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Week 9 Notes for NET2003
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-Ian! D. Allen - idallen@idallen.ca

Remember - knowing how to find out an answer is more important than
memorizing the answer.  Learn to fish!  RTFM!  (Read The Fine Manual)

Keep up on your readings (Course Outline: average 5 hours/week homework)

Linux Networking Start-Up
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BIOS executes the boot block from the first available disk drive.

The boot block redirects to a bootloader program such as LILO or GRUB.

LILO/GRUB allow a choice of systems to boot.

Choosing a Linux kernel, the Linux kernel loads
- loader can pass options to the kernel using a "kernel command line"
  - a copy is available as /proc/cmdline when the system is running
- services can look in this command line for keywords/values
- e.g. the word "single" boots Linux in single-user maintenance mode

Linux kernel runs Process #1 - "init"
- all subsequent processes fork/exec and are descendants of this one

init runs the "rc" scripts corresponding to the default run level
- see initdefault in /etc/inittab
- /etc/rc?.d/* or /etc/rc.d/rc?.d/* are symlinks to /etc/init.d/*

Auto-config RC scripts may probe hardware
- locate drivers for discovered hardware
  - boot-time hardware detection scripts: kudzu, harddrake, etc.
- without auto-config, you have to know what drivers match which hardware
- drivers load into the running kernel using "insmod", usually called
  from an easier-to-use wrapper routine named "modprobe"
- use command "lsmod" to see loaded drivers
- use "rmmod" to remove a loaded kernel module
- Note: most drivers can be compiled into the base kernel instead of loaded
  as modules at run time; lsmod will not show these compiled-in drivers!

boot time usually executes an RC "network" start-up script
- may be called /etc/init.d/network
- script may load special modules (e.g. ipv6)
- script may set network kernel options using sysctl
  - see /etc/sysctl.conf
- may use helper scripts such as "ifup" and "ifdown" to start network
  - see files in /etc/sysconfig/  and  /etc/sysconfig/network-scripts/
  - each device has a config file:
    e.g. /etc/sysconfig/network-scripts/ifcfg-{lo,eth0,eth1}
- enabling the device also adds a route for that device

start-up scripts in /etc/rc?.d/ or /etc/rc.d/rc?.d/ start "daemons"
- programs that listen on well-known TCP and UDP network ports
- /etc/services is a local list of names of many well-known ports
  - see also http://www.iana.org/assignments/port-numbers

major service port numbers (often seen in trace output):
   TCP 20 ftp-data
   TCP 21 ftp (control)
   TCP 22 SSH
   TCP 23 telnet
   TCP 25 SMTP
   UDP/TCP 53 domain (DNS)
   TCP 80 HTTP
   TCP 110 POP3
   TCP 113 ident
   TCP 119 NNTP
   UDP/TCP 123 NTP
   UDP/TCP 137-139 netbios (SMB)
   TCP 443 HTTPS