IPv4 over IPv6 preference hints

Datum: 01.02.2011 22:26

Early adopters using some tunnel (6to4, sixxs, ...) usually prefer to use IPv4 (available native) over IPv6 (tunneled through somewhere not too near).

Furthermore, some applications don't even try IPv4 once IPv6 is or seems available locally.

Here are some hints on how to invite or force applications resp. operatings systems to prefer IPv4:

• Linux: Edit /etc/gai.conf, add or uncomment the line containing
  precedence ::ffff:0:0/96  100.
   
• Java: Most JVMs have the very bad habit to not even try IPv4 for i.e. URLConnections if IPv6 seems or is available.
  These JVM parameters help:
  -Djava.net.preferIPv4Stack=true -Djava.net.preferIPv6Addresses=false 
   
• Java in Ubuntu/Debian: Sometimes these JVM parameters are not enough. As documented in Ubuntu bug 486215 and Eclipse bug 298515 it may also be required to set net.ipv6.bindv6only to 0 using /sbin/sysctl net.ipv6.bindv6only=0 respectively editing /etc/sysctl.d/bindv6only.conf