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FAQ: Frequently Asked Questions about CGI Programming

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Frequently Asked Questions on CGI Programming

0.   Preamble
0.1. Changes
0.2. Notice and Disclaimer
0.3. Where to get this document
0.4. How to contribute to this document?
0.5. Can I email the author my questions?
0.6. What's up with posting to comp.infosystems.www.authoring.cgi?
0.7. Credits

1.   Basic Questions
1.1. What is CGI?
1.2. Is it a script or a program?
1.3. When do I need to use CGI?
1.4. Should I use CGI or JAVA?
1.5. Should I use CGI or SSI or ... { PHP/ASP/... }
1.6. Should I use CGI or an API?
1.7. So what are in a nutshell the options for webserver programming?
1.8. What do I absolutely need to know?
1.9. Does CGI create new security risks?
1.10. Do I need to be on Unix?
1.11. Do I have to use Perl?
1.12. What languages should I know/use?
1.13. Do I have to put it in cgi-bin?
1.14. Do I have to call it *.cgi?  *.pl?
1.15. What is the "CGI Overhead", and should I be worried about it?
1.16. What do I need to know about file permissions and "chmod"?
1.17. What is CGIWrap, and how does it affect my program?
1.18. How do I decode the data in my Form?

2.   HTTP Headers and NPH Scripts
2.1. What is HTTP (HyperText Transfer Protocol)?
2.2. What HTTP request headers can I use?
2.3. What Environment variables are available to my application?
2.4. Why doesn't my script get REMOTE_USER?  My page is password-protected.
2.5. What HTTP response headers do I need to know about?
2.6. What is NPH?
2.7. Must/should/can I write nph scripts?
2.8. Do I have to call it nph-*
2.9. What is the difference between GET and POST?

3.   Techniques: "How do I..."
3.1. Can I get information about who is visiting?
3.2. Can I get the email of visitors?
3.3. 	"But I saw display my email address..."
3.4. Can I verify the email addresses people enter in my Form?
3.5. Subject: How can I get the hostname of the remote user?
3.6. Can I get browser details and return different pages?
3.7. Can I trace where a user has come from/is going to?
3.8. Can I launch a long process and return a page before it's finished?
3.9. Can I launch a long process which the user interacts with?
3.10. Can I password-protect my pages?
3.11. Can I do HTTP authentication using CGI?
3.12. Can I identify users/sessions without password protection?
3.13. Can I redirect users to another page?
3.14. Can I run a CGI script without returning a new page to the browser?
3.15. Can I write output to a different Netscape frame?
3.16. Can I write output to several frames at once?
3.17. Can I use a CGI script to generate both text and inline images?
3.18. How can I use Caches to make CGI scripts faster and more Net-friendly?
3.19. How can I avoid users hitting "submit" twice?
3.20. How can I stop my CGI script reading and writing files as "nobody"?
3.21. How can I prevent my CGI results being cached by the browser?
3.22. How can I control the default filename when downloading a file via CGI?

4.   Troubleshooting a CGI application
4.1. Are there some interactive debugging tools and services available?
4.2. I'm having trouble with my headers.   What can I do?
4.3. Why do I get Error 500 ("the script misbehaved", or "Internal Server Error")
4.4. I tried to use (Content-Type|Location|whatever), but it appears in my Browser?
4.5. How can I run my CGI program 'live' in a debugger?
4.6. I'm using CGI with QUERY_STRING embedded in my HTML, but it gets corrupted?

5.   Further Reading
5.1. Other FAQs/collections
5.2. Reference Pages


Subject: SECTION 0 - PREAMBLE NOTE: the numbering in this document is automatically generated by my posting software, and will change between postings if new questions are added (as _may_ happen when I see - or someone contributes - a FAQ I've previously overlooked :-)
Subject: 0.1 Changes Last Modified: July 2000. Updated several links reported by Site Valet as moved. Otherwise unchanged.
Subject: 0.2 Notice and Disclaimer Copyright 1996-2000 Nick Kew. You are free to copy or distribute this document in whole or in part for any purpose and on any medium you choose, provided you include this notice and disclaimer in full. Disclaimer: This information is offered in good faith and in the hope that it may be of use, but is not guaranteed to be correct, up to date or suitable for any particular purpose. The author accepts no liability in respect of this information or its use.
Subject: 0.3 Where to get this document The official homes of this document on the Web are now URL URL NOTE - If you want to mirror the FAQ on your WWW site on a publicly-visible server, please make sure you keep it up-to-date. Other known sources are: (1) USENET: posted to newsgroups (TEXT) news:comp.infosystems.www.authoring.cgi news:comp.answers news:news.answers (2) RTFM and mirror sites (TEXT) (3) RTFM WWW mirror sites, including (Partial HTML) Europe - America - (4) By EMAIL from the FAQserver at RTFM (TEXT) Send email to with send usenet/news.answers/www/cgi-faq in the body of your message
Subject: 0.4 How to contribute to this document? I have removed the InterFAQ from this answer, as it has become clear that people prefer the familiar approach of emailing me to that of contributing via the web, and (in turn) the InterFAQ contents has not been maintained for some time. Thomas Boutell has since introduced a somewhat similar project, the OpenFAQ. Just mail me. ( )
Subject: 0.5 Can I email the author my questions? Please don't. Post them to an appropriate newsgroup, where they'll be seen and possibly answered by a whole lot more people than just me. And remember: bad (or incoherent) questions get bad answers, so think carefully before posting. If you have an actual programming job to do, I might be interested However, I am unlikely to be interested in jobs below $1000. If you think something already in the FAQ needs clarifying, feel free to mail me: don't expect a personal reply, but I *might* add something to the answer in question, so check the next posting (or three).
Subject: 0.6 What's up with posting to comp.infosystems.www.authoring.cgi? This is now a moderated newsgroup. The moderator is a bot run by Thomas Boutell ( ). The charter for moderation is as follows: This newsgroup is self-moderated. Your first posting will not appear until you have read and responded to an automatic welcome mailing, at which point your posting will appear with no further delay. Provision will also be made to automatically approve first postings that contain a header requesting this. Subsequent postings are approved automatically. If posting normally doesn't work - as could be the case if your newsfeed has trouble with moderated groups - you can post articles by emailing them to: Provided the return address in your mail is correct, you will then receive precise instructions for having your post(s) automatically approved. Alternative means of posting are detailed in the WWW FAQ, posted regularly by Thomas Boutell.
