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Date:      Mon, 7 Sep 2020 22:57:21 +0200
From:      Michael Tuexen <Michael.Tuexen@lurchi.franken.de>
To:        Doug Hardie <bc979@lafn.org>
Cc:        freebsd-net@freebsd.org
Subject:   Re: Address Differences between UDP and SCTP
Message-ID:  <7CF5C0CF-A173-4253-9F93-70199578A8F7@lurchi.franken.de>
In-Reply-To: <6A9D0A4B-F35C-4012-A868-5450D60EC13B@mail.sermon-archive.info>
References:  <6A9D0A4B-F35C-4012-A868-5450D60EC13B@mail.sermon-archive.info>

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> On 7. Sep 2020, at 22:48, Doug Hardie <bc979@lafn.org> wrote:
>=20
> I was quite surprised to discover that the sockaddr structure returned =
from recv_fd and recvfrom handle IPv4 addresses differently when using =
an INET6 socket.  I don't know if this was intended, or a side effect.  =
I started using SCTP because of the need for accessing multi-homed =
servers.  Some would be on IPv6 and others on IPv4.  SCTP handles that =
nicely if you use an INET6 socket.  When a transaction is received, if =
it is to an IPv4 address, then the returned sockaddr will have a =
inet_family of IPv4 and the IPv4 structure.  If it was sent to an IPv6 =
address, then the inet6_family is used.  A simple test of the family =
tells you which address format was provided and the address is in IPv4 =
or IPv6 format accordingly.
>=20
> However, A new site needed to be added and it is behind a NAT router.  =
The problem with SCTP is that most (possibly all) NAT routers only work =
with TCP and UDP.  They will not port forward SCTP.  So I have no way to =
get through to the machine.  So I added code to check for that situation =
and use UDP instead.  This will work because I don't thing it is at all =
likely that a machine behind NAT can be multi-homed.
Would using SCTP/UDP/IPv[46] be an option? It is supported by the =
FreeBSD kernel.
See https://tools.ietf.org/html/rfc6951#section-6 for the socket API for =
it.
>=20
> However, the code to obtain the remote IP address failed miserably.  =
It turns out that if you have v6only set to 1, you will never see the =
IPv4 packets.  If you set it to 0, then you get the packets, but the =
sockaddr format with UDP is different than that for SCTP.  If it is an =
IPv6 address, everything is the same.  However, if it is an IPv4 =
address, then the family remains IPv6, and the address is in sin6_addr =
and it is in the format ::ffff:n.n.n.n.  This makes it interesting as I =
need to obtain the IPv4 address as part of the verification process that =
the transaction is authorized.
For UDP and TCP you always get IPv6 addresses on AF_INET6 sockets. If =
you are actually using IPv4, IPv4-mapped IPv6 addresses are used. For =
SCTP you an choose if you want IPv4-mapped IPv6 addresses or IPv4 =
address. It is controlled by
the socket option specified in =
https://tools.ietf.org/html/rfc6458#section-8.1.15
>=20
> Was this difference intended, or is it likely to change in the future?
I think it is intended.

Best regards
Michael
>=20
> -- Doug
>=20
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