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NetworkManager/docs/internal/hostname.md
Beniamino Galvani 54bee733de docs: document the hostname management workflow 
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Hostname management

NetworkManager can update the system hostname via different mechanisms. The following diagram describes the workflow:

Hostname management workflow

A few notes on the diagram:

  • if there is a static hostname set in /etc/hostname, no action is taken. NetworkManager only manages the transient hostname. See the systemd-hostnamed documentation about the distinction between static and transient hostname.

  • When there is no static hostname set, NetworkManager builds a sorted list of active devices that are eligible for the hostname evaluation. When doing so, the hostname setting of the connection active on each device is taken into account. Properties from-dhcp and from-dns-lookup determine if the two methods must be used for the device; property only-from-default determines whether devices without a default route should be considered, and priority is used to sort the entries. In case of a tie of the priority, a device with the default route comes first.

  • When evaluating a given device, first NM checks if the device received an hostname via DHCP and then if the first IPv4 and IPv6 addresses configured on the interface can be resolved to a name via DNS. The mechanism used to perform the reverse DNS lookup of a specific address is described in the diagram on the right.

  • In the "spawn helper" step, NM starts a separate process that performs the reverse DNS lookup using the NSS services specified (see man nsswitch.conf). Service dns does the lookup via a DNS query, while files returns results from /etc/hosts.

  • In case there is no valid result from DHCP or DNS, from any device, if there a valid transient hostname set outside of NetworkManager, that hostname is honored.