The story began a long time ago, back when CentOS 7 (RHEL 7) was released. If you had used disk encryption on CentOS 6, there were no issues with the automatic unlocking of disks when connecting a USB flash drive with the necessary keys. However, upon the release of version 7, everything suddenly stopped working as you were used to. At that time, a solution could be found by reverting dracut to sysvinit with a simple line in the config: echo 'omit_dracutmodules+=" systemd "' > /etc/dracut.conf.d/luks-workaround.conf
This immediately stripped away all the benefits of systemd—the fast and parallel startup of system services, which significantly reduced boot time.
And it's still the same:
Not waiting for a solution, I created one for myself, and now I’m sharing it with the public; for those interested, read on.
Introduction
When I first started working with CentOS 7, systemd didn’t evoke any strong emotions, as I didn't initially notice significant differences apart from minor syntax changes in service management. Over time, I grew fond of systemd, but my first impression was somewhat tarnished since the dracut developers didn’t pay much attention to supporting the boot process with systemd alongside disk encryption. Overall, it worked, but entering the disk password at every boot was server — not the most enjoyable task.
Having tried numerous recommendations and studying the manual, I realized that in systemd mode, USB configuration is possible, but only through manual association of each disk with the key on the USB drive. Moreover, the USB drive can only be linked by its UUID; LABEL did not work. Maintaining such a setup was not very convenient, so ultimately, I plunged into waiting, and after nearly 7 years, I understood—no one intends to solve the issue.
Issues
Of course, anyone can write their own plugin for dracut, but making it functional is not so easy. It turned out that due to the parallel nature of systemd's startup, incorporating one's code and changing the boot process is not straightforward. The dracut documentation explained far from everything. However, after extensive experimentation, I was able to solve the problem.
How it works
At its core, there are three units:
- luks-auto-key.service — searches for storage devices with keys for LUKS
- luks-auto.target — acts as a dependency for the built-in systemd-cryptsetup units
- luks-auto-clean.service — cleans up temporary files created by luks-auto-key.service
luks-auto-generator.sh is a script that is executed by systemd and generates units based on kernel parameters. Similar generators create fstab units and so on.
luks-auto-generator.sh
The behavior of the standard systemd-cryptsetup can be modified with drop-in.conf, adding a dependency on luks-auto.target.
luks-auto-key.service and luks-auto-key.sh
This unit runs the script luks-auto-key.sh, which, based on the keys rd.luks.*, finds media with keys and copies them to a temporary directory for further use. After the process is completed, keys from the temporary directory are removed by luks-auto-clean.service.
Sources:
/usr/lib/dracut/modules.d/99luks-auto/module-setup.sh
#!/bin/bash
check () {
if ! dracut_module_included "systemd"; then
