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I am running one Ubuntu Server 14.04 with network configuration using /etc/network/interfaces and so ifup/ifdown scripts.

I realised that if I configure one interface in DHCP mode, then I unplug the cable from the interface and the IP address configured via DHCP is maintained until the DHCP lease expires. What I need is dhclient to flush addresses and routes configured via DHCP in the interface when the link goes down immediately.

How can I get dhclient to flush IP addresses and routes when the link status is DOWN?

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You can use ifplugd to run a command when an Ethernet cable is plugged or unplugged. I'm not sure whether installing ifplugd is necessary — it's possible that there's a built-in way with udev and upstart — but I can't find it in the upstart documentation.

Since you're using ifupdown, call ifdown when the cable is unplugged. In /etc/ifplugd/ifplugd.action, write something like

#!/bin/sh
if [ "$1" = "eth0" ] && [ "$2" = "down" ]; then
  ifdown "$1"
fi
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  • Great! You provided me the key point.
    – chrpinedo
    Commented Mar 25, 2015 at 13:41

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