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DRACUT.CMDLINE(7) dracut DRACUT.CMDLINE(7)

NAME

dracut.cmdline - dracut kernel command line options

DESCRIPTION

The root device used by the kernel is specified in the boot configuration file on the kernel command line, as always.
The traditional root=/dev/sda1 style device specification is allowed, but not encouraged. The root device should better be identified by LABEL or UUID. If a label is used, as in root=LABEL=<label_of_root> the initramfs will search all available devices for a filesystem with the appropriate label, and mount that device as the root filesystem. root=UUID=<uuidnumber> will mount the partition with that UUID as the root filesystem.
In the following all kernel command line parameters, which are processed by dracut, are described.
"rd.*" parameters mentioned without "=" are boolean parameters. They can be turned on/off by setting them to {0|1}. If the assignment with "=" is missing "=1" is implied. For example rd.info can be turned off with rd.info=0 or turned on with rd.info=1 or rd.info. The last value in the kernel command line is the value, which is honored.

Standard

init=<path to real init>
specify the path to the init program to be started after the initramfs has finished
root=<path to blockdevice>
specify the block device to use as the root filesystem.
Example. 
root=/dev/sda1
root=/dev/disk/by-path/pci-0000:00:1f.1-scsi-0:0:1:0-part1
root=/dev/disk/by-label/Root
root=LABEL=Root
root=/dev/disk/by-uuid/3f5ad593-4546-4a94-a374-bcfb68aa11f7
root=UUID=3f5ad593-4546-4a94-a374-bcfb68aa11f7
root=PARTUUID=3f5ad593-4546-4a94-a374-bcfb68aa11f7
rootfstype=<filesystem type>
"auto" if not specified.
Example. 
rootfstype=ext3
rootflags=<mount options>
specify additional mount options for the root filesystem. If not set, /etc/fstab of the real root will be parsed for special mount options and mounted accordingly.
ro
force mounting / and /usr (if it is a separate device) read-only. If none of ro and rw is present, both are mounted according to /etc/fstab.
rw
force mounting / and /usr (if it is a separate device) read-write. See also ro option.
rootfallback=<path to blockdevice>
specify the block device to use as the root filesystem, if the normal root cannot be found. This can only be a simple block device with a simple file system, for which the filesystem driver is either compiled in, or added manually to the initramfs. This parameter can be specified multiple times.
rd.auto rd.auto=1
enable autoassembly of special devices like cryptoLUKS, dmraid, mdraid or lvm. Default is off as of dracut version >= 024.
rd.hostonly=0
removes all compiled in configuration of the host system the initramfs image was built on. This helps booting, if any disk layout changed, especially in combination with rd.auto or other parameters specifying the layout.
rd.cmdline=ask
prompts the user for additional kernel command line parameters
rd.fstab=0
do not honor special mount options for the root filesystem found in /etc/fstab of the real root.
resume=<path to resume partition>
resume from a swap partition
Example. 
resume=/dev/disk/by-path/pci-0000:00:1f.1-scsi-0:0:1:0-part1
resume=/dev/disk/by-uuid/3f5ad593-4546-4a94-a374-bcfb68aa11f7
resume=UUID=3f5ad593-4546-4a94-a374-bcfb68aa11f7
rd.skipfsck
skip fsck for rootfs and /usr. If you’re mounting /usr read-only and the init system performs fsck before remount, you might want to use this option to avoid duplication.

iso-scan/filename

Using iso-scan/filename with a Fedora/Red Hat/CentOS Live iso should just work by copying the original kernel cmdline parameters.
Example. 
menuentry 'Live Fedora 20' --class fedora --class gnu-linux --class gnu --class os {
    set isolabel=Fedora-Live-LXDE-x86_64-20-1
    set isofile="/boot/iso/Fedora-Live-LXDE-x86_64-20-1.iso"
    loopback loop $isofile
    linux (loop)/isolinux/vmlinuz0 boot=isolinux iso-scan/filename=$isofile root=live:LABEL=$isolabel ro rd.live.image quiet rhgb
    initrd (loop)/isolinux/initrd0.img
}

Misc

rd.emergency=[reboot|poweroff|halt]
specify, what action to execute in case of a critical failure. rd.shell=0 also be specified.
rd.driver.blacklist=<drivername>[,<drivername>,...]
do not load kernel module <drivername>. This parameter can be specified multiple times.
rd.driver.pre=<drivername>[,<drivername>,...]
force loading kernel module <drivername>. This parameter can be specified multiple times.
rd.driver.post=<drivername>[,<drivername>,...]
force loading kernel module <drivername> after all automatic loading modules have been loaded. This parameter can be specified multiple times.
rd.retry=<seconds>
specify how long dracut should retry the initqueue to configure devices. The default is 30 seconds. After 2/3 of the time, degraded raids are force started. If you have hardware, which takes a very long time to announce its drives, you might want to extend this value.
rd.timeout=<seconds>
specify how long dracut should wait for devices to appear. The default is 0, which means forever. Note that this timeout should be longer than rd.retry to allow for proper configuration.
rd.noverifyssl
accept self-signed certificates for ssl downloads.
rd.ctty=<terminal device>
specify the controlling terminal for the console. This is useful, if you have multiple "console=" arguments.

