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Configuration - The silo.conf file

silo.conf is a configuration file for SILO which is read during booting by the second stage boot loader.

It provides instructions for SILO, including which kernels or other operating systems to load and what options to pass to them. Unlike the LILO bootloader on the Intel platform, SILO reads and parses the configuration file at boot time. This means that any changes can be made to the configuration file and if the system is shut down properly or the config file makes it to the disk, SILO will use it on next bootup.

SILO is able to boot kernels even without this configuration file or if this file is crippled or contains syntax errors, but the user has to enter full PROM names and full path of the images to load and all options required for them manually.


The configuration file consists of comments and variable assignments:

Comments start with a "#" character, and continue to the end of the line.

Flag variables consist of a single keyword and are followed by whitespace or the end of the file.

String variables consist of the variable name, optional architecture scope, optional whitespace, an "=" character, optional whitespace, the value and required whitespace, or the end of the file.

Architecture scope is allowed only for variable names image and other. It must directly follow a variable name without any whitespace and consists of the "[" character, a subset of case-insensitive tokens sun4, sun4c, sun4d, sun4e, sun4m and/or sun4u, separated by the "," character (no whitespace is allowed between the comma character), and ends with the "]" character.

An example:


This will declare image with the specified file name only on sun4c, sun4d or sun4m.

Device names depend on the PROM version of the machine used during bootup.

For older PROM's (v0), the device name syntax is:


  • <xx> is one of sd, st, xd, xy, fd, le or ie
  • <m> is the controller number as decimal number
  • <n> is the device id as decimal number
  • <o> is the zero based partition number.


  • sd(0,3,0)
  • fd(0,0,0)

For newer PROMs (v2) and all PROM's in the 64bit SPARC machines the syntax is:


where the fully qualified PROM device name starts with a "/" character for the root of the PROM device tree, and contains all the nodes from the root in the tree up to the disk device node. If some node contains more than one child node with the same name and the path has to go through such node, an "@" character followed by the hexadecimal address pair is desirable to resolve the ambiguity. Optional partition number is a ":" character followed by a letter a (for first partition) through h (for last, 8th, partition). v2 PROM device names might look like:


v2 PROM device names if specified as part of the file name (see File Names below) should be followed by the ";" character to separate the device name from the optional Linux partition number (note that there can be two partition numbers specified, the latter takes precedence) and from the required pathname. Variable names are case insensitive, values of string variables are case sensitive.

Variable names are case insensitive, values of string variables are case sensitive.

Blanks and equal signs may only be part of a variable name or a value if they are escaped by a backslash or if the value is embedded in double quotes. An equal sign may not be the only character in a name or value.

An escaped tab is converted to an escaped blank. An escaped newline is removed from the input stream. An escaped backslash (i.e. two backslashes) is converted to a backslash. Inside quoted strings, only double quotes, backslashes and newlines can be escaped.

File names

Some string variables are expecting file names. A file name format in SILO is:




The first form refers to a file or directory on some supported filesystem (currently ext2fs, ext3fs, ufs, romfs or iso9660), the latter refers to a range of 512B blocks on a device.

For a device block range, either <devicename>, or <part>, or [<m>-<n>] must be given. If the last part is not given, it defaults to [1-16] (ie. 7680 bytes from offset 512 in the device, the normal SPARC boot block location).

Optional <devicename> is the PROM name of the device where the file or range is located on. See Device Names above for its syntax. For v2 and P1275 PROM's, the device name must be immediately followed by the ";" character. The default is the boot device SILO was booted from.

Optional <part> is the 1 based partition number on the device. First partition is 1 (e.g. on /dev/sda in Linux is this /dev/sda1). The default is the default partition (the value of the partition variable in the config file).

<absolute_pathname> must start with a "/" character and is the pathname from the root of the file system on that device (unless it is the root filesystem; This is different to the pathname you will see in Linux).

<m> is the first block number (in blocksize 512 bytes) of the range to be loaded and <n> is the last block number plus one.

The filename can also include two special tokens: cat and ls. Both have to be followed by whitespace and some filename (directory name in the ls case). ls can have options between the ls token and the directory name. cat causes the filename to be loaded and printed to the screen instead of being executed, ls causes a directory listing to be generated to the screen. ls uses syntax:

ls [-[l][t][r]] <dirname>

Option l generates long listing instead of listing only filenames, option t sorts by mtime instead of name and r reverses the sorting. As both cat and ls require white space between the token and the filename, you should surround them with double quotes:

image="cat /etc/passwd"
image="ls /lib/modules/"
image="ls -lt /lib/"

Global Options

/etc/silo.conf begins with a possibly empty global options section. This section contains all variable assignments up to the first encountered image or other setting.