Subject: 0.7 Credits This FAQ was written by Nick Kew, and has been considerably improved with the help of comments and criticisms, newsgroup posts and miscellaneous suggestions from correspondents including Nathan Neulinger, Maurice L. Marvin, Matthew Healy, Alan J. Flavell, Don Libes, Alain Deckers, David S. Jackson, J.M. Ivler, and no doubt others I've forgotten to credit (please remind me if necessary).
Subject: SECTION 1 - BASIC QUESTIONS This section aims to deal with basic questions, addressing the role and nature of CGI, and its place in Web programming. Questions/answers which just don't appear to 'fit' under any other section may also be included here.
Subject: 1.1 What is CGI? [ from the CGI reference ] The Common Gateway Interface, or CGI, is a standard for external gateway programs to interface with information servers such as HTTP servers. A plain HTML document that the Web daemon retrieves is static, which means it exists in a constant state: a text file that doesn't change. A CGI program, on the other hand, is executed in real-time, so that it can output dynamic information.
Subject: 1.2 Is it a script or a program? The distinction is semantic. Traditionally, compiled executables (binaries) are called programs, and interpreted programs are usually called scripts. In the context of CGI, the distinction has become even more blurred than before. The words are often used interchangably (including in this document). Current usage favours the word "scripts" for CGI programs.
Subject: 1.3 When do I need to use CGI? There are innumerable caveats to this answer, but basically any Webpage containing a form will require a CGI script or program to process the form inputs.
Subject: 1.4 Should I use CGI or JAVA? [answer to this non-question hopes to try and reduce the noise level of the recurrent "CGI vs JAVA" threads]. CGI and JAVA are fundamentally different, and for most applications are NOT interchangable. CGI is a protocol for running programs on a WWW server. Whilst JAVA can also be used for that, and even has a standardised API (the servlet, which is indeed an alternative to CGI), the major role of JAVA on the Web is for clientside programming (the applet). In certain instances the two may be combined in a single application: for example a JAVA applet to define a region of interest from a geographical map, together with a CGI script to process a query for the area defined.
Subject: 1.5 Should I use CGI or SSI or ... { PHP/ASP/... } CGI and SSI (Server-Side Includes) are often interchangable, and it may be no more than a matter of personal preference. Here are a few guidelines: 1) CGI is a common standard agreed and supported by all major HTTPDs. SSI is NOT a common standard, but an innovation of NCSA's HTTPD which has been widely adopted in later servers. CGI has the greatest portability, if this is an issue. 2) If your requirement is sufficiently simple that it can be done by SSI without invoking an exec, then SSI will probably be more efficient. A typical application would be to include sitewide 'house styles', such as toolbars, netscapeised <body> tags or embedded CSS stylesheets. 3) For more complex applications - like processing a form - where you need to exec (run) a program in any case, CGI is usually the best choice. 4) If your transaction returns a response that is not an HTML page, SSI is not an option at all. Many more recent variants on the theme of SSI are now available. Probably the best-known are PHP which embeds server-side scripting in a pre-html page, and ASP which is Microsoft's version of a similar interface.
Subject: 1.6 Should I use CGI or an API? APIs are proprietary programming interfaces supported by particular platforms. By using an API, you lose all portability. If you know your application will only ever run on one platform (OS and HTTPD), and it has a suitable API, go ahead and use it. Otherwise stick to CGI.
Subject: 1.7 So what are in a nutshell the options for webserver programming? Too many to enumerate - but I'll try and summarise. Briefly, there are several decisions you have to make, including: * Power. Is it up to a complex task? * Complexity. How much programming manpower is it worth? * Portability. Might you want to run your program on another system? So here's an overview of the main options. It's inevitably subjective, but may be helpful to someone: Basic SSI: Simple interface for basic dynamic content. Non-standard - read your server docs. Enhanced SSI[1]: Suitable for more complex tasks within an HTML page. CGI: The standardised, portable general-purpose API, not limited to working with HTML pages. Enhanced CGI-like[2]: Typically gain efficiency but lose portability compared to standard CGI. Servlets: An alternative API for JAVA, that overcomes the limitation of JAVA not supporting environment variables. Server API: Generally the most powerful and most complex option. [1] For example, PHP, ASP. [2] For example, CGI adapted to mod_perl or fastcgi.
Subject: 1.8 What do I absolutely need to know? If you're already a programmer, CGI is extremely straightforward, and just three resources should get you up to speed in the time it takes to read them: 1) Installation notes for your HTTPD. Is it configured to run CGI scripts, and if so how does it identify that a URL should be executed? (Check your manuals, READMEs, ISP webpages/FAQS, and if you still can't find it ask your server administrator). 2) The CGI specification at NCSA tells you all you need to know to get your programs running as CGI applications. 3) WWW Security FAQ. This is not required to 'get it working', but is essential reading if you want to KEEP it working! If you're NOT already a programmer, you'll have to learn. If you would find it hard to write, say, a 'grep' or 'cat' utility to run from the commandline, then you will probably have a hard time with CGI. Make sure your programs work from the commandline BEFORE trying them with CGI, so that at least one possible source of errors has been dealt with.
Subject: 1.9 Does CGI create new security risks? Yes. Period. There is a lot you can do to minimise these. The most important thing to do is read and understand Lincoln Stein's excellent WWW security FAQ, at
Subject: 1.10 Do I need to be on Unix? No, but it helps. The Web, along with the Internet itself, C, Perl, and almost every other Good Thing in the last 20 years of computing, originated in Unix. At the time of writing, this is still the most mature and best-supported platform for Web applications.
Subject: 1.11 Do I have to use Perl? No - you can use any programming language you please. Perl is simply today's most popular choice for CGI applications. Some other widely- used languages are C, C++, TCL, BASIC and - for simple tasks - even shell scripts. Reasons for choosing Perl include its powerful text manipulation capabilities (in particular the 'regular' expression) and the fantastic WWW support modules available.