Debug

If you are dropped to an emergency shell, the file /run/initramfs/rdsosreport.txt is created, which can be saved to a (to be mounted by hand) partition (usually /boot) or a USB stick. Additional debugging info can be produced by adding rd.debug to the kernel command line. /run/initramfs/rdsosreport.txt contains all logs and the output of some tools. It should be attached to any report about dracut problems.
rd.info
print informational output though "quiet" is set
rd.shell
allow dropping to a shell, if root mounting fails
rd.debug
set -x for the dracut shell. If systemd is active in the initramfs, all output is logged to the systemd journal, which you can inspect with "journalctl -ab". If systemd is not active, the logs are written to dmesg and /run/initramfs/init.log. If "quiet" is set, it also logs to the console.
rd.memdebug=[0-4]
Print memory usage info at various points, set the verbose level from 0 to 4. 
Higher level means more debugging output:

    0 - no output
    1 - partial /proc/meminfo
    2 - /proc/meminfo
    3 - /proc/meminfo + /proc/slabinfo
    4 - /proc/meminfo + /proc/slabinfo + tracekomem
        NOTE: tracekomem is a shell script utilizing kernel trace to track
              the rough total memory consumption of kernel modules during
              loading. It may override other trace configurations.
rd.break
drop to a shell at the end
rd.break={cmdline|pre-udev|pre-trigger|initqueue|pre-mount|mount|pre-pivot|cleanup}
drop to a shell on defined breakpoint
rd.udev.info
set udev to loglevel info
rd.udev.debug
set udev to loglevel debug

I18N

rd.vconsole.keymap=<keymap base file name>
keyboard translation table loaded by loadkeys; taken from keymaps directory; will be written as KEYMAP to /etc/vconsole.conf in the initramfs.
Example. 
rd.vconsole.keymap=de-latin1-nodeadkeys
rd.vconsole.keymap.ext=<list of keymap base file names>
list of extra keymaps to bo loaded (sep. by space); will be written as EXT_KEYMAP to /etc/vconsole.conf in the initramfs
rd.vconsole.unicode
boolean, indicating UTF-8 mode; will be written as UNICODE to /etc/vconsole.conf in the initramfs
rd.vconsole.font=<font base file name>
console font; taken from consolefonts directory; will be written as FONT to /etc/vconsole.conf in the initramfs.
Example. 
rd.vconsole.font=LatArCyrHeb-16
rd.vconsole.font.map=<console map base file name>
see description of -m parameter in setfont manual; taken from consoletrans directory; will be written as FONT_MAP to /etc/vconsole.conf in the initramfs
rd.vconsole.font.unimap=<unicode table base file name>
see description of -u parameter in setfont manual; taken from unimaps directory; will be written as FONT_UNIMAP to /etc/vconsole.conf in the initramfs
rd.locale.LANG=<locale>
taken from the environment; if no UNICODE is defined we set its value in basis of LANG value (whether it ends with ".utf8" (or similar) or not); will be written as LANG to /etc/locale.conf in the initramfs.
Example. 
rd.locale.LANG=pl_PL.utf8
rd.locale.LC_ALL=<locale>
taken from the environment; will be written as LC_ALL to /etc/locale.conf in the initramfs

LVM

rd.lvm=0
disable LVM detection
rd.lvm.vg=<volume group name>
only activate the volume groups with the given name. rd.lvm.vg can be specified multiple times on the kernel command line.
rd.lvm.lv=<logical volume name>
only activate the logical volumes with the given name. rd.lvm.lv can be specified multiple times on the kernel command line.
rd.lvm.conf=0
remove any /etc/lvm/lvm.conf, which may exist in the initramfs

