While
it is probably better to run a PSGI based
server, than a CGI-based server it can be also very useful to learn
how to write CGI scripts. Especially if you need to maintain one.
This article will help you set
up an Apache web server to run CGI scripts.
Background start Linux, install Apache2
After creating the Droplet,
ssh to the server and update the installed packaged to the latest and
then reboot the machine.
(The droplet I was using to
implement this had an IP address of 107.170.93.222 )
ssh root@107.170.93.222 # aptitude update # aptitude safe-upgrade
Once the machine restarted ssh
to it again and and install the MPM preforking version of Apache2.
ssh root@107.170.93.222 # aptitude install apache2-mpm-prefork
You can check if apache is
running by executing ps axuw:
# ps axuw | grep apache
The output on my instance
looked like this:
root 1961 0.0 0.5 71284 2608 ? Ss 14:16 0:00 /usr/sbin/apache2 -k start www-data 1964 0.0 0.4 360448 2220 ? Sl 14:16 0:00 /usr/sbin/apache2 -k start www-data 1965 0.0 0.4 360448 2220 ? Sl 14:16 0:00 /usr/sbin/apache2 -k start root 2091 0.0 0.1 9452 908 pts/0 S+ 14:16 0:00 grep --color=auto apache
Now you can browse to the
web-site by pointing your browser to the IP address of the machine.
In my case it was http://107.170.93.222/ Please note, some browsers
will not work properly if you don't put the http:// in front of the
IP address.
If everything works fine you
will see something like this in the browser:
This is the content of the
/var/www/index.html file on the server.
You can edit that file, and
reload the page in the browser.
Creating the first CGI script in Perl
Create the /var/cgi-bin
directory
(Please note, we don't create
this inside the /var/www directory on purpose. This way, even if
misconfigured the server it won't serve the source code of the
script. Which is a good thing.)
mkdir /var/cgi-bin
and create a file
called /var/cgi-bin/echo.pl with the following content:
#!/usr/bin/perl use strict; use warnings; print qq(Content-type: text/plain\n\n); print "hi\n";
Make the file executable by
chmod +x /var/cgi-bin/echo.pl
and run it on the command
line:
# /var/cgi-bin/echo.pl
It should print:
Content-type: text/plain hi
This is your first CGI-script
in Perl.
Now we need to configure the
Apache web server to server it properly.
Configure Apache to serve CGI files
Open the configuration file of Apache /etc/apache2/sites-enabled/It has the following in it with a bunch of comments between the lines:
<VirtualHost *:80 >
ServerAdmin webmaster@localhost DocumentRoot /var/www ErrorLog ${APACHE_LOG_DIR}/error.log CustomLog ${APACHE_LOG_DIR}/access.log combined
< /VirtualHost >
Add the following lines after the DocumentRoot line:
ScriptAlias /cgi-bin/ /var/cgi-bin/
<Directory "/var/cgi-bin" >
AllowOverride None
Options +ExecCGI -MultiViews +SymLinksIfOwnerMatch
Require all granted
</Directory >
Now you should have:
<VirtualHost *:80 > ServerAdmin webmaster@localhost DocumentRoot /var/www ScriptAlias /cgi-bin/ /var/cgi-bin/
AllowOverride None Options +ExecCGI -MultiViews +SymLinksIfOwnerMatch Require all granted ErrorLog ${APACHE_LOG_DIR}/error.log CustomLog ${APACHE_LOG_DIR}/access.log combined
</VirtualHost >
By default this Apache instance does not have the CGI module enabled. This we can see by noticing that the mods-enabled directory does not have any of the cgi files that are available in the mods-available directory:
# ls -l /etc/apache2/mods-enabled/ | grep cgi # ls -l /etc/apache2/mods-available/ | grep cgi -rw-r--r-- 1 root root 115 Jul 20 2013 cgid.conf -rw-r--r-- 1 root root 60 Jul 20 2013 cgid.load -rw-r--r-- 1 root root 58 Jul 20 2013 cgi.load -rw-r--r-- 1 root root 89 Jul 20 2013 proxy_fcgi.load -rw-r--r-- 1 root root 89 Jul 20 2013 proxy_scgi.load
We create symbolic links from
the mods-enabled to the mods-available directory for the two cgid.*
files, and then check again if the symbolic links were created
properly by running ls -l again:
# ln -s /etc/apache2/mods-available/cgid.load /etc/apache2/mods-enabled/ # ln -s /etc/apache2/mods-available/cgid.conf /etc/apache2/mods-enabled/ # ls -l /etc/apache2/mods-enabled/ | grep cgi lrwxrwxrwx 1 root root 37 Mar 19 14:39 cgid.conf -> /etc/apache2/mods-available/cgid.conf lrwxrwxrwx 1 root root 37 Mar 19 14:39 cgid.load -> /etc/apache2/mods-available/cgid.load
At this point we can reload
the Apache web server by the following command:
# service apache2 reload
This will tell Apache to read
the configuration files again.
