Perl and Networking
Why would you want to write networking
applications in Perl?
The Internet is based on Transmission Control
Protocol/Internet Protocol (TCP/IP), and most networking
applications are based on a straightforward application
programming interface (API) to the protocol known as Berkeley
sockets. The success of TCP/IP is due partly to the ubiquity
of the sockets API, which is available for all major languages
including C, C++, Java, BASIC, Python, COBOL, Pascal, FORTRAN,
and, of course, Perl. The sockets API is similar in all these
languages. There may be a lot of work involved porting a
networking application from one computer language to another,
but porting the part that does the socket communications is
usually the least of your problems.
For dedicated Perl programmers, the answer to
the question that starts this chapter is clear�because you
can! But for those who are not already members of the choir,
one can make a convincing argument that not only is networking
good for Perl, but Perl is good for networking.
A Language Built for Interprocess
Communication
Perl was built from the ground up to make it
easy to do interprocess communication (the thing that happens
when one program talks to another). As we shall see later in
this chapter, in Perl there is very little difference between
opening up a local file for reading and opening up a
communications channel to read data from another local
program. With only a little more work, you can open up a
socket to read data from a program running remotely on another
machine somewhere on the Internet. Once the communications
channel is open, it matters little whether the thing at the
other end is a file, a program running on the same machine, or
a program running on a remote machine. Perl's input/output
functions work in the same way for all three types of
connections.
A Language Built for Text
Processing
Another Perl feature that makes it good for
network applications is its powerful integrated regular
expression-matching and text-processing facilities. Much of
the data on the Internet is text based (the Web, for
instance), and a good portion of that is unpredictable,
line-oriented data. Perl excels at manipulating this type of
data, and is not vulnerable to the type of buffer overflow and
memory overrun errors that make networking applications
difficult to write (and possibly insecure) in languages like C
and C++.
An Open Source Project
Perl is an Open Source project, one of the
earliest. Examining other people's source code is the best way
to figure out how to do something. Not only is the source code
for all of Perl's networking modules available, but the whole
source tree for the interpreter itself is available for your
perusal. Another benefit of Perl's openness is that the
project is open to any developer who wishes to contribute to
the library modules or to the interpreter source code. This
means that Perl adds features very rapidly, yet is stable and
relatively bug free.
The universe of third-party Perl modules is
available via a distributed Web-based archive called CPAN, for
Comprehensive Perl Archive Network. You can search CPAN for
modules of interest, download and install them, and contribute
your own modules to the archive. The preface to this book
describes CPAN and how to reach it.
Object-Oriented Networking
Extensions
Perl5 has object-oriented extensions, and
although OO purists may express dismay over the fast and loose
way in which Perl has implemented these features, it is
inarguable that the OO syntax can dramatically increase the
readability and maintainability of certain applications.
Nowhere is this more evident than in the library modules that
provide a high-level interface to networking protocols. Among
many others, the IO::Socket modules provide a clean and
elegant interface to Berkeley sockets; Mail::Internet provides
cross-platform access to Internet mail; LWP gives you
everything you need to write Web clients; and the Net::FTP and
Net::Telnet modules let you write interfaces to these
important protocols.
Security
Security is an important aspect of network
application development, because by definition a network
application allows a process running on a remote machine to
affect its execution. Perl has some features that increase the
security of network applications relative to other languages.
Because of its dynamic memory management, Perl avoids the
buffer overflows that lead to most of thesecurity holes in C
and other compiled languages. Of equal importance, Perl
implements a powerful "taint" check system that prevents
tainted data obtained from the network from being used in
operations such as opening files for writing and executing
system commands, which could be dangerous.
Performance
A last issue is performance. As an
interpreted language, Perl applications run several times more
slowly than C and other compiled languages, and about par with
Java and Python. In most networking applications, however, raw
performance is not the issue; the I/O bottleneck is. On
I/O-bound applications Perl runs just as fast (or as slowly)
as a compiled program. In fact, it's possible for the
performance of a Perl script to exceed that of a compiled
program. Benchmarks of a simple Perl-based Web server that we
develop in Chapter
12 are several times better than the C-based Apache Web
server.
If execution speed does become an issue, Perl
provides a facility for rewriting time-critical portions of
your application in C, using the XS extension system. Or you
can treat Perl as a prototyping language, and implement the
real application in C or C++ after you've worked out the
architectural and protocol details.
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