crypto LUKS

rd.luks=0
disable crypto LUKS detection
rd.luks.uuid=<luks uuid>
only activate the LUKS partitions with the given UUID. Any "luks-" of the LUKS UUID is removed before comparing to <luks uuid>. The comparisons also matches, if <luks uuid> is only the beginning of the LUKS UUID, so you don’t have to specify the full UUID. This parameter can be specified multiple times.
rd.luks.allow-discards=<luks uuid>
Allow using of discards (TRIM) requests for LUKS partitions with the given UUID. Any "luks-" of the LUKS UUID is removed before comparing to <luks uuid>. The comparisons also matches, if <luks uuid> is only the beginning of the LUKS UUID, so you don’t have to specify the full UUID. This parameter can be specified multiple times.
rd.luks.allow-discards
Allow using of discards (TRIM) requests on all LUKS partitions.
rd.luks.crypttab=0
do not check, if LUKS partition is in /etc/crypttab

crypto LUKS - key on removable device support

rd.luks.key=<keypath>:<keydev>:<luksdev>
keypath is a path to key file to look for. It’s REQUIRED. When keypath ends with .gpg it’s considered to be key encrypted symmetrically with GPG. You will be prompted for password on boot. GPG support comes with crypt-gpg module which needs to be added explicitly.
keydev is a device on which key file resides. It might be kernel name of devices (should start with "/dev/"), UUID (prefixed with "UUID=") or label (prefix with "LABEL="). You don’t have to specify full UUID. Just its beginning will suffice, even if its ambiguous. All matching devices will be probed. This parameter is recommended, but not required. If not present, all block devices will be probed, which may significantly increase boot time.
If luksdev is given, the specified key will only be applied for that LUKS device. Possible values are the same as for keydev. Unless you have several LUKS devices, you don’t have to specify this parameter. The simplest usage is:
Example. 
rd.luks.key=/foo/bar.key
As you see, you can skip colons in such a case.
 
Note
 
 
dracut pipes key to cryptsetup with -d - argument, therefore you need to pipe to crypsetup luksFormat with -d -, too!
Here follows example for key encrypted with GPG: 
gpg --quiet --decrypt rootkey.gpg | \
cryptsetup -d - -v --cipher serpent-cbc-essiv:sha256 \
--key-size 256 luksFormat /dev/sda3
If you use plain keys, just add path to -d option: 
cryptsetup -d rootkey.key -v --cipher serpent-cbc-essiv:sha256 \
 --key-size 256 luksFormat /dev/sda3

MD RAID

rd.md=0
disable MD RAID detection
rd.md.imsm=0
disable MD RAID for imsm/isw raids, use DM RAID instead
rd.md.ddf=0
disable MD RAID for SNIA ddf raids, use DM RAID instead
rd.md.conf=0
ignore mdadm.conf included in initramfs
rd.md.waitclean=1
wait for any resync, recovery, or reshape activity to finish before continuing
rd.md.uuid=<md raid uuid>
only activate the raid sets with the given UUID. This parameter can be specified multiple times.

DM RAID

rd.dm=0
disable DM RAID detection
rd.dm.uuid=<dm raid uuid>
only activate the raid sets with the given UUID. This parameter can be specified multiple times.

MULTIPATH

rd.multipath=0
disable multipath detection

FIPS

rd.fips
enable FIPS
boot=<boot device>
specify the device, where /boot is located.
Example. 
boot=/dev/sda1
boot=/dev/disk/by-path/pci-0000:00:1f.1-scsi-0:0:1:0-part1
boot=UUID=<uuid>
boot=LABEL=<label>
rd.fips.skipkernel
skip checksum check of the kernel image. Useful, if the kernel image is not in a separate boot partition.

