Red Hat Enterprise Linux 4, 5 and 6 enable Internet Protocol Version 6 (IPv6) by default. However, in certain situations, some users may find it desirable to disable IPv6 support or to re-enable it after it has been disabled.

For Red Hat Enterprise Linux 6, the tech brief Preparing for IPv6 Networking in Red Hat Enterprise Linux provides a broad introduction to IPv6 networking.

Disabling IPv6 support in Red Hat Enterprise Linux 6

1. Create a file /etc/modprobe.d/ipv6.conf with the following contents:

   options ipv6 disable=1
   
   

2. For completeness, it is a good idea to configure the ip6tables service not to start at boot by issuing the following command:

   # chkconfig ip6tables off
   
   

3. Edit /etc/sysconfig/network and add the line NETWORKING_IPV6=no

4. Disable ipv6 support in the kernel through /etc/sysctl.conf :

   # ipv6 support in the kernel, set to 0 by default
   
   net.ipv6.conf.all.disable_ipv6 = 1
   
   net.ipv6.conf.default.disable_ipv6 = 1
   
   

5. Optionally to prevent rpc.nfsd setting up IPv6 sockets while NFS server is running, edit /etc/netconfig for the lines starting with udp6 and tcp6; change the "v" in the third column to "-"(hyphen/dash)

6. Reboot the system to disable IPv6 support.

Note: There is a special case where this might not work, please see The "ipv6 disable=1" option does not seem to work on Red Hat Enterprise Linux 6 system.
Disabling ipv6 in the sysctl.conf will ensure ipv6 isn't used even if the ipv6 module is loaded and can work as a short term solution (until a full reboot)

Re-enabling IPv6 support in Red Hat Enterprise Linux 6

1. Review the files under /etc/modprobe.d/ and remove (or comment out) any of the following lines:

   options ipv6 disable=1
   
   install ipv6 /bin/true
   
   blacklist ipv6
   
   

2. Configure the ip6tables service to start at boot by issuing the following command:

   # chkconfig ip6tables on
   
   

3. Edit /etc/sysconfig/network and add the line NETWORKING_IPV6=yes

4. Make sure the following options to your /etc/sysctl.conf are

   # ipv6 support in the kernel, set to 0 by default
   
   net.ipv6.conf.all.disable_ipv6 = 0
   
   net.ipv6.conf.default.disable_ipv6 = 0
   
   

5. Edit /etc/netconfig for the lines starting with udp6 and tcp6; change the "-"(hyphen/dash) in the third column to "v"

6. Reboot the system to activate IPv6 support.

Disabling IPv6 support in Red Hat Enterprise Linux 5

1. Remove the following line (if present) from the /etc/modprobe.conf file:

   alias net-pf-10 ipv6
   
   

2. Add the following line to the /etc/modprobe.conf file:

   alias net-pf-10 off
   
   

3. In versions of Red Hat Enterprise Linux before 5.4, add the following line to the /etc/modprobe.conf file:

   alias ipv6 off
   
   

4. In Red Hat Enterprise Linux 5.4 and later, add the following line to the /etc/modprobe.conf file:

   options ipv6 disable=1
   
   

5. To prevent errors during the network initscript start routine, change the NETWORKING_IPV6 parameter in the /etc/sysconfig/network file to the following:

   NETWORKING_IPV6=no
   
   

6. For completeness, it is a good idea to configure the ip6tables service not to start at boot by issuing the following command:

   # chkconfig ip6tables off
   
   

7. Reboot the system to disable IPv6 support.

Re-enabling IPv6 support in Red Hat Enterprise Linux 5

1. Remove the following lines (if present) from the /etc/modprobe.conf file:

   alias net-pf-10 off
   
   alias ipv6 off
   
   options ipv6 disable=1
   
   

2. Add the following line to the /etc/modprobe.conf file (if not present already):

   alias net-pf-10 ipv6
   
   

3. Change the NETWORKING_IPV6 parameter in the /etc/sysconfig/network file to the following:

   NETWORKING_IPV6=yes
   
   

4. For completeness, it is a good idea to configure the ip6tables service to start at boot by issuing the following command:

   # chkconfig ip6tables on
   
   

5. Reboot the system to re-enable IPv6 support.

Disabling IPv6 support in Red Hat Enterprise Linux 4

1. Remove the following line (if present) from the /etc/modprobe.conf file:

   alias net-pf-10 ipv6
   
   

2. Add the following line to the /etc/modprobe.conf file:

   alias net-pf-10 off
   
   

3. Reboot the system to disable IPv6 support.

Re-enabling IPv6 support in Red Hat Enterprise Linux 4

1. Remove the following line from the /etc/modprobe.conf file:

   alias net-pf-10 off
   
   

2. Add the following line to the /etc/modprobe.conf file:

   alias net-pf-10 ipv6
   
   

3. Reboot the system to re-enable IPv6 support.