dhcpd ignores $CONFIG_FILE in /etc/default/isc-dhcp-server

[Norman Wilson]

Package isc-dhcp-server 4.3.3 on Ubuntu 16.04 LTS.

On earlier (pre-systemd) systems, one could point dhcpd at a configuration file in any location by setting CONFIG_FILE=/new/path/dhcpd.conf in /etc/default/isc-dhcp-server. In this version such setting is ignored; dhcpd always uses the default /etc/dhcp/dhcpd.conf.

In more formulaeic terms:

Put CONFIG_FILE=/var/dhcpconf/mydhcp.conf in /etc/default/isc-dhcp-server.
I expect that when service isc-dhcp-server is next started, it will use that configuration file.
In fact it still uses /etc/dhcp/dhcpd.conf.

The problem can be worked around by making /etc/dhcp/dhcpd.conf a symlink to the desired config file, but why have /etc/default/isc-dhcp-server at all if it doesn't work?

The offending code is in /lib/systemd/system/isc-dhcp-server.service. Here is a relevant extract:

[Service]
EnvironmentFile=/etc/default/isc-dhcp-server
RuntimeDirectory=dhcp-server
# The leases files need to be root:dhcpd even when dropping privileges
ExecStart=/bin/sh -ec '\
    CONFIG_FILE=/etc/dhcp/dhcpd.conf; \
    if [ -f /etc/ltsp/dhcpd.conf ]; then CONFIG_FILE=/etc/ltsp/dhcpd.conf; fi; \
    [ -e /var/lib/dhcp/dhcpd.leases ] || touch /var/lib/dhcp/dhcpd.leases; \
    chown root:dhcpd /var/lib/dhcp /var/lib/dhcp/dhcpd.leases; \
    chmod 775 /var/lib/dhcp ; chmod 664 /var/lib/dhcp/dhcpd.leases; \
    exec dhcpd -user dhcpd -group dhcpd -f -4 -pf /run/dhcp-server/dhcpd.pid -cf $CONFIG_FILE $INTERFACES'

/etc/default/isc-dhcp-server is correctly read as an EnvironmentFile, but the ExecStart command unconditionally sets CONFIG_FILE, discarding any value from the /etc/default file.

One fix might be to change the first few lines of the ExecStart to check whether $CONFIG_FILE has been set, and perhaps whether it is still the default value:

ExecStart=/bin/sh -ec '\
    case $CONFIG_FILE in \
    ""|/etc/dhcp/dhcpd.conf) \
        CONFIG_FILE=/etc/dhcp/dhcpd.conf; \
        if [ -f /etc/ltsp/dhcpd.conf ]; then CONFIG_FILE=/etc/ltsp/dhcpd.conf; fi; \
        ;; \
    esac

Or it might make more sense to let whatever package is in charge of ltsp configuration append CONFIG_FILE=/etc/ltsp/dhcpd.conf to /etc/defaults/isc-dhcp-server, and replace the lines above with a simple

ExecStart=/bin/sh -ec '\
    CONFIG_FILE=${CONFIG_FILE:-/etc/dhcp/dhcpd.conf}; \

Either way, a fix would be appreciated.