Ports and Services
When a client process wants to connect a server, the client must have a way of identifying the server that it wants to connect. SO if the client knows the 32-bit Internet address of the host on which the server resides it can contact that host. But how does the client identify the particular server process running on that host ?
To resolve the problem of identifying a particular server process running on a host, both TCP and UDP have defined a group of well known ports.
For our purposes, a port will be defined as an integer number between 1024 and 65535. This is because all port numbers smaller than 1024 are considered well-known -- for example, telnet uses port 23, http uses 80, ftp uses 21, and so on.
The port assignments to network services can be found in the file /etc/services. If you are writing your own server then care must be taken to assign a port to your server. You should make sure that this port should already be not assigned to any other server.
Normally its a practice to assign any port number more than 5000. But there are many organizations who has written servers having port number more than 5000. For example Yahoo Messanger runs on 5050, SIP Server runs on 5060 etc.
Example Ports and Services:
Here is a small list of services and associated ports. You can find most updated list of internet ports and associated service at IANA - TCP/IP Port Assignments.
|Service||Port Number||Service Description|
|echo||7 ||UDP/TCP sends back what it receives|
|discard|| 9|| UDP/TCP throws away input|
|daytime ||13|| UDP/TCP returns ASCII time|
|chargen ||19|| UDP/TCP returns characters|
|ftp ||21 ||TCP file transfer|
|telnet|| 23|| TCP remote login|
|smtp|| 25 ||TCP email|
|daytime ||37|| UDP/TCP returns binary time|
|tftp|| 69 ||UDP trivial file transfer|
|finger|| 79 ||TCP info on users|
|http ||80 ||TCP World Wide Web|
|login ||513 ||TCP remote login|
|who|| 513|| UDP different info on users|
|Xserver|| 6000|| TCP X windows (N.B. >1023)|
Port and Service Functions:
Unix provides following functions to fetch service name from the /etc/services file.
struct servent *getservbyname(char *name, char *proto): - This call takes service name and protocol name and returns corresponding port number for that service.
struct servent *getservbyport(int port, char *proto): - This call takes port number and protocol name and returns corresponding service name.
The return value for each function is a pointer to a structure with the following form:
Here is the description of the member fields:
|s_name||http||This is official name of the service. For example SMTP, FTP POP3 etc.|
|s_aliases||ALIAS||This will hold list of service aliases. Most of the time this will be set to NULL.|
|s_port||80||This will have associated port number. For example for HTTP this will be 80.|
|This will be set to the protocol used. Internet services are provided using either TCP or UDP.|