inetd...Server of Servers

• Looks at a number of ports

• Determines when a service is needed on any of those ports

• Calls the appropriate server

• Restarts new server

– for each client ( nowait )

– when the previous server dies (wait )

• Interacts with– syslog to record activity

– inetd.conf to configure itself

– /etc/services to determine which ports are associated with which server

inetd configuration

inetd.conf: (example entry.. one for each server)

telnet stream tcp nowait root /etc/telnetd telnetd

name of the service

type of connection (steam tcp or dgram udp)

when to start a new server (wait or nowait)

user id to run the server

path name to the executable

command line and options/parameters

/etc/services:(example entry)telnet 23 / tcp

protocolport numberservice

INETD

superserver

configuration

Daemons

• Consider the overhead of starting a server for inetd

• Frequent requests for service may result in overhead which is less efficient than leaving the server running

• These servers which run continuously are daemons

• Daemons don’t– recover easily or automatically if they die

– they don’t reconfigure themselves for each request for service where a server spawned by inetd can reconfigure itself on each spawning of a new server

How many connections?

• Different servers handle this question differently

• Gopher– Destroys a TCP connection for each file transmitted

• WWW servers– new TCP connection for each file

– each page is potentially composed of a number of files

• FTP– one connection for control

– one connection for data

Launching Servers• Manually (at the command line)

• At boot (part of a boot script)

• Inetd (automatically)– serially

– parallel

• Programmatically– iteratively (serial)

– parallel (unix fork)

– preallocation

• Inetd

Sustaining Servers

Programmatically iterative

While (true) Accept connection While client wants service

get request process send response Close Connection

Only ONE client serviced at a time!How would you like a web server to make you wait

until another client was done?

Programmatically Parallel

Server(idle)

ServerClientrequest

Server

Server (clone)Client

(idle)

Server(idle)

Main server only fields requests, doesn’t really “serve”

Programmatically Preallocate

Server(idle)

Server(clone)(idle)

Server(clone)(idle)

Server

Server(clone)

(idle)

Server(clone)(idle)

Client

Main Server chooses which clone will actually serve

Request made by a client to Server

Simple means of providing information services

• Chapter 3 discusses some simple approaches to defining services

– fingerd

– telnetd

– inetd

• These represent a means of starting the process of information serving

finger and fingerd

• Build a services user and put a .plan file in the services home directory

User Home Directory:

.planLook in my public directory for info

finger dgame

client:

server:

fingerd reads .planand appends

output

user dgame Real Name:David Game

......

Look in my public directory for info

GNU finger

• allows you to extend the idea of a .plan file

• instead of a text displayed, it executes a script in a .fingerrc file in the home directory

• considerations– standard input of .fingerrc script is finger output

– three arguments

» domain name or ip address of remote host

» local/remote keyword indicating same host or not

» name of user fingering (or null)

– don’t use setuid or setgid in the script

– run by the user nobody

• get from ftp.gnu.ai.mit.edu as a tar file

updating your finger info

• Making the finger data current

• Perhaps putting todays schedule in it

• program in the archive to allow for uptodate reporting

• replaces the .plan with a FIFO (more later)

• must recompile the program and install it

• must start the program running on the machine, from a startup file if possible

• example p. 40

simple uses of inetd/telnet

• You supply a program which reads/writes standard input/standard output

• inetd supplies the network connection

• Example: in inetd.conf

stock stream tcp nowait nobody /bin/cat cat /.../stockquote

in /etc/services

stock 2000/tcp• Use telnet as the client, a stdin/stdout match on client side.

telnet yourmachine.where.it.is. 2000• Provide an alias name to the machine like “stock”

– p. 41

telnet/(d) ... it’s default

• daemon sends client a login prompt

• client logs in

• daemon looks in /etc/password and gets the login shell to use

• login shell can be any program that read/writes standard in/standard out

• replace the shell for this service (user) with your own program instead of a shell

• program which serves should not allow escape to a full-featured shell

• consider replacing login shells with text-based– gopher or web browsers

Captured shell

• Provides access to a limited number of commands• perl script example on p. 44

X clients• We don’t really cover x-windows in this track

• The idea is to start the x-client (running as an x-windows application) but to display the results on the x-server (the service client)

• x-client on service server

• x-server on service client

Downside of not providing a central (telnet) login

• If you provide the service on your own port number, how do you let others know where it is.

• The more services you offer, the more you have to publicize

• Providing a services login at a standard port can then allow you to provide other services from that point