boot, bootrc – connect to the root file server




Boot is the first program run after a kernel has been loaded. It mounts bootfs.paq, a compressed filesystem contained in root(3), sets up the rc(1) environment and executes bootrc passing on its arguments. Bootrc connects to the file server that will serve the root, performs any authentication needed to connect to that server, and executes the init(8) program. Both boot and bootrc are started by the kernel, never run directly by the user. See booting(8) for information about the process of loading the kernel (and boot) into memory.

Once loaded, the kernel initializes its data structures and devices. It sets the two environment variables /env/cputype and /env/terminal to describe the processor. It then binds a place-holder file server, root(3), onto / and crafts an initial process whose sole function is to exec(2) /boot/boot, a binary which is compiled into root(3).

The command line passed depends on the information passed from boot ROM to kernel. Machines that boot directly from ROM (that is, most machines other than PCs) pass the boot line given to the ROM directly to boot.

On the PC, each line in the file plan9.ini(8) of the form name=value is passed to the boot program as an environment variable with the same name and value.

After boot passed its execution to bootrc, it must determine the fileserver to use and a method with which to connect to it. Typically device will name a local disk partition or ethernet interface. The complete list of methods is given below.

Bootrc must also set a user name to be used as the owner of devices and all console processes and an encryption key to be used when challenged. Bootrc will prompt for these.

Method and device are prompted for first. The prompt lists all valid methods, with the default in brackets, for example:


bootargs is (tcp, il, local!device) [local!/dev/sdC0/fscache]

A newline picks the default. Entering !rc breaks into the rc(1) shell. Other possible responses are method, method!device or method!device args.

The other interactions depend on whether the system is a terminal or a CPU server.


A terminal must have a username set. If not specified with the user= plan9.ini(8) parameter, boot will prompt for one on the console:



The user will also be prompted for a password to be used as an encryption key on each attach(5):



Once connected, boot mounts the root file system before / and makes the connection available as #s/boot for subsequent processes to mount (see bind(2)). Boot completes by exec(2)’ing /$cputype/init -t. If the -m option is given it is also passed as an option to init. If the environment variable init is set (via plan9.ini(8)), it is used as an argument to exec(2) instead.

If the bootfs.paq has been built with the cache file system, cfs(4), the local disk partition /dev/sdXX/cache (where XX is a unit specifier) exists, and the root file system is from a remote server, then bootrc will insert a user level cache process between the remote server and the local namespace that caches all remote accesses on the local partition.

CPU Servers

The user owning devices and console processes on CPU servers and that user’s domain and encryption key are read from NVRAM on all machines except PCs. PCs keep the information in the disk partition /dev/sdXX/nvram.


authid: bootes

The key is used for mutual authentication of the server and its clients. The domain and id identify the owner of the key.

Once connected, boot behaves as on the terminal except for exec(2)’ing /$cputype/init -c.

Booting Methods

The methods available to any system depend on what was compiled into the kernel. The complete list of booting methods are listed below.


connect via Ethernet using the TCP protocol. The device and args are passed to ipconfig(8) when configuring the IP stack. The plan9.ini(8) variables fs and auth override the file server and authentication servers obtained (if any) from DHCP during ipconfig(8).  


the same as tcp but uses the IL protocol to connect to the fileserver.  


connect to the local file system. The device is a disk partition file holding a file system. Bootrc inspects the disk partition with fstype(1) to determine the file system type and starts the appropriate server with args.  


On PCs, the default arguments to boot are constructed using the bootargs variable in plan9.ini(8).

Start cwfs(4) in config mode:


bootargs=local!/dev/sdC0/fscache -c

Use an IP stack on an alternate ethernet interface with a static address and fixed file server and authentication server addresses.


bootargs=tcp!-g ether /net/ether1 \

(The bootargs line is split only for presentation; it is one line in the file.)






root(3), dhcpd(8), init(8)


The use of bootargs in general is odd.


Boot first appeared in Plan 9 from Bell Labs. It was rewritten in rc for 9front (April, 2011).