NetworkManager dispatcher scripts let you run local tasks in response to network events. To use them safely, install a root-owned executable in the dispatcher directory, branch on the event and validated inputs, and keep blocking work short. Network events can be delayed or superseded, so check the current connection state before making consequential changes.
Install scripts with the required protections
NetworkManager runs dispatcher scripts from /etc/NetworkManager/dispatcher.d and /usr/lib/NetworkManager/dispatcher.d, including subdirectories. When files in those locations have the same name, the /etc entry takes precedence. Scripts run alphabetically.
The NetworkManager dispatcher reference says each script should be a regular executable file owned by root, must not be writable by group or others, and must not have the setuid bit set. Review the parent directory and any symlink targets as well: they must not let an untrusted user replace code that will run with administrator authority.
- Place the script in a documented dispatcher directory and make it executable.
- Set root ownership, remove group and other write permissions, and ensure setuid is not set.
- Use a distinctive filename; remember that scripts run alphabetically and that an identically named file in
/etcoverrides one in/usr/lib. - Keep credentials out of logs, and handle event values as data rather than interpolating them unsafely into shell commands.
Handle the event and its inputs explicitly
NetworkManager passes two arguments to a dispatcher script: the interface name and the action. The interface argument is not always a conventional device name. Depending on the event, it may be VPN_IP_IFACE, DEVICE_IP_IFACE, or DEVICE_IFACE. For hostname, the device argument is none; for connectivity-change and dns-change, it is empty. Do not assume the first argument is always non-empty.
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Documented actions include pre-up, up, pre-down, down, dhcp4-change, dhcp6-change, connectivity-change, reapply, dns-change, hostname, and device-add. The device-add path has a special generic-device handler behavior in which only one script is executed. Branch on the action and validate the values required by that branch rather than assuming every event has identical context.
The environment can include NM_DISPATCHER_ACTION, CONNECTION_UUID, CONNECTION_ID, CONNECTION_DBUS_PATH, CONNECTION_FILENAME, CONNECTION_EXTERNAL, DEVICE_IFACE, and DEVICE_IP_IFACE. VPN events can expose VPN-prefixed address variables. Connection user data is exported as encoded CONNECTION_USER_... variables; for example, the manual maps test.foo-Bar2 to CONNECTION_USER_TEST__FOO_055_BAR2. Values depend on the event, so test whether a variable is unset or empty before using it.
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Choose an event that fits the task
Prefer ordinary up and down events unless the task genuinely must happen before activation or disconnection. A pre-event handler delays the network transition while it runs, making it a poor fit for slow or unpredictable work.
Use pre-up or pre-down only for short, necessary work
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Keep ordinary handlers short; offload work that may take a long time
Dispatcher scripts run one at a time, asynchronously from the main NetworkManager process, and are killed if they run too long. The manual recommends starting a child process and letting the dispatcher parent return when work may take an arbitrary amount of time. The documentation does not establish a universal timeout duration; check the target system’s package documentation or configuration rather than relying on a fixed number.
Use no-wait.d only when overlap is safe
Symlinks into /etc/NetworkManager/dispatcher.d/no-wait.d/ run immediately without waiting for preceding scripts and can run in parallel. This can reduce waiting, but changes ordering and allows concurrent invocations. Use it only if the task remains correct under overlap and does not rely on another dispatcher script having already completed.
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Make handlers resilient to repeated and stale events
A queued event may still be delivered after a later event has made its original state obsolete. For instance, an up event may be processed after the interface has gone down again. Treat the action as a notification, not a guaranteed snapshot of current network state.
- Make repeated runs harmless where possible, such as by checking whether the desired change is already in place.
- Before consequential work, verify the current device or connection state instead of relying only on the event arguments.
- If using
no-wait.d, protect shared files and other resources against concurrent runs. - Design for missing or empty event-specific variables and for changes that occur between notification and execution.
Troubleshoot a script that does not run or behaves unexpectedly
- Check installation and permissions. Confirm the exact dispatcher path, regular-file status, root ownership, executable bit, absence of group/other write permissions, and absence of setuid.
- Record the event safely. Log the action and arguments while excluding secrets, then compare them with the event your handler expects. Do not assume an interface argument is present for every action.
- Look for a blocking handler. A
pre-uporpre-downscript can hold activation or disconnection; a long-running handler may be terminated. - Check for stale events or overlap. Compare the current interface state with the triggering event and determine whether a
no-wait.dsymlink allowed another invocation to run at the same time. - Confirm the local version and behavior. Consult the manual for the installed distribution package. The Ubuntu Noble NetworkManager manual identifies its packaged version as 1.46.0-1ubuntu2.8 and describes the general sequential execution, timeout, and no-wait behavior. The RHEL 8 networking guide also directs readers to the NetworkManager manual for dispatcher actions and environment variables.
Distribution packaging and NetworkManager versions can differ. The project reference documents the interface; the installed distribution’s manual is the better check for local version context.
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