[Dnsmasq-discuss] IPv6 - Router Advertisement

Tsachi tsachi.kimel at gmail.com
Thu Nov 14 17:38:41 GMT 2013

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

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?

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.

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