Stable IPv6 address on Fedora

Question

Unlike other systems, Fedora 25 workstation doesn't use stable IPv6 addresses, by default.

For example, with CentOS 7 or Fedora 23, a stable IPv6 is automatically configured (in an IPv6 enabled network where a IPv6 router is present) - i.e. one that is derived from the MAC-address.

That IPv6 address then can be used in an DNS AAAA-record.

In contrast to that, the IPv6 address of a Fedora 25 workstation system doesn't have any relation to its MAC address and doesn't seem to be stable.

How to configure deterministic and stable IPv6 addresses on Fedora 25?

Answer 1

On Fedora 25 Workstation, NetworkManager (NM) configures all network interfaces, by default. That means also the wired ones. And the NetworkManager doesn't create EUI-64 derived IPv6 addresses. Instead it generates so called 'stable-privacy' ones. Apparently to not disclose the MAC address to each IPv6 destination.

This can be changed for a given interface $i via changing the IPV6_ADDR_GEN_MODE key in the /etc/sysconfig/network-scripts/ifcfg-$i configuration file. For example via:

sed -i 's/^IPV6_ADDR_GEN_MODE=stable-privacy/IPV6_ADDR_GEN_MODE=eui64/' \
  /etc/sysconfig/network-scripts/ifcfg-$i

The change is effective after NetworkManager rereads its configuration and after a reconnect:

nmcli con reload
nmcli con down $i
nmcli con up $i

Notes

• this option isn't exposed via the NM settings GUI
• the interface configuration files under /etc/sysconfig/network-scripts read by NM are Fedora/Redhat specific, but the configuration key is not - i.e. on other distributions NM just reads the interface configurations from different locations/configuration files

---

Fedora also comes with systemd-networkd which doesn't disable EUI64 generation, by default. Thus, a simpler way to get stable IPv6 addresses under Fedora is just to remove NetworkManager and configure/enable systemd-networkd, instead.

Or one can set the interface in question to unmanaged in NetworkManager and then configure it in systemd-networkd.

In any case, the networkd config is pretty minimal then, e.g.:

cat /etc/systemd/network/20-wired.network
[Match]
# manage all matching interfaces
#Name=en*
# just manage one:
Name=eno1
    
[Network]
DHCP=ipv4

Answer 2

The addresses that you think are not stable, actually are.

These are RFC 7217 stable privacy addresses, which are designed to make devices less trackable as they roam across networks, while maintaining the same address on any given network.

Stable privacy addresses are assigned instead of EUI-64 derived addresses when SLAAC is in use on a link. They do not replace RFC 4941 privacy addresses.

This all means that in order to have stable addresses, you need to do nothing. You can also use stateful DHCPv6 and get rid of SLAAC entirely, and in this way you can centrally control the assignment of addresses. But then you lose the benefits of stable privacy addresses.

Answer 3

You can configure this per-connection. Tested on Fedora 35.

nm-settings manual: nm-settings-nmcli - ipv6 setting

1. Edit /etc/NetworkManager/system-connections/<connection_name>.nmconnection and set addr-gen-mode to eui64. The default is stable-privacy.

   [ipv6]
   addr-gen-mode=eui64
   

2. Reboot.