"ifdown wlan0" brings down multiple interfaces, not just the specified one

[Tessa Lau]

I have a server with multiple internet connections using several different network interfaces (e.g. wlan0 for wireless and lte0 for an LTE dongle). Both are configured in /etc/network/interfaces to use DHCP like so:

allow-hotplug lte0
iface lte0 inet dhcp

iface wlan0 inet dhcp
    wpa-ssid MY_NETWORK
    wpa-psk XXXX
    wpa-driver nl80211
    wpa-key-mgmt WPA-PSK

If both interfaces are up and I try to bring just one of them down using "ifdown wlan0" I would expect the other interface to remain up. However, this actually brings down both interfaces and does a DHCP release on both.

% sudo ifdown wlan0
Internet Systems Consortium DHCP Client 4.2.4
Copyright 2004-2012 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/

Listening on LPF/lte0/94:b9:b4:0a:c6:e0
Sending on LPF/lte0/94:b9:b4:0a:c6:e0
Listening on LPF/wlan0/80:86:f2:b1:c3:3f
Sending on LPF/wlan0/80:86:f2:b1:c3:3f
Sending on Socket/fallback
DHCPRELEASE on lte0 to 100.79.128.1 port 67 (xid=0x33b5bd5b)
Connection closed.

This is actually catastrophic for us since we are connecting to the server via the lte0 interface, and we have just shut down our access route to the server.

This seems like a bug in ifdown, where it is bringing down the wrong interface despite what I told it to do on the command line.

[Tessa Lau (tessa-lau)]

I've discovered that this is actually a bug (or badly designed "feature") in the isc-dhcp package.

I upstreamed the bug here: https://bugs.launchpad.net/ubuntu/+source/isc-dhcp/+bug/1524093