Re: slirp: Can I get IPv6-only DHCP working?

[Thomas Huth <thuth@redhat.com>]

On 26/08/2022 01.15, Peter Delevoryas wrote:
> On Fri, Aug 26, 2022 at 12:56:10AM +0200, Samuel Thibault wrote:
>> Hello,
>>
>> Peter Delevoryas, le jeu. 25 août 2022 15:38:53 -0700, a ecrit:
>>> It seems like there's support for an IPv6 dns proxy, and there's literally a
>>> file called "dhcpv6.c" in slirp, but it has a comment saying it only supports
>>> whatever is necessary for TFTP network boot I guess.
>>
>> For which DNS support is welcome :)
>>
>>> Maybe there's no support then?
>>
>> It seems there is:
>>
>>      if (ri.want_dns) {
>>          *resp++ = OPTION_DNS_SERVERS >> 8; /* option-code high byte */
>>          *resp++ = OPTION_DNS_SERVERS; /* option-code low byte */
>>          *resp++ = 0; /* option-len high byte */
>>          *resp++ = 16; /* option-len low byte */
>>          memcpy(resp, &slirp->vnameserver_addr6, 16);
>>          resp += 16;
>>      }
> 
> Well, that's great, but actually I just care about whether slirp supports DHCPv6
> address requests. Sorry if I didn't explain that properly.
> 
> My goal is to run:
> 
>      qemu-system-arm -machine fby35-bmc -nographic -mtdblock image-bmc \
>          -net nic,model=ftgmac100,netdev=nic \
>          -netdev user,id=nic,hostfwd=::2222-:22
> 
> And then see that the BMC received an IPv6 address assignment.
> 
> But, slirp currently just supports IP address assignment through BOOTP?  I
> didn't realize that until looking a little closer at the code.

No, slirp support "IPv6 *stateless* address assignment" (if you haven't 
heard about that before, I suggest to google it). That means that IPv6 
addresses are not administered by a DHCP server, but that each client can 
built its own IPv6 address. It basically works like this (don't quote me on 
that, it's been a while since I worked with this stuff): Once the network 
interface of the client gets activated, the OS creates a link-local IPv6 
that can be only used for activating on the link (based on the MAC address). 
It then uses this address for sending a "router solicitation" message, and 
if there is a router on the link, it replies with a "router advertisment" 
that contains a routable prefix. The client then can take this prefix to 
form a unique IPv6 address and assign it to its interface. QEMU/slirp 
support this way of address assignment, see ndp_send_na() in ip6_icmp.c.
So if your BMC code supports stateless IPv6 address (it certainly should), 
you should be fine already).

  HTH,
   Thomas