Devuan Jessie beta released

The original configuration was with DHCP, and the interface was up, so dhclient was already running. After editing the interfaces file the configuration was static, so at that point ifdown didn't expect dhclient to be running and consequently failed to terminate it.

The problem is modifying the configuration file while the interface is up, combined with scripts that use the *current* interfaces file for ifdown rather than the configuration which was in place when ifup was executed.

Under systemd any background processes like dhclient associated with an interface (in the corresponding cgroup) would automatically be terminated when the interface is stopped, regardless of the current configuration. This is not a perfect solution to the problem of changing configurations, but it does mitigate the specific issue of ifdown failing to stop dhclient.