Network

 
Important
 
 
It is recommended to either bind an interface to a MAC with the ifname argument, or to use the systemd-udevd predictable network interface names.
Predictable network interface device names based on:
•firmware/bios-provided index numbers for on-board devices
•firmware-provided pci-express hotplug slot index number
•physical/geographical location of the hardware
•the interface’s MAC address
See: http://www.freedesktop.org/wiki/Software/systemd/PredictableNetworkInterfaceNames
Two character prefixes based on the type of interface:
en
ethernet
wl
wlan
ww
wwan
Type of names:
o<index>
on-board device index number
s<slot>[f<function>][d<dev_id>]
hotplug slot index number
x<MAC>
MAC address
[P<domain>]p<bus>s<slot>[f<function>][d<dev_id>]
PCI geographical location
[P<domain>]p<bus>s<slot>[f<function>][u<port>][..][c<config>][i<interface>]
USB port number chain
All multi-function PCI devices will carry the [f<function>] number in the device name, including the function 0 device.
When using PCI geography, The PCI domain is only prepended when it is not 0.
For USB devices the full chain of port numbers of hubs is composed. If the name gets longer than the maximum number of 15 characters, the name is not exported. The usual USB configuration == 1 and interface == 0 values are suppressed.
PCI ethernet card with firmware index "1"
•eno1
PCI ethernet card in hotplug slot with firmware index number
•ens1
PCI ethernet multi-function card with 2 ports
•enp2s0f0
•enp2s0f1
PCI wlan card
•wlp3s0
USB built-in 3G modem
•wwp0s29u1u4i6
USB Android phone
•enp0s29u1u2
ip={dhcp|on|any|dhcp6|auto6}
dhcp|on|any
get ip from dhcp server from all interfaces. If root=dhcp, loop sequentially through all interfaces (eth0, eth1, ...) and use the first with a valid DHCP root-path.
auto6
IPv6 autoconfiguration
dhcp6
IPv6 DHCP
ip=<interface>:{dhcp|on|any|dhcp6|auto6}[:[<mtu>][:<macaddr>]]
This parameter can be specified multiple times.
dhcp|on|any|dhcp6
get ip from dhcp server on a specific interface
auto6
do IPv6 autoconfiguration
<macaddr>
optionally set <macaddr> on the <interface>. This cannot be used in conjunction with the ifname argument for the same <interface>.
ip=<client-IP>:[<peer>]:<gateway-IP>:<netmask>:<client_hostname>:<interface>:{none|off|dhcp|on|any|dhcp6|auto6|ibft}[:[<mtu>][:<macaddr>]]
explicit network configuration. If you want do define a IPv6 address, put it in brackets (e.g. [2001:DB8::1]). This parameter can be specified multiple times. <peer> is optional and is the address of the remote endpoint for pointopoint interfaces and it may be followed by a slash and a decimal number, encoding the network prefix length.
<macaddr>
optionally set <macaddr> on the <interface>. This cannot be used in conjunction with the ifname argument for the same <interface>.
ip=<client-IP>:[<peer>]:<gateway-IP>:<netmask>:<client_hostname>:<interface>:{none|off|dhcp|on|any|dhcp6|auto6|ibft}[:[<dns1>][:<dns2>]]
explicit network configuration. If you want do define a IPv6 address, put it in brackets (e.g. [2001:DB8::1]). This parameter can be specified multiple times. <peer> is optional and is the address of the remote endpoint for pointopoint interfaces and it may be followed by a slash and a decimal number, encoding the network prefix length.
ifname=<interface>:<MAC>
Assign network device name <interface> (ie "bootnet") to the NIC with MAC <MAC>.
 
Warning
 
 
Do not use the default kernel naming scheme for the interface name, as it can conflict with the kernel names. So, don’t use "eth[0-9]+" for the interface name. Better name it "bootnet" or "bluesocket".
rd.route=<net>/<netmask>:<gateway>[:<interface>]
Add a static route with route options, which are separated by a colon. IPv6 addresses have to be put in brackets.
Example. 
    rd.route=192.168.200.0/24:192.168.100.222:ens10
    rd.route=192.168.200.0/24:192.168.100.222
    rd.route=192.168.200.0/24::ens10
    rd.route=[2001:DB8:3::/8]:[2001:DB8:2::1]:ens10
bootdev=<interface>
specify network interface to use routing and netroot information from. Required if multiple ip= lines are used.
BOOTIF=<MAC>
specify network interface to use routing and netroot information from.
rd.bootif=0
Disable BOOTIF parsing, which is provided by PXE
nameserver=<IP> [nameserver=<IP> ...]
specify nameserver(s) to use
rd.peerdns=0
Disable DNS setting of DHCP parameters.
biosdevname=0
boolean, turn off biosdevname network interface renaming
rd.neednet=1
boolean, bring up network even without netroot set
vlan=<vlanname>:<phydevice>
Setup vlan device named <vlanname> on <phydeivce>. We support the four styles of vlan names: VLAN_PLUS_VID (vlan0005), VLAN_PLUS_VID_NO_PAD (vlan5), DEV_PLUS_VID (eth0.0005), DEV_PLUS_VID_NO_PAD (eth0.5)
bond=<bondname>[:<bondslaves>:[:<options>]]
Setup bonding device <bondname> on top of <bondslaves>. <bondslaves> is a comma-separated list of physical (ethernet) interfaces. <options> is a comma-separated list on bonding options (modinfo bonding for details) in format compatible with initscripts. If <options> includes multi-valued arp_ip_target option, then its values should be separated by semicolon. Bond without parameters assumes bond=bond0:eth0,eth1:mode=balance-rr
team=<teammaster>:<teamslaves>
Setup team device <teammaster> on top of <teamslaves>. <teamslaves> is a comma-separated list of physical (ethernet) interfaces.
bridge=<bridgename>:<ethnames>
Setup bridge <bridgename> with <ethnames>. <ethnames> is a comma-separated list of physical (ethernet) interfaces. Bridge without parameters assumes bridge=br0:eth0

