[PATCH] doc: add some examples for IPv6 NAT configuration

[Ian Wienand]

Add some expanded examples for the nat ipv6 introduced with
927acaedec7effbe67a154d8bfa0e67f7d08e6c7.

Unfortunately while for IPv4 it's well-known what addresses ranges are
useful for NAT, with IPv6 unless you enjoy digging through RFC's going
back-and-forth over unique local addresses and the meaning of the word
"site" it's generally much less obvious.  I've tried to add some
details on choosing a range inline with RFC 4193 and then some
pointers for when it maybe doesn't work in the guest as you first
expect despite you doing what the RFC's say!

Signed-off-by: Ian Wienand <iwienand at redhat.com>
---
 docs/formatnetwork.html.in | 47 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/docs/formatnetwork.html.in b/docs/formatnetwork.html.in
index fb740111b1..94a4cab4d1 100644
--- a/docs/formatnetwork.html.in
+++ b/docs/formatnetwork.html.in
@@ -1209,6 +1209,53 @@
   </ip>
 </network></pre>
 
+    <h3><a id="examplesNATv6">IPv6 NAT based network</a></h3>
+
+    <p>
+      Below is a variation for also providing IPv6 NAT.  This can be
+      especially useful when using multiple interfaces where some,
+      such as WiFi cards, can not be bridged (usually on a laptop),
+      making it difficult to provide end-to-end IPv6 routing.
+    </p>
+
+    <pre>
+<network>
+  <name>default6</name>
+  <bridge name="virbr0"/>
+  <forward mode="nat">
+    <nat ipv6='yes'>
+      <port start='1024' end='65535'/>
+    </nat>
+
+  <ip address="192.168.122.1" netmask="255.255.255.0">
+    <dhcp>
+      <range start="192.168.122.2" end="192.168.122.254"/>
+    </dhcp>
+  </ip>
+  <ip family="ipv6" address="fdXX:XXXX:XXXX:NNNN:: prefix="64"/>
+  </ip>
+</network></pre>
+
+    <p>IPv6 NAT addressing has some caveats over the more straight
+    forward IPv4 case.
+    <a href="https://tools.ietf.org/html/rfc4193">RFC 4193</a>
+    defines the address range <tt>fd00::/8</tt> for <tt>/48</tt> IPv6
+    private networks.  It should be concatenated with a random 40-bit
+    string (i.e. 10 random hexadecimal digits replacing the <tt>X</tt>
+    values above, RFC 4193 provides
+    an <a href="https://tools.ietf.org/html/rfc4193#section-3.2.2">algorithm</a>
+    if you do not have a source of sufficient randomness).  This
+    leaves <tt>0</tt> through <tt>ffff</tt> for subnets (<tt>N</tt>
+    above) which you can use at will.</p>
+
+    <p>Many operating systems will not consider these addresses as
+    preferential to IPv4, due to some practial history of these
+    addresses being present but unroutable and causing networking
+    issues.  On many Linux distributions, you may need to
+    override <tt>/etc/gai.conf</tt> with values
+    from <a href="https://www.ietf.org/rfc/rfc3484.txt">RFC 3484</a>
+    to have your IPv6 NAT network correctly preferenced over IPv4.</p>
+
     <h3><a id="examplesRoute">Routed network config</a></h3>
 
     <p>
-- 
2.26.2