cf-socket: turn off IPV6_V6ONLY on Windows if it is supported

[lifenjoiner]

It makes support for IPv4-mapped IPv6 addresses.

IPV6_V6ONLY refs:
https://en.wikipedia.org/wiki/IPv6#IPv4-mapped_IPv6_addresses
https://github.com/golang/go/blob/master/src/net/ipsock_posix.go
https://en.wikipedia.org/wiki/Unix-like
off:
https://www.kernel.org/doc/html/latest/networking/ip-sysctl.html#proc-sys-net-ipv6-variables
https://man.netbsd.org/inet6.4
https://man.freebsd.org/cgi/man.cgi?query=inet6
https://github.com/apple-oss-distributions/xnu/blob/main/bsd/man/man4/inet6.4
on:
https://learn.microsoft.com/en-us/windows/win32/winsock/ipproto-ipv6-socket-options
acts like off, but returns 1 and dummy setting:
https://man.dragonflybsd.org/?command=inet6
https://man.dragonflybsd.org/?command=ip6
unsupported and read-only returns 1:
https://man.openbsd.org/inet6.4

default value refs:
https://datatracker.ietf.org/doc/html/rfc3493#section-5.3
https://www.kernel.org/doc/html/latest/networking/ip-sysctl.html#proc-sys-net-ipv6-variables

[bagder]

Quoting section 5.3 of RFC 3493:

Note - This option has no effect on the use of IPv4 Mapped addresses

Which sounds like the opposite to the reason for this PR?

[lifenjoiner]

The whole note in section 5.3 of RFC 3493:

Note - This option has no effect on the use of IPv4 Mapped addresses
which enter a node as a valid IPv6 addresses for IPv6 communications
as defined by Stateless IP/ICMP Translation Algorithm (SIIT) [5].

That is confusing. I guess it means in separate translator "boxes" by the SIIT (rfc2765) abstract section:

The algorithm
translates between IPv4 and IPv6 packet headers (including ICMP
headers) in separate translator "boxes" in the network without
requiring any per-connection state in those "boxes".

Some facts on IPv4-mapped IPv6 addresses support:

• Turning off IPV6_V6ONLY does make it work on IPv4-mapped IPv6 addresses with dual-stack socket.
• Most OSes works on it by default. while Windows don't.
• Browsers like Chrome can visit them, while IE and Firefox don't, on Windows.
• Windows, ping works, while nslookup doesn't. dig does.

Edit: I had a golang compiled proxy for Firefox. Golang runtime has the capability by default.

[lifenjoiner]

The problem is Windows doesn't follow the RFC to set IPV6_V6ONLY off by default, but requires users to do it when they want to create dual-stack socket for IPv4-mapped IPv6 addresses support.
Search IPV6_V6ONLY in the options table.

[jay]

Does it really need a separate function?

#if defined(IPV6_V6ONLY) && defined(WIN32)
  /* Allow IPv4 with AF_INET6 sockets on Windows. Other OSes allow this by default. */
  {
    int on = 0;
    setsockopt(ctx->sock, IPPROTO_IPV6, IPV6_V6ONLY, (void *)&on, sizeof(on));
  }
#endif

[lifenjoiner]

Does it really need a separate function?

Not really.

A function is for other place to reuse it. And, if not, the line is longer than 79 chars with (void) return. Compilers would optimize it for binaries :p

[lifenjoiner]

Does it really need a separate function?

Not really.

A function is for other place to reuse it. And, if not, the line is longer than 79 chars with (void) return. Compilers would optimize it for binaries :p

The real reason is, I saw "we write C89 code", just recalled.

curl/docs/CODE_STYLE.md

Line 50 in 6b1e4dc

Since we write C89 code, **//** comments are not allowed. They were not

[bagder]

Thanks!