Pound

Pound is a reverse proxy, load balancer and HTTPS front-end for Web servers. It was developed to enable distributing the load among several Web-servers and to allow for a convenient SSL wrapper for those Web servers that do not offer it natively. Pound is distributed under the GNU General Public License, Version 3, or (at your option) any later version.

The original version of pound was written by Robert Segall roseg@apsis.ch, at Apsis GmbH. In 2018, Sergey Poznyakoff added support for OpenSSL 1.x to the then current version of the program (2.8). This version of pound, hosted on github was further modified by Rick O'Sullivan and Frank Schmirler, who added WebSocket support.

On April 2020, Apsis started development of pound 3.0 - essentially an attempt to rewrite pound from scratch, introducing dependencies on some third-party software.

On 2022-09-19, Robert announced that he stops further development and maintenance of pound. Following that, Sergey decided to continue development of the program starting from his fork.

What Pound Is

1. a reverse-proxy: it passes requests from client browsers to one or more backend servers.
2. a load balancer: it distributes the requests from client browsers among several backend servers, while keeping session information.
3. an SSL wrapper: it decrypts HTTPS requests from client browsers and passes them as plain HTTP to the back-end servers.
4. an HTTP/HTTPS sanitizer: it verifies requests for correctness and accepts only well-formed ones.
5. a fail-over server: should a backend server fail, pound will take note of the fact and stop passing requests to it until it recovers.
6. a request redirector: requests may be distributed among servers according to the requested URL.

Pound is a very small program, easily audited for security problems. It can run as setuid/setgid and/or in a chroot jail. Pound does not access the hard-disk at all (except for reading certificate files on start, if required) and should thus pose no security threat to any machine.

What Pound Is Not

1. Pound is not a Web server: it serves no content itself, it only passes requests and responses back and forth between clients and actual web servers (backends).
2. Pound is not a Web accelerator: no caching is done -- every request is passed to a backend server "as is".

Notice On Project Versioning

I took over pound development at its 2.x branch. The branch 3.x, which emerged for a short time before the original project was abandoned, I consider to be a failed experiment. To ensure consistent versioning and avoid confusion, my versioning of pound starts with 4.0.

Documentation

Documentation in manpage format is available in the distribution. A copy of the documentation is available online.

Build requirements

To build, pound needs OpenSSL version 1.1.x or 3.0.x.

As of current release, pound still supports OpenSSL 1.0, but this support will soon be discontinued.

If you compile it on a Debian-based system, you need to install the libssl-dev package prior to building pound:

Compilation

If you cloned pound from the repository, you will need the following tools in order to build it:

• GNU Autoconf, version 2.69 or later.
• GNU Automake, version 1.15 or later.

First, run

 ./bootstrap

This will prepare the necessary infrastructure files (Makefile.in's etc.)

If you are building pound from a tarball, the above step is not needed, since all the necessary files are included in the tarball.

To prepare pound for compilation, run ./configure. Its command line options will decide where on the filesystem the binary will be installed, where it will look for its configuration file, etc. When run without options, the binary will be installed at /usr/local/sbin and it will look for its configuration in file /usr/local/etc/pound.cfg.

If you run it as:

 ./configure --prefix=/usr --sysconfdir=/etc

then the binary will be installed at /usr/sbin/pound and it will read its configuration from /etc/pound.cfg.

For a detailed discussion of --prefix, --sysconfdir, and other generic configure options, refer to Autoconf documentation.

Apart from the generic ones, there are also several pound-specific configuration options:

• --enable-pcreposix or --disable-pcreposix

Enable or disable the use of the pcreposix library. This is a library that makes it possible to use both POSIX extended and Perl-compatible regular expressions in the pound configuration file.

By default, its presence is determined automatically.

• --enable-super or --disable-super

Enable (the default) or disable compilation of the supervisor process code. This feature is used when pound runs in daemon mode. If it is enabled, an extra supervisor process is launched that controls execution of the pound daemon process. If the daemon process terminates abnormally, the supervisor immediately restarts it and reflects the fact in the log file.

This can also be disabled or enabled on runtime, by the configuration settings.

