How do I disable or enable the IPv6 protocol in Red Hat Enterprise Linux / CentOS

Disabling IPv6 support in Red Hat Enterprise Linux 8

Disable ipv6 built-in kernel module.
1. Edit /etc/default/grub and append ipv6.disable=1 to GRUB_CMDLINE_LINUX like the following sample:
```
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:
```
# 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.
Note: While following above method if you notice any Selinux denied messages in audit.log file such as avc: denied { module_request } then disable the ipv6 from /etc/sysctl.d/ipv6.conf file shown below instead.
Alternatively, this can be done via sysctl settings. Please be aware that this breaks SSH Xforwarding unless sshd_config contains AddressFamily inet.
1. Create a new file named /etc/sysctl.d/ipv6.conf and add the following options:
```
# First, disable for all interfaces
net.ipv6.conf.all.disable_ipv6 = 1
# If using the sysctl method, the protocol must be disabled all specific interfaces as well. 
net.ipv6.conf.<interface>.disable_ipv6 = 1
```
2. The new settings would then need to be reloaded with:
```
# sysctl -p /etc/sysctl.d/ipv6.conf
```
3. Create a backup of the initramfs:
```
# cp /boot/initramfs-$(uname -r).img /boot/initramfs-$(uname -r).bak.$(date +%m-%d-%H%M%S).img
```
4. Then rebuild the Initial RAM Disk Image using:
```
# dracut -f -v
```
  4.1 Verifying file inclusion:
```
# lsinitrd /boot/initramfs-<version>.img  | grep 'etc/sysctl.d/ipv6.conf'
```
5. Comment out any IPv6 addresses found in /etc/hosts, including ::1 localhost address
```
# cp -p /etc/hosts /etc/hosts.disableipv6
# sed -i 's/^[[:space:]]*::/#::/' /etc/hosts
```
Optionally to prevent rpc* messages output after disabling ipv6, edit /etc/netconfig for the lines starting with udp6 and tcp6; change the “v” in the third column to “-“(hyphen/dash).

Re-enabling IPv6 support in Red Hat Enterprise Linux 8

1. Edit /etc/default/grub and delete the entry ipv6.disable=1 from the GRUB_CMDLINE_LINUX, like the following sample:
```
GRUB_CMDLINE_LINUX="rd.lvm.lv=rhel/swap crashkernel=auto rd.lvm.lv=rhel/root"
```
2. Run the grub2-mkconfig command to regenerate the grub.cfg file:
```
# grub2-mkconfig -o /boot/grub2/grub.cfg
```
  Alternatively, on UEFI systems, run the following:
```
# grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg
```
3. Delete the file /etc/sysctl.d/ipv6.conf which contains the entry:
```
# First, disable for all interfaces
net.ipv6.conf.all.disable_ipv6 = 1
# If using the sysctl method, the protocol must be disabled all specific interfaces as well. 
net.ipv6.conf.<interface>.disable_ipv6 = 1
```
4. If the Initial RAM Disk image was created earlier while disabling IPv6 , only then carry out this step.
```
# dracut -f
```
5. Check the content of the file /etc/ssh/sshd_config and make sure the AddressFamily line is commented:
```
#AddressFamily inet
```
6. Make sure the following line exists in /etc/hosts, and is not commented out:
```
::1         localhost localhost.localdomain localhost6 localhost6.localdomain6
```
7. Reboot the system to enable IPv6 support.

Disabling IPv6 support in Red Hat Enterprise Linux 8

Re-enabling IPv6 support in Red Hat Enterprise Linux 8

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