Then we can brows to
http://107.170.93.222/cgi-bin/echo.pl (after replacing the IP address
by yours) and you will see it shows the word "hi".
Congratulations, your first
CGI script is running.
Dynamic results
This simple CGI script will
show exactly the same content no matter who access it and when. Let's
make a little change to the script so that we can see it is really
generated dynamically.
#!/usr/bin/perl use strict; use warnings; print qq(Content-type: text/plain\n\n); print scalar localtime;
After changing the script if
you access the page now it will display the current time on the
server. If you reload the page, it will show the new current time.
Trouble shooting
If when you access the
http://107.170.93.222/cgi-bin/echo.pl URL you see the content of the
script instead of the word "hi" then you probably put the
cgi-bin directory inside the /var/www and/or probably forgot to
create the symbolic links for the cgid.* files. Move the cgi-bin
directory outside the /var/www, update the configuration files, set
up the symbolic links, and reload the server.
500 Internal Server Error
If you get an 500
Internal Server Error look at the error log
in /var/log/apache2/error.log
[Wed Mar 19 15:19:15.740781 2014] [cgid:error] [pid 3493:tid 139896478103424] (8)Exec format error: AH01241: exec of '/var/cgi-bin/echo.pl' failed [Wed Mar 19 15:19:15.741057 2014] [cgid:error] [pid 3413:tid 139896186423040] [client 192.120.120.120:62309] End of script output before headers: echo.pl
This can happen if the script
does not start with a sh-bang line pointing to a correctly installed
perl. The first line of the file should be
#!/usr/bin/perl [Wed Mar 19 15:24:33.504988 2014] [cgid:error] [pid 3781:tid 139896478103424] (2)No such file or directory: AH01241: exec of '/var/cgi-bin/echo.pl' failed [Wed Mar 19 15:24:33.505429 2014] [cgid:error] [pid 3412:tid 139896261957376] [client 192.120.120.120:58087] End of script output before headers: echo.pl
This probably indicates that
the file is in DOS format. This can happen if you have ftp-ed the
file from a Windows machine in binary mode. (You should not use ftp
anyway.) You can fix this by running
dos2unix /var/cgi-bin/echo.pl [Wed Mar 19 15:40:31.179155 2014] [cgid:error] [pid 4796:tid 140208841959296] (13)Permission denied: AH01241: exec of '/var/cgi-bin/echo.pl' failed
[Wed Mar 19 15:40:31.179515 2014] [cgid:error] [pid 4702:tid 140208670504704] [client 192.120.120.120:60337] End of script output before headers: echo.pl
The above lines in the
error.log, probably indicated that the script does not have the Unix
executable bit.
chmod +x /var/cgi-bin/echo.pl Wed Mar 19 16:02:20.239624 2014] [cgid:error] [pid 4703:tid 140208594970368] [client 192.120.120.120:62841] malformed header from script 'echo.pl': Bad header: hi
This will be received if there
is an output before the the Content-type. For example if you have
written the two print-lines in reversed order like this:
#!/usr/bin/perl use strict; use warnings; print "hi\n"; print qq(Content-type: text/plain\n\n); [Wed Mar 19 16:08:00.342999 2014] [cgid:error] [pid 4703:tid 140208536221440] [client 192.120.120.120:59319] End of script output before headers: echo.pl
This probably means that the
script died before it managed to print the Content-type. The
error.log, probably contains the exception that was not caught just
before the above line.
$| = 1;
I am not sure, but I
think Premature end of script headers is the same
as End of script output before headers.
503 Service Unavailable
After I created the symbolic
links to the cgid.* files and reloaded the Apache server, I got 503
Service Unavailable in the browser and the following line in
the log file:
[Wed Mar 19 15:30:22.515457 2014] [cgid:error] [pid 3927:tid 140206699169536] (22)Invalid argument: [client 192.120.120.120:58349] AH01257: unable to connect to cgi daemon after multiple tries: /var/cgi-bin/echo.pl
I am not sure why did this
happen, but it after restarting the apache server it started to work
properly:
# service apache2 restart
In most of the situations a
reload should be enough, but maybe not when a module is
enabled/disabled.
404 Not Found
If you get a 404 Not
Found error in the browser and
[Wed Mar 19 15:35:13.487333 2014] [cgid:error] [pid 4194:tid 139911599433472] [client 192.120.120.120:58339] AH01264: script not found or unable to stat: /usr/lib/cgi-bin/echo.pl
in the error log, then maybe
the ScriptAlias line is missing, or not pointing to the
proper directory.
403 Forbidden
If you get a 403
Forbidden error, the probably the Directory directive
was not correctly configure or does not point to the correct path.
Summary
That's it. Hopefully, you have
your first CGI script in Perl running.
