preseed.cfg

[...]

d-i preseed/late_command string \
echo -e "\n# The secondary network interface" >> /target/etc/network/interfaces; \
echo       "allow-hotplug eth1"               >> /target/etc/network/interfaces; \
echo       "iface eth1 inet dhcp"             >> /target/etc/network/interfaces
d-i preseed/late_command string echo -e "\n# The public network interface\nallow-hotplug eth2\niface eth2 inet static\n  address XXX\n  netmask XXX\n  gateway XXX" >> /target/etc/network/interfaces

d-i finish-install/keep-consoles boolean true
d-i finish-install/reboot_in_progress note