How to open a port early in boot process to unlock LUKS via SSH

The subject line is wrong. The problem isn't a closed port, it's a port which wasn't bound. SSHd hasn't started yet; that's the reason you can't connect to it.

The dropbear (ssh server) is supposed to be started very early during boot phase - earlier than the init (rcN.d) sequence and firewall init scripts; even earlier than / is mounted (it is encrypted too, right?). So it comes to initramfs, the pre-/ userland loaded for kernel by boot loader. The image is (re)generated by update-initramfs -u from contents of /etc/initramfs-tools/, including dropbear configuration in /etc/initramfs-tools/etc/dropbear/. To play with dropbear config, play with that one.

Thus, few points to check:

• dropbear doesn't start: it haven't been plugged into initramfs sequence well;
• default firewall denies all.

I got this same problem a few weeks ago (Debian Wheezy 7.6) and after some days of troubleshooting I found out that there was a config file missing which was preventing to the cryptroot script on init-top to run correctly, hence it was not stopping to ask the password via ssh, killing the dropbear at the end of the sequence (init-bottom).

The config file is called cryptroot and should be under /etc/initramfs-tools/conf.d/ If I am not mistaken that config file should have been created automatically during install (I have read just one tutorial talking about that config file) but somehow it did not (tested in a physical server and in a VM, same OS and versions)

It took me a couple of tries to configure it properly, since I could not find the proper syntax at that time. My cryptroot config file is as follows:

target=crypt-root,source=/dev/vg0/root,lvm=root

Once created the config file just update the initramfs and try again:

update-initramfs -u