In buster /etc/init.d/isc-dhcp-server changed. It now starts both v4 and v6 servers if INTERFACES, INTERFACESv4 and INTERFACESv6 are unset in /etc/default/isc-dhcp-server (as is the case in jessie).
INTERFACES is deprecated and replaced by INTERFACES{v4,v6}.
/etc/init.d/isc-dhcp-server:
start)
if test -n "$INTERFACES" -a -z "$INTERFACESv4"; then
echo "DHCPv4 interfaces are no longer set by the INTERFACES variable in" >&2
echo "/etc/default/isc-dhcp-server. Please use INTERFACESv4 instead." >&2
echo "Migrating automatically for now, but this will go away in the future." >&2
INTERFACESv4="$INTERFACES"
fi
if test -n "$INTERFACESv4"; then
echo "Launching IPv4 server only."
start_daemon "-4" "$DHCPDv4_CONF" "$NAME4" \
"$DHCPDv4_PID" "$DESC4" "$INTERFACESv4"
fi
if test -n "$INTERFACESv6"; then
echo "Launching IPv6 server only."
start_daemon "-6" "$DHCPDv6_CONF" "$NAME6" \
"$DHCPDv6_PID" "$DESC6" "$INTERFACESv6"
fi
if test -z "$INTERFACESv4" -a -z "$INTERFACESv6"; then
echo "Launching both IPv4 and IPv6 servers (please configure INTERFACES in /etc/default/isc-dhcp-server if you only want one or the other)."
start_daemon "-4" "$DHCPDv4_CONF" "$NAME4" \
"$DHCPDv4_PID" "$DESC4" ""
start_daemon "-6" "$DHCPDv6_CONF" "$NAME6" \
"$DHCPDv6_PID" "$DESC6" ""We need to set the interfaces to listen to in /etc/default or modify the init script to have a working dhcp-server startup.