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-rw-r--r--lib/kernel/doc/src/inet.xml62
1 files changed, 62 insertions, 0 deletions
diff --git a/lib/kernel/doc/src/inet.xml b/lib/kernel/doc/src/inet.xml
index 32b4a429dd..d0ed26a18d 100644
--- a/lib/kernel/doc/src/inet.xml
+++ b/lib/kernel/doc/src/inet.xml
@@ -520,6 +520,68 @@ fe80::204:acff:fe17:bf38
</p>
</item>
+ <tag><c>{ipv6_v6only, Boolean}</c></tag>
+ <item>
+ <p>
+ Restricts the socket to only use IPv6, prohibiting any
+ IPv4 connections. This only applicable for
+ IPv6 sockets (option <c>inet6</c>).
+ </p>
+ <p>
+ On most platforms this option has to be set on the socket
+ before associating it to an address. Therefore it is only
+ reasonable to give it when creating the socket and not
+ to use it when calling the function
+ (<seealso marker="#setopts/2">setopts/2</seealso>)
+ containing this description.
+ </p>
+ <p>
+ The behaviour of a socket with this socket option set to
+ <c>true</c> is becoming the only portable one. The original
+ idea when IPv6 was new of using IPv6 for all traffic
+ is now not recommended by FreeBSD (you can use
+ <c>{ipv6_v6only,false}</c> to override the recommended
+ system default value),
+ forbidden by OpenBSD (the supported GENERIC kernel)
+ and impossible on Windows (that has separate
+ IPv4 and IPv6 protocol stacks). Most Linux distros
+ still have a system default value of <c>false</c>.
+ This policy shift among operating systems towards
+ separating IPv6 from IPv4 traffic has evolved since
+ it gradually proved hard and complicated to get
+ a dual stack implementation correct and secure.
+ </p>
+ <p>
+ On some platforms the only allowed value for this option
+ is <c>true</c>, e.g. OpenBSD and Windows. Trying to set
+ this option to <c>false</c> when creating the socket
+ will in this case fail.
+ </p>
+ <p>
+ Setting this option on platforms where it does not exist
+ is ignored and getting this option with
+ <seealso marker="#getopts/2">getopts/2</seealso>
+ returns no value. On Windows the option acually does
+ not exist but it is emulated as being a read-only option
+ with the value <c>true</c>.
+ </p>
+ <p>
+ So it boils down to that setting this option to <c>true</c>
+ when creating a socket will never fail except possibly
+ (at the time of this writing) on a platform where you
+ have customized the kernel to only allow <c>false</c>,
+ which might be doable (but weird) on e.g. OpenBSD.
+ </p>
+ <p>
+ If you read back the option value using
+ <seealso marker="#getopts/2">getopts/2</seealso>
+ and get no value the option does not exist in the host OS
+ and all bets are off regarding the behaviour of both
+ an IPv6 and an IPv4 socket listening on the same port
+ as well as for an IPv6 socket getting IPv4 traffic.
+ </p>
+ </item>
+
<tag><c>{keepalive, Boolean}</c>(TCP/IP sockets)</tag>
<item>
<p>Enables/disables periodic transmission on a connected