NFS

root=[<server-ip>:]<root-dir>[:<nfs-options>]
mount nfs share from <server-ip>:/<root-dir>, if no server-ip is given, use dhcp next_server. If server-ip is an IPv6 address it has to be put in brackets, e.g. [2001:DB8::1]. NFS options can be appended with the prefix ":" or "," and are seperated by ",".
root=nfs:[<server-ip>:]<root-dir>[:<nfs-options>], root=nfs4:[<server-ip>:]<root-dir>[:<nfs-options>], root={dhcp|dhcp6}
root=dhcp alone directs initrd to look at the DHCP root-path where NFS options can be specified.
Example. 
    root-path=<server-ip>:<root-dir>[,<nfs-options>]
    root-path=nfs:<server-ip>:<root-dir>[,<nfs-options>]
    root-path=nfs4:<server-ip>:<root-dir>[,<nfs-options>]
root=/dev/nfs nfsroot=[<server-ip>:]<root-dir>[: <nfs-options>]
Deprecated! kernel Documentation_/filesystems/nfsroot.txt_ defines this method. This is supported by dracut, but not recommended.
rd.nfs.domain=<NFSv4 domain name>
Set the NFSv4 domain name. Will override the settings in /etc/idmap.conf.
rd.net.dhcp.retry=<cnt>
If this option is set, dracut will try to connect via dhcp <cnt> times before failing. Default is 1.
rd.net.timeout.dhcp=<arg>
If this option is set, dhclient is called with "-timeout <arg>".
rd.net.timeout.iflink=<seconds>
Wait <seconds> until link shows up. Default is 60 seconds.
rd.net.timeout.ifup=<seconds>
Wait <seconds> until link has state "UP". Default is 20 seconds.
rd.net.timeout.route=<seconds>
Wait <seconds> until route shows up. Default is 20 seconds.
rd.net.timeout.ipv6dad=<seconds>
Wait <seconds> until IPv6 DAD is finished. Default is 50 seconds.
rd.net.timeout.ipv6auto=<seconds>
Wait <seconds> until IPv6 automatic addresses are assigned. Default is 40 seconds.
rd.net.timeout.carrier=<seconds>
Wait <seconds> until carrier is recognized. Default is 5 seconds.

CIFS

root=cifs://[<username>[:<password>]@]<server-ip>:<root-dir>
mount cifs share from <server-ip>:/<root-dir>, if no server-ip is given, use dhcp next_server. if server-ip is an IPv6 address it has to be put in brackets, e.g. [2001:DB8::1]. If a username or password are not specified as part of the root, then they must be passed on the command line through cifsuser/cifspass.
 
Warning
 
 
Passwords specified on the kernel command line are visible for all users via the file /proc/cmdline and via dmesg or can be sniffed on the network, when using DHCP with DHCP root-path.
cifsuser=<username>
Set the cifs username, if not specified as part of the root.
cifspass=<password>
Set the cifs password, if not specified as part of the root.
 
Warning
 
 
Passwords specified on the kernel command line are visible for all users via the file /proc/cmdline and via dmesg or can be sniffed on the network, when using DHCP with DHCP root-path.

iSCSI

root=iscsi:[<username>:<password>[:<reverse>:<password>]@][<servername>]:[<protocol>]:[<port>][:[<iscsi_iface_name>]:[<netdev_name>]]:[<LUN>]:<targetname>
protocol defaults to "6", LUN defaults to "0". If the "servername" field is provided by BOOTP or DHCP, then that field is used in conjunction with other associated fields to contact the boot server in the Boot stage. However, if the "servername" field is not provided, then the "targetname" field is then used in the Discovery Service stage in conjunction with other associated fields. See rfc4173[1].
 
