CS 596: Client Server Programming
Server Stuff

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San Diego State University -- This page last updated March 8, 1995

1. References for this lecture :
2. inetd -- The internet super daemon
3. /etc/services
4. /etc/inetd.conf
5. Sample use of fortune service
6. Sample fortune client
7. Servers started by inetd
8. Standard Services provided by inetd
9. Tcpwrappers
10. Tcpwrappers (continued)
11. Server Configuration files
12. Location of server parameters
13. Configuration files
14. Example of BAD configuration file
15. Configuration file issues
16. Server code issues

References for this lecture:

• Comer Chapters 9 - 12.
• Manual pages for inetd, inetd.conf, services

inetd -- The internet super daemon

Starts servers on demand

Started from /etc/rc

Configuration file in /etc/inetd.conf

Other information from /etc/services

/etc/services

format:

service-name port/protocol aliases

example (from sciences.sdsu.edu):

fortune 6665/tcp

ofortune 6666/tcp

/etc/inetd.conf

format:

service-name socket-type protocol wait-status uid server-program server-arguments

service-name

The name found in /etc/services.

socket-type

Something like stream or dgram.

protocol

A valid protocol the system knows about

wait-status

Either wait or nowait

uid

The user the program will run as.

server-program

Full path the executable

server-arguments

The full argv array (5 max)

Sample use of fortune service

example entry for fortune server on sciences.sdsu.edu:

fortune stream tcp nowait nobody\ /usr/local/bin/fortune.real \ fortune

ofortune stream tcp nowait \ nobody /usr/local/bin/fortune.real\ fortune -o

% telnet sciences 6665

Trying 130.191.224.2...

Connected to sciences.

Escape character is '^]'.

Just because your doctor has a name for your condition doesn't mean he knows what it is.

Connection closed by foreign host.

%

Sample fortune client

The /usr/etc/mconnect command is similar to telnet, but I/O can be redirected from it.

#!/bin/sh
if [ X$1 = X ]; then	
/usr/etc/mconnect -p 6665 sciences | /usr/ucb/tail + 3
else	
/usr/etc/mconnect -p 6666 sciences | /usr/ucb/tail + 3
fi

Usage:

% fortune

or

% fortune foo

Servers started by inetd

The connected socket is assigned to both stdin, stdout, and stderr.

Use the getpeername() call on fd 0 to find the peer's name.

Normally a server has an option which allows it to be started from inetd.

Is there a way to automatically tell if a server is started from inetd or not?

Standard Services provided by inetd

inetd itself provides a number of simple TCP services:

• echo (port 7)
• discard (port 9)
• chargen (port 19)
• daytime (port 13)
• time (port 37)

Tcpwrappers

Server security without modification to the server.

Sample /etc/inetd.conf entry:

telnet stream tcp nowait root \ /usr/etc/tcpd telnetd

inetd runs tcpd instead of telnetd

/usr/etc/tcpd uses the following Files:

• /etc/hosts.allow
• /etc/hosts.deny

Tcpwrappers (continued)

Format of /etc/hosts.allow and /etc/hosts.deny:

daemon-list : client-list : [: shell-command ]

For example if we want to block all incoming telnet sessions from off-campus we do the following in /etc/hosts.allow:

telnetd : 130.191. :

and /etc/hosts.deny:

telnetd : ALL :

tcpwrappers can use the IDENT protocol to check for

incoming users.

Server Configuration files

Most servers need lots of information to work properly:

• Location of data files
• Port number to use
• Hostname to advertise
• Security parameters
• etc.

This information doesn't change from server start to server start

Location of server parameters

Assignment of values for parameters:

• inetd -- Limit of 4 arguments!
• interactive server startup
• options compiled into the server
• configuration file

Configuration files

Helpful guidelines for configuration file design:

• Use plain ASCII text
• Allow for comments
• If possible, use name/value pairs
• Extendable (remember protocols?)
• Consistency

Examples of good configuration file formats

• Microsoft .INI files:

[GROUP Identifier]

name=value

• X11 defaults files:

class.application.name: value

*.name: value

• Gopherd configuration file:

name: value

Example of BAD configuration file

Apple Macintosh Resources.

You have to use either Apple's 'resedit' or the application has to have its own interface.

Prime example of bad use of this is Mac Eudora (old)

The modem dialup script is entered using Resedit.

Configuration file issues

Allow for comments:

• In a template configuration file comments can describe the meaning of options.
• Keep track of a modification history

Use name/value pairs to set options:

• Allows for extendability
• Allows for server defaults
• Is easy to read and modify by users

Consistency is important!

database = /usr/local/lib/database

helpdatabase: /usr/local/lib/database.help

Server code issues

Configuration file location is only one option

Server needs a standard interface to the configuration once it has read it.

If using consistent name/value pairs, a Dictionary or symbol table is a good interface.

The server can preload the dictionary with its own compiled defaults and then overwrite these with the configuration file.

This code should be reusable.