Disabling IPv6 support in Red Hat Enterprise Linux

source from: https://access.redhat.com/solutions/8709

Disabling IPv6 support in Red Hat Enterprise Linux 7

• Disable ipv6 built-in kernel module.

1. Edit /etc/default/grub and append ipv6.disable=1 to GRUB_CMDLINE_LINUX like the following sample:
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   GRUB_CMDLINE_LINUX="rd.lvm.lv=rhel/swap crashkernel=auto rd.lvm.lv=rhel/root ipv6.disable=1"
   

2. Run the grub2-mkconfig command to regenerate the grub.cfg file:
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   # grub2-mkconfig -o /boot/grub2/grub.cfg
   
   Alternatively, on UEFI systems, run the following:        
   # grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg
   

3. Reboot the system to disable IPv6 support.

• Alternatively, this can be done via sysctl settings. Please be aware that this breaks SSH Xforwarding unless sshd_config containsAddressFamily inet.

1. Create a new file named /etc/sysctl.d/ipv6.conf and add the following options:
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   # To disable for all interfaces
   net.ipv6.conf.all.disable_ipv6 = 1
   # the protocol can be disabled for specific interfaces as well.
   net.ipv6.conf..disable_ipv6 = 1
   

2. The new settings would then need to be reloaded with:
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   # sysctl -p /etc/sysctl.d/ipv6.conf
   

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Disabling IPv6 support in Red Hat Enterprise Linux 6

1. Create a file /etc/modprobe.d/ipv6.conf with the following contents:
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   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:
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   # chkconfig ip6tables off
   

3. Alternatively, the IPv6 support can also be disabled in the kernel through /etc/sysctl.conf by setting following parameter:
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   # IPv6 support in the kernel, set to 0 by default
   net.ipv6.conf.all.disable_ipv6 = 1
   net.ipv6.conf.default.disable_ipv6 = 1
   

Please be aware that this breaks SSH Xforwarding unless AddressFamily inet is set in sshd_config.

1. Comment out any IPv6 addresses found in /etc/hosts, including ::1 localhost address
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   # cp -p /etc/hosts /etc/hosts.disableipv6
   # sed -i 's/^[[:space:]]*::/#::/' /etc/hosts
   

2. 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)
3. Reboot the system to disable IPv6 support.

Note: The solution above only disables(unhooks all of the calls) the ipv6 module, but doesn't prevent it from loading. Unloading the module isn't recommended due to the dependency factors it has with other modules(for example: bonding) and services. To prevent the module from loading, modify/etc/modprobe.d/ipv6.conf with the following only:

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install ipv6 /bin/true

• 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)
• 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.

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:
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   options ipv6 disable=1
   install ipv6 /bin/true
   blacklist ipv6
   alias ipv6 off
   

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

3. Make sure the following options to your /etc/sysctl.conf are
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   # ipv6 support in the kernel, set to 0 by default
   net.ipv6.conf.all.disable_ipv6 = 0
   net.ipv6.conf.default.disable_ipv6 = 0
   

4. Make sure the following line exists in /etc/hosts, and is not commented out.
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   ::1         localhost localhost.localdomain localhost6 localhost6.localdomain6
   

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.

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Disabling IPv6 support in Red Hat Enterprise Linux 5

1. Remove the following line (if present) from the /etc/modprobe.conf file:
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   alias net-pf-10 ipv6
   

2. Add the following line to the /etc/modprobe.conf file:
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   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:
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   alias ipv6 off
   

4. In Red Hat Enterprise Linux 5.4 and later, add the following line to the /etc/modprobe.conf file:
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   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:
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   NETWORKING_IPV6=no
   

6. Comment out any IPv6 addresses found in /etc/hosts, including ::1 localhost address
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   # cp -p /etc/hosts /etc/hosts.disableipv6
   # sed -i 's/^[[:space:]]*::/#::/' /etc/hosts
   

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

8. 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:
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   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):
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   alias net-pf-10 ipv6
   

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

4. Make sure the following line exists in /etc/hosts, and is not commented out.
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   ::1         localhost localhost.localdomain localhost6 localhost6.localdomain6
   

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

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

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Disabling IPv6 support in Red Hat Enterprise Linux 4

1. Remove the following line (if present) from the /etc/modprobe.conf file:
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   alias net-pf-10 ipv6
   

2. Add the following line to the /etc/modprobe.conf file:
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   alias net-pf-10 off
   

3. Comment out any IPv6 addresses found in /etc/hosts, including ::1 localhost address
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   # cp -p /etc/hosts /etc/hosts.disableipv6
   # sed -i 's/^[[:space:]]*::/#::/' /etc/hosts
   

4. 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:
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   alias net-pf-10 off
   

2. Add the following line to the /etc/modprobe.conf file:
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   alias net-pf-10 ipv6
   

3. Make sure the following line exists in /etc/hosts, and is not commented out.
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   ::1         localhost localhost.localdomain localhost6 localhost6.localdomain6
   

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

Additional notes

Disabling IPv6 by blacklisting the module invalidates bonding in the system.