Subject: 1.12 What languages should I know/use? It isn't really that important. Use what you're comfortable with, or what you're constrained (eg by your manager) to use. If you're just dabbling with programming, Perl is a good choice, simply because of the wealth of ready-to-run Perl/CGI resources available. If you're serious about programming, you should be at home in a range of languages. C, the industry standard, is a must (at least to the level of comfortably reading other people's code). You'll certainly want at least one scripting language such as Perl, Python or Tcl. C++ is also a good idea. In response to a Usenet newbie question: > I am seriously wanting to learn some CGI programming languages J.M. Ivler wrote some eloquent words of wisdom: > If you want to learn a programming language, learn a programming language. > If you want to learn how to do CGI programming, learn a programming > language first. > > My book is one of the few that tackles two languages at the same time. > Why? because it's not about languages (which are just syntax for logic). > CGI programming is about programming, and how to leverage the experience > for the person coming to the site, or maintaining the site, or in some way > meeting some requirements. Language is just a tool to do so.
Subject: 1.13 Do I have to put it in cgi-bin? see next question
Subject: 1.14 Do I have to call it *.cgi? *.pl? Maybe. It depends on your server installation. These types of filenames are commonly used conventions - no more. It is up to the server administrator whether or not CGI scripts are enabled, and (if so) what conventions tell the server to run or to print them. If you are running your own server, read the manual. If you're on ISP or other rented webspace, check their webpages for information or FAQs. As a last resort, ask the server administrator.
Subject: 1.15 What is the "CGI Overhead", and should I be worried about it? The CGI Overhead is a consequence of HTTP being a stateless protocol. This means that a CGI process must be initialised for every "hit" from a browser. In the first instance, this usually means the server forking a new process. This in itself is a modest overhead, but it can become important on a heavily-used server if the number of processes grows to problem levels. In the second place, the CGI program must initialise. In the case of a compiled language such as C or C++ this is negligible, but there is a small penalty to pay for scripting languages such as Perl. Thirdly, CGI is often used as 'glue' to a backend program, such as a database, which may take some considerable time to initialise. This represents a major overhead, which must be avoided in any serious application. The most usual solution is for the backend program to run as a separate server doing most of the work, while the actual CGI simply carries messages. Fourthly, some CGI scripts are just plain inefficient, and may take hundreds of times the resources they need. Programs using system() or `backtick` notation often fall into this category. Note that there are ways to reduce or eliminate all these overheads, but these tend to be system- or server-specific. The best-supported server is probably Apache, as commercial server-vendors may prefer to push their proprietary solutions in preference to CGI.
Subject: 1.16 What do I need to know about file permissions and "chmod"? Unix systems are designed for multiple users, and include provision for protecting your work from unauthorised access by other users of the system. The file permissions determine who is permitted to do what with your programs, data, and directories. The command that sets file permissions is chmod. Web servers typically run as user "nobody". That means that, setting aside serious bugs (such as those in certain versions of the Frontpage extensions), your files are absolutely secure from damage through the webserver. It also means that you may have to make explicit changes to enable the server to access them in a CGI context. There are two ways to run CGI: - by default they run as the webserver user (nobody) For most purposes this is safest, as your programs and data are protected by the operating system from unauthorised access through possible bugs in your CGI. However, when the CGI has to write to a file, that file must be writable to every web user on the system, and is therefore completely unprotected. - setuid, they run under your own userid. This means that files written by your CGI can be secure. On the other hand, any bugs in your CGI could now compromise *all* your programs and data on the server. As an elementary security precaution, scripts (e.g. Perl) are prevented from running setuid by most OSs. The "cgiwrap" program offers a workaround for this. A third way you should *never* permit CGI to be run is: - as root or setuid root, they can run as any user. This is extremely dangerous, as any bugs could compromise the entire server, including every user's files. Fortunately only the system administrator can install setuid root programs. If you are *at all* concerned about security, make sure that no such programs (in particular Frontpage extensions) are installed, regardless of whether you use them yourself. For a proper overview, "man chmod". Some modes that may be useful in a typical CGI context are: * CGI programs, 0755 * data files to be readable by CGI, 0644 * directories for data used by CGI, 0755 * data files to be writable by CGI, 0666 (data has absolutely no security) * directories for data used by CGI with write access, 0777 (no security) * CGI programs to run setuid, 4755 * data files for setuid CGI programs, 0600 or 0644 * directories for data used by setuid CGI programs, 0700 or 0755 * For a typical backend server process, 4750 Finally, if this answer tells you anything you didn't already know, don't even think about trying to set up a secure server!
Subject: 1.17 What is CGIWrap, and how does it affect my program? [ quoted from ] > CGIWrap is a gateway program that allows general users to use CGI scripts > and HTML forms without compromising the security of the http server. > Scripts are run with the permissions of the user who owns the script. In > addition, several security checks are performed on the script, which will not > be executed if any checks fail. > > CGIWrap is used via a URL in an HTML document. As distributed, cgiwrap > is configured to run user scripts which are located in the > ~/public_html/cgi-bin/ directory. See
Subject: 1.18 How do I decode the data in my Form? The normal format for data in HTTP requests is URLencoded. All Form data is encoded in a string, of the form param1=value1&param2=value2&...paramn=valuen Many non-alphanumeric characters are "escaped" in the encoding: the character whose hexadecimal number is "XY" will be represented by the character string "%XY". Decoding this string is a fundamental function of every CGI library. Another format is "multipart/form-data", also known as "file upload". You will get this from the HTML markup <form method="POST" enctype="multipart/form-data"> (but note you must accept URLencoded input in any case, since not all browsers support multipart forms). Most(?) CGI libraries will handle this transparently.
Subject: SECTION 2 - HTTP HEADERS AND NPH SCRIPTS This is a fairly technical section dealing with HTTP, the protocol of the Web. It also includes NPH, the mechanism by which CGI programs can return HTTP header information directly to the Client.
