Dracut initramfs module to start Dropbear sshd on early boot to enter encryption passphrase remotely or just connect and debug.
There are a number of reasons why you would want to do this:
- It provides a way of entering the encryption keys for a number of servers without console switching
- It allows remote booting of (externally-hosted) encrypted servers
This is based heavily on the work of others, in particular mk-fg. The major changes between this version are: adaption for RHEL 6 and the old version of dracut installed there; additional options for replicated the system host key or a user provided one (see the "dropbear_rsa_key" option, documented in earlyssh.conf); an additional utility (unlock) for automating the unlock process.
Users are strictly authenticated by provided SSH public keys. These can be either: root's ~/.ssh/authorized_keys or a custom file ("dropbear_acl" option). Depending on your environment, it may make sense to make the preboot authorized_keys file quite different to the normal one.
See dropbear(8) manpage for full list of supported restrictions there (which are fairly similar to openssh). If using in combination with the unlock utility (see below), a useful restriction may be to make /bin/unlock a 'forced command' in SSH.
First of all, you must have dropbear. CentOS/RHEL users can get this from EPEL.
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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_keyswill be used for ACL. -
See dracut.cmdline(7) manpage for info on how to setup "network" module (otherwise sshd is kinda useless).
Simplest way might be just passing
ip=dhcp rd.neednet=1on cmdline, if dhcp can assign predictable ip and pass proper routes.On older Dracut versions (e.g. 004 in RHEL6), networking is only configured if you have configured a network root. In order to work around this, dracut-earlyssh system will install a dummyroot script (if it detects dracut v004 at build-time). The cmdline for these versions should be
ip=dhcp netroot=dummy. -
Put 60dropbear in to dracut modules directory, usually /lib/dracut/modules.d
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Run dracut to build initramfs with the thing.
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Probably you need to build the initramfs from the installer. See https://bugzilla.redhat.com/show_bug.cgi?id=524727#c23 in how to boot the installer via grub to allow ssh logins. chmod into encrypted system before rebuilding with dracut --kver kernelverinboot --force
On boot, sshd will be started with:
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Port: ${dropbear_port} (dracut.conf) or 2222 (default).
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User (to allow login as-): root
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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).
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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 in most dracut builds, but can probably be replaced with ash (busybox) or bash (heavy) using appropriate modules.
After the system starts booting, sshd should be killed during dracut "cleanup" phase, once main os init is about to run.
It essentially just calls /usr/bin/systemd-tty-ask-password-agent --query But therer is a severe Bug to consider: xxxx
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dropbear_port
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dropbear_rsa_key
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dropbear_acl
See above.
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 <host> <port>, 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.
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.