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 > _______________________________________________ > freebsd-net@freebsd.org mailing list > https://lists.freebsd.org/mailman/listinfo/freebsd-net > To unsubscribe, send any mail to "freebsd-net-unsubscribe@freebsd.org"
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