The following global options are recognized:

  • default=name
    uses the specified image as the default boot image. If default is omitted, the image appearing first in the configuration file is used.

  • message=message_filename
    specifies a file containing a message that is displayed before the boot prompt.

  • partition-boot
    This flag causes the same functionality as the -t command line option for the silo program, which stores the boot block into the same partition as the second stage loader.

  • password=password
    This protects booting by a password. The password is given in cleartext in the configuration file. Because of that, the configuration file should be only readable by the super user and the password should differ if possible from other passwords on the system.

  • restricted
    A password is only required to boot the image specified in /etc/silo.conf if parameters are specified on the command line or if the image is not specified in the configuration file at all (ie. arbitrary file load).

  • secondary=filename
    Forces the second stage boot loader to be other than /boot/second.b. Same as the -b command line option. This can be overriden by the command line option.

  • timeout=tsecs
    Sets a timeout (in tenths of a second) for keyboard input. If no key is pressed for the specified time, the first image is automatically booted.

In addition to these global options, the following per-image options can be specified in the global section:

  • append
  • device
  • fill-reboot-cmd
  • image
  • initrd-prompt
  • initrd-size
  • initrd
  • other
  • partition
  • pause-after
  • pause-message
  • ramdisk
  • read-only
  • read-write
  • root

They are used as defaults if they aren't specified in the configuration sections of the respective kernel images and will be used also for the arbitrary images specified on the input line and not mentioned in the configuration file (unless overridden by input line options).

Per-Image Options

A per-image section starts with either a line


for booting from files, or


for booting from device ranges.

The image or other tokens can be directly followed by an architecture scope. In that case, the image declaration will be only available if the architecture of the booting machine is listed in the scope. If it is not listed, the whole image or other line and the following related section will be treated as not being present in the configuration file.

partition_name in the other setting is a normal filename, as described in the section File Names, but without the <absolute_pathname> part. So it is either a single partition number (starting with 1), or a device name (for v2 and P1275 PROMs followed by a ";" character) followed by a partition number.


# Example with device for v0 PROM:

# Example with device for v2 or P1275 PROM:

From the image or other line on until next image or other line are variable assignments and flags for this image's section. The following options and flags are recognized:

  • label=name
    The boot loader uses the main file name (without it's path of each image specification to identify that image. A different name can be used by setting the variable label.

  • alias=name
    A second name for the same entry can be used by specifying an alias.

  • partition=part_no
    Specifies the default partition number (a digit between 1 and 8, /dev/sda1 is part_no 1) to be used if some filename does not explicitely specify a partition number.

  • device=string
    Specifies the default device name to be used if some filename does not specify a partition number explicitely. This defaults to the device SILO has been booted from if you don't specify device in either the global section or per-image section of the configuration file.

  • append=string
    Appends the options specified to the parameter line passed to the kernel. This is typically used to specify parameters of hardware that can't be entirely auto-detected or for which probing may be dangerous, for example:


  • literal=string
    Like append, but removes all other options such as setting the root device. Because vital options can be removed unintentionally with literal, this option cannot be set in the global options section.

  • ramdisk=size
    This specifies the size of the optional RAM disk. A value of zero indicates that no RAM disk should be created. If this variable is omitted, the RAM disk size configured into the boot image is used.

  • read-only
    This specifies that the root file system should be mounted read-only. Typically, the system startup procedure re-mounts the root file system read-write later (e.g. after fsck'ing it).

  • read-write - This specifies that the root file system should be mounted read-write.

  • root=root-device
    This specifies the device that should be mounted as root file system.

  • proll=filename
    This should be used only for JavaStation flash installations. It specifies a file that will be loaded at boot time and executed in order to emulate old Sun v2 PROM on top of IEEE P1275 called PROLL.

  • initrd=filename
    This specifies the file that will be loaded at boot time as the initial RAM disk. Example:


    SILO will not decompress the initial ramdisk, the Linux kernel will do that. If the initial ramdisk does not fit on one media (usually floppy), you can split it into several pieces and separate the file names in the list by "|" characters. In this case, you have to provide a non-zero initrd-size and, if the images reside on different media, initrd-prompt as well. Example (on the first floppy is initrd1.img, on the second one initrd2.img. Both are always in the root directory, and the sum of the sizes of both images is 1700000 Bytes:

  • initrd-size=size
    When more than one initial ramdisk part is specified in the initrd setting, this option is required to be the sum of both images mentioned on that line, in Bytes. This is required so that SILO can reserve space for the image, eventhough size of some parts has not been determined yet.