Warning
 
 
Passwords specified on the kernel command line are visible for all users via the file /proc/cmdline and via dmesg or can be sniffed on the network, when using DHCP with DHCP root-path.
Example. 
root=iscsi:192.168.50.1::::iqn.2009-06.dracut:target0
If servername is an IPv6 address, it has to be put in brackets:
Example. 
root=iscsi:[2001:DB8::1]::::iqn.2009-06.dracut:target0
root=??? netroot=iscsi:[<username>:<password>[: <reverse>: <password>]@][<servername>]:[ <protocol>]:[ <port>][:[<iscsi_iface_name>]:[ <netdev_name>]]:[ <LUN>]:<targetname> ...
multiple netroot options allow setting up multiple iscsi disks:
Example. 
root=UUID=12424547
netroot=iscsi:192.168.50.1::::iqn.2009-06.dracut:target0
netroot=iscsi:192.168.50.1::::iqn.2009-06.dracut:target1
If servername is an IPv6 address, it has to be put in brackets:
Example. 
netroot=iscsi:[2001:DB8::1]::::iqn.2009-06.dracut:target0
 
Warning
 
 
Passwords specified on the kernel command line are visible for all users via the file /proc/cmdline and via dmesg or can be sniffed on the network, when using DHCP with DHCP root-path. You may want to use rd.iscsi.firmware.
root=??? rd.iscsi.initiator=<initiator> rd.iscsi.target.name= <target name> rd.iscsi.target.ip= <target ip> rd.iscsi.target.port= <target port> rd.iscsi.target.group= <target group> rd.iscsi.username= <username> rd.iscsi.password=<password> rd.iscsi.in.username=<in username> rd.iscsi.in.password= <in password>
manually specify all iscsistart parameter (see iscsistart --help)
 
Warning
 
 
Passwords specified on the kernel command line are visible for all users via the file /proc/cmdline and via dmesg or can be sniffed on the network, when using DHCP with DHCP root-path. You may want to use rd.iscsi.firmware.
root=??? netroot=iscsi rd.iscsi.firmware=1
will read the iscsi parameter from the BIOS firmware
rd.iscsi.param=<param>
<param> will be passed as "--param <param>" to iscsistart. This parameter can be specified multiple times.
Example. 
"netroot=iscsi rd.iscsi.firmware=1 rd.iscsi.param=node.session.timeo.replacement_timeout=30"
will result in 
iscsistart -b --param node.session.timeo.replacement_timeout=30
rd.iscsi.ibft rd.iscsi.ibft=1: Turn on iBFT autoconfiguration for the interfaces
rd.iscsi.waitnet=0: Turn off waiting for all interfaces to be up before trying to login to the iSCSI targets.
rd.iscsi.testroute=0: Turn off checking, if the route to the iSCSI target IP is possible before trying to login.

FCoE

fcoe=<edd|interface|MAC>:{dcb|nodcb}
Try to connect to a FCoE SAN through the NIC specified by <interface> or <MAC> or EDD settings. For the second argument, currently only nodcb is supported. This parameter can be specified multiple times.
 
Note
 
 
letters in the MAC-address must be lowercase!

NBD

root=??? netroot=nbd:<server>:<port/exportname>[: <fstype>[: <mountopts>[:<nbdopts>]]]
mount nbd share from <server>.
NOTE: If "exportname" instead of "port" is given the standard port is used. Newer versions of nbd are only supported with "exportname".
root=dhcp with dhcp root-path=nbd:<server>:<port/exportname>[: <fstype>[:<mountopts>[:<nbdopts>]]]
root=dhcp alone directs initrd to look at the DHCP root-path where NBD options can be specified. This syntax is only usable in cases where you are directly mounting the volume as the rootfs.
NOTE: If "exportname" instead of "port" is given the standard port is used. Newer versions of nbd are only supported with "exportname".

DASD

rd.dasd=....
same syntax as the kernel module parameter (s390 only)

ZFCP

rd.zfcp=<zfcp adaptor device bus ID>,<WWPN>,<FCPLUN>
rd.zfcp can be specified multiple times on the kernel command line.
Example. 
rd.zfcp=0.0.4000,0x5005076300C213e9,0x5022000000000000
rd.zfcp.conf=0
ignore zfcp.conf included in the initramfs

ZNET

rd.znet=<nettype>,<subchannels>,<options>
rd.znet can be specified multiple times on the kernel command line.
Example. 
rd.znet=qeth,0.0.0600,0.0.0601,0.0.0602,layer2=1,portname=foo
rd.znet=ctc,0.0.0600,0.0.0601,protocol=bar

