Since I experienced slow DNS lookup

Since I experienced slow DNS lookup with Debian testing/unstable, I felt I need to disable IPv6 — this has largely been blamed for that, and it's not really needed by most of the people.

When MDLog:/sysadmin posted Disable IPV6 module on default kernels, this remembered me there is more than one way to do it — and it was painful to some people.

Here's the summary of what I can think of, with alternate methods in case the first one doesn't work with you:

• Debian kernel 2.4:

In /etc/modutils, change the line
alias net-pf-10 ipv6
to
alias net-pf-10 off
alias ipv6 off
Reboot.

• Debian kernel 2.6/Ubuntu ("official" method)/Fedora Core (method 1):

In /etc/modprobe.d/aliases, change the line
alias net-pf-10 ipv6
to

alias net-pf-10 off
alias ipv6 off
Reboot.

• Debian kernel 2.6/Ubuntu (alternate method):

In /etc/modprobe.d/blacklist, add a line
blacklist ipv6
Reboot.

• RHEL/CentOS:

In /etc/modprobe.conf, make sure you have
alias net-pf-10 off
alias ipv6 off
Reboot.

• RHEL/CentOS/Fedora Core (method 2)/Mandriva (method 1):

In /etc/sysconfig/network, add a line
NETWORKING_IPV6="no"
Reboot.

• SuSE 9.0:

In /etc/modules.conf, change the line
alias net-pf-10 ipv6
to
alias net-pf-10 off
alias ipv6 off
Reboot.

• SuSE 9.1/9.2/9.3/10.0 ("official" method):

In /etc/modprobe.conf, change the line
alias net-pf-10 ipv6
to
alias net-pf-10 ipv6
install ipv6 /bin/true
Reboot.

• SuSE 9.1/9.2/9.3/10.0 (alternate method)/Mandriva (method 2)/Slackware:

In /etc/modprobe.conf, make sure you change
alias net-pf-10 ipv6
to
alias net-pf-10 off
alias ipv6 off
Reboot.

• Gentoo:

In /etc/modules.d/aliases, make sure you change
alias net-pf-10 ipv6
to
alias net-pf-10 off
alias ipv6 off
Reboot.

NOTES:

In distros using /etc/modprobe.conf, beware that you might get overriden by /etc/modprobe.conf.local or by /etc/modprobe.d/*.
If you disable IPv6, I suppose you don't need tunnels either. You could also add there:
install sit0 /bin/true
A last "anti-IPv6" issue: don't forget the old trick of checking if your Firefox has IPv6 disabled:
- CTRL+L, about:config (Enter)
- Filter by ipv6.
- Set network.dns.disableIPv6 to true.

reported from : http://beranger.org