Recipe 19.8 Debugging the Raw HTTP Exchange

19.8.1 Problem

Your CGI script is misbehaving strangely with your browser, and you suspect something in the HTTP header is missing. You want to find out exactly what your browser is sending to the server in the HTTP header.

19.8.2 Solution

Create your own fake web server and point your browser at it, as shown in Example 19-6.

Example 19-6. dummyhttpd
  #!/usr/bin/perl -w
  # dummyhttpd - start an HTTP daemon and print what the client sends
  use strict;
  use LWP 5.32;  # minimal good version
  use HTTP::Daemon;
  my $server = HTTP::Daemon->new(Timeout => 60, LocalPort => 8989);
  print "Please contact me at: <URL:", $server->url, ">\n";
  while (my $client = $server->accept) {
      while (my $answer = $client->get_request) {
          print $answer->as_string;
          while (<STDIN>) {
              last RESPONSE   if $_ eq ".\n";
              last CONNECTION if $_ eq "..\n";
              print $client $_;
          print "\nEOF\n";
      print "CLOSE: ", $client->reason, "\n";
      undef $client;

19.8.3 Discussion

It's hard to keep track of which versions of all browsers still have which bugs. The fake server program can save you days of head scratching, because sometimes a misbehaving browser doesn't send the server the right thing. Historically, we have seen aberrant browsers lose their cookies, mis-escape a URL, send the wrong status line, and other blunders even less obvious.

The fake server is best run on the same machine as the real server. That way your browser still sends any cookies destined for that domain. Then instead of pointing your browser at:

use the alternate port given in the new constructor in the Solution. You don't need to be the superuser to run the server if you use the alternate port.

If you convince yourself that the client is behaving properly but wonder about the server, it's easiest to use the telnet program to manually talk to the remote server.

% telnet 80
GET /bogotic HTTP/1.0
<<blank line here>>
HTTP/1.1 404 File Not Found
Date: Tue, 21 Apr 1998 11:25:43 GMT
Server: Apache/1.2.4
Connection: close
Content-Type: text/html

<TITLE>404 File Not Found </TITLE>
<H1>File Not Found</H1>
The requested URL /bogotic was not found on this server.<P>

If you have LWP installed on your system, you can use the GET alias for the lwp-request program. This will follow any redirection chains, which can shed light on your problem. For example:

% GET -esuSU
User-Agent: lwp-request/1.32

GET --> 302 Moved Temporarily
GET --> 302 Moved Temporarily
GET --> 404 File Not Found
Connection: close
Date: Tue, 21 Apr 1998 11:29:03 GMT
Server: Apache/1.2.4
Content-Type: text/html
Client-Date: Tue, 21 Apr 1998 12:29:01 GMT
Title: Broken Links

<HEAD><TITLE>An Error Occurred</TITLE></HEAD>
<H1>An Error Occurred</h1>
404 File Not Found

19.8.4 See Also

The documentation for the standard CGI module; Recipe 19.9; lwp-request(1)