• --with-maxbuf=n

Sets the value of MAXBUF parameter - the size of a generic buffer used internally by pound for various needs. The default is 4096. You will probably not want to change it.

• --with-owner=user

Name of the system user who will own the pound executable file. When not supplied, the first name from the following list that exists in the /etc/passwd file will be used: proxy, www, daemon, bin, sys, root.

• --with-group=group

Name of the system group who will own the pound executable. When not supplied, the first name from the following list that exists in the /etc/passwd file will be used: proxy, www, daemon, bin, sys, root.

• --with-dh=n

Default DH parameter length. Allowed values for n are 2048 (the default) and 1024.

This option has no effect when compiling with OpenSSL 1.1 or later.

• --with-ssl=directory

Directory under which OpenSSL is installed. You will seldom need this option. Most of the time configure is able to detect that location automatically.

• --with-t_rsa=n

Sets default time interval for regeneration of RSA ephemeral keys.

This option has no effect when compiling with OpenSSL 1.1 or later.

When configuration is finished, run

 make

When building from a git clone, the first run of this command can take considerable time, if you are compiling with OpenSSL 1.0. That's because it involves generating DH parameters.

Testing

Testing a reverse proxy in general, and pound in particular, is not a trivial task. Testsuite in pound was implemented quite recently and is still somewhat experimental. Notwithstanding this, it has already helped to discover several important bugs that lurked in the code.

To test pound you will need Perl version 5.26.3 or later and the IO::FDPass module. To install the later on a debian-based system, run

 apt install libio-fdpass-perl

On other systems you may need to install it directly from cpan by running

 cpan -i IO::FDPass

To run tests, type

 make check

from the top-level source directory. On success, you will see something like that:

## -------------------------- ##
## pound 4.0 test suite.      ##
## -------------------------- ##
  1: Configuration file syntax                       ok
  2: Basic request processing                        ok
  3: xHTTP                                           ok
  4: CheckURL                                        ok
  5: Custom Error Response                           ok
  6: MaxRequest                                      ok
  7: HeadRemove                                      ok
  8: AddHeader                                       ok
  9: RewriteLocation                                 ok
 10: HeadRequire                                     ok
 11: URL                                             ok

## ------------- ##
## Test results. ##
## ------------- ##

All 11 tests were successful.

If a test results in something other than ok, it leaves the detailed diagnostics in files in the tests/*N*/testsuite.dir directory, where N is the ordinal number of the test. Pack them all into a single tarball and send it over to gray@gnu.org for investigation. See also the Bug Reporting section below.

Installing

If both building and testing succeeded, install pound. To do so, run the following command as root:

 make install

Configuration

Pound looks for its configuration file in a location defined at compile time, normally /etc/pound.cfg, or /usr/local/etc/pound.cfg. It's syntax is discussed in detail in the manual. Here we will describe some example configurations.

Any pound configuration must contain at least two parts: a ListenHTTP (or ListenHTTPS) section, that declares a frontend, i.e. the end of the proxy that is responsible for connection with the outside world, and Service section with one or more Backend sections within, which declares where the incoming requests should go. The Service section can be global or it can be located within the ListenHTTP block. Global Service sections can be shared between two or more ListenHTTP sections. Multiple Service sections can be supplied, in which case the Service to use when handling a particular HTTP request will be selected using the supplied criteria, such as source IP address, URL, request header or the like.

Simplest configuration

The following configuration instructs pound to listen for incoming HTTP requests on 192.0.2.1:80 and to pass it to single backend on 10.10.0.1:8080.

ListenHTTP
	Address 192.0.2.1
	Port 80
	Service
		Backend
			Address 10.10.0.1
			Port 8080
		End
	End
End

Notice, that the two statements Address, and Port are in general mandatory both in ListenHTTP and in Backend. There are two exceptions, however: if Address is a file name of a UNIX socket file, or if an already opened socket is passed to pound via the SocketFrom statement. These two cases are discussed below.

Argument to the Address statement can be an IPv4 or IPv6 address, a hostname, that will be resolved at program startup, or a full pathname of a UNIX socket file.