Subject: 2.1 What is HTTP (HyperText Transfer Protocol)? HTTP is the protocol of the Web, by which Servers and Clients (typically browsers) communicate. An HTTP transaction comprises a Request sent by the Client to the Server, and a Response returned from the Server to the Client. Every HTTP request and response includes a message header, describing the message. These are processed by the HTTPD, and may often be mostly ignored by CGI applications (but see below). A message body may also be included: 1) A HEAD or GET request sends only a header. Any form data is encoded in an HTTP_QUERY_STRING header field, which is available to the CGI program as an environment variable QUERY_STRING. 2) A POST request sends both header and body. The body typically comprises data entered by a user in a form. 3) A HEAD request does not expect a body in the response. 4) A GET or POST request will accept a response with or without a body, according to the header. The body of a response is typically an HTML document.
Subject: 2.2 What HTTP request headers can I use? Most HTTP request headers are passed to the CGI script as environment variables. Some are guaranteed by the CGI spec. Others are server, browser and/or application dependent. To see what _your_ browser and server are telling each other, just use a trivial little CGI script to print out the environment. In Unix: #!/bin/sh echo "Content-type: text/plain" echo set (Just call it "env.cgi" or something, and put it where your server will execute it. Then point your browser at http://your.server/path/to/env.cgi ). This enables you to see at-a-glance what useful server variables are set. Note that dumping the environment like this within a more complex script can be a useful debugging technique. For details, see the CGI Environment Variables specification at (which also includes a version of the above script - somewhat more nicely formatted - online).
Subject: 2.3 What Environment variables are available to my application? See previous question. Those you can rely on are documented in NCSA's pages; those associated with your particular server and browser can be determined using the above script.
Subject: 2.4 Why doesn't my script get REMOTE_USER? My page is password-protected. You will get REMOTE_USER if the _script_ is password protected. That's all. The page the user is coming from has nothing to do with it.
Subject: 2.5 What HTTP response headers do I need to know about? Unless you are using NPH, the HTTPD will insert necessary response headers on your behalf, always provided it is configured to do so. However, it is conventional for servers to insert the Content-Type header based on a page's filename, and CGI scripts cannot rely on this. Hence the usual advice is to print an explicit Content-Type header. At least one of "Content-Type", "Status" and "Location" is almost always required. A few other headers you may wish to use explicitly are: Status (to set HTTP return code explicitly. Caveats: (1) Behaviour is undefined if it conflicts with another header. (2) This is NOT an HTTP header.) Location (to redirect the user to another URI, which may or may not be on your own server) Set-cookie (Netscape/Nonstandard) Set a cookie Refresh (Netscape/Nonstandard) Clientpull You can also use general MIME headers: eg "Keywords" for the benefit of indexers (although in this instance some major search robots have regrettably introduced a new protocol to do the same thing). For a detailed reference, see RFC1945 (HTTP/1.0) or RFC2068 (HTTP/1.1).
Subject: 2.6 What is NPH? NPH = No Parsed Headers. The script undertakes to print the entire HTTP response including all necessary header fields. The HTTPD is thereby instructed not to parse the headers (as it would normally do) nor add any which are missing.
Subject: 2.7 Must/should/can I write nph scripts? Generally, no. It is usually better to save yourself hassle by letting the HTTPD produce the headers for you. If you are going to use NPH, be sure to read and understand the HTTP spec at Your headers should be complete and accurate, because you're instructing the HTTPD not to correct them or insert what's missing. Possible circumstances where the use of NPH is appropriate are: * When your headers are sufficiently unusal that they might be differently parsed by different HTTPDs (eg combining "Location:" with a "Status:" other than 302). * When returning output over a period of time (eg displaying unbuffered results of a slow operation in 'real' time). See RFC1945 (HTTP/1.0) or RFC2068 (HTTP/1.1) for detail
Subject: 2.8 Do I have to call it nph-* According to NCSA's reference pages, this is the standard for telling the server that your script is NPH, so this should be a fully portable convention.
Subject: 2.9 What is the difference between GET and POST? Firstly, the the HTTP protocol specifies differing usages for the two methods. GET requests should always be idempotent on the server. This means that whereas one GET request might (rarely) change some state on the Server, two or more identical requests will have no further effect. This is a theoretical point which is also good advice in practice. If a user hits "reload" on his/her browser, an identical request will be sent to the server, potentially resulting in two identical database or guestbook entries, counter increments, etc. Browsers may reload a GET URL automatically, particularly if cacheing is disabled (as is usually the case with CGI output), but will typically prompt the user before re-submitting a POST request. This means you're far less likely to get inadvertently-repeated entries from POST. GET is (in theory) the preferred method for idempotent operations, such as querying a database, though it matters little if you're using a form. There is a further practical constraint that many systems have builtin limits to the length of a GET request they can handle: when the total size of a request (URL+params) approaches or exceeds 1Kb, you are well-advised to use POST in any case. In terms of mechanics, they differ in how parameters are passed to the CGI script. In the case of a POST request, form data is passed on STDIN, so the script should read from there (the number of bytes to be read is given by the Content-length header). In the case of GET, the data is passed in the environment variable QUERY_STRING. The content-type (application/x-www-form-urlencoded) is identical for GET and POST requests.
Subject: SECTION 3 - TECHNIQUES: "HOW DO I..." This section comprises programming hints and tips for a number of popular tasks. Also included are a number of common questions to which the answer is "you can't", with the reasons why.
Subject: 3.1 Can I get information about who is visiting? *sigh* Many people keep mailing me questions or suggested hacks to get visitor information, particularly email addresses. It seems they won't take "NO" for an answer. The bottom line is that whatever information is available to _you_ is _equally_ available to every spammer on the net. Therefore when a browser bug _does_ permit personal data to be collected, it gets reported and fixed very quickly (one short-lived Netscape 2.0.x release reportedly had such a bug in its Javascript engine). You can get some limited information from the environment variables passed to you by the browser. Relatively few of these are guaranteed to be available, and some may be misleading. For particular types of information, see below. For full details, see NCSA's reference pages.
Subject: 3.2 Can I get the email of visitors? Why do you want to do this? The best information available is the REMOTE_ADDR and REMOTE_HOST, which tell you nothing about the user. Techniques such as "finger@" are not reliable, are widely disliked, and generally serve only to introduce long delays in your CGI. Better - as well as more polite - just to ask your users to fill in a form. BTW: the "From:" header line (HTTP_FROM variable) is usually only set by robots, since human visitors to your webpage will not normally want their addresses collected without permission, and browsers respect this.
