                                 Pound
                                 =====

    Pound is a reverse proxy, load balancer and HTTPS front-end for Web
    servers. It was initially developed to distribute the load among
    several Web-servers and to allow for a convenient SSL wrapper for
    those Web servers that do not offer it natively. Since then, it has
    acquired a lot more features. 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 at Apsis
    GmbH[0]. In 2018, Sergey Poznyakoff added support for OpenSSL 1.x
    to the then current version of the program (2.8). That 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 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 backend 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. Although pound is able to serve static content, it is not a Web server:
       its main purpose is to pass 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 texinfo and manpage formats is available in the
    distribution. A copy of the documentation is available online[1].

                           Build requirements
                           ------------------

    To build, pound needs OpenSSL[2] 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.

    If you plan using Perl-compatible regular expressions, install
    PCRE2[3] (the libpcre2-dev package, on debian).

    You may wish to install Lua[4] for extending pound functionality,
    (lua5.4-dev).

    Additional software is needed for testing. See Testing, for
    details.

                              Compilation
                              -----------

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

    * GNU Autoconf[5], version 2.71 or later.
    * GNU Automake[6], version 1.16.5 or later.
    * GNU Libtool[7], version 2.4.2 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 already included in it.

    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[8].

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

    * --enable-dns-tests

    Enable tests of DNS-based dynamic backends. The dynamic backends
    are enabled using the --enable-dynamic-backends option (see below).
    The corresponding tests involve the use of an experimental
    pre-loaded library and are by default disabled. Use this option to
    enable them. These tests also require the Net::DNS perl module. See
    below, for details.

    * --enable-dynamic-backends or --disable-dynamic-backends

    Enable or disable support for DNS-based dynamic backends. You will
    need the GNU adns[9] library to be able to use it. On debian-based
    systems, it can be installed by

             apt-get install libadns1-dev

    By default, dynamic backends are enabled whenever this library is
    present.

    * --enable-lua or --disable-lua

    Enable or disable use of Lua[4] extension modules. By default, it
    is enabled if the Lua library and header files are installed and
    are of suitable version. You will need Lua version 5.3 or 5.4 for
    this to work. Use --disable-lua, if you are don't need this feature
    or want to get a smaller binary without extra dependencies.

    * --enable-pcre or --disable-pcre

    Enable or disable use of the libpcre2 or libpcre library. This
    library allows you to use Perl-compatible regular expressions in
    pound configuration file.

    By default, its presence is determined automatically; libpcre2 is
    preferred over libpcre. To force compiling with the older libpcre,
    use --enable-pcre=1. To disable pcre support, use --disable-pcre.

    * --enable-pthread-cancel-probe or --disable-pthread-cancel-probe

    Pound calls the pthread_cancel function as part of its shutdown
    sequence. In GNU libc, this function tries to load shared library
    libgcc_s.so.1. It will fail to do so, if the program is running in
    chroot (the RootJail statement is given), unless the library has
    previously been copied to the chroot directory. To avoid this,
    pound will do a temptative call to pthread_cancel early, before
    chrooting, so that the necessary library will be loaded and remain
    available after chroot. To determine whether to do this
    pthread_cancel probe hack, configure checks if the program is going
    to be linked with GNU libc.

    These two options allow you to forcefully enable or disable this
    probe. For instance, you may wish to enable it, if another libc
    implementation exhibits a similar behavior.

    * --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-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 that, it has
    already helped to discover several important bugs that lurked in
    the code.

    To test pound you will need Perl[10] version 5.26.3 or later, with
    JSON[11] module (it is normally part of Perl), and the
    IO::FDPass[12] module. To install the latter on a reasonably recent
    debian-based system, run

         apt-get install libio-fdpass-perl

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

         cpan -i IO::FDPass

    Testing HTTPS requires additionally Perl modules IO::Socket::SSL
    and Net::SSLeay. If these are not installed, HTTPS tests will be
    skipped. To install them on a debian-based system, run:

         apt-get install libio-socket-ssl-perl libnet-ssleay-perl

    Additionally, to test DNS-based dynamic backends functionality, the
    Net::DNS module is needed. On debian-based systems it is installed
    with the following command:

         apt-get install libnet-dns-perl

    To run tests, type

         make check

    from the top-level source directory. On success, you will see
    something like that (ellipsis indicating output omitted for
    brevity):