HTTPS frontend

This example shows how to configure HTTPS frontend and redirect all plain HTTP requests to it. It assumes the domain name of the site is www.example.org and its IP address is 192.0.2.1.

# Declare HTTP frontend
ListenHTTP
	Address 192.0.2.1
	Port 80
	Service
		# Redirect all requests to HTTPS.  The redirection
		# target has no path component, which means that the
		# path (and query parameters, if any) from the request
		# will be preserved.
		Redirect 301 https://www.example.org
	End
End

# Declare HTTPS frontend.
ListenHTTPS
	Address 192.0.2.1
	Port 443
	# Certificate file must contain the certificate, optional
	# certificate chain and the signature, in that order.
	Cert "/etc/ssl/priv/example.pem"
	# List of certificate authority certificates.
	CAlist /etc/ssl/acme/lets-encrypt-root.pem"
	# Disable obsolete protocols (SSLv2, SSLv3 and TLSv1).
	Disable TLSv1
	# Inform backend that it's HTTPS
	AddHeader "X-Forwarded-Proto: https"
	Service
		Backend
			Address 10.10.0.1
			Port 8080
		End
	End
End

Virtual Hosts

To implement virtual hosts, one needs to instruct pound to route requests to different services depending on the value of their Host: headers. In previous versions of pound it was achieved using the HeadRequire directive. Since version 4.1 pound provides the Host directive for this purpose.

Let's assume that you have internal server 192.168.0.10 that is supposed to serve the needs of virtual host www.server0.com and 192.168.0.11 that serves www.server1.com. You want pound to listen on address 192.0.2.1 and separate the requests to each host. The configuration file would look like this:

ListenHTTP
	Address 192.0.2.1
	Port    80

	Service
		Host "www.server0.com"
		Backend
			Address 192.168.0.10
			Port    80
		End
	End

	Service
		Host "www.server1.com"
		Backend
			Address 192.168.0.11
			Port    80
		End
	End
End

The same can be done using ListenHTTPS.

If you want to use the same service for both the hostname and the hostname prefixed with www., use the Match statement, as in:

	Service
		Match OR
			Host "server0.com"
			Host "www.server0.com"
		End
		Backend
			Address 192.168.0.10
			Port    80
		End
	End

Sessions

Pound is able to keep track of sessions between a client browser and a backend server. Unfortunately, HTTP is defined as a stateless protocol, which complicates matters: many schemes have been invented to allow keeping track of sessions, and none of them works perfectly. What's worse, sessions are critical in order to allow web-based applications to function correctly - it is vital that once a session is established all subsequent requests from the same browser be directed to the same back-end server.

Six possible ways of detecting a session have been implemented in pound (hopefully the most useful ones): by client address, by Basic authentication (user id/password), by URL parameter, by cookie, by HTTP parameter and by header value.

Session tracking is declared using the Session block in Service section. Only one Session can be used per Service. The type of session tracking is declared with the Type statement.

• Type IP: Session tracking by address

  In this scheme pound directs all requests from the same client IP address to the same back-end server. Put the lines

  Session
      Type    IP
      TTL     300
  End
  

  in the configuration file to achieve this effect. The value indicates what period of inactivity is allowed before the session is discarded.

• Type Basic: by Basic Authentication

  In this scheme pound directs all requests from the same user (as identified in the Basic Authentication header) to the same backend server. Put the lines

  Session
      Type    Basic
      TTL     300
  End
  

  in configuration file to achieve this effect. The value indicates what period of inactivity is allowed before the session is discarded.

  This type is a special case of the Type Header, described below.

  WARNING: given the constraints of the HTTP protocol it may very well be that the authenticated request will go to a different back-end server than the one originally requesting it. Make sure all your servers support the same authentication scheme!

• Type URL: by URL parameter

  Quite often session information is passed through URL parameters (the browser is pointed to something like http://xxx?id=123). Put the lines

  Session
      Type    URL
      ID      "id"
      TTL     300
  End
  

  to support this scheme and the sessions will be tracked based on the value of the id parameter.

• Type Cookie: by cookie value

  Applications that use this method pass a certain cookie back and forth. Add the lines

  Session
      Type    Cookie
      ID      "sess"
      TTL     300
  End
  

  to your configuration file - the sessions will be tracked by the value of the sess cookie.

