Re: (web server) serving on both ipv6 and ipv4?

[Chris Vine <vine35792468@gmail.com>]

On Wed, 19 Jan 2022 13:44:13 +0000
Chris Vine <vine35792468@gmail.com> wrote:
> On Wed, 19 Jan 2022 13:07:33 +0000
> Chris Vine <vine35792468@gmail.com> wrote:
> [snip]
> > As I understand it, with linux IPv6 sockets are dual stack capable, and
> > in earlier kernel versions this was be enabled by default.  I believe
> > with current versions that is no longer the case, and that you have to
> > specifically enable dual stack by turning off IPV6_V6ONLY using
> > setsockopt before binding on the socket.
> > 
> > Then, if receiving a IPv4 connection from address 1.2.3.4, this would be
> > mapped as ::::ffff:1.2.3.4.
> > 
> > I do not know about other OSes.  I have half a memory that some earlier
> > versions of windows did not support dual stack sockets (XP?).
> 
> By the way I did use dual stack some years ago, and I cannot now
> remember all the details, but I think I may have had to bind on
> in6addr_any (which in dual stack would cover INADDR_ANY) or on ::1
> (which would cover 127.0.0.1) to get dual stack to work.  I suggest you
> play around with it to see.
> 
> One other correction: when I said there was a mapping to ::::ffff:
> 1.2.3.4 I meant ::ffff:1.2.3.4.

You have stimulated my interest and this is what I have found.  First,
in C the correct call in linux to do what you want to obtain a dual
stack socket is to set the IPV6_V6ONLY option in the IPPROTO_IPV6 level
to 0 (off). However neither IPV6_V6ONLY nor IPPROTO_IPV6 is defined in
guile, so you have to enter the numeric values for your OS by hand.  In
linux this will do it in guile, but of course it is non-portable:

  (setsockopt [sock] 41 26 0)

However, this only actually seems to accept a connection from a IPv4
address if the socket binds to :: (which is in6addr_any, but
the in6addr_any symbol also appears not to be defined in guile).
Binding to ::1 (localhost) will not enable you to connect from
127.0.0.1 on my computer.  Whether binding to :: and so permitting any
interface to access the socket is OK for you depends on what your needs
are.  If not then it looks as if you are stuck with having two sockets,
one for IPv4 and one for IPv6.