[Dnsmasq-discuss] IPv6 - Router Advertisement

Simon Kelley simon at thekelleys.org.uk
Fri Nov 15 10:18:30 GMT 2013

On 14/11/13 17:38, Tsachi wrote:
> Hi,
> I have a question regarding dnsmasq behavior regarding IPv6 RA message
> while using the "constructor" config option.
> Version details: Kernel 2.6.35, Dnsmasq ver 2.67
> I am working on a small Linux router device which obtains a single
> dynamic global \64 prefix from a network (the prefix is changing every
> time the device connects to the network).
> This prefix is published (RA) on the device LAN interface (eth0) for
> the purpose of hosts Global address generation (using SLAAC).
> The following dnsmasq ipv6 configurations are set:
> dhcp-range=::,constructor:eth0, ra-only,64,60
> log-dhcp
> dhcp-option=option6:23,[::]
> ra-param=eth0,20
> The LAN interface (eth0) starts with link local address.
> After the device connects to the network, a new ipv6 global address is
> manually assigned to the eth0 interface ( \64 constructed from the
> prefix that the device received).
> ip addr add 2002::21a:2bff:fe3c:4d5e/64 dev eth0
> As expected, this triggers periodic unsolicited RA messages (and RA
> responses to RS).
> Log shows the following RA messages - "RTR-ADVERT(eth0) 2002::"
> After some time the device disconnects from the network and then the
> eth0 IPv6 address is removed:
> ip addr del 2002::21a:2bff:fe3c:4d5e/64 dev eth0
> The log shows the following RA messages " router advertisement on
> 2002::, old prefix for eth0"
> But the log doesn't show – "RTR-ADVERT" messages (nor the device sends
> any RA after, and will not response to RS requests).
> I was hoping to see an unsolicited RA massage (RTR-ADVERT) announcing
> this prefix with preferred-time=0 and maybe also "router lifetime=0",
> so hosts are aware of the disconnection.
> What do you think regarding unsolicited RA massage when the interface
> global ip address is deleted?

What you describe (RA with preferred time zero) is exactly what should 
happen, so this looks like a bug.
> By the way, In a different scenario, if I delete the interface ipv6
> address and add a new one, the log shows:
> router advertisement on 2012::, old prefix for eth0
> router advertisement on 2055::, constructed for eth0
> RTR-ADVERT(eth0) 2055::
> RTR-ADVERT(eth0) 2012:: old prefix
> The second prefix is sent with "preferred-time=0", as expected.

OK, that's useful infromation. I'll investigate further.



> Thanks,
> Tsachi
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