systemd-resolved stub resolver vs NetworkManager direct resolv.conf management

Answer

When multiple network interfaces are active, systemd‑resolved maintains a merged list of DNS servers from all interfaces and queries them in a priority order that stays stable as long as the interface metrics do not change. In contrast, NetworkManager configured to manage /etc/resolv.conf directly writes a single static file containing only the DNS servers from the connection it currently considers active (lowest route metric), so the effective DNS server list—and thus the priority—can change whenever the active connection changes.

Confirmed facts

• systemd‑resolved runs a local stub listener on 127.0.0.53, caches responses, and keeps per‑link DNS settings obtained from DHCP or static configuration, merging servers from all active interfaces.
• When NetworkManager is set to manage /etc/resolv.conf directly (dns=default), it writes a single static file with the DNS servers from the connection judged active by lowest route metric, overwriting any previous content and performing no caching or merging.

Likely explanation

systemd‑resolved assigns a priority to each DNS server based on the interface's DNS priority (or a global fallback order) and queries servers in that order, falling back to lower‑priority servers only after timeouts. This provides consistent priority across multiple interfaces. NetworkManager's direct‑file mode cannot express multiple DNS sources; it selects the 'best' connection per NM's active‑device rules and writes only its DNS servers, causing the effective DNS priority to change whenever the active connection changes.

Verification steps

1. Check NetworkManager's DNS plugin setting: grep ^dns /etc/NetworkManager/NetworkManager.conf (look for dns=default, dns=systemd-resolved or dns=none).
2. Confirm that systemd-resolved is running: systemctl status systemd-resolved.
3. Inspect /etc/resolv.conf: ls -l /etc/resolv.conf to see if it is a symlink to /run/systemd/resolve/stub-resolv.conf (stub resolver) or a plain file (direct NM management).
4. Compare the DNS server lists per interface: nmcli device show | grep IP4.DNS for each active NIC.

Missing diagnostic detail

To determine whether the stub resolver or direct file management is in effect, please provide the current NetworkManager dns= setting (e.g., dns=default, dns=systemd-resolved, dns=none). This influences which behavior applies and thus the recommendation.