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NetworkManager dispatcher scripts let you run local tasks in response to connection and device events. To use them safely, install a root-owned executable in the dispatcher directory, branch on the event and validate its inputs, keep blocking work short, and account for delayed or overlapping events. The precise behavior can vary with the NetworkManager build and distribution.
Install scripts with the required protections
NetworkManager runs scripts from /etc/NetworkManager/dispatcher.d and /usr/lib/NetworkManager/dispatcher.d, including subdirectories. An entry in /etc takes precedence over an identically named entry in /usr/lib; scripts run in alphabetical order. The NetworkManager reference manual says each script should be a regular executable file owned by root, not writable by group or others, and not setuid.
- Create a regular script in a documented dispatcher directory, using a clear filename.
- Set root ownership and executable permissions. For example, an administrator could use
chown root:root /etc/NetworkManager/dispatcher.d/20-local-taskandchmod 755 /etc/NetworkManager/dispatcher.d/20-local-task; ensure the resulting file is not group- or other-writable and has no setuid bit. - Review the parent directory and any symlink targets as well as the script itself. Prevent untrusted users from replacing code that will run with administrator authority.
- Keep secrets out of logs, and treat event values as data rather than interpolating them unsafely into shell commands.
Choose an event and validate its inputs
Each script receives two arguments: the interface name and the action. The interface argument is not always a conventional interface name. Depending on the event, it may correspond to 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.
Documented actions include pre-up, up, pre-down, down, dhcp4-change, dhcp6-change, connectivity-change, reapply, dns-change, hostname, and device-add. The reference manual describes a special generic-device handler path for device-add in which only one script is executed. Handle each action deliberately rather than assuming every invocation represents a connection with a populated interface name.
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Use environment variables defensively
Depending on the event, 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-related events can provide VPN-prefixed address variables. Connection user data is exported in encoded CONNECTION_USER_... variables; for example, the manual maps test.foo-Bar2 to CONNECTION_USER_TEST__FOO_055_BAR2. Availability depends on context, so test whether variables are unset or empty before using them.
Example: branch on the action
A minimal shell handler can reject unexpected or incomplete input before doing work:
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#!/bin/sh
iface=$1
action=$2
case "$action" in
up)
[ -n "$iface" ] || exit 0
# Validate current state before consequential work.
;;
down)
[ -n "$iface" ] || exit 0
;;
connectivity-change|dns-change)
# The interface argument may be empty for these actions.
;;
*)
exit 0
;;
esacThis illustrates input handling only; add the task-specific state checks and actions you actually need.
Choose timing that will not block network changes
| Action or mode | Effect | When it fits |
|---|---|---|
up or down |
Runs in response to activation or disconnection without making that transition wait for the task. | Use for most follow-up work that need not happen before the transition. |
pre-up |
Runs before NetworkManager reports the interface fully activated; NetworkManager waits for the script. | Only when work must complete before activation and can finish promptly. |
pre-down |
Runs before disconnection; NetworkManager waits. It is not emitted for forced disconnections such as carrier loss or fading wireless signal. | Only for work that must happen before a cleanly handled disconnection. |
Symlink into no-wait.d |
Runs immediately without waiting for preceding scripts, and in parallel with them. | Only when the handler is safe under concurrent execution and altered ordering. |
Scripts run one at a time asynchronously from the main NetworkManager process and are killed if they run too long. The manual recommends spawning a child and allowing the dispatcher parent to return when work may take an arbitrary amount of time. It does not establish one universal timeout duration, so do not assume a fixed limit across distributions or builds.
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Design for repeated, concurrent, and stale events
A queued script can run after another event has made its triggering event obsolete—for example, an up handler may run after the interface has gone down again. The event is therefore not a guarantee that the reported state remains current when your code acts.
- Make operations idempotent where possible, so a repeated invocation does not cause harmful duplicate effects.
- Before consequential changes, query the current device or connection state and act on that state rather than trusting the old event alone.
- If using
no-wait.d, protect shared files and other shared resources against overlapping runs, and do not depend on alphabetical completion order. - Keep pre-event work short because it delays activation or disconnection.
Troubleshoot a script that did not run or delayed networking
- Check installation and permissions. Confirm the exact dispatcher path, regular-file status, executable bit, root ownership, group/other write permissions, and setuid state. Check parent-directory and symlink-target protections too.
- Confirm the event inputs. Log the action and arguments safely, without secrets, then verify that the action and interface value match the case your handler expects. Account for an empty or special device argument.
- Look for blocking work. A
pre-uporpre-downscript delays the corresponding transition. A handler that runs too long can be terminated; the timeout is not a universal documented number. - Check for stale events or overlap. Compare the current interface state with the event that triggered the script. If the script is linked into
no-wait.d, account for parallel execution. - Check your distribution’s installed version and manual. Ubuntu’s Noble manual identifies its packaged version as NetworkManager
1.46.0-1ubuntu2.8and describes the general sequential, timeout, and no-wait behavior. Red Hat’s RHEL 8 networking guide also directs readers to the NetworkManager manual for dispatcher actions and environment variables.
The project reference manual documents the dispatcher interface; consult the manual for the NetworkManager package installed on your system when distribution-specific details matter.
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