[Dnsmasq-discuss] Make RA_INTERVAL configureable? Deprecate old prefixes?

[Simon Kelley]

On 25/07/13 12:36, Uwe Schindler wrote:
> Hi again,
>
> one addition, because its too hot here... The reality is again
> different (very complicated with IPv6):

Hot there too? Reply at the end.

>
>>> On 24/07/13 19:16, Uwe Schindler wrote:
>>>> Hi,
>>>>
>>>>> RA's are not the same as DHCP, there's no such thing as a
>>>>> "lease time". What there is, is preferred and valid
>>>>> lifetimes. What should happen is that when old address is
>>>>> about to go away, the address for that prefix on the network
>>>>> adaptor should have its preferred lifetime set to zero, and
>>>>> dnsmasq will then notice that and send RA's with the
>>>>> preferred lifetime also set to zero.
>>>>
>>>> I meant the life time, it was just a typo somehow :-) The idea
>>>> here was to send preferred_lifetime=0 in RAs for prefixes that
>>>> disappeared from the interface to "inform" the clients that
>>>> they are no longer useable (my provider does not route old
>>>> prefixes after a reconnect with PPP). Currently you have to set
>>>> the lifetime very low (which produces more traffic on the wire)
>>>> or manually delete or switch on/off the network adaptors once
>>>> the router reconnects or the prefix changes for some reason.
>>>>
>>>> My proposal is to keep a list of "previous prefixes" from the
>>>> constructor:ethX code and also send those prefixes, but with
>>>> lifetime=0 to inform all client to no longer use that address.
>>>> Those 0-lifetime RAs should be sent at least for the length of
>>>> the original lifetime. Radvd has code to handle this (although
>>>> it’s a bit buggy).
>>>>
>>>
>>>
>>> My understanding of this is that it's something that should
>>> happen in the DHCP client and/or ISP DHCP server.
>>>
>>> Ie, when the prefix has to change, the ISP's DHCP server renews
>>> the existing prefix delegation, but with the preferred
>>> lifetime==0. It also offers a new prefix delegation. The old and
>>> new prefixes get installed in the network adaptor with the
>>> preferred lifetime values. Dnsmasq starts to advertise the new
>>> prefix and continues to advertise the old prefix, but with the
>>> pref- lifetime at zero taken from the value in
>> the adaptor.
>>
>> The problem is different in most cases: When you have a dhcp client
>> waiting on an PPPoE connection (e.g. wide-dhcpc, like most consumer
>> routers use - ISC dhcp cannot listen on ppp interfaces
>> unfortunately) and the PPPoE connection gets
>> reconnected/disconnected, the ppp interface is disappears and is
>> later recreated. When its disappearing, the whole set of IP
>> addresses gets lost and they are not deprecated, so dnsmasq will
>> not recognize any deprecation. I have seen your code that
>> propagates the deprecation of existing interfaces in the RAs, but
>> this only works if the prefix/IP address is still existing on the
>> interface, just with preferred lifetime==0. With PPP interfaces
>> disappearing and being recreated, this does not help, because the
>> address is gone (it’s a new instance of the network interface, just
>> with same name). As dnsmasq is mainly for consumer routers which
>> are mostly working with PPP connections, it would be nice to have.
>
> Indeed ist more complicated but effect ist he same: For DSL lines you
> ahev in most cases the following setup: - PPP connection (PPPoE),
> creating network adaptor "ppp0" - this adaptor is coming on going
> when connections are created/released - LAN interface (on which
> dnsmasq is listening), "eth0", with a static IPv4 address and a
> dynamic IPv6 address prefix - wide-dhcpc client listening on the ppp0
> interface and requesting prefix delegation. The delegated prefix is
> assigned as new address with given suffix (::1 or from mac address)
> to the eth0 interface.
>
> In fact, as you say: When ppp0 disconnects or reconnects, the
> wide-dhcp client should not actually remove the delegated prefix from
> the eth0 interface, but unfortunately it does. The reason for this is
> the following: the dhcp client stops when the ppp0 interface
> disappears, so it is restarted in the ifup scripts after the ppp
> connection was reconnected. When the wide-dhcpc client exits, it will
> "clean up" so it removes all addresses assigned ->  that's the
> problem.

I think this is even more broken than that. Surely, what should happen 
if the PPPoE link goes down and then comes up again is that the DHCP 
client should attempt to renew existing DHCP leases, including prefix 
delegations. Any sensible DHCP server at the ISP should give back the 
same prefix delegations, so not only should the prefixes not be removed 
from eth0 just because ppp0 goes down, they will probably not even be 
deprecated once the PPP link is back. Using dnsmasq to deprecate them 
once the PPP link goes is the last thing you should do.

>
> Maybe this is different with ISC dhcp (because you can script the
> whole prefix delegation), but ISC dhcp is never used, because current
> versions of isc-dhcp cannot listen on ppp interfaces (refuses to
> start). There is no reason for this, on the mailing list they say
> it’s a relic from earlier IPv4 days. There is nothing against
> listening for DHCP on a ppp interface. Because of this most consumer
> routers (like the Fritzbox) use wide-dhcpc as client for prefix
> delegation on ppp interfaces (and easier to setup). Unfortunately
> wide-dhcp seems dead, son unless ISC updates their DHCP client to
> allow listening on ppp interfaces, too, we cannot use it. With ISC
> dhcp you could script the prefix delegation and indeed keep the
> address on eth0, but deprecated (preferred_lifetime==0).

Unless you can renew the delegation, see comments above.

I think we're coming to the conclusion that this is not a dnsmasq 
problem, and we should be cautious in bending dnsmasq to try and fix it.


Cheers,

Simon.



>
>>> Again, this is new territory, I'm not saying has to be done like
>>> that, but that's the process I was thinking of when I wrote the
>>> existing code.
>>
>> That's fine!
>>
>> See also:
>> http://manpages.ubuntu.com/manpages/precise/man5/radvd.conf.5.html
>>
>> DeprecatePrefix on|off
>>
>> Upon  shutdown,  this  option  will cause radvd to deprecate the
>> prefix by announcing it in the radvd shutdown  RA  with  a  zero
>> preferred  lifetime and a valid lifetime slightly greater than 2
>> hours. This  will  encourage  end-nodes  using  this  prefix  to
>> deprecate  any  associated addresses immediately. Note that this
>> option should only be used when only one  router  is  announcing
>> the  prefix  onto  the  link, otherwise end-nodes will deprecate
>> associated addresses despite the prefix still  being  valid  for
>> preferred use.
>>
>> See  RFC4862, section 5.5.3., "Router Advertisement Processing",
>> part (e).
>>
>> Default: off
>>
>> Shutdown means in that case shutdown of interface, too. At least
>> this is how it works for radvd.
>>
>> Uwe
>>
>>> Cheers,
>>>
>>> Simon.
>>>
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>>
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>
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