I have few servers, which have automatically enabled IPv6 protocols (IPv6 gets automatically enabled on Debian), as well as on most latest Linux distribituions nowdays.
Disabling IPv6 network protocol on Linux if not used has 2 reasons:
1. Security (It's well known security practice to disable anything not used on a server)
Besides that IPv6 has been known for few criticil security vulnerabilities, which has historically affected the Linux kernel.
2. Performance (Sometimes disabling IPv6 could have positive impact on IPv4 especially on heavy traffic network servers).
I've red people claiming disabling IPv6 improves the DNS performance, however since this is not rumors and did not check it personally I cannot positively confirm this.
Disabling IPv6 on all GNU / Linuces can be achieved by changing the kernel sysctl settings net.ipv6.conf.all.disable_ipv6 by default net.ipv6.conf.all.disable_ipv6 equals 1 which means IPv6 is enabled, hence to disable IPv6 I issued:
server:~# sysctl net.ipv6.conf.all.disable_ipv6=0
To set it permanently on system boot I put the setting also in /etc/sysctl.conf :
server:~# echo 'net.ipv6.conf.all.disable = 1 >> /etc/sysctl.conf
The aforedescribed methods should be working on most Linux kernels version > 2.6.27 in that number it should work 100% on recent versions of Fedora, CentOS, Debian and Ubuntu.
To disable IPv6 protocol on Debian Lenny its necessery to blackist the ipv6 module in /etc/modprobe.d/blacklist by issuing:
echo 'blacklist ipv6' >> /etc/modprobe.d/blacklist
On Fedora / CentOS there is a another universal "Redhat" way disable IPv6.
On them disabling IPv6 is done by editting /etc/sysconfig/network and adding:
I would be happy to hear how people achieved disabling the IPv6, since on earlier and (various by distro) Linuxes the way to disable the IPv6 is probably different.