Boot is the first program run after a kernel has been loaded.
It connects to the file server that will serve the root, performs
any authentication needed to connect to that server, and exec(2)'s
the init(8) program. It is started by the kernel, never run directly
by the user. See booting(8) for information about the process
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 DOS file plan9.ini of the form name=value
is passed to the boot program as an environment variable with
the same name and value. The command line is
(The first argument is ignored by boot.) Boot must determine the
file server to use and a method with which to connect to it. Typically
this will name a file server on the network, or state that the
root file system is on local disk and name the partition. The
complete list of methods is given below.
Boot 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. Boot will prompt for these.
Method and address are prompted for first. The prompt lists all
valid methods, with the default in brackets, for example:
A newline picks the default. Other possible responses are method
or method!address. To aid in automatic reboot, the default is
automatically taken on CPU servers if nothing is typed within
root is from (tcp, local!#S/sdC0/fs)[tcp]:
The other interactions depend on whether the system is a terminal
or a CPU server.
The user will also be prompted for a password to be used as an
encryption key on each attach(5):
The terminal must have a username to set. If none is specified
with the –u option, boot will prompt for one on the console:
With most methods boot can now connect to the file server. However,
with the serial line methods 9600 and 19200, the actual mechanics
of setting up the complete connection are too varied to put into
the boot program. Instead boot lets the user set up the connection.
It prints a prompt on the console and then
simulates a dumb terminal between the user and the serial line:
The user can now type at the modem to dial the number. What is
typed depends on the modem and is beyond this discussion.
Connect to file system now, type ctrl–d when done.
(Use the view or down arrow key to send a break)
When the user types a control–D, boot stops simulating a terminal
and starts the file system protocol over the serial line.
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 a command line to exec instead.
If the kernel 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
the kernel will insert a user level cache process between the
remote server and the local namespace that caches all remote
accesses on the local partition. The –f flag commands cfs to reformat
the cache partition.
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 PC's. PC's keep the information in the disk partition
/dev/sdXX/nvram. If a –k option is given or if no stored information
is found boot will prompt for all three
items and store them.
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
The methods available to any system depend on what was compiled
into the kernel. The complete list of booting methods are listed
tcp connect via Ethernet using the TCP protocol. The 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
server IP addresses obtained (if any) from DHCP during ipconfig(8).
localconnect to the local file system. The first argument is a
disk holding a file system. Boot inspects the disk. If the disk
is a fossil(4) file system, it invokes /boot/fossil to serve it.
If the venti environment variable (really, plan9.ini(8) variable)
is set, boot first arranges for fossil to be able to contact the
named venti(8) server. The variable's value can take the following
the file should be a venti partition with a configuration stored
on it using venti/conf (see venti–fmt(8)). Boot will start a loopback
IP interface on 127.0.0.1 and start venti announcing on tcp!127.1!17034
for venti service and tcp!127.1!8000 for web service, using the
configuration stored in that
same as the last but specify an alternate venti service address.
In this example, using * will announce on all available IP interfaces
(even ones configured later) rather than just the loopback device.
The loopback interface is still configured.
/dev/sdC0/arenas tcp!*!17034 tcp!*!80
same as the last but specify alternate venti service and web addresses.
The loopback interface is still configured.
tcp!18.104.22.168!17034 [ args ]
the network address of a venti server running on a separate machine.
Boot will configure the IP stack by passing args, if any, to ipconfig(8).
Fossil is invoked as
/boot/fossil –f partition –c 'srv –A fboot' –c 'srv –p fscons'
and boot then renames /srv/fboot to /srv/boot, so fossil.conf
should not use the srv command to create fboot, boot, nor fscons.
If the disk is not a fossil(4) partition, boot invokes /boot/kfs.
A variety of programs, like 9660srv and dossrv(4) masquerade as
kfs to allow booting from alternate media formats, so as long
as the disk is not a fossil disk, no check is made that the disk
is in fact a kfs disk. The args are passed to kfs(4).
For the tcp method, the address must be a numeric IP address.
If no address is specified, a file server address will be found
from another system on the network using the BOOTP protocol and
the Plan 9 vendor–specific fields.