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https://github.com/mk-fg/dracut-crypt-sshd

dracut initramfs module to start sshd on early boot to enter encryption passphrase from across the internets
https://github.com/mk-fg/dracut-crypt-sshd

boot dm-crypt dracut dropbear initramfs module shell sshd

Last synced: 6 months ago
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dracut initramfs module to start sshd on early boot to enter encryption passphrase from across the internets

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README 

          
dracut-crypt-sshd

--------------------



**Deprecation notice**: there is a much improved and generalized version of this

module, maintained at [dracut-crypt-ssh/dracut-crypt-ssh](https://github.com/dracut-crypt-ssh/dracut-crypt-ssh/).

This project here is mostly for historical reasons.



[Dracut initramfs](https://dracut.wiki.kernel.org/index.php/Main_Page) module

to start [Dropbear sshd](https://matt.ucc.asn.au/dropbear/dropbear.html)

on early boot to enter encryption passphrase from across the internets or just

connect and debug whatever stuff there.



Idea is to use the thing on remote VDS servers, where full-disk encryption is

still desirable (if only to avoid data leaks when disks will be decomissioned

and sold by VDS vendor) but rather problematic due to lack of KVM or whatever

direct console access.



Authenticates users strictly by provided authorized_keys ("dropbear_acl" option) file.



See dropbear(8) manpage for full list of supported restrictions there (which are

fairly similar to openssh).



### Obligatory warning



Please think about your

[threat model](https://en.wikipedia.org/wiki/Threat_model) first,

and security/usability trade-off second.



This module is very unlikely to help at all against malicious hosting provider

or whatever three-letter-agency that will coerce it into cooperation, should it

take interest in your poor machine - they can just extract keys from RAM image

(especially if it's a virtualized container), backdoor kernel/initramfs and

force a reboot, or do whatever else to get encryption keys via hardware/backdoor.



It can help, as mentioned, against attacks on the data after you're done with it

completely (i.e. shut the machine/container in question down for good), or

against rather clumsy and incompetent "power off first and think second"

attacks.



If this benefit is worth the hassle of this extra ssh'ing, some maintenance

overhead and the possibility of loosing the LUKS key/header (and all access to

data with it), only then (I think) this module might be useful to you.



Please also check out "Bad Things" section below before use.



### Usage



First of all, it needs dropbear (at least its sshd, I tested only version built

without pam support, both static and shared should work) and gcc installed (to

build auth.c tool).



- Copy or symlink `60dropbear-sshd` into `/usr/lib/dracut/modules.d/`.



- Add `dracutmodules+="dropbear-sshd"` to dracut.conf

  (will pull in "network" module as dependency).



- Check out supported dracut.conf options below.

  With no extra options, ad-hoc server rsa key will be generated (and its

  fingerprint/bbcode will be printed to dracut log),

  `/root/.ssh/authorized_keys` will be used for ACL.



- See dracut.cmdline(7) manpage for info on how to setup "network" module

  (otherwise sshd is kinda useless).



  Simpliest way might be just passing `ip=dhcp rd.neednet=1` on cmdline, if dhcp

  can assign predictable ip and pass proper routes.



  Example of luks (uuid starts with "7a476ea0") + lvm (vg named "lvmcrypt", with lv

  there having fs with "root" label) + static-net (see manpage above for syntax)

  grub.cfg entry (wrapped for readability):



        menuentry "My Linux" {

          linux /vmlinuz ro root=LABEL=root

            rd.luks.uuid=7a476ea0 rd.lvm.vg=lvmcrypt rd.neednet=1

            ip=88.195.61.177::88.195.61.161:255.255.255.224:myhost:enp0s9:off

          initrd /dracut.xz

        }



- Run dracut to build initramfs with the thing.



On boot, sshd will be started with:



- Port: ${dropbear_port} (dracut.conf) or 2222 (default).



- User (to allow login as-): root



- Host key: ${dropbear_rsa_key} (dracut.conf) or generated

  (fingerprint echoed during generation and to console on sshd start).

  DSA keys are not supported (and shouldn't generally be used with ssh).



- Client key(s): ${dropbear_acl} (dracut.conf) or `/root/.ssh/authorized_keys`



- Password auth and port forwarding explicitly disabled.



Dropbear should echo a few info messages on start (unless rd.quiet or similar

options are used) and print host ssh key fingerprint to console, as well as any

logging (e.g. errors, if any) messages.



Do check the fingerprints either by writing them down on key generation, console

or through network perspectives at least.



To login:



    % ssh -p2222 root@some.remote.host.tld



Shell is /bin/sh, which should be

[dash](http://gondor.apana.org.au/~herbert/dash/) in most dracut builds, but can

probably be replaced with ash (busybox) or bash (heavy) using appropriate modules.



Once inside:



```console



% console_peek   # to see what's on the console (e.g. which dev prompt is for)

...

% console_auth    # queries passphrase and sends it to console

Passphrase:

%

```



Boot should continue after last command, which should send entered passphrase to

cryptsetup, waiting for it on the console, assuming its correctness.



sshd should be killed during dracut "cleanup" phase, once main os init is about to run.