• Type Parm: by HTTP parameter value

  Applications that use this method pass an HTTP parameter (http://x.y/z;parameter) back and forth. Add the lines

  Session
      Type    PARM
      TTL     300
  End
  

  To your configuration file - sessions will be tracked by the value of the parameter.

• Type Header: by header value

  Applications that use this method pass a certain header back and forth. Add the lines

  Session
      Type    Header
      ID      "X-sess"
      TTL     300
  End
  

  to your configuration file - the sessions will be tracked by the value of the X-sess header.

Please note the following restrictions on session tracking:

• Session tracking is always associated with a certain Service. Thus, each group may have other methods and parameters.
• There is no default session: if you have not defined any sessions, no session tracking will be done.
• Only one session definition is allowed per Service. If your application has alternative methods for sessions you will have to define a separate Service for each method.

A note on cookie injection: some applications have no session-tracking mechanism at all but would still like to have the client always directed to the same backend time after time. Some reverse proxies use a mechanism called cookie injection in order to achieve this: a cookie is added to backend responses and tracked by the reverse proxy.

Pound was designed to be as transparent as possible, therefore this mechanism is not supported. If you really need this sort of persistent mapping use the client address session mechanism (Type IP), which achieves the same result without changing the contents in any way.

Logging

If pound operates in daemon mode (the default), all diagnostics goes to syslog, facility daemon. Pound switches to syslog right before it disconnects from the controlling terminal. Until then, it sends its messages to the standard error.

By default only error and informative messages are logged. The amount of information logged is controlled by the LogLevel configuration statement. Possible settings are:

• 0 No logging.

• 1 Regular logging: only error conditions and important informative messages are logged.

• 2 Extended logging: show chosen backend servers as well.

• 3 Log requests using Apache-style Combined Log format.

• 4 Same as 3, but without the virtual host information.

• 5 Same as 4 but with information about the Service and Backend used.

The LogLevel statement can be global (effective for all listeners), as well as per-listener.

Socket Passing

Pound can obtain socket to listen on from another program via a UNIX socket. This mode of operation is requested by the following statement in ListenHTTP section:

  SocketFrom "/path/to/socket"

When this statement is present, neither Address nor Port may be used in this listener. Pound will connect to the named socket and obtain the socket descriptor from it. Then it will start listening for incoming requests on that socket.

This can be used both in ListenHTTP and ListenHTTPS sections.

Currently it is used in pound testsuite.

Request Modification

Normally, pound passes all incoming requests to backends verbatim, with the following exceptions:

1. X-Forwarded-For: header is added to each request. The value of this header is the actual IP address of the client machine that sent the request.

2. If a client connects via HTTPS, the following HTTP headers are added to the request:

• X-SSL-Subject: information about the certificate owner
• X-SSL-Issuer: information about the certificate issuer (CA)
• X-SSL-notBefore: start of validity date for the certificate
• X-SSL-notAfter: end od validity date for the certificate
• X-SSL-serial: certificate serial number (in decimal)
• X-SSL-cipher: the cipher currently in use
• X-SSL-certificate: the full client certificate (multi-line)

3. Any additions and removals requested by the HeaderAdd and HeaderRemove configuration statements in the listener section.

Using RootJail

The RootJail configuration directive instructs pound to chroot to the given directory at startup. Normally, its use should be quite straightforward:

RootJail "/var/pound"

Several points to note:

When using RootJail, pound does not remove its PID file before shutting down.

Pound tries to open all files and devices it needs before chrooting. There might be cases, however, when it is not enough and you would need to copy certain system files to the chroot directory. In particular, if you get the following message when stopping pound:

libgcc_s.so.1 must be installed for pthread_cancel to work

then you need to copy that library to the RootJail directory, e.g.:

mkdir /var/pound/lib64
cp /usr/lib64/libgcc_s.so.1 /var/pound/lib64

Bug-reporting

If you think you found a bug in pound or in its documentation, please send a mail to Sergey Poznyakoff gray@gnu.org (or gray+pound@gnu.org.ua), or use the github issue tracker.