[Dnsmasq-discuss] multiple dhcp6 servers

[Simon Kelley]

On 12/10/12 13:56, Gene Czarcinski wrote:
> On 10/12/2012 06:44 AM, Simon Kelley wrote:
>>> >OK, it is what is. Is there some practical way to "tell" dnsmasq an
>>> >association for a hostname and an IPv6 address? I cannot see something
>>> >like that used to update an authoritative name server but it would be
>>> >useful to have some kind of an update capability with dnsmasq.
>> If I've understood your question right, that's what dhcp-host does.
> I guess I am not making myself clear.
>
> First of all, my problem is running qemu/kvm/libvirt virtual
> networks/guests and will not likely occur in a real/hardware-only
> situation. It is also possible that my problem is unique but then, it
> may or may not be ... I might be just out front and getting some arrows
> in my back.
>
> With the cost and availability of today's hardware, it is possible to
> create large networks (multiple networks) of virtual guests ... my
> virtualization host runs Fedora 17 on hardware consisting of an 8-core
> AMD processor, 16GB memory, an SDD for root and home, and a couple of
> large SATA-III disks for data storage [those virtual systems run faster
> than real systems did a few years ago].
>
> So, I wanted to run some IPv6 virtual networks (currently, I have 16
> virtual networks defined). Now, if dns was invented because those little
> IPv4 numbers were hard to remember, then trying to remember IPv6
> addresses is impossible. One of the things I want to do is access some
> of those guests from the virtualization host and (sometimes) from other
> hosts. Host names are much easier to remember and, using dnsmasq, it all
> works for IPv4. Well, I wanted the same thing for IPv6.
>
> One of the things libvirt does is to start a dnsmasq process for every
> virtual network -- you have 10 virtual networks started, you have 10
> separate dnsmasqs started. If you add an IPv6 address to your virtual
> network definition, then libvirt adds a --listen-address=<ip6-address>
> to the dnsmasq command line and also starts radvd for that network (yes,
> you now have lots of radvd processes running too).
>
> I wanted libvirt to support dhcp6 for virtual networks. Before investing
> a lot of time into adding this support to libvirt, I thought I would run
> a little test to see how things worked. I created a little patch to
> modify the radvd parameters to use "AdvManagedFlag on" and a test-only
> kludge to add --dhcp-range <ip6-address-range> to the dnsmasq command
> line. Installed the modified libvirt and started the first virtual IPv6
> network ... looks good. Started the second IPv6 network ... bang! The
> dnsmasq did not start because it could not get exclusive use of server
> port 547. [BTW, the first dnsmasq did not work because of the
> listen-address=/interface= which I covered in another message]
>
> Now if the networks were pretty static then it is possible to have one,
> "big" dnsmasq service all of the virtual networks. I do not know about
> others but I am bringing networks up and down. defining new networks,
> etc. in my test/research environment (like I said, maybe my situation is
> unique). A single dnsmasq is not practical and would be a significant
> change to libvirt.
>
> As things currently exist, I am using SLAAC IPv6 addresses and radvd for
> the default route. This works in that, via IPv6, I can access the
> virtualization host as well as other systems on my local network. But, I
> cannot go the other way because I have no idea what the IPv6 address are
> for individual virtual systems. Yes, I can use manual configuration for
> IPv6 and add entries in the /etc/hosts file for each of those systems
> but this does not scale.
>
> Note -- On second and third level virtual networks, I run my own virtual
> services with radvd and dnsmasq and that works fine.
>
> So, as i see it, here are the alternatives:
>
> 1. Get multiple copies of dnsmasq to run on a single hardware system so
> that I can use dhcp6.
>
> 2. Have some way to update the dnsmasq cache with the hostname and
> related IPv6 address.
>
> Any suggestions/comments?


All understood.

It's worth trying the following, to see if makes DHCPv6 with multiple 
instance work.

In src/dhcp.c, the function make_fd() has the following code:

   if (option_bool(OPT_NOWILD) || option_bool(OPT_CLEVERBIND))
     {
#ifdef SO_REUSEPORT
       int rc = setsockopt(fd, SOL_SOCKET, SO_REUSEPORT, &oneopt, 
sizeof(oneopt));
#else
       int rc = setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &oneopt, 
sizeof(oneopt));
#endif
       if (rc == -1)
         die(_("failed to set SO_REUSE{ADDR|PORT} on DHCP socket: %s"), 
NULL, EC_BADNET);
     }


Add morally equivalent code to dhcp6_init() in src/dhcp6.c. That might 
be enough.

I'm hoping that since you already have a test system in place, you can 
check this quickly, and see if does the trick.



Cheers,

Simon.