Connection won't be closed, but nothing should work there, as initramfs gets destroyed.



### dracut.conf parameters



- dropbear_port



- dropbear_rsa_key



- dropbear_acl



See above.



### Common issues and non-issues



- `Dropbear sshd failed to start`



Only means what it says, see output of dropbear *before* it died - it should

print some specific errors which led to it exiting like that.



- `Failed reading '-', disabling DSS`



Will *always* be printed and should be ignored - DSA keys are not generated/used

in these scripts, and probably shouldn't be.



- Host hangs in initramfs, but can't be pinged (e.g. `ping my.host.tld`) from outside.



Either no network configuration parameters were passed to dracut, or it failed

to configure at least one IP address.



Don't forget `rd.neednet=1` on cmdline, as dracut will ignore specified network

settings without nfs (or whatever net-) root otherwise.



Read up dracut.cmdline(7), "Network" section and/or see why/if dracut failed to

configure net as requested with `rd.debug`.

See also "Debugging tips" section below.



- Host pings, but ssh can't connect.



Try `nc -v  `, or "ncat" instead of "nc" there.

"ncat" can be found in "nmap" package, "nc" usually comes pre-installed.



If it hangs without printing "Connected to ..." line - can be some firewall

before host or dropbear failed to start/listen.



If there's no "SSH-2.0-dropbear_..." after "Connected to ..." line - some issue

with dropbear.



- `lastlog_perform_login: Couldn't stat /var/log/lastlog: No such file or directory`



Pops up when logging in, can be safely ignored.



### Debugging tips



If (or rather "when") something goes wrong and you can't access just-booted

machine over network and can't get to console (hence sshd in initramfs), don't

panic - it's fixable if machine can be rebooted into some rescue system

remotely.



Usually it's some dhcp+tftp netboot thing from co-located machine (good idea to

setup/test in advance) plus whoever is there occasionally pushing the power

button, or maybe some fancy hw/interface for that (e.g. hetzner "rescue" interface).



To see what was going on during initramfs, open

"modules.d/99base/rdsosreport.sh" in dracut, append this (to the end):



	set -x

	netstat -lnp

	netstat -np

	netstat -s

	netstat -i

	ip addr

	ip ro

	set +x



	exec >/dev/null 2>&1

	mkdir /tmp/myboot

	mount /dev/sda2 /tmp/myboot

	cp /run/initramfs/rdsosreport.txt /tmp/myboot/

	umount /tmp/myboot

	rmdir /tmp/myboot



Be sure to replace `/dev/sda2` with whatever device is used for /boot, rebuild

dracut and add `rd.debug` to cmdline (e.g. in grub.cfg's "linux" line).



Upon next reboot, *wait* for at least a minute, since dracut should give up on

trying to boot the system first, then it will store full log of all the stuff

modules run ("set -x") and their output in "/boot/rdsosreport.txt".



Naturally, to access that, +1 reboot into some "rescue" system might be needed.



In case of network-related issues - e.g. if "rdsosreport.txt" file gets created

with "rd.debug", but host can't be pinged/connected-to for whatever reason -

either enable "debug" dracut module or add `dracut_install netstat ip` line to

`install()` section of "modules.d/60dropbear-sshd/module-setup.sh" and check

"rdsosreport.txt" or console output for whatever netstat + ip commands above

(for "rdsosreport.sh") show - there can be no default route, whatever interface

naming mixup, no traffic (e.g. unrelated connection issue), etc.



### Bad Things



- Has quite a few rather unnecessary assumptions about environment baked-in

  (e.g. `/lib/libnss_files.so.2` path), and does `gcc -std=gnu99 -O2 -Wall

  "$moddir"/auth.c -o "${tmp_file}"` for that "console_auth" binary on dracut

  run.



  Better way to handle these and compile whatever binaries would be allowing for

  proper `./configure && make && make install` process, which I don't know

  enough about to support.



  Luckily, there are people that do, check out these links for better integrated

  versions of a similar thing:



  * https://github.com/dracut-crypt-ssh/dracut-crypt-ssh/

  * https://github.com/artem-sidorenko/dracut-earlyssh/

  * https://github.com/haraldh/dracut/pull/43

  * https://github.com/mdcurtis/dracut-earlyssh

  * https://github.com/philfry/dracut-earlyssh



- Only tested with customized source-based distro

  ([Exherbo](http://exherbo.org/)), no idea how easy it is to use with generic

  debian or ubuntu.



- `check()` in module_setup.sh should probably not be empty no-op.



- Should probably have `set -e` or something alike (dracut-specific?) in install().



- No idea how to sanely run `ssh-keygen` (openssh) from a script, maybe use

  openssl instead?



### Based on code, examples and ideas from



- https://bugzilla.redhat.com/show_bug.cgi?id=524727

- http://roosbertl.blogspot.de/2012/12/centos6-disk-encryption-with-remote.html

- https://bitbucket.org/bmearns/dracut-crypt-wait