RE: sctp doesn't honour net.ipv6.bindv6only

[David Laight <David.Laight@ACULAB.COM>]

From: Marcelo Ricardo Leitner
> Sent: 19 May 2020 20:47
> 
> On Tue, May 19, 2020 at 10:47:17AM +0000, David Laight wrote:
> > The sctp code doesn't use sk->sk_ipv6only (which is initialised
> > from net.ipv6.bindv6only) but instead uses its own flag
> 
> It actually does, via [__]ipv6_only_sock() calls since 7dab83de50c7
> ("sctp: Support ipv6only AF_INET6 sockets.").
> 
> > sp->v4mapped which always defaults to 1.
> >
> > There may also be an expectation that
> >   [gs]etsockopt(sctp_fd, IPPROTO_IPV6, IPV6_V6ONLY,...)
> > will access the flag that sctp uses internally.
> > (Matching TCP and UDP.)
> 
> My understanding is that these are slightly different.
> 
> v4mapped, if false, will allow the socket to deal with both address
> types, without mapping. If true, it will map v4 into v6.
> v6only, if false, it will do mapping for tcp/udp, but sctp won't use
> it. If true, it will deny using v4, which is complementary to v4mapped
> for sctp.
> 
> Did I miss anything?

Possibly I did, I wasn't looking much beyond the [sg]etsockopt code.
Although our code supports SCTP/IPv6 and I have tested it a bit
I don't think any of our customers use it (yet).
We default to creating IPv6 listening sockets but all the connections
are IPv4.

I think I'm still confused though:

IIRC v6only (mainly) affects listening sockets.
If 0 (the default on Linux) an IPv4 connection will 'attach to' an
IPv6 socket and the application will see v4mapped addresses [1].
If 1 the application needs to create two separate sockets to receive
both IPv4 and IPV6 connections.

I can't see how SCTP would be any different to TCP and UDP.
It can't make any sense to dual-home with a mixture of IPv4/6 addresses.

So does v4mapped just control the format of the addresses on the socket
interface when an IPv4 connection is using an IPv6 socket? 

[1] Actually, thinking further I can't remember whether this is true.
All our code allows for v4mapped addresses and decodes them for printing.

	David

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