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

Steven Barth cyrus at openwrt.org
Mon Mar 30 12:50:34 EDT 2015

> 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.
Nothing is favored, DHCPv6 just hands out ULAs only (if one is there). 
Client can still decide what it uses to connect to anywhere, besides it 
can't use an ULA to connect to the internet anyway.

>> 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.
First of all the O-Flag is ignored whenever the M-Flag is set. Plus 
there cannot be clients that only do DHCPv6 but not RAs, since DHCPv6 
does not transmit any kind of routes thus a client cannot ignore RAs.
There are no assumption of unreliability of a prefix, ULAs aren't used 
for internet communication since they are not routed. Sorry, I don't see 
your point.
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