The Althttpd Webserver

Althttpd is a simple webserver that has run the https://sqlite.org/ website since 2004. Althttpd strives for simplicity, security, and low resource usage.

As of 2018, the althttpd instance for sqlite.org answers about 500,000 HTTP requests per day (about 5 or 6 per second) delivering about 50GB of content per day (about 4.6 megabits/second) on a $40/month Linode. The load average on this machine normally stays around 0.1 or 0.2. About 19% of the HTTP requests are CGI to various Fossil source-code repositories.

Design Philosophy

Althttpd is usually launched from xinetd or stunnel4. A separate process is started for each incoming connection, and that process is wholly focused on serving that one connection. A single althttpd process will handle one or more HTTP requests over the same connection. When the connection closes, the althttpd process exits.

Althttpd can also operate stand-alone. Althttpd itself listens on port 80 for incoming HTTP requests, then forks a copy of itself to handle each inbound connection. Each connection is still handled using a separate process. The only difference is that the connection-handler process is now started by a master althttpd instance rather than by xinetd or stunnel4.

Althttpd has no configuration file. All configuration is handled using a few command-line arguments. This helps to keep this configuration simple and mitigate worries about about introducing a security vulnerability through a misconfigured web server.

Althttpd does not itself handle TLS connections. For HTTPS, althttpd relies on stunnel4 to handle TLS protocol negotiation, decryption, and encryption.

Because each althttpd process only needs to service a single connection, althttpd is single threaded. Furthermore, each process only lives for the duration of a single connection, which means that althttpd does not need to worry too much about memory leaks. These design factors help keep the althttpd source code simple, which facilitates security auditing and analysis.

Source Code

The complete source code for althttpd is contained within a single C-code file with no dependences outside of the standard C library. The source code file is named "althttpd.c". To build and install althttpd, run the following command:

 gcc -Os -o /usr/bin/althttpd althttpd.c

The althttpd source code is heavily commented and accessible. It should be relatively easy to customize for specialized needs.

Setup Using Xinetd

Shown below is the complete text of the /etc/xinetd.d/http file on sqlite.org that configures althttpd to server unencrypted HTTP requests on both IPv4 and IPv6. You can use this as a template to create your own installations.

service http
{
  port = 80
  flags = IPv4
  socket_type = stream
  wait = no
  user = root
  server = /usr/bin/althttpd
  server_args = -logfile /logs/http.log -root /home/www -user www-data
  bind = 45.33.6.223
}

service http
{
  port = 80
  flags = REUSE IPv6
  bind = 2600:3c00::f03c:91ff:fe96:b959
  socket_type = stream
  wait = no
  user = root
  server = /usr/bin/althttpd
  server_args = -logfile /logs/http.log -root /home/www -user www-data
}

The key observation here is that each incoming TCP/IP connection on port 80 launches a copy of /usr/bin/althttpd with some additional arguments that amount to the configuration for the webserver.

Notice that althttpd is run as the superuser. This is not required, but if it is done, then althttpd will move itself into a chroot jail at the root of the web document hierarchy (/home/www in the example) and then drop all superuser privileges prior to reading any content off of the wire. The -user option tells althttpd to become user www-data after entering the chroot jail.

The -root option tells althttpd where to find the document hierarchy. In the case of sqlite.org, all content is served from /home/www. At the top level of this document hierarchy is a bunch of directories whose names end with ".website". Each such directory is a separate website. The directory is chosen based on the Host: parameter of the incoming HTTP request. A partial list of the directories on sqlite.org is this:

3dcanvas_tcl_lang_org.website
3dcanvas_tcl_tk.website
androwish_org.website
canvas3d_tcl_lang_org.website
canvas3d_tcl_tk.website
cvstrac_org.website
default.website
fossil_scm_com.website
fossil_scm_hwaci_com.website
fossil_scm_org.website
system_data_sqlite_org.website
wapp_tcl_lang_org.website
wapp_tcl_tk.website
www2_alt_mail_net.website
www_androwish_org.website
www_cvstrac_org.website
www_fossil_scm_com.website
www_fossil_scm_org.website
www_sqlite_org.website

For each incoming HTTP request, althttpd takes the text of the Host: parameter in the request header, converts it to lowercase, and changes all characters other than ASCII alphanumerics into "_". The result determines which subdirectory to use for content. If nothing matches, the "default.website" directory is used as a fallback.

For example, if the Host parameter is "www.SQLite.org" then the name is translated into "www_sqlite_org.website" and that is the directory used to serve content. If the Host parameter is "fossil-scm.org" then the "fossil_scm_org.website" directory is used. Oftentimes, two or more names refer to the same website. For example, fossil-scm.org, www.fossil-scm.org, fossil-scm.com, and www.fossil-scm.com are all the same website. In that case, typically only one of the directories is a real directory and the others are symbolic links.

On a minimal installation that only hosts a single website, it suffices to have a single subdirectory named "default.website".

Within the *.website directory, the file to be served is selected by the HTTP request URI. Files that are marked as executable are run as CGI. Non-executable files with a name that ends with ".scgi" and that have content of the form "SCGI hostname port" relay an SCGI request to hostname:port. All other non-executable files are delivered as-is.

If the request URI specifies the name of a directory within *.website, then althttpd appends "/home", "/index.html", and "/index.cgi", in that order, looking for a match.

If a prefix of a URI matches the name of an executable file then that file is run as CGI. For as-is content, the request URI must exactly match the name of the file.

For content delivered as-is, the MIME-type is deduced from the filename extension using a table that is compiled into althttpd.

Setup For HTTPS Using Stunnel4

Althttpd itself does not do any encryption. To set up an encrypted website using althttpd, the recommended technique is to use stunnel4.