Subject: 3.3 "But I saw display my email address..." Some sites will play party tricks, which can get *some users* email addresses. Possible tell-tale signs of this are inordinate delays loading a page (fingering @REMOTE_HOST - doesn't often work but probably can't be detected from the webpage), or a submit button that appears to do nothing at all (a mailto: form - works well with some browsers but trivially detectable). As a "snoop" party trick that's fine, but if you find someone abusing these facilities (eg they send you junkmail), alert their service provider!
Subject: 3.4 Can I verify the email addresses people enter in my Form? Unfortunately people will sometimes enter an incorrect or invalid email address in your Form. Worse, they may enter a valid but incorrect email address that will deliver to someone who doesn't want your mail. Proposed regexps to match email addresses are sometimes posted. Most of these will fail against perfectly valid email addresses, like "S=N.OTHER/OU1=X12345A/RECIPNUM=1/" (which is what your address looks like if you are connected to the Internet via X400 - and if you think that example is too easy, check the ones at the end of Eli the Bearded's Email Addressing FAQ). Probably the most complete parser and checker available for download is Tom Christiansen's, at Of course, this still says nothing about deliverability. A frequently-suggested hack that doesn't work is to use SMTP EXPN or VRFY commands. Modern versions of sendmail permit administrators to disable these commands, and many sites take advantage of this facility to protect their users' privacy. Probably the best way to verify an email address is to send mail to it, asking the user to respond. Include a clause like "if you have received this mail in error, please accept our apologies..."
Subject: 3.5 Subject: How can I get the hostname of the remote user? You can't. Well, not always. IF it is available, you'll find it in the REMOTE_HOST environment variable. However, this will more often than not contain the numerical IP address rather than the IP name of the remote host. Remember that not all IP addresses have a hostname associated with them; this is the case of most IP addresses assigned to dialup users, for example. Your web server may also not perform a reverse lookup on incoming connections, in which case REMOTE_HOST will contain the IP address even if it has a corresponding IP name. In the second case, you can do a reverse lookup yourself in your script, but this is expensive and should probably be avoided unless absolutely necessary. Even if you do manage to obtain a hostname, you should be aware that it may not correspond to the hostname the user is accessing your page from. It may instead be that of an intervening proxy host. The short answer is therefore that there is no reliable way of finding out what the remote user's hostname is.
Subject: 3.6 Can I get browser details and return different pages? Why do you want to do this? Well-written HTML will display correctly in any browser, so the correct answer to this question is to design a template for your output in good HTML, and make sure your output is correct. If you insist on a different answer, you can use the HTTP_USER_AGENT environment variable. This requires care, and can lead to unexpected results. For example, checking for "Mozilla" and serving a frameset to it ensures that you *also* serve the frameset to early (Non-Frame) Netscapes, me-too browsers (notably Microsoft[1]) and others who have chosen to lie to you about their browser. Note also that not every User Agent is a browser. Your page may be read by a user agent you've never heard of, and then displayed by 100 different browsers. Or retrieved by different browsers from a cache. Another reason to write good HTML, and not try to devise a clever or koool substitute. [1] At the time of writing, only Netscape 2+ supported frames, and some authors considered them koool. That's changed, but the same general principle still holds.
Subject: 3.7 Can I trace where a user has come from/is going to? HTTP_REFERER might or might not tell you anything. By all means use it to collect partial statistics if you participate in (say) an advertising banner scheme. But it is not always set, and may be meaningless (eg if a user has accessed your page from a bookmark, and the browser is too dumb to cope with this). The HTTP protocol forbids relying on Referer information for functionality in your programs, so don't try it. You cannot trace outgoing links at all. If you really must try, point all the external links to your HTTPD and use its redirection facility (which gives you generally-reliable logs). This is much less inefficient than using a CGI script. BTW: don't even think about asking Javascript to send you information on some event: it's a violation of privacy which Netscape fixed as soon as complaints about its abuse started coming in. If it works with *your* browser, you should upgrade!
Subject: 3.8 Can I launch a long process and return a page before it's finished? [UNIX] You have to fork/spawn the long-running process. The important thing to remember is to close all its file descriptors; otherwise nothing will be returned to the browser until it's finished. The standard trick to accomplish this is redirection to/from /dev/null: "long_process < /dev/null > /dev/null 2>&1 &" print HTML page as usual
Subject: 3.9 Can I launch a long process which the user interacts with? This does not fit well with the basic mechanics of the Web, in which each transaction comprises a single request and response. If your processing can be done on the Client machine, you can use a clientside application; for example a Java applet. For processing on the server, one trick that works well for Clients running an X server (and far more efficient than a JAVA solution) is: if ( fork() ) { print HTML page explaining what's going on and advising about xhost } else { exec ("xterm -display THEIR_DISPLAY -title MY_APP -e MY_PROG ARGS < /dev/null > /dev/null 2>&1 &") ; } NOTE: THEIR_DISPLAY is not necessarily the same as REMOTE_HOST or REMOTE_ADDR. You have to ask users to supply their display (set REMOTE_HOST as default). A JAVA terminal program will accomplish something similar for the many users with platforms that support JAVA but not X.
Subject: 3.10 Can I password-protect my pages? Yes. Use your HTTPD's authentication, just as you would a basic HTML page. Now you'll have the identity of every visitor in REMOTE_USER.
Subject: 3.11 Can I do HTTP authentication using CGI? It depends on which version of the question you asked. Yes, you can use CGI to trigger the browser's standard Username/Password dialogue. Send a response code 401, together with a "WWW-authenticate" header including details of the the authentication scheme and realm: e.g. (in a non-NPH script) Status: 401 Unauthorized to access the document WWW-authenticate: Basic realm="foobar" Content-type: text/plain Unauthorised to access this document The use you can make of this is server-dependent, and harder, since most servers expect to deal with authentication before ever reaching the CGI (eg through .www_acl or .htaccess). Thus it cannot usefully replace the standard login sequence, although it can be applied to other situations, such as re-validating a user - e.g after a certain timeout period or if the same person may need to login under more than one userid. What you can never get in CGI is the credentials returned by the user. The HTTPD takes care of this, and simply sets REMOTE_USER to the username if the correct password was entered. For a much longer but outdated discussion of this question, see my discussion at
Subject: 3.12 Can I identify users/sessions without password protection? The most usual (but browser-dependent) way to do this is to set a cookie. If you do this, you are accepting that not all users will have a 'session'. An alternative is to pass a session ID in every GET URL, and in hidden fields of POST requests. This can be a big overhead unless _every_ page requires CGI in any case. Another alternative is the Hyper-G[1] solution of encoding a session-id in the URLs of pages returned: http://hyper-g.server/session_id/real/path/to/page This has the drawback of making the URLs very confusing, and causes any bookmarked pages to generate old session_ids. Note that a session ID based solely on REMOTE_HOST (or REMOTE_ADDR) will NOT work, as multiple users may access your pages concurrently from the same machine. [1] Actually I don't think that's been true of Hyper-G since sometime in '96. However, general advances in web server technology, such as Apache's mod_alias or mod_rewrite, make it straightforward without the need for CGI.
