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 2022, the althttpd instance for sqlite.org answers about 500,000 HTTP requests per day (about 5 or 6 per second) delivering about 200GB of content per day (about 18 megabits/second) on a $40/month Linode. The load average on this machine normally stays around 0.5. About 10% of the HTTP requests are CGI to various Fossil source-code repositories.

Design Philosophy

Althttpd is usually launched from xinetd or similar. 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 (or 443 for incoming HTTPS 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.

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

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.

For serving TLS connections there are two options:

1. althttpd can be built with the ENABLE_TLS macro defined and linked to -lssl -lcrypto, then started with the --cert fullchain.pem and --pkey privkey.pem flags.

2. althttpd can be started via an external connection service such as stunnel4, passing the -https 1 flag to althttpd to tell it that it is "indirectly" operating in HTTPS mode via that service.

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.

To build althttpd with built-in TLS support using libssl:

gcc -Os -o /usr/bin/althttpd -fPIC -DENABLE_TLS \
althttpd.c -lssl -lcrypto

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.

Supporting HTTPS using Xinetd

Beginning with version 2.0 (2022-01-16), althttpd optionally support TLS-encrypted connections. Setting up an HTTPS website using Xinetd is very similar to an HTTP website. The appropriate configuration for xinetd is a single file named "https" in the /etc/xinetd.d directory with content like the following:

service https
{
  port = 443
  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 -cert /etc/letsencrypt/live/sqlite.org/fullchain.pem -pkey /etc/letsencrypt/live/sqlite.org/privkey.pem
  bind = 45.33.6.223
}
service https
{
  port = 443
  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 -cert /etc/letsencrypt/live/sqlite.org/fullchain.pem -pkey /etc/letsencrypt/live/sqlite.org/privkey.pem
}

You will, of course, want to adjust pathnames and IP address so that they are appropriate for your particular installation.

This https configuration file is the same as the previous http configuration file with just a few changes:

• Change the service name from "http" to "https"
• Change the port number from 80 to 443
• Add -cert and -pkey options to althttpd so that it will know where to find the appropriate certificate and private-key.

After creating the new https configuration file, simply restart xinetd (usually with the command "/etc/init.d/xinetd restart") and immediately an HTTPS version of your existing website will spring into existance.

Setup For HTTPS Using Stunnel4

Older versions of althttpd did not support encryption. The recommended way of encrypting website using althttpd was to use stunnel4. This advice has now changed. We now recommend that you update your althttpd to version 2.0 or later and use the xinetd technique described in the previous section. This section is retained for historical reference.

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.

Stand-alone with HTTPS

If althttpd is built with TLS support then it can be told to operate in HTTPS mode with one of the following options:

althttpd -root ~/www --port 8043 --cert unsafe-builtin

this option uses a compiled-in self-signed SSL certificate which is wildly insecure and is intended for testing purposes only. Use the --cert option to specify your own PEM-format SSL certificate. The argument to --cert can be the concatenation of the SSL private key (often named "privkey.pem") and the certificate chain (often named "fullchain.pem"). Alternatively, the --cert can point to just the fullchain.pem file and the separate --pkey option can point to the privkey.pem file.

Using your own certificate:

althttpd -root ~/www --port 8043 --cert fullchain.pem --pkey privkey.pem

Note that the certificate is read before althttpd drops root privileges, so the certificate may live somewhere inaccessible to the non-root user under which the althttpd process will run.

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 it is 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 "-".

When althttpd returns a 404, it tries to determine whether the request is malicous and, if it believes so, it may optionally temporarily block the client's IP.

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/". File or directory names below "/.well-known/" are allowed to begin with "." or "-" (but not with ".."). 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. The "-auth" file is read and processes sequentially, so lines below the "http-redirect" line are never seen or processed for http requests.

• 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

Client IP Blocking

If the --ipshun DIRECTORY option is included to althttpd and DIRECTORY is an absolute pathname (begins with "/") accessible from within the chroot jail, and if the IP address of the client appears as a file within that directory, then althttpd might return 503 Service Unavailable rather than process the request.

• If the file is zero bytes in size, then 503 is always returned. Thus you can "touch" a file that is an IP address name to permanently banish that client.

• If the file is N bytes in size, then 503 is returned if the mtime of the file is less than 60*N seconds ago. In other words, the client is banished for one minute per byte in the file.

Banishment files are automatically created if althttpd gets a request that would have resulted in a 404 Not Found, and upon examining the REQUEST_URI the request looks suspicious. Any request that include /../ is considered a hack attempt, for example. There are other common vulnerability probes that are also checked. Probably this list of vulnerability probes will grow with experience.

The banishment files are automatically unlinked after 5 minutes/byte.

Banishment files are initially 1 byte in size. But if a banishment expires and then a new request arrives prior to 5 minutes per byte of block-file size, then the file grows by one byte and the mtime is reset.

GZip Content Compression

Althttpd has basic support for server-side content compression, which often reduces the over-the-wire cost of files by more than half. Rather than add a dependency on a compression library to althttpd, it relies on the client to provide content in both compressed and uncompressed forms.

When serving a file, if the client expresses support for gzip compression and a file with the same name plus a .gz extension is found, the gzipped copy of the file is served to the client with a response header indicating that it is gzipped. To the user, it appears as if the originally-requested file is served compressed. Under the hood, however, a different file is served.

Note that this feature only supports static files, not CGI.