Re: [PATCH 0/2] option to disable default gateway in IPv6 RA

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Laine, many thanks for such a detailed reply.

On 06/29/2016 08:55 PM, Laine Stump wrote:

* Beyond that, I think it would make more sense to have the option defined in the <ip> element for the IPv6 address rather than at the toplevel
Why may we need it? We are talking about isolated networks, so what is the need for a gateway if all guests are in the same subnet? This is just what you fixed in a related commit 013427e6e733f7a662f4e8a9c11f7dad4cd65e3f.

As I understand, the difference to IPv4 is that IPv6 RA cannot have empty default gateway. The link-local address of the source of RA is implicitly considered a gateway. And the only thing you can do is to set its lifetime to 0 to disable it.

It occured to me that these fixes can be treated as an extension of aforementioned commit, and we should just add "ra-param=*,0,0" to dnsmasq config if we have a new enough version.

(I know there is already an option called "ipv6" at the toplevel, but that is a special case because it's telling what to do wrt IPv6 when there *aren't any* ipv6 <ip> elements in the network definition). A question: would it be possible to set multiple IPv6 addresses, and mark one of them as the default? If so, how would that be configured?

From "man dnsmasq":
"When RA is enabled, dnsmasq will advertise a prefix for each dhcp-range, with default router and recursive DNS server as the relevant link-local address on the machine running dnsmasq."

So it looks like this is impossible, at least for dnsmasq (I have not manage to make it work). A little of googling gave me that radvd supports default route, but it is not the case.


* When you're checking for whether or not dnsmasq is able to support the option you're using, you base this on a dnsnasq version number. Is there any chance that the necessary info could be learned from the output of dnsmasq --help? Would it be adequate to just check for the presence of the string "--ra-param=" in the help output? This is already done to check for dnsmasq's use of SO_BINDTODEVICE - see dnsmasqCapsSetFromBuffer(). I'm guessing you based your addition on the existing code for DNSMASQ_DHCPv6_SUPPORT() and DNSMASQ_RA_SUPPORT(), but I think those were probably put in before the patches that added parsing of --help output to learn dnsmasq capabilities.
OK

--
Your sincerely,
Maxim Perevedentsev

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