IPv6: hardcoded address `fd00::2` violates RFC 4193

[AkihiroSuda]

https://github.com/rootless-containers/slirp4netns/blob/v1.1.9/slirp4netns.1.md

Default configuration:

* MTU:               1500
* CIDR:              10.0.2.0/24
* Gateway/Host:      10.0.2.2    (network address + 2)
* DNS:               10.0.2.3    (network address + 3)
* IPv6 CIDR:         fd00::/64
* IPv6 Gateway/Host: fd00::2
* IPv6 DNS:          fd00::3

According to RFC 4193, the addresses in fd00::/8 seems to need to be random

https://tools.ietf.org/html/rfc4193
https://networkengineering.stackexchange.com/questions/30128/ipv6-ula-vs-link-local

(slirp4netns --enable-ipv6 is still experimental and we can have breaking changes)

[AkihiroSuda]

@giuseppe Could you take a look? What should we do with this?

[giuseppe]

is it really a risk for slirp4netns though? IPv6 is not routed so there is not really risk of collisions, or do you think we should just pick a random IP to honor the RFC? I am fine if we go this way (and if any problem arises users of slirp4netns can force the IP addr)

[AkihiroSuda]

I think honoring RFC is not necessarily, but probably we will eventually need to support specifying random addr.
Not an urgent task.

[Luap99]

I think it should pick a random one. For users who want a fixed subnet, a --cidr6 option should be added.

[AkihiroSuda]

@pfandl @Luap99 Anybody of you interested in opening a PR?

[Luap99]

If I can find some time, sure, but probably not in the next few weeks.

[pfandl]

@AkihiroSuda do you want just commit pfandl@8cc216f ?