Client & Server in UNIX

	What is  a server 　 A server is a services proviser and is  always listening and waiting to provide service in according to the service request from remote sites. 　 From O.S. point of view, server is a special program which always running on the background. In UNIX system, we call such program a daemon. 　 UNIX中的 network server程式係以daemon的形式存在，該由何server program提供服務取決於service request中的port number及protocol number. 　 UNIX中常見的Server 程式: sendmail : SMTP server httpd : Web Server mountd & nfsd : NFS Server inetd : TCP/IP Network super daemon
	UNIX TCP/IP Super Daemon -- inetd A super daemon which manages other nework daemons. Used to improve UNIX network client/server performance. Manages other network daemons by its configuration file /etc/inetd.conf User-developed network services can also be installed as inetd clients. inetd是BSD引進的觀念，BSD 4.3版以前，個別network service均以獨立的 daemon方式存在；因此如果一個伺服器提供10種service，就必須至少啟動10個以上的daemon，以應付隨時到來的service request。這衍生了效率與管理的問題，尤其當network service種類快速的增加及user自行開發network program風氣大開的網路時代，這種運作方式更顯得捉襟見肘。 BSD 4.4開始，inetd成為大多數network server deamon(ftp、telnet、rsh、talk ?)的代言人，inet會根據/etc/inetd.conf的service port定義，在Service request到來時，叫起適當的program以提供必要的service. 經過適當的定義，使用者自行開發的client/server程式，也可以被整合至inetd中，以整體且有效率的方式提供服務。
	How /etc/inetd works - An Eample

Ientd must ne restarted when any of the configuration files(/etc/inetd.conf, /etc/services & /etc/protocols) updated。

定義inetd運作方式的的兩個設定檔: /etc/services  &  /etc/inetd /etc/services --> 定義network services(protocol) name與tcp (transport layer) port number的對應關係。 An Example of /etc/services: ... chargen 19/tcp ttytst source chargen 19/udp ttytst source ftp-data 20/tcp ftp  21/tcp fsp 21/udp fspd ssh  22/tcp  # SSH Remote Login Protocol ssh   22/udp # SSH Remote Login Protocol telnet  23/tcp # 24 - private smtp   25/tcp mail # 26 - unassigned time   37/tcp timserver time   37/udp timserver rlp    39/udp resource # resource location nameserver   42/tcp name  # IEN 116 whois   43/tcp nicname re-mail-ck   50/tcp # Remote Mail Checking Protocol re-mail-ck   50/udp # Remote Mail Checking Protocol domain    53/tcp nameserver # name-domain server domain   53/udp nameserver mtp   57/tcp # deprecated ... ### Note that it is presently the policy of IANA to assign a single well-known port number ### for both TCP and UDP; hence, most entries here have two entries even if the protocol ### doesn't support UDP operations. /etc/inetd.conf --> 定義network services(protocol) 特性與其相關應用程式啟動方式。一個service daemon被 inetd 叫起前，設定檔參考順序是：/etc/services --> /etc/inetd.conf。因此disable一個services，理論上可以在這兩個檔的任一個中，編輯其相關設定；不過習慣上，我們通常會修改/etc/inetd.conf。 　 An Example of /etc/inetd.conf #echo  stream  tcp  nowait  root  internal #echo  dgram  udp  wait  root  internal #echo  dgram  udp  wait  root  internal #discard  stream  tcp  nowait  root  internal #discard  stream  tcp  nowait  root  internal #discard  stream  tcp  nowait  root  internal #discard   dgram  udp  wait  root  internal #daytime stream  tcp  nowait  root  internal #daytime dgram  udp  wait  root  internal #chargen stream  tcp  nowait  root  internal #chargen dgram  udp  wait  root  internal #time stream  tcp  nowait  root  internal #time dgram  udp  wait  root  internal # These are standard services. # ftp  stream  tcp  nowait  root  /usr/sbin/tcpd in.ftpd -l -a telnet stream  tcp  nowait  root  /usr/sbin/tcpd in.telnetd # shell  stream  tcp  nowait  root  /usr/sbin/tcpd in.rshd login  stream  tcp  nowait  root  /usr/sbin/tcpd in.rlogind syslog  dgram  udp  wait  root  /usr/sbin/tcpd  syslogd  -m 0 ...