I've noticed that if I try to connect with firefox on Debian/Linux to a website with an AAAA-record and a A-record and the IPv6-connectivity of the site isn't working, the timeout for trying the IPv4-address is really long. It seems like it's never going to happen. When I try telnet to port 80, it takes more then 3 minutes before telnet will try the IPv4-address. To me it looks like this is handled by the application, not by the TCP-stack. Is this correct ? I've used about:config as a workaround to not use IPv6 to connect to some sites. The site I have most problems with is: http://undeadly.org/ Is there any place where the timeout in Linux can be set ? in /proc somewhere ? I couldn't find it. Atleast some server applications seem to do the right thing: For example in Postfix I found this: smtp_connect_timeout (default: 30s) The SMTP client time limit for completing a TCP connection, or zero (use the operating system built-in time limit). When no connection can be made within the deadline, the Postfix SMTP client tries the next address on the mail exchanger list. Specify 0 to disable the time limit (i.e. use whatever timeout is implemented by the operating system). Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks). The default time unit is s (seconds).