On the sqlite.org website, the relevant lines of the /etc/stunnel/stunnel.conf file are:

cert = /etc/letsencrypt/live/sqlite.org/fullchain.pem
key = /etc/letsencrypt/live/sqlite.org/privkey.pem
[https]
accept       = :::443
TIMEOUTclose = 0
exec         = /usr/bin/althttpd
execargs     = /usr/bin/althttpd -logfile /logs/http.log -root /home/www -user www-data -https 1

This setup is very similar to the xinetd setup. One key difference is the "-https 1" option is used to tell althttpd that the connection is encrypted. This is important so that althttpd will know to set the HTTPS environment variable for CGI programs.

It is ok to have both xinetd and stunnel4 both configured to run althttpd, at the same time. In fact, that is the way that the SQLite.org website works. Requests to http://sqlite.org/ go through xinetd and requests to https://sqlite.org/ go through stunnel4.

Stand-alone Operation

On the author's desktop workstation, in his home directory is a subdirectory named ~/www/default.website. That subdirectory contains a collection of files and CGI scripts. Althttpd can serve the content there by running the following command:

althttpd -root ~/www -port 8080

The "-port 8080" option is what tells althttpd to run in stand-alone mode, listening on port 8080.

The author of althttpd has only ever used stand-alone mode for testing. Since althttpd does not itself support TLS encryption, the stunnel4 setup is preferred for production websites.

Security Features

To defend against mischief, there are restrictions on names of files that althttpd will serve. Within the request URI, all characters other than alphanumerics and ",-./:~" are converted into a single "_". Furthermore, if any path element of the request URI begins with "." or "-" then althttpd always returns a 404 Not Found error. Thus is it safe to put auxiliary files (databases or other content used by CGI, for example) in the document hierarchy as long as the filenames being with "." or "-".

An exception: Though althttpd normally returns 404 Not Found for any request with a path element beginning with ".", it does allow requests where the URI begins with "/.well-known/". This exception is necessary to allow LetsEncrypt to validate ownership of the website.

Basic Authentication

If a file named "-auth" appears anywhere within the content hierarchy, then all sibling files and all files in lower-level directories require HTTP basic authentication, as defined by the content of the "-auth" file. The "-auth" file is plain text and line oriented. Blank lines and lines that begin with "#" are ignored. Other lines have meaning as follows:

• http-redirect

  The http-redirect line, if present, causes all HTTP requests to redirect into an HTTPS request.

• https-only

  The https-only line, if present, means that only HTTPS requests are allowed. Any HTTP request results in a 404 Not Found error. The https-only line normally occurs after an http-redirect line.

• realm NAME

  A single line of this form establishes the "realm" for basic authentication. Web browsers will normally display the realm name as a title on the dialog box that asks for username and password.

• user NAME LOGIN:PASSWORD

  There are multiple user lines, one for each valid user. The LOGIN:PASSWORD argument defines the username and password that the user must type to gain access to the website. The password is clear-text - HTTP Basic Authentication is not the most secure authentication mechanism. Upon successful login, the NAME is stored in the REMOTE_USER environment variable so that it can be accessed by CGI scripts. NAME and LOGIN are usually the same, but can be different.

• anyone

  If the "anyone" line is encountered, it means that any request is allowed, even if there is no username and password provided. This line is useful in combination with "http-redirect" to cause all ordinary HTTP requests to redirect to HTTPS without requiring login credentials.

Basic Authentication Examples

The http://www.sqlite.org/ website contains a "-auth" file in the toplevel directory as follows:

 http-redirect
 anyone

That -auth file causes all HTTP requests to be redirected to HTTPS, without requiring any further login. (Try it: visit http://sqlite.org/ and verify that you are redirected to https://sqlite.org/.)

There is a "-auth" file at https://fossil-scm.org/private/ that looks like this:

 realm Access To All Fossil Repositories
 http-redirect
 user drh drh:xxxxxxxxxxxxxxxx

Except, of course, the password is not a row of "x" characters. This demonstrates the typical use for a -auth file. Access is granted for a single user to the content in the "private" subdirectory, provided that the user enters with HTTPS instead of HTTP. The "http-redirect" line is strongly recommended for all basic authentication since the password is contained within the request header and can be intercepted and stolen by bad guys if the request is sent via HTTP.

Log File

If the -logfile option is given on the althttpd command-line, then a single line is appended to the named file for each HTTP request. The log file is in the Comma-Separated Value or CSV format specified by RFC4180. There is a comment in the source code that explains what each of the fields in this output line mean.

The fact that the log file is CSV makes it easy to import into SQLite for analysis, using a script like this:

CREATE TABLE log(
  date TEXT,             /* Timestamp */
  ip TEXT,               /* Source IP address */
  url TEXT,              /* Request URI */
  ref TEXT,              /* Referer */
  code INT,              /* Result code.  ex: 200, 404 */
  nIn INT,               /* Bytes in request */
  nOut INT,              /* Bytes in reply */
  t1 INT, t2 INT,        /* Process time (user, system) milliseconds */
  t3 INT, t4 INT,        /* CGI script time (user, system) milliseconds */
  t5 INT,                /* Wall-clock time, milliseconds */
  nreq INT,              /* Sequence number of this request */
  agent TEXT,            /* User agent */
  user TEXT,             /* Remote user */
  n INT,                 /* Bytes of url that are in SCRIPT_NAME */
  lineno INT             /* Source code line that generated log entry */
);
.mode csv
.import httplog.csv log

The filename on the -logfile option may contain time-based characters that are expanded by strftime(). Thus, to cause a new logfile to be used for each day, you might use something like:

 -logfile /var/logs/althttpd/httplog-%Y%m%d.csv