Subject: 3.13 Can I redirect users to another page? For permanent and simple redirection, use the HTTPD configuration file: it's much more efficient than doing it yourself. Some servers enable you to do this using a file in your own directory (eg Apache) whereas others use a single configuration file (eg CERN). For more complicated cases (eg process form inputs and conditionally redirect the user), use the "Location:" response header. If the redirection is itself a CGI script, it is easy to URLencode parameters to it in a GET request, but don't forget to escape the URL!
Subject: 3.14 Can I run a CGI script without returning a new page to the browser? Yes, but think carefully first: How are your readers going to know that their "submit" has succeeded? They may hit 'submit' many times! The correct solution according to the HTTP specification is to return HTTP status code 204. As an NPH script, this would be: #!/bin/sh # do processing (or launch it as background job) echo "HTTP/1.0 204 No Change" echo (as non-NPH, you'd simply replace HTTP/1.0 with the Status: CGI header). Alan J Flavell has pointed out that this will fail with certain popular browsers, and suggests a workaround to accommodate them: [ May 1998 update[1]: I'm deleting Alan's suggestion, because the problem is mainly of historical interest, and the workaround is no longer recommended. See his page for a a detailed survey and recommendations. ] His survey is at [1] With apologies to Alan for having left it in so long.
Subject: 3.15 Can I write output to a different Netscape frame? Yep. The fact you're using CGI makes no difference: use "target=" in your links as usual. Alternatively, the script can print a "Window-target:" header. Read Netscape's pages for detail: these answer all the questions about things like "getting rid of" or "breaking out of" frames, too.
Subject: 3.16 Can I write output to several frames at once? A single CGI script can only ever print to one frame. However, this limitation may be overcome by using more than one script. The first script (the URL of the "submit" button) prints a frameset, typically to a "_parent" or "_top" target. The sources for one or more of the frames thus generated may also be CGI scripts, to which you can easily pass parameters (eg encoded in URLs with method GET). This hack is definitely not recommended. If you find yourself wanting to update several frames from a single user event, it probably means you should review the design of your application at a higher level. Warnings: 1. Don't forget to escape your URLs. 2. This technique results in your server being hit by multiple concurrent CGI requests. You'll need LOTS of memory, especially if you use a memory-hog like Perl. It can be a good recipe for bringing a server to its knees. Javascript is often a valid alternative here, but note just how silly it can (and often does) look in a different browser.
Subject: 3.17 Can I use a CGI script to generate both text and inline images? Not directly. One script generates one response to one request. If you want to generate a dynamic page including dynamic images (say, a report including graphs, all of which depend on user input) then your primary script will print the usual <img src="[script-to-generate-image]" alt="[what you asked for]"> and, just as in the multiple frames case, you can pass data to the image-generating program encoded in a GET URL. Of course, the same caveats apply: see above.
Subject: 3.18 How can I use Caches to make CGI scripts faster and more Net-friendly? This is currently beyond the scope of this FAQ. However, there is an excellent introduction to net-friendly webpages, including CGI pages, at A sample cacheing perl/cgi script by Andrew Daviel is available at
Subject: 3.19 How can I avoid users hitting "submit" twice? You can't. You just have to deal with it when they do. You can avoid re-processing a submission by embedding a unique ID in your Form each time it is displayed. When you process the form, you enter the ID in a database. Or, if it's already there, you don't repeat the processing. You probably want to expire your database entries after a little time: an hour should be fine in a typical situation. If you're already using cookies (e.g. a shoppingcart), an alternative is to use the cookie as a unique identifier. This means you also have to handle the situation where a user deliberately "goes round twice" and submits the same form with different contents. If your script may take some time to process, you should also consider running it as a background job, and returning an immediate acknowledgement to the user (see above if your "immediate" response gets delayed until processing is complete in any case).
Subject: 3.20 How can I stop my CGI script reading and writing files as "nobody"? CGI scripts are run by the HTTPD, and therefore by the UID of the HTTPD process, which is (by convention) usually a special user "nobody". There are two basic ways to run a script under your own userid: (1) The direct approach: use a setuid program. (2) The double-server approach: have your CGI script communicate with a second process (e.g. a daemon) running under your userid, which is responsible for the actual file management. The direct approach is usually faster, but the client-server architecture may help with other problems, such as maintaining integrity of a database. When running a compiled CGI program (e.g. C, C++), you can make it setuid by simply setting the setuid bit: e.g. "chmod 4755 myprog.cgi" For security reasons, this is not possible with scripting languages (eg Perl, Tcl, shell). A workaround is to run them from a setuid program, such as cgiwrap. In most cases where you'd want to use the client-server approach, the server is a finished product (such as an SQL server) with its own CGI interface. A lightweight alternative to this is Don Libes' "expect" package. Note that any program running under your userid has access to all your files, and could do serious damage if hacked. Take care!
Subject: 3.21 How can I prevent my CGI results being cached by the browser? Firstly, we need to debunk a myth. People asking this question usually add that they tried "Pragma: no-cache". Whilst this is not actively wrong, there is no requirement on browsers to take any notice of it, and most of them don't. The "Pragma: no-cache" header (now superseded by HTTP/1.1 Cache-Control) is a directive to proxies. The browser sends it with an HTTP request to indicate that it wants the request to be dealt with by the original server and will not accept a proxy's cached document (e.g. when you use a reload button). The server may send it to tell a proxy not to cache the document. Having said all that, a practical hack to get round cacheing is to use a different URL for your CGI script each time it's called. This can easily be accomplished by adding a unique identifier such as current time in the QUERY_STRING or PATH_INFO. The browser will see a different URL, but the script can just ignore it. Note that this can be very inefficient, and should be avoided where possible.
