In Debian, the DHCP client daemon binary is called dhcpcd-bin and it is invoked via a wrapper called dhcpcd. Dhcpcd is normally invoked by
but ifup only allows the setting of a limited number of dhcpcd command line flags. The dhcpcd wrapper reads extra flags from /etc/default/dhcpcd and then invokes
It also adds the hostname as an argument to the command line, unless
it is already set by ifup or /etc/default/dhcpcd or SET_HOSTNAME is
configured. In that case there is no point in sending the old
hostname with the DHCP request when it will be overridden by a new
name supplied by the DHCP server.
Note that the interface argument must be supplied, there is no default
to eth0 like the unadorned dhcpcd binary.