"luks-auto needs systemd in the initramfs"
return 1
fi
return 255
}
depends () {
echo "systemd"
return 0
}
install () {
inst "$systemdutildir/systemd-cryptsetup"
inst_script "$moddir/luks-auto-generator.sh" "$systemdutildir/system-generators/luks-auto-generator.sh"
inst_script "$moddir/luks-auto-key.sh" "/etc/systemd/system/luks-auto-key.sh"
inst_script "$moddir/luks-auto.sh" "/etc/systemd/system/luks-auto.sh"
inst "$moddir/luks-auto.target" "${systemdsystemunitdir}/luks-auto.target"
inst "$moddir/luks-auto-key.service" "${systemdsystemunitdir}/luks-auto-key.service"
inst "$moddir/luks-auto-clean.service" "${systemdsystemunitdir}/luks-auto-clean.service"
ln_r "${systemdsystemunitdir}/luks-auto.target" "${systemdsystemunitdir}/initrd.target.wants/luks-auto.target"
ln_r "${systemdsystemunitdir}/luks-auto-key.service" "${systemdsystemunitdir}/initrd.target.wants/luks-auto-key.service"
ln_r "${systemdsystemunitdir}/luks-auto-clean.service" "${systemdsystemunitdir}/initrd.target.wants/luks-auto-clean.service"
}
/usr/lib/dracut/modules.d/99luks-auto/luks-auto-generator.sh
#!/bin/sh
# -*- mode: shell-script; indent-tabs-mode: nil; sh-basic-offset: 4; -*-
# ex: ts=8 sw=4 sts=4 et filetype=sh
. /lib/dracut-lib.sh
SYSTEMD_RUN='/run/systemd/system'
CRYPTSETUP='/usr/lib/systemd/systemd-cryptsetup'
TOUT=$(getargs rd.luks.key.tout)
if [ ! -z "$TOUT" ]; then
mkdir -p "${SYSTEMD_RUN}/luks-auto-key.service.d"
cat > "${SYSTEMD_RUN}/luks-auto-key.service.d/drop-in.conf" <<EOF
[Service]
Type=oneshot
ExecStartPre=/usr/bin/sleep $TOUT
EOF
fi
mkdir -p "$SYSTEMD_RUN/luks-auto.target.wants"
for argv in $(getargs rd.luks.uuid -d rd_LUKS_UUID); do
_UUID=${argv#luks-}
_UUID_ESC=$(systemd-escape -p $_UUID)
mkdir -p "${SYSTEMD_RUN}/systemd-cryptsetup@luksx2d${_UUID_ESC}.service.d"
cat > "${SYSTEMD_RUN}/systemd-cryptsetup@luksx2d${_UUID_ESC}.service.d/drop-in.conf" <<EOF
[Unit]
After=luks-auto.target
ConditionPathExists=!/dev/mapper/luks-${_UUID}
EOF
cat > "${SYSTEMD_RUN}/luks-auto@${_UUID_ESC}.service" <<EOF
[Unit]
Description=luks-auto Cryptography Setup for %I
DefaultDependencies=no
Conflicts=umount.target
IgnoreOnIsolate=true
Before=luks-auto.target
BindsTo=dev-disk-byx2duuid-${_UUID_ESC}.device
After=dev-disk-byx2duuid-${_UUID_ESC}.device luks-auto-key.service
Before=umount.target
[Service]
Type=oneshot
RemainAfterExit=yes
TimeoutSec=0
ExecStart=/etc/systemd/system/luks-auto.sh ${_UUID}
ExecStop=$CRYPTSETUP detach 'luks-${_UUID}'
Environment=DRACUT_SYSTEMD=1
StandardInput=null
StandardOutput=syslog
StandardError=syslog+console
EOF
ln -fs ${SYSTEMD_RUN}/luks-auto@${_UUID_ESC}.service $SYSTEMD_RUN/luks-auto.target.wants/luks-auto@${_UUID_ESC}.service
done
/usr/lib/dracut/modules.d/99luks-auto/luks-auto-key.service
[Unit]
Description=LUKS AUTO key searcher
After=cryptsetup-pre.target
Before=luks-auto.target
DefaultDependencies=no
[Service]
Environment=DRACUT_SYSTEMD=1
Type=oneshot
ExecStartPre=/usr/bin/sleep 1
ExecStart=/etc/systemd/system/luks-auto-key.sh
RemainAfterExit=true
StandardInput=null
StandardOutput=syslog
StandardError=syslog+console
/usr/lib/dracut/modules.d/99luks-auto/luks-auto-key.sh
#!/bin/sh
