Emacs Web Server

As part of my quest of developing solid knowledge of GNU Emacs lisp, I have implemented a pseudo-HTTP/1.0 web server within Emacs. Behold,

git clone git://github.com/skeeto/emacs-http-server.git

To all other non-emacsen text editors, can your text editor do that?! Ha! Even though elisp is a slow, closure-less, dynamically scoped, ugly cousin of more popular lisps, it's still a lot of fun to write.

To fire it up, load it into Emacs and run the extended command (M-x) httpd-start. By default it will serve files from "~/public_html". To change this, change the variable httpd-root to the desired web root. You can stop the server with httpd-stop.

It's about 200 lines of code and can serve static websites made of small, static files. I say small files because it serves files from buffers, meaning it has to read the entire file in first.

For a simple, text editor based server it can hold up to a pretty decent load. At one point I hit it with 8 wget instances all making rapid recursive downloads and my manual navigation wasn't slowed down noticeably. Despite running in the slow elisp interpreter, I think it can have much better performance by caching commonly served files in buffers.

It should run, unmodified, anywhere a modern Emacs can run, so I expect that it's already very portable. I can imagine it being useful in a situation where someone needs to temporarily host some files but there isn't a web server on the machine. Just grab this script and throw it at Emacs.

Well, it only does IPv4 right now, though I expect IPv6 only requires changing one number (namely, 4 to 6). I don't have any IPv6 systems to test it on.

When writing it I also had security in mind so, as far as I know, it should be safe to use. It cleans up the GET from the client so that no files underneath the serving root can be accessed.

The server log is lisp itself. Here is an example log starting the server, serving one request, and halting,

  (start "Wed May 13 23:33:34 2009")
   (date "Wed May 13 23:36:25 2009")
   (address "")
   (get "/0001.html")
    ("Referer" "")
    ("Connection" "keep-alive")
    ("Keep-Alive" "300")
    ("Accept-Charset" "ISO-8859-1,utf-8;q=0.7,*;q=0.7")
    ("Accept-Encoding" "gzip,deflate")
    ("Accept-Language" "en-us,en;q=0.5")
    ("Accept" "image/png,image/*;q=0.8,*/*;q=0.5")
    ("User-Agent" "Mozilla/5.0 [...] Iceweasel/3.0.9 (Debian-3.0.9-1)")
    ("Host" "")
    ("GET" "/0001.html" "HTTP/1.1"))
   (path "~/public_html/0001.html")
   (status 200))
  (stop "Wed May 13 23:38:17 2009"))

The log is alists of alists, making a hierarchical tree structure that can be explored with some simple lisp functions. Normally this sort of thing is done with XML, but lisp already has its own structured format: lists!

When GET is a directory, it looks for "index.html" and serves that if it exists. More indexes can be added to the variable httpd-indexes. This can actually be done in a special ".htaccess.el" file.

If a ".htaccess.el" exists in the directory from which a file is being served, Emacs will first load/execute it. You see, it's just a lisp program. If you wanted to add a new index file name, the hypertext access file could contain this,

(add-to-list 'httpd-indexes "0001.html")

It's a bit like a .emacs file.

But I think one of the coolest things about having a lisp-based server is that the server can be modified in place without disrupting or restarting it. In my Emacs web server, the only change that requires a restart is changing the server port. In fact, I wrote most of it while the server was running and tested my changes from a browser right as I made them — all on the same instance of the server.

If you want to look into the AI side of this, the server could modify its own code in response to its use.

I also had the idea of creating dynamic websites with elisp, in the same way PHP or Perl does. If a .el file (or .elc) is accessed, the server would pass the GET/POST arguments as an alist to a function in the elisp file. The server would also provide some nifty HTML generation macros. A dynamic script might look like this,

(defun script (get)
    (title "My Script"))
    (h1 "Your Query")
    (p (concat "Your query was "
               (html-sanitize (cdr (assoc "q" get)) "."))))))

However, this is not (yet?) implemented. Just an idea.

I will continue to work on it, though I don't expect to add much more to it. I will mostly improve the code and documentation.

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Chris Wellons

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