Precedence for IPv4 vs IPv6

You can use /etc/gai.conf to set up your IPV4/IPV6 precedence as documented here and here.


Say we have two hosts www.he.net and www.ripe.net

$ host www.he.net
www.he.net is an alias for he.net.
he.net has address 216.218.186.2
he.net has IPv6 address 2001:470:0:76::2


$ host www.ripe.net
www.ripe.net has address 193.0.6.139
www.ripe.net has IPv6 address 2001:67c:2e8:22::c100:68b

Case 1: Prefer IPV4
Append the following to /etc/gai.conf

precedence ::ffff:0:0/96  100

then we have:

$ telnet www.ripe.net 80
Trying 193.0.6.139...
^C
$ telnet www.he.net 80
Trying 216.218.186.2...

Case 1: Prefer IPV6 for specific hosts
If we append

precedence 2001:470::/32 100

then we have

$ telnet www.ripe.net 80
Trying 193.0.6.139...
^C
$ telnet www.he.net 80
Trying 2001:470:0:76::2...
^C

So we seem to prefer that network for ipv6 and ipv4 everywhere else.


Case 3: Prefer ipv4 for specific hosts
Wondering if we invert the mask the reverse will be true.