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Accepted
dargaud
Asked: 2019-12-13 12:12:38 +0800 CST

Taking an action after connecting a USB-to-Ethernet adapter

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When I connect a USB-to-Ethernet adapter (that has been previously used), I receive a MAC address, and then if there's something connected on the other end I receive also an IP address (192.168.1.1 in my case, configured in static in NetworkManager). On the other end there are various instruments that wait for the DHCP server running on that PC to give them an IP.

The problem is that the DHCP server refuses to start if the subnet is not found, so it cannot be launched before the connection is up. How can I do systemctl restart isc-dhcp-server.service after I receive the IP 192.168.1.1 from NetworkManager ?

I thought of using a udev rule with ATTR{address}=="f0:1f:af:3d:6c:e7" RUN+="systemctl restart isc-dhcp-server.service" but it runs before the IP is assigned. Is there a udev option I can use or do I have to use another method via systemd or NetworkManager or other ?

services systemd dhcpd udev

3 Answers

  1. UDEV triggering a systemd service is the way to do this.

    you can match your usb adapter by usb id , its too late when you can see a mac address.

    this post has the instructions

    https://unix.stackexchange.com/questions/229987/udev-rule-to-match-any-usb-storage-device

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  2. Your system is configured with systemd .

    So one solution is to add a dependency between isc-dhcp-server and the usb device .

    1. identify the name of device under systemd system
    systemctl | grep -i sys-subsystem-net-devices.*6c.*e7

    ps 6c & e7 are the last 2 bytes of your mac from you usb device

    you must have something like

     sys-subsystem-net-devices-enxf01faf3d6ce7.device
    1. amend the unit file for isc-dhcp-server
    export EDITOR=emacs # <= change to what you want 
systemctl edit isc-dhcp-server.service

    and add these lines

    [Unit]
After=sys-subsystem-net-devices-enxf01faf3d6ce7.device
Wants=sys-subsystem-net-devices-enxf01faf3d6ce7.device

    1. reboot

    2. check status and configuration of isc-dhcp-server by running these commands

    systemctl show --property=After isc-dhcp-server.service 

systemctl status isc-dhcp-server.service
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  3. Best Answer

    Finally spent some time on this problem and solved it with a script I named restart-dhcpd.sh and placed in /etc/NetworkManager/dispatcher.d/. (Note: you may need to systemctl restart network-manager.service and/or systemctl daemon-reload afterwards):

    #! /bin/bash
IF=$1
STATUS=$2
case "$2" in
    dhcp4-change|up)
            logger -s "NM Script $STATUS $IF triggered"
            if [ "$1" = "enx6cf0496c6947" ]; then
                    /bin/systemctl restart isc-dhcp-server.service
            fi
    ;;
esac
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