unlocking rootfs via ssh login in initramfs
You can unlock your rootfs on bootup from remote, using ssh to log in
to the booting system while it's running with the initramfs mounted.
Setup
For remote unlocking to work, the following packages have to be
installed before building the initramfs: dropbear busybox
The file /etc/initramfs-tools/initramfs.conf holds the configuration
options used when building the initramfs. It should contain BUSYBOX=y
(this is set as the default when the busybox package is installed) to
have busybox installed into the initramfs, and should not contain
DROPBEAR=n, which would disable installation of dropbear to initramfs.
If set to DROPBEAR=y, dropbear will be installed in any case; if
DROPBEAR isn't set at all, then dropbear will only be installed in
case of an existing cryptroot setup.
The host keys used for the initramfs are dropbear_dss_host_key and
dropbear_rsa_host_key, both located
in /etc/initramfs-tools/etc/dropbear/. If they do not exist when the
initramfs is compiled, they will be created automatically. Following
are the commands to create them manually:
dropbearkey -t dss -f /etc/initramfs-tools/etc/dropbear/dropbear_dss_host_key
dropbearkey -t rsa -f /etc/initramfs-tools/etc/dropbear/dropbear_rsa_host_key
As the initramfs will not be encrypted, publickey authentication is
assumed. The key(s) used for that will be taken from
/etc/initramfs-tools/root/.ssh/authorized_keys. If this file doesn't
exist when the initramfs is compiled, it will be created and
/etc/initramfs-tools/root/.ssh/id_rsa.pub will be added to it. If the
latter file doesn't exist either, it will be generated automatically -
you will find the matching private key which you will later need to
log in to the initramfs under /etc/initramfs-tools/root/.ssh/id_rsa
(or id_rsa.dropbear in case you need it in dropbear format). Following
are the commands to do the respective steps manually:
To create a key (in dropbear format):
dropbearkey -t rsa -f /etc/initramfs-tools/root/.ssh/id_rsa.dropbear
To convert the key from dropbear format to openssh format:
/usr/lib/dropbear/dropbearconvert dropbear openssh \
/etc/initramfs-tools/root/.ssh/id_rsa.dropbear \
/etc/initramfs-tools/root/.ssh/id_rsa
To extract the public key:
dropbearkey -y -f /etc/initramfs-tools/root/.ssh/id_rsa.dropbear | \
grep "^ssh-rsa " > /etc/initramfs-tools/root/.ssh/id_rsa.pub
To add the public key to the authorized_keys file:
cat /etc/initramfs-tools/root/.ssh/id_rsa.pub >> /etc/initramfs-tools/root/.ssh/authorized_keys
In case you want some interface to get configured using dhcp, setting
DEVICE= in /etc/initramfs-tools/initramfs.conf should be sufficient.
The initramfs should also honour the ip= kernel parameter. In case you
use grub, you probably might want to set it in /boot/grub/menu.lst,
either in the '# kopt=' line or appended to specific 'kernel' line(s).
The ip= kernel parameter is documented in Documentation/nfsroot.txt in
the kernel source tree.
Issues
Don't forget to run update-initramfs when you changed the config to
make it effective!
Collecting enough entropy for the ssh daemon sometimes seems to be an
issue. Startup of the ssh daemon might be delayed until enough entropy
has been retrieved. This is non-blocking for the startup process, so
when you are at the console you won't have to wait for the sshd to
complete its startup.
Unlocking procedure
To unlock from remote, you could do something like this:
ssh -o "UserKnownHostsFile=~/.ssh/known_hosts.initramfs" \
-i "~/id_rsa.initramfs" [email protected] \
"echo -ne \"secret\" >/lib/cryptsetup/passfifo"
This example assumes that you have an extra known_hosts file
"~/.ssh/known_hosts.initramfs" which holds the cryptroot system's
host-key, that you have a file "~/id_rsa.initramfs" which holds the
authorized-key for the cryptroot system, that the cryptroot system's
name is "initramfshost.example.com", and that the cryptroot passphrase
is "secret"
-- <[email protected]>, Wed, 30 Sep 2009