# -*- mode: shell-script; indent-tabs-mode: nil; sh-basic-offset: 4; -*-
# ex: ts=8 sw=4 sts=4 et filetype=sh
export DRACUT_SYSTEMD=1
. /lib/dracut-lib.sh
MNT_B="/tmp/luks-auto"
ARG=$(getargs rd.luks.key)
IFS=$':' _t=(${ARG})
KEY=${_t[0]}
F_FIELD=''
F_VALUE=''
if [ ! -z $KEY ] && [ ! -z ${_t[1]} ];then
IFS=$'=' _t=(${_t[1]})
F_FIELD=${_t[0]}
F_VALUE=${_t[1]}
F_VALUE="${F_VALUE%"}"
F_VALUE="${F_VALUE#"}"
fi
mkdir -p $MNT_B
finding_luks_keys(){
local _DEVNAME=''
local _UUID=''
local _TYPE=''
local _LABEL=''
local _MNT=''
local _KEY="$1"
local _F_FIELD="$2"
local _F_VALUE="$3"
local _RET=0
blkid -s TYPE -s UUID -s LABEL -u filesystem | grep -v -E -e "TYPE=".*_member"" -e "TYPE="crypto_.*"" -e "TYPE="swap"" | while IFS=$'' read -r _line; do
IFS=$':' _t=($_line);
_DEVNAME=${_t[0]}
_UUID=''
_TYPE=''
_LABEL=''
_MNT=''
IFS=$' ' _t=(${_t[1]});
for _a in "${_t[@]}"; do
IFS=$'=' _v=(${_a});
temp="${_v[1]%"}"
temp="${temp#"}"
case ${_v[0]} in
'UUID')
_UUID=$temp
;;
'TYPE')
_TYPE=$temp
;;
'LABEL')
_LABEL=$temp
;;
esac
done
if [ ! -z "$_F_FIELD" ];then
case $_F_FIELD in
'UUID')
[ ! -z "$_F_VALUE" ] && [ "$_UUID" != "$_F_VALUE" ] && continue
;;
'LABEL')
[ ! -z "$_F_VALUE" ] && [ "$_LABEL" != "$_F_VALUE" ] && continue
;;
*)
[ "$_DEVNAME" != "$_F_FIELD" ] && continue
;;
esac
fi
_MNT=$(findmnt -n -o TARGET $_DEVNAME)
if [ -z "$_MNT" ]; then
_MNT=${MNT_B}/KEY-${_UUID}
mkdir -p "$_MNT" && mount -o ro "$_DEVNAME" "$_MNT"
_RET=$?
else
_RET=0
fi
if [ "${_RET}" -eq 0 ] && [ -f "${_MNT}/${_KEY}" ]; then
cp "${_MNT}/${_KEY}" "$MNT_B/${_UUID}.key"
info "Found ${_MNT}/${_KEY} on ${_UUID}"
fi
if [[ "${_MNT}" =~ "${MNT_B}" ]]; then
umount "$_MNT" && rm -rfd --one-file-system "$_MNT"
fi
done
return 0
}
finding_luks_keys $KEY $F_FIELD $F_VALUE
/usr/lib/dracut/modules.d/99luks-auto/luks-auto.target
[Unit]
Description=LUKS AUTO target
After=systemd-readahead-collect.service systemd-readahead-replay.service
After=cryptsetup-pre.target luks-auto-key.service
Before=cryptsetup.target
/usr/lib/dracut/modules.d/99luks-auto/luks-auto.sh
#!/bin/sh
# -*- mode: shell-script; indent-tabs-mode: nil; sh-basic-offset: 4; -*-
# ex: ts=8 sw=4 sts=4 et filetype=sh
export DRACUT_SYSTEMD=1
. /lib/dracut-lib.sh
MNT_B="/tmp/luks-auto"
CRYPTSETUP='/usr/lib/systemd/systemd-cryptsetup'
for i in $(ls -p $MNT_B | grep -v /);do
info "Trying $i on $1..."
$CRYPTSETUP attach "luks-$1" "/dev/disk/by-uuid/$1" $MNT_B/$i 'tries=1'
if [ "$?" -eq "0" ]; then
info "Found $i for $1"
exit 0
fi
done
warn "No key found for $1. Fallback to passphrase mode."
/usr/lib/dracut/modules.d/99luks-auto/luks-auto-clean.service
[Unit]
Description=LUKS AUTO key cleaner
After=cryptsetup.target
DefaultDependencies=no
[Service]
Type=oneshot
ExecStart=/usr/bin/rm -rfd --one-file-system /tmp/luks-auto
/etc/dracut.conf.d/luks-auto.conf
add_dracutmodules+=" luks-auto "Installation
mkdir -p /usr/lib/dracut/modules.d/99luks-auto/
# place almost all files here
chmod +x /usr/lib/dracut/modules.d/99luks-auto/*.sh
# create the file /etc/dracut.conf.d/luks-auto.conf
# and generate a new initramfs
dracut -f
Conclusion
For convenience, I maintained compatibility with kernel command line parameters for sysvinit mode, which simplifies usage in older installations.
Source: habr.com