Subject: 3.22 How can I control the default filename when downloading a file via CGI? (from a newsgroup post by Matthew Healy) One option, assuming you aren't already using the PATH_INFO environment variable, is just to call your CGI script with extra path information. For example, suppose the URL to your script is actually Instead, try calling it as and note that you need to escape the URL if it's in an HTML page:;name2=value2 And probably the browser will assign the name given in the last chunk as the suggested filename for downloading. This works because the http server looks for the program file to run, then passes any extra path to the program as PATH_INFO variable; the browser cannot tell where the SCRIPT_NAME part ends and the PATH_INFO part begins. This can also be very useful if you want one script to generate more than one filename -- the script can check the PATH_INFO value and alter its response accordingly...
Subject: SECTION 4 - TROUBLESHOOTING A CGI APPLICATION Since this subject is quite well covered by other documents, this FAQ has relatively little to say. Eric Wienke has a page "Debugging CGI Scripts 101" at Tom Christiansen's "Idiot's guide to solving Perl/CGI problems" is a slightly tongue-in-cheek list of common problems, and how to track them down. Much of what Tom covers is not specifically Perl, but applies equally to CGI programming in other languages. Marc Hedlund's CGI FAQ and Thomas Boutell's WWW FAQ also deal with this subject. See "Further Reading" below (if you don't already know where to find these documents).
Subject: 4.1 Are there some interactive debugging tools and services available? (1) Several CGI programming libraries offer powerful interactive debugging facilities. These include: - for Perl, Lincoln Stein's (now part of the standard Perl distribution) - for Tcl, Don Libes' cgi.tcl - for C++, Nick Kew's CGI++ (2) Nathan Neulinger's cgiwrap is another package with debugging aids. (3) The "mod_cgi" Apache module (new with Apache 1.2) enables you to capture script output and errors for diagnosis. See also the next question.
Subject: 4.2 I'm having trouble with my headers. What can I do? For simple cases, examining your response headers "by hand" may suffice: (1) telnet to the host and port where the server is running - e.g. telnet 80 (2) Enter HTTP request. The most useful for this purpose is usually HEAD; eg HEAD /index.html HTTP/1.0 (optional HTTP headers) (followed by a blank line) Now you'll get a full HTTP response header back. For complex cases, such as sending a request with headers (as a browser does) or POSTing a form, this author's free online diagnosis cg-eye is included in the respective toolkits at This combines an offline cgi "linter" with two online services: (a) Interactive mode permits you to formulate an HTTP request, which is then sent to your server. (b) Live mode submits your form, exactly as it gets it from your browser. In both cases, it will print a detailed report of the transaction, and optionally (if the CGI is producing an HTML page) validate it.
Subject: 4.3 Why do I get Error 500 ("the script misbehaved", or "Internal Server Error") Your script must follow the CGI interface, which requires it to print: (1) One or more Header lines. (2) A blank line (3) (optional, but strongly advised) a document body. This error means it didn't. The Header lines can include anything that's valid under HTTP, but must normally include at least one of the three special CGI headers: Content-Type Location Status Example (a very minimal HTML page via CGI) Content-Type: text/html <= Header <= Blank Line <title>HelloWorld</title>Hello World <= Document Body A common reason for a script to fail is that it crashed before printing the header and blank line (or while these are buffered). Or that it didn't run at all: you _did_ try it from the commandline as well as check the file permissions and server configuration, didn't you? Another possible reason is that it printed something else - like an error message - in the Headers. Check error logs, put a dummy header right at the top (for debugging only), check the "Idiot's Guide", and use the debug mode of your CGI library.
Subject: 4.4 I tried to use (Content-Type|Location|whatever), but it appears in my Browser? That means you put the line in the wrong place. It must appear in the CGI Header, not the document body. See previous question. It's also possible that you didn't print a header at all, or had a blank line or other noise before or in the header, but that the HTTPD has corrected this error for you (servers which correct your errors may give rise to the "works on A not on B" phenomenon). See previous question.
Subject: 4.5 How can I run my CGI program 'live' in a debugger? David S. Jackson offers the following tip: > I have a very good trick for debugging CGIs written in C/C++ running on > UNIX. You might want to add it to the debugging section of your CGI faq. > > First, in your CGI code, at it's start, add "sleep(30);". This will cause > the CGI to do nothing for thiry seconds (you may need to adjust this > time). Compile the CGI with debuging info ("-g" in gcc) and install the > CGI as normal. Next, using your web browser, activate the CGI. It will of > course just sit there doing nothing. While it is 'sleeping', find it's PID > (ps -a | grep <cgi name>). Load your debugger and attach to that PID > ("attach <pid>" in gdb). You will also need to tell it where to find the > symbol definitions ("symbol-file <cgi>" in gdb). Then set a break point > after the invocation of the sleep function and you are ready to debug. Do > be aware that your browser will eventually timeout if it doesn't recieve > anything. (Anyone know similar tricks for scripting languages)?
Subject: 4.6 I'm using CGI with QUERY_STRING embedded in my HTML, but it gets corrupted? The problem is the & character, which has two separate special meanings: - In HTTP (and hence CGI) it is a separator in your QUERY_STRING - In HTML it is an escape character So when it appears in an HTML context, it should be encoded. If you need a link to myprog.cgi with QUERY_STRING "a=1&b=2" you should write <a href="myprog.cgi?a=1&amp;b=2">my program</a> which the browser's HTML parser will convert to what you wanted. There are possible browser problems here, although they appear to be limited to older browsers. Some other approaches are: - Use a different separator character in CGI programs when called in this manner. Or even a completely different encoding. This is safe, but may be much more work unless your CGI library supports setting a different separator character. - Avoid any parameters whose names include that of any HTML entity. This runs a possible risk if the set of entities changes in future, or when browsers introduce proprietary 'extensions'.
