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Enable IPv4 Address Conflict Detection by default

Summary

Enable IPv4 Address Conflict Detection by default in NetworkManager.

Owner


Current status

Detailed Description

A common source of networking issues is the presence of duplicate IPv4 addresses in the same physical network. Such problems are quite common, and at the same time hard to diagnose for users.

To the rescue comes RFC 5227 (“IPv4 Address Conflict Detection”) which provides a mechanism to detect address conflicts. A host implementing Address Conflict Detection (from now on “ACD”) sends ARP probes for each IP address it wants to use; if another host replies, the address is already in use and can’t be configured on the interface.

Note that this mechanism applies to both static and DHCP addresses. It might seem unnecessary for DHCP, as a well-behaving server should give out unique leases; however, there could be hosts on the network not using DHCP. Indeed, RFC 2131 (Dynamic Host Configuration Protocol) specifies that the client should probe the newly received address and should send a DHCPDECLINE to the DHCP server if the address is already in use.

In Fedora 39, ACD is disabled by default; it can be enabled by setting property “ipv4.dad-timeout” to a positive value in a connection profile. The property name contains “DAD” which stands for “duplicate address detection” and is another name of ACD. The property specifies the maximum timeout in milliseconds used to check for the presence of duplicate IP addresses on the network. If a duplicate is found, a warning is logged; in the DHCP case, NetworkManager tries to get a different lease, while in the static case, the address is just skipped.

This change aims at enabling ACD by default in Fedora 40, by setting the default value to 200ms. Note that this change is only about IPV4; IPv6 always performs a duplicate check for each address that is configured, as specified by RFC 4862.


Benefit to Fedora

NetworkManager will not configure IPv4 addresses that are detected as duplicate. This will save users from having to debug weird connectivity issues. Instead, NetworkManager will report an error and will indicate the MAC of the conflicting host.

Scope

  • Proposal owners: change the default value, test that no regression is seen in the upstream test suite.
  • Other developers: N/A (not needed for this Change)
  • Policies and guidelines: N/A (not needed for this Change)
  • Trademark approval: N/A (not needed for this Change)


Upgrade/compatibility impact

The change in default behavior will affect all users that install or upgrade to the new Fedora release.


How To Test

To test the effect of the change on F39, add the following configuration snippet to file /etc/NetworkManager/conf.d/20-ipv4-dad.conf and then restart the NetworkManager service: 

[connection-dad-default]
ipv4.dad-timeout=200

To trigger a conflict, configure the local machine with a static address that is already in use by another host. When bringing up the connection, it will fail and report an address conflict.


User Experience

Enabling ACD will cause an additional delay when bringing up interfaces, because NetworkManager needs first to probe the address. The delay is between 100ms and 200ms, because RFC 5227 requires that the probe interval is randomized. The delay will affect both static and DHCP connections.

In case users want to avoid this delay, ACD can be disabled for the specific connection profile by setting property ipv4.dad-timeout=0, or globally by adding the following configuration snippet to /etc/NetworkManager/conf.d/20-ipv4-dad.conf: 

[connection-dad-default]
ipv4.dad-timeout=0

Apart from this small delay, the big advantage of this change is that users will be able to discover the potential conflict immediately. If the address is static, the activation will fail and report an error. For DHCP, NetworkManager will send a DHCPDECLINE message to the server and it will try to get a different lease. In all cases, the conflicting address will be skipped and the network will not be brought in an inconsistent state.


Dependencies

N/A


Contingency Plan

  • Contingency mechanism: Revert the change, try again the next Fedora release.
  • Contingency deadline: Beta freeze
  • Blocks release? No


Documentation

The “nm-settings” man page will indicate the new default value. No other documentation changes are required.


Release Notes

The change needs to be mentioned in the release notes.

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