation. There may be reasons other  
than broken IPv6 uplinks why network administrators choose to set both  
M- and O-flags. I think the other reasons are far more common (the  
lack of DHCPv6 support in Android is just one of them). Even with the  
changes pushed out today, odhcpd makes the assumption that the  
ip6prefix can't be relied upon if a ULA prefix is provided and I think  
that is just incorrect.

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