  • initrd-prompt
    If more than one initial ramdisk part is specified, wait for the user pressing a key between loading the different images, so that the user can exchange media. This flag is needed if some initrd parts reside on the same device, but different removable media. On the other side, if you e.g. load one part from a floppy and the second part from a hard disk, this option is not needed (the question is who'd write something like that into silo.conf).

  • bootblock=filename
    For the other image section, if booting from some partition is desired but the boot block on that partition has been saved off to some file and the boot block in that partition contains something else, such as a SILO boot block), this option specifies the name of the file where the bootblock has been saved to. SILO loads the image from that file, pretends as if it had loaded it from the specified partition, and executes it.
    This is for instance useful if you have Sun Solaris installed on the first partition of the disk and want to install SILO into the master boot record (boot block of the first partition). In that case you should first save the Solaris boot block into a file. Example:

    dd if=/dev/sda of=/boot/old.b bs=512 count=15 skip=1

    After this, install SILO and add the following to /etc/silo.conf:


    Note that this does not work correctly on UltraSPARC based machines at this moment.

  • fill-reboot-cmd
    This flag should be used only with Linux kernels. If specified for an image, it causes the file /proc/sys/kernel/reboot-cmd to be initialized by the device SILO booted from, image name of the kernel being loaded and all arguments to it specified either in the configuration file, or in the input line. This means that if the user does not modify that file and reboots the system, it should load the same kernel as last time and pass it the same arguments.

  • pause-after
    If this flag is specified, SILO will stop after loading the kernel (and initial ramdisks if specified) and will ask the user to press a key before continuing.

  • pause-message=string
    If pause-after is specified, this variable specifies the string to print to the user when asking him or her to press a key. The default pause-message is:

    Press ENTER to continue.

  • single-key
    This enables booting the image by hitting a single key when the cursor is located at the first character in the input line,without the need to press <ENTER> afterwards. single-key requires that either the label of the image or it's alias (or both) is a single character. If you need to specify parameters for such an image, or if you want to boot some other image which happens to start with the same character you will need to start the input line with at least one space which will be removed before processing, but will disable this single-key feature.

  • solaris
    This flag tells SILO that the image to be loaded is a Solaris kernel. This has been implemented so that users don't have to save the old Solaris boot block, or in case it has been lost. SILO then tries to use the Solaris ufsboot second stage loader to load the specified kernel image. The usual kernel name is /kernel/unix. SILO automatically prepends the path /platform/<platform> when needed. SILO also recognises the special path name /kernel/unix with any device and/or partition, and assumes the solaris option by default.

    Note that this method of loading Solaris might not work for you, as it is quite error prone due to different versions of Solaris and PROM's.

    Updates for the OpenBoot PROM (OBP) should be installed by loading the flash update image from a TFTP server, similar to installing an operating system from the network.

    The recommended solution for sharing Solaris and Linux on the same disk is to install SILO into the Linux partition boot block as opposed to the master boot block, and add PROM aliases to boot from the different devices. For instance, if Solaris has been installed on Linux device name /dev/sda1, and Linux on /dev/sda4, then silo.conf could look like this:


    (See also the Examples section). After this, SILO needs to be invoked with the -t option, to store the boot block into the same partition as the second stage loader.

    An example to set the fourth partition to be the default boot device (from the EPROM):

    setenv boot-device /sbus/espdma/esp/sd@0,0:d

    nvalias linux /sbus/espdma/esp/sd@0,0:d
    nvalias solaris /sbus/espdma/esp/sd@0,0:a

    so that one could already boot from the PROM either with "boot linux", or with "boot solaris". This also ensures that you can re-install an operating system without bothering another installation.

SILO configuration examples

A basic silo.conf file which can load the images with label "linux", and "old" from the fifth partition on the first found SCSI hard disk drive:

# basic silo.conf example

A silo.conf that can boot the images with label "linux" and "solaris". "linux" here is on the first partition of the third SCSI hard disk drive, and "solaris" on the first partition of the SCSI disk drive with SCSI ID 3.

# silo.conf example for dual boot linux and solaris
Last update: Friday January 05, 2007 - pkrul at