It may well be that for some reason the program shown in the previous example does not run on your machine. When looking at possible reasons for this, you will learn much about typical problems that arise in network programming.
For the rest of this chapter, we will assume you work on a POSIX-style system that supports TCP/IP. If the previous example program does not run on your machine, it may help to replace the value assigned to the variable ‘daytime_server’ with the name (or the IP address) of another server from the list mentioned above. Now you should see the date and time being printed by the program, otherwise you may have run out of servers that support the ‘daytime’ service.
Try changing the service to ‘chargen’ or ‘ftp’. This way, the program connects to other services that should give you some response. If you are curious, you should have a look at your /etc/services file. It could look like this:
# /etc/services: # # Network services, Internet style # # Name Number/Protocol Alternate name # Comments echo 7/tcp echo 7/udp discard 9/tcp sink null discard 9/udp sink null daytime 13/tcp daytime 13/udp chargen 19/tcp ttytst source chargen 19/udp ttytst source ftp 21/tcp telnet 23/tcp smtp 25/tcp mail finger 79/tcp www 80/tcp http # WorldWideWeb HTTP www 80/udp # HyperText Transfer Protocol pop-2 109/tcp postoffice # POP version 2 pop-2 109/udp pop-3 110/tcp # POP version 3 pop-3 110/udp nntp 119/tcp readnews untp # USENET News irc 194/tcp # Internet Relay Chat irc 194/udp …
Here, you find a list of services that traditional Unix machines usually
support. If your GNU/Linux machine does not do so, it may be that these
services are switched off in some startup script. Systems running some
flavor of Microsoft Windows usually do not support these services.
Nevertheless, it is possible to do networking with gawk on
Microsoft
Windows.6
The first column of the file gives the name of the service, and
the second column gives a unique number and the protocol that one can use to connect to
this service.
The rest of the line is treated as a comment.
You see that some services (‘echo’) support TCP as
well as UDP.
Microsoft preferred to ignore the TCP/IP family of protocols until 1995. Then came the rise of the Netscape browser as a landmark “killer application.” Microsoft added TCP/IP support and their own browser to Microsoft Windows 95 at the last minute. They even back-ported their TCP/IP implementation to Microsoft Windows for Workgroups 3.11, but it was a rather rudimentary and half-hearted implementation. Nevertheless, the equivalent of /etc/services resides under C:\WINNT\system32\drivers\etc\services on Microsoft Windows 2000 and Microsoft Windows XP. On Microsoft Windows 7, 8 and 10 there is a directory %WinDir%\System32\Drivers\Etc that holds the hosts file and probably also a services file.