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

Steven Barth cyrus at openwrt.org
Fri Mar 27 08:03:37 EDT 2015

> The problem is that you try to be "smart" by abusing the ability to set
> the address preferred lifetime lower than T1.  I don't dispute that it
> is allowed by the RFC, but it is definitely not recommended.
There is no formal requirement (not even a SHOULD) that says otherwise. 
The recommendation made is usually based on the basis that DHCPv6 is 
your only source of addresses which it isn't for us.

> Based on this, it should not come as an surprise that a number of
> clients start behaving erratically if you set T1 much higher than the
> preferred lifetime.  Don't do that. It causes problems.
> You can of course continue to argue that not honouring T1 is a bug, but
> that will not fix any of the failing clients.
Now we know that they actually don't. The clients behave well, that 
seemed to be a misunderstanding.
The OP just tried with a version of our server which had a buggy T1 

> I have a real hard time understanding what makes a DHCPv6 IA_NA address
> any different from a RA based address wrt lifetimes.  If you choose to
> provide both RA and DHCPv6 service to the lan clients using the same
> assigned prefix, then the lifetimes should be kept the same as well.
> Choosing between DHCPv6 and SLAAC is really up to the clients in this
> case.  If you want to prefer one or the other from the server side, then
> I don't see any reason to provide both.
The big difference is that as a router I can send RAs whenever I want 
and clients will react immediately. Most clients however do not support 
the same when doing DHCP and only do dumb polling based on T1/T2. So I 
have to be "smart" in some way to workaround. I would gladly get rid of 
DHCPv6 after all for clients but there is no good way to collect 
hostnames otherwise.

> And wrt the fear of sudden renumbering: The upstrem source, where you get
> these addresses assigned, will include lifetimes.  Why don't you reuse
> those (after some basic sanitation)? If the problem is that the upstream
> lies to you, then I suggest fixing that instead.
We do propagate them as-is through RAs. However imagine manual or 
automatic failover to a different ISP / connection or simply an ISP 
doing 6rd where your IPv6 prefix is mapped from your IPv4-address and th 
connection flaps and you get a different v4-address or a VPN-connection 
being brought up or down. In all these scenarios there needs to be a way 
to tell clients to stop using an address for global traffic near 
instantaneously otherwise they might lose connectivity.


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