Booting live images

Dracut offers multiple options for live booted images:
SquashFS with read-only filesystem image
The system will boot with a read only filesystem from the SquashFS and apply a writable device-mapper snapshot over the read only filesystem. Using this method ensures a relatively fast boot and lower RAM usage. Users must be careful to avoid writing too many blocks to the snapshot volume. Once the blocks of the snapshot overlay are exhausted, the root filesystem becomes unusable and requires a reboot. A persistent overlay is marked Invalid, and requires a difficult recovery. Non-persistent overlays default to 512 MiB in RAM, but the size can be adjusted with the rd.live.overlay.size= kernel command line option.
The filesystem structure is expected to be: 
squashfs.img          |  Squashfs from LiveCD .iso downloaded via network
   !(mount)
   /LiveOS
       |- ext3fs.img  |  Filesystem image to mount read-only
            !(mount)
            /bin      |  Live filesystem
            /boot     |
            /dev      |
            ...       |
Dracut uses this method of live booting by default. No additional command line options are required other than root=live:<URL> to specify the location of your squashed filesystem.
•The compressed SquashFS image can be copied during boot to RAM at /run/initramfs/squashed.img by using the rd.live.ram=1 option.
•A device with a persistent overlay can be booted read only by using the rd.live.overlay.readonly option on the kernel command line. This will cause a temporary, writable overlay to be stacked over a read-only snapshot of the root filesystem.
Uncompressed live filesystem image
When the live system was installed with the --skipcompress option of the livecd-iso-to-disk installation script for Live USB devices, the root filesystem image, ext3fs.img, is expanded on installation and no SquashFS is involved during boot.
•If rd.live.ram=1 is used in this situation, the full, uncompressed root filesystem is copied during boot to /run/initramfs/rootfs.img in the /run tmpfs.
•If rd.live.overlay=none is provided as a kernel command line option, a writable, linear device-mapper target is created on boot with no overlay.
writable filesystem image
The system will retrieve a compressed filesystem image, extract it to /run/initramfs/fsimg/rootfs.img, connect it to a loop device, create a writable, linear device-mapper target at /dev/mapper/live-rw, and mount that as a writable volume at /. More RAM is required during boot but the live filesystem is easier to manage if it becomes full. Users can make a filesystem image of any size and that size will be maintained when the system boots. There is no persistence of root filesystem changes between boots with this option.
The filesystem structure is expected to be: 
rootfs.tgz            |  Compressed tarball containing fileystem image
   !(unpack)
   /rootfs.img        |  Filesystem image at /run/initramfs/fsimg/
      !(mount)
      /bin            |  Live filesystem
      /boot           |
      /dev            |
      ...             |
To use this boot option, ensure that rd.writable.fsimg=1 is in your kernel command line and add the root=live:<URL> to specify the location of your compressed filesystem image tarball or SquashFS image.
rd.writable.fsimg=1
Enables writable filesystem support. The system will boot with a fully writable (but non-persistent) filesystem without snapshots (see notes above about available live boot options). You can use the rootflags option to set mount options for the live filesystem as well (see documentation about rootflags in the Standard section above). This implies that the whole image is copied to RAM before the boot continues.
 
Note
 
 
There must be enough free RAM available to hold the complete image.
This method is very suitable for diskless boots.
root=live:<url>
Boots a live image retrieved from <url>. Valid handlers: http, https, ftp, torrent, tftp.
Example. 
root=live:http://example.com/liveboot.img
root=live:ftp://ftp.example.com/liveboot.img
root=live:torrent://example.com/liveboot.img.torrent
rd.live.debug=1
Enables debug output from the live boot process.
rd.live.dir=<path>
Specifies the directory within the squashfs where the ext3fs.img or rootfs.img can be found. By default, this is LiveOS.
rd.live.squashimg=<filename of SquashFS image>
Specifies the filename for a SquashFS image of the root filesystem. By default, this is squashfs.img.
rd.live.ram=1
Copy the complete image to RAM and use this for booting. This is useful when the image resides on, i.e., a DVD which needs to be ejected later on.
rd.live.overlay=<devspec>:(<pathspec>|auto)|none
Allow the usage of a permanent overlay.
•<devspec> specifies the path to a device with a mountable filesystem.
•<pathspec> is the path to a file within that filesystem, which shall be used to persist the changes made to the device specified by the root=live:<url> option.
•none specifies no overlay when an uncompressed live root filesystem is available.
Example. 
rd.live.overlay=/dev/sdb1:persistent-overlay.img
rd.live.overlay.size=<size_MiB>
Specifies a non-persistent overlay size in MiB. The default is 512.
rd.live.overlay.readonly=1
Specifies a non-persistent, writable snapshot overlay to be stacked over a read-only snapshot of the root filesystem, /dev/mapper/live-ro.
rd.live.overlay.reset=1
Specifies that a persistent overlay should be reset on boot. All root filesystem changes are vacated by this action.
rd.live.overlay.thin=1
Enables the usage of thin snapshots instead of classic dm snapshots. The advantage of thin snapshots is that they support discards, and will free blocks that are not claimed by the filesystem. In this use case, this means that memory is given back to the kernel when the filesystem does not claim it anymore.

