how a network device comes alive on Linux

 

executable file

script file or argument

environment variable file

directory

The following init activities are focussed on how a network device

comes alive and are deduced from a functioning RedHat 5.0 installation:

 

 

 

As the kernel boots into memory, it discovers network interfaces

such as eth and pi. The final step is to launch

/sbin/init

which (as the man page says) is "the father of all processes".

init

is responsible for initializing the whole show, much like

autoexec.bat did for DOS. Primarily, it creates processes using

the script file

/etc/inittab.

/etc/inittab

:

has information for the various runlevels but before dropping into the

default, id:n:initdefault: (run level<n> is 3 for full multiuser

or 5 for X11) it executes the line si:sysinit:

/etc/rc.d/rc.sysinit

This is where system initialization activities occur.

/etc/rc.d/rc.sysinit

:

sets env variables from

/etc/sysconfig/network if it exists,

else it sets NETWORKING=no, HOSTNAME=localhost.

/sbin/swapon -a starts swapping

sets and echos HOSTNAME (got it from

/etc/sysconfig/network)

domainname sets the NIS domain name (or none)

/sbin/fsck checks root filesystem, than mounts read only

/sbin/quoaton sets user/group quotas

mount mounts /proc

/sbin/isapnp sets up ISA PNP devices

mount remounts the / filesystem rw

enters / and /proc into mtab

if kernel uses modules it does

/sbin/depmod -a -n

uses

/etc/conf.modules (or modules.conf) to load sound modules

/sbin/kerneld is started to handle module requests

/sbin/fsck checks remaining filesystems

/bin/mount -a -t nonfs.proc mounts them

/sbin/quoaton -a turns on user and group quotas

and cleans up after itself (/etc/~mtab etc)

/etc/sysconfig/clock is sourced

if /sbin/

hwclock exists it sets clock, else /sbin/clock(symlink to hwclock)

swapon enaples swap space

/etc/rc.d/rc.serial is sourced for serial ports

/etc/rc.d/rc.modules?? loads modules

/sbin/modprobe loads any SCSI tape module

/etc/rc.d/init.d/random starts

inittab then instructs init to use the files in /etc/rc.d/rc<n>.d to

change to the runlevel shown in its id:(n):initdefault line

/etc/rc,d/rc<n>.d

directories have K* or S* links which normally

point to /etc/rc.d/init.d scripts which can Kill or Start

the appropriate processes by passing

start or stop as an

argument to them. The order these scripts are called

in depends upon the number following the K's (called first) or

S's. Lower numbers are called first. The runlevel editor in

X plays with these.

 

 

Network start

For the sake of these notes, I'm now only following the networking

S10network

(start) process on my installation.

It is a symlink to

/etc/rc.d/init.d/network.

/etc/rc.d/init.d/network

(start)

executes

/etc/rc.d/init.d/functions

sources

/etc/sysconfig/network (if it exists else exits)

sources

/etc/sysconfig/pcmcia if it exists

checks that

NETWORKING is set and /sbin/ifconfig exists (else exits)

cd's into directory /etc/sysconfig/network-scripts and

searches (with

/bin/ls) for each interface(except loopback)

with a config file of the form '1fcfg-xyz0', where xyz is the device

type (xyz) and 0 is the number. eg. eth0 is type eth and number 0:

(

start)

runs on every interface found (in the order

that /bin/ls found them above, this might be important)

now, following imaginary interface xyz0:

run .

/ifup ifcfg-xyz0 boot

sources

network-functions

sets

CONFIG = ifcfg-xyz0

if

./ifcfg-xyz0 exists and user not root then execs

/usr/sbin/usernetctl CONFIG up to determine if user has

permission to bring thatinterface up

/etc/rc.d/init/d/functions:source_config

sources the

ifcfg-xyz0 file (if exists else complains.

since it found it before.)

now,

DEVICE=xyz0

IPADDRESS= a.b.c.d

etc.

if variable

ifcfg-xyz0:ONBOOT = 'no' then exits

runs

./ifup-xyz ifcfg-xyz0 boot if ifup-xyz exists

(notice no trailing number, this is a

file which (generally by using variables

from ifcfg-xyz0) does anything special that

the interface needs, beyond what happens

below). eth doesn't happen to need one.

special handling for

ALIAS, BOOTPROTO,DHCP and SLAVE (not followed)

/sbin/ifconfig xyz0 ($MACADDR for eth if set) $IPADDRESS netmask $NETMASK broadcast $BROADCAST

(looks like IPADRESS, NETMASK and BROADCAST better all be valid)

/sbin/route add -host $IPADDR xyz0

/sbin/route add default gw $GATEWAY xyz0 (if GATEWAY set you get a default)

/sbin/update /etc/resolv.conf based on $DNSSRVS and $SEARCH if set

if no

$HOSTNAME set HOSTNAME=$BOOTPHOSTNAME

runs .

/ifup-post ifcfg-xyz0

source

./network-functions

sources

ifcfg-xyz0

run

./ifup-routes xyz0

if it it exists, reads

/etc/sysconfig/static-routes

static-routes has entries: $device $arguments

/sbin/route

add $device $arguments for lines with device=xyz0

(

ISALIAS=yes not followed)

$

host > /etc/HOSTNAME (under some condition or other, I'm too tired

to figure this one out...)

/sbin/netreport notifies programs that have requested it

/bin/touch /ver/lock/subsys/network (timestamp on file shows time networking up)

DONE

with network start (for one interface anyway)