Subject: 5.1 Other FAQs/collections **** Lincoln Stein's FAQ is probably the most **** **** important WWW document you will ever read. **** Web Authoring FAQs For general WWW issues, the World Wide Web FAQ by Thomas Boutell Perl/CGI programming FAQ, by Shishir Gundavaram and Tom Christiansen The Idiot's Guide to solving Perl/CGI problems by Tom Christiansen The WWW Security FAQ by Lincoln Stein CGI Resources Library The WWW Virtual Library
Subject: 5.2 Reference Pages CGI Internet Draft - the official spec The Common Gateway Interface (CGI) - old de facto spec HyperText Transfer Protocol (HTTP) HyperText Markup Language (HTML)
Subject: INDEX The index is generated from an arbitrary list of keywords. If I've missed anything obvious that should be here, please let me know. APACHE 1.15, 3.12, 3.13, 4.1 ASP 1.5, 1.7 AUTHENTICATION 3.10, 3.11 BACKGROUND 3.14, 3.19 BASIC 1, 1.7, 1.11, 3.4, 3.9, 3.10, 3.11, 3.20 BROWSER 1.15, 2.2, 2.3, 2.9, 3.1, 3.6, 3.7, 3.8, 3.11, 3.12, 3.16, 3.21, 3.22, 4.2, 4.5, 4.6 C 1.10, 1.11, 1.12, 1.15, 3.20, 4.1, 4.5 CACHE 3.6, 3.21 CERN 3.13 CGI 0.3, 0.6, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.11, 1.12, 1.14, 1.15, 1.16, 1.17, 1.18, 2.1, 2.2, 2.5, 2.9, 3.2, 3.7, 3.11, 3.12, 3.13, 3.14, 3.15, 3.16, 3.18, 3.20, 3.21, 3.22, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 5.1, 5.2 CGIWRAP 1.16, 1.17, 3.20, 4.1 CHMOD 1.16, 3.20 COOKIE 2.5, 3.12, 3.19 CREDENTIALS 3.11 DATABASE 1.15, 2.9, 3.19, 3.20 DEBUG 4.3, 4.5 EMAIL 0.3, 3.1, 3.3, 3.4 ENVIRONMENT 1.7, 2.1, 2.2, 2.9, 3.1, 3.5, 3.6, 3.22 ERROR 3.4, 4.3, 4.4 EXPECT 0.5, 2.1, 3.11, 3.20, 4.1 FAQ 0, 0.3, 0.5, 0.6, 0.7, 1.8, 1.9, 3.4, 3.18, 4.5, 5.1 FORK 3.8, 3.9 FRAMES 3.6, 3.15, 3.16, 3.17 GET 0.1, 0.5, 1.8, 1.18, 2.1, 2.4, 2.9, 3.1, 3.3, 3.11, 3.12, 3.13, 3.16, 3.17, 3.21, 4.2 HEAD 2.1, 4.2 HEADER 0.6, 2, 2.1, 2.5, 2.6, 2.9, 3.2, 3.11, 3.13, 3.14, 3.15, 3.21, 4.2, 4.3, 4.4 HTML 0.3, 1.1, 1.5, 1.7, 1.8, 1.9, 1.17, 1.18, 2.1, 2.2, 3.6, 3.8, 3.9, 3.10, 3.11, 3.14, 3.22, 4, 4.2, 4.3, 4.6, 5.1, 5.2 HTTP 0.3, 1.1, 1.8, 1.9, 1.15, 1.17, 1.18, 2, 2.1, 2.2, 2.5, 2.6, 2.7, 2.9, 3.4, 3.7, 3.11, 3.12, 3.14, 3.18, 3.21, 3.22, 4.1, 4.2, 4.3, 4.6, 5.1, 5.2 HTTPD 1.5, 1.6, 1.8, 2.1, 2.5, 2.6, 2.7, 3.7, 3.10, 3.11, 3.13, 3.20, 4.4 IMAGE 3.17 JAVA 1.4, 1.7, 3.9 JAVASCRIPT 3.1, 3.7, 3.16 LOCATION 2.5, 2.7, 3.13, 4.3 MICROSOFT 1.5, 3.6 MOZILLA 3.6 MULTIPART 1.18 NCSA 1.1, 1.5, 1.8, 2.2, 2.3, 2.8, 3.1, 5.2 NETSCAPE 2.5, 3.1, 3.6, 3.7, 3.15 NOBODY 1.16, 3.20 NPH 2, 2.5, 2.6, 2.7, 2.8, 3.11, 3.14 PASSWORD 2.4, 3.11 PERL 1.10, 1.11, 1.12, 1.15, 1.16, 3.4, 3.16, 3.18, 3.20, 4.1, 5.1 PERMISSIONS 1.16, 1.17, 4.3 PHP 1.5, 1.7 POST 0.5, 0.6, 1.18, 2.1, 2.9, 3.12, 3.22 PRAGMA 3.21 REDIRECT 2.5, 3.13 REFRESH 2.5 REQUEST 2.1, 2.2, 2.9, 3.9, 3.13, 3.17, 3.21, 4.2 RESPONSE 1.5, 1.12, 2.1, 2.5, 2.6, 3.9, 3.11, 3.13, 3.17, 3.19, 3.22, 4.2 SECURITY 1.8, 1.9, 1.16, 1.17, 3.20, 5.1 SERVER 0.3, 1.4, 1.5, 1.7, 1.8, 1.14, 1.15, 1.16, 1.17, 2.1, 2.2, 2.3, 2.5, 2.8, 2.9, 3.5, 3.9, 3.11, 3.12, 3.16, 3.20, 3.21, 3.22, 4.2, 4.3 SSI 1.5, 1.7 STATUS 2.5, 2.7, 3.11, 3.14, 4.3 TCL 1.11, 1.12, 3.20, 4.1 UNIX 1.10, 1.16, 2.2, 3.8, 4.5 URL 0.3, 1.8, 1.17, 2.9, 3.12, 3.13, 3.16, 3.17, 3.21, 3.22 URLENCODE 3.13 WWW 0.3, 0.6, 1.4, 1.8, 1.9, 1.11, 1.17, 2.7, 2.9, 3.4, 3.11, 4.1, 4.2, 5.1, 5.2 .

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