Plymouth Boot Splash

plymouth.enable=0
disable the plymouth bootsplash completely.
rd.plymouth=0
disable the plymouth bootsplash only for the initramfs.

Kernel keys

masterkey=<kernel master key path name>
Set the path name of the kernel master key.
Example. 
masterkey=/etc/keys/kmk-trusted.blob
masterkeytype=<kernel master key type>
Set the type of the kernel master key.
Example. 
masterkeytype=trusted
evmkey=<EVM key path name>
Set the path name of the EVM key.
Example. 
evmkey=/etc/keys/evm-trusted.blob
ecryptfskey=<eCryptfs key path name>
Set the path name of the eCryptfs key.
Example. 
ecryptfskey=/etc/keys/ecryptfs-trusted.blob

Deprecated, renamed Options

Here is a list of options, which were used in dracut prior to version 008, and their new replacement.
rdbreak
rd.break
rd_CCW
rd.ccw
rd_DASD_MOD
rd.dasd
rd_DASD
rd.dasd
rdinitdebug rdnetdebug
rd.debug
rd_NO_DM
rd.dm=0
rd_DM_UUID
rd.dm.uuid
rdblacklist
rd.driver.blacklist
rdinsmodpost
rd.driver.post
rdloaddriver
rd.driver.pre
rd_NO_FSTAB
rd.fstab=0
rdinfo
rd.info
check
rd.live.check
rdlivedebug
rd.live.debug
live_dir
rd.live.dir
liveimg
rd.live.image
overlay
rd.live.overlay
readonly_overlay
rd.live.overlay.readonly
reset_overlay
rd.live.overlay.reset
live_ram
rd.live.ram
rd_NO_CRYPTTAB
rd.luks.crypttab=0
rd_LUKS_KEYDEV_UUID
rd.luks.keydev.uuid
rd_LUKS_KEYPATH
rd.luks.keypath
rd_NO_LUKS
rd.luks=0
rd_LUKS_UUID
rd.luks.uuid
rd_NO_LVMCONF
rd.lvm.conf
rd_LVM_LV
rd.lvm.lv
rd_NO_LVM
rd.lvm=0
rd_LVM_SNAPSHOT
rd.lvm.snapshot
rd_LVM_SNAPSIZE
rd.lvm.snapsize
rd_LVM_VG
rd.lvm.vg
rd_NO_MDADMCONF
rd.md.conf=0
rd_NO_MDIMSM
rd.md.imsm=0
rd_NO_MD
rd.md=0
rd_MD_UUID
rd.md.uuid
rd_NO_MULTIPATH: rd.multipath=0
rd_NFS_DOMAIN
rd.nfs.domain
iscsi_initiator
rd.iscsi.initiator
iscsi_target_name
rd.iscsi.target.name
iscsi_target_ip
rd.iscsi.target.ip
iscsi_target_port
rd.iscsi.target.port
iscsi_target_group
rd.iscsi.target.group
iscsi_username
rd.iscsi.username
iscsi_password
rd.iscsi.password
iscsi_in_username
rd.iscsi.in.username
iscsi_in_password
rd.iscsi.in.password
iscsi_firmware
rd.iscsi.firmware=0
rd_NO_PLYMOUTH
rd.plymouth=0
rd_retry
rd.retry
rdshell
rd.shell
rd_NO_SPLASH
rd.splash
rdudevdebug
rd.udev.debug
rdudevinfo
rd.udev.info
rd_NO_ZFCPCONF
rd.zfcp.conf=0
rd_ZFCP
rd.zfcp
rd_ZNET
rd.znet
KEYMAP
vconsole.keymap
KEYTABLE
vconsole.keymap
SYSFONT
vconsole.font
CONTRANS
vconsole.font.map
UNIMAP
vconsole.font.unimap
UNICODE
vconsole.unicode
EXT_KEYMAP
vconsole.keymap.ext

Configuration in the Initramfs

/etc/conf.d/
Any files found in /etc/conf.d/ will be sourced in the initramfs to set initial values. Command line options will override these values set in the configuration files.
/etc/cmdline
Can contain additional command line options. Deprecated, better use /etc/cmdline.d/*.conf.
/etc/cmdline.d/*.conf
Can contain additional command line options.

AUTHOR

Harald Hoyer

SEE ALSO

dracut(8) dracut.conf(5)

NOTES

1.
rfc4173
http://tools.ietf.org/html/rfc4173#section-5
01/24/2017 dracut