        ## -------------------------- ##
        ## pound 4.14 test suite.     ##
        ## -------------------------- ##
          1: Configuration file syntax                       ok
          2: Traditional log levels (compilation)            ok
          3: Basic request processing                        ok
          4: Traditional log levels (output)                 ok
          5: HTTP log formats                                ok
          6: Log suppression                                 ok
          7: xHTTP                                           ok
          8: CheckURL                                        ok
          9: Custom Error Response                           ok
         10: MaxRequest                                      ok
         11: MaxURI                                          ok
         12: RewriteLocation                                 ok
         13: RewriteLocation (https)                         ok
         14: Named backends                                  ok
         15: Chunked encoding                                ok
         16: Invalid Transfer-Encoding headers               ok
        
        Listener request modification
        
          8: Basic set directives                            ok
        
          ...
        
        ## ------------- ##
        ## Test results. ##
        ## ------------- ##
        
        All 71 tests were successful.

    If a test results in something other than ok, it leaves detailed
    diagnostics in directory tests/testsuite.dir/NN, where NN is the
    ordinal number of the test. If you encounter such failed tests,
    please tar the contents of tests/testsuite.dir directory and send
    the resulting tarball over to <gray@gnu.org> for investigation. See
    also section Bug Reporting below.

                              Installation
                              ------------

    If both building and testing succeeded, it's time to 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.
    The configuration file syntax is discussed in detail in the
    manual[1]. 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 pass them 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

    The Address, and Port statements are present in both ListenHTTP and
    Backend sections. For ListenHTTP they specify the IP address and
    port to listen on. Argument to the Address statement can be an IPv4
    or IPv6 address, or a full pathname of a UNIX socket file. The Port
    statement takes as its argument a port number or its symbolic name
    from /etc/services. Both statements can be omitted: if Port is
    missing, the default port number (80 for HTTP and 443 for HTTPS) is
    assumed. If Address statement is omitted, the program will listen
    on all available interfaces.

    There is also an alternative way of configuring the socket to
    listen on: the SocketFrom statement. It is described in detail
    below.

    For the Backend statement, both Address and Port are mandatory.
    They specify the IP address and port of the remote backend to which
    all requests received by the listener will be forwarded.

                             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
        	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 values of their
    Host: headers. To do so, use the Host statement in the Service
    section.

    The argument to Host specifies the host name. When an incoming
    request arrives, its headers are analyzed. The Service section will
    be used only if the value of the Host: header from the request
    matches the argument to the Host statement. By default, exact
    case-insensitive comparison is used.

    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. 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., you can either use the Match
    statement, or a regular expression.

    A Match statement groups several conditions using boolean shortcut
    evaluation. In the following example, boolean or is used to group
    two Host statements:

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

    When this service is considered, the value of the Host: header from
    the incoming request is matched against each host listed in the
    Match OR statement. If any value compares equal, the match succeeds
    and the service is selected for processing the request.

    By default, the Host directive uses exact case-insensitive string
    comparison. This can be altered by supplying one or more options to
    it. In the example below, we use regular expression matching to
    achieve the same result as in the configuration above:

        	Service
        		Host -re "^(www\\.)?server0\\.com$"
        		Backend
        			Address 192.168.0.10
        			Port    80
        		End
        	End

    Notice double-slashes: a slash is an escape character and must be
    escaped if intended to be used literally. By default, the -re
    option assumes POSIX extended regular expressions. If pound is
    compiled with PCRE2 (or its predecedssor, PCRE), Perl-compatible
    regular expressions may be used as well.

                            Dynamic backends
                            ................

    Dynamic backends are created and updated on the fly based on the
    information from DNS, such as A (AAAA) or SRV records. The
    following dynamic backend types are defined:

    * first Resolve the symbolic host name from the Address directive
      and use first IP from the DNS response as the address of the
      created dynamic backend. Thus, at most one dynamic backend will
      be created. 
    * all Resolve the symbolic host name and create one backend for
      each address from the DNS response. This enables load balancing
      between created backends. Each backend will be assigned the same
      priority. 
    * srv Obtain SRV records for the host name and use them to generate
      dynamic backends. Each record produces new dynamic backend of
      Resolve all type, which in turn creates regular backends as
      described above. The weight field of the SRV record is mapped to
      the priority field of each generated backend. The SRV priority
      field determines the balancer group where the backend will be
      hosted.

    An example of a dynamic backend definition:

        	Service
        		Backend
        			Address "_proxy._tcp.example.org"
        			Resolve srv
        			Family any
        		End
        	End

                            Special backends
                            ................

    Backends discussed above define where incoming requests should be
    routed. There are also several other types of backends, which are
    collectively called special, because requests sent to them are
    handled by pound itself. It can be said that special backends
    implement parts of the HTTP server functionality. These are:

    * Error status [file]

    Replies with the HTTP status code status. If file is given, it
    supplies content of the response.

    * Redirect [status] "url"

    Returns a redirect to url. If status is omitted, 302 is assumed.

    * SendFile dir

    Serves static files from directory dir. The path part of the
    request URL gives the file name. Request rewriting[13] can be used
    to modify it.

    * Control

    Provides access to pound management interface[14].

    * Metrics

    Generates Openmetric telemetry response.

    * Lua

    Special backends can be implemented in Lua language, if its use was
    enabled at compile time.

                                Sessions
                                ........

    Pound is able to keep track of sessions between client browser and
    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 backend 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 backend server. Put the lines

          Session
        	  Type    IP
        	  TTL     300
          End

    in the configuration file to achieve this effect. The TTL value
    indicates the period of inactivity that 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 meaning of TTL is
    as discussed above.

    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
    backend 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 directly supported. It can, however, be
    implemented using Lua extensions[15].

                                Logging
                                .......

    If pound operates in daemon mode (the default), all diagnostics
    goes to the 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 or "null" No logging. 
    * 1 or "regular" Regular logging: only error conditions and
      important informative messages are logged. 
    * 2 or "extended" Extended logging: show chosen backend servers as
      well. 
    * 3 or "vhost_combined" Log requests using Apache-style Combined
      Log format. 
    * 4 or "combined" Same as 3, but without the virtual host
      information. 
    * 5 or "detailed" Same as 4, but with information about the Service
      and Backend used.

    You can also define your own logging format using the LogFormat[16]
    statement, and then use its name as the argument to LogLevel
    statement.

    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.
    Several request modification directives are provided, that allow
    you to add or remove headers from the request. The following two
    groups of headers are added by default. Each of them can be turned
    off using the HeaderOption directive.

    1. The forwarded headers:

    * X-Forwarded-For: header passes the actual IP address of the
      client machine that sent the request. 
    * X-Forwarded-Proto: header contains the original protocol (http or
      https). 
    * X-Forwarded-Port: header contains the port on the server that the
      client connected to.

    1. The second group contains ssl headers that are added only if the client
       connected using HTTPS. The X-SSL-Cipher header is always present
       if this header group is enabled. The rest of headers below is
       added only if the client certificate was supplied:

    * X-SSL-Cipher: SSL version followed by a slash and active cipher
      algorithm.
    * X-SSL-Certificate: the full client certificate (multi-line).
    * X-SSL-Issuer: information about the certificate issuer (CA).
    * X-SSL-Subject: information about the certificate owner.
    * X-SSL-notAfter: end od validity date for the certificate.
    * X-SSL-notBefore: start of validity date for the certificate.
    * X-SSL-serial: certificate serial number (in decimal).

    The HeaderOption directive can be used (either globally or in
    listener block) to disable any or both of these groups, e.g.: ``
    HeaderOption no-ssl forwarded ``

    Any number of headers can be added or removed using the HeaderAdd
    and HeaderRemove directives in the listener section. The order in
    which these directives are applied is:

    1. Headers controlled by the HeaderOption directive are added.
    2. Headers requested by HeaderRemove directives are removed.
    3. Headers from HeaderAdd directives are added.

                                  ACME
                                  ----

    Pound offers built-in support for ACME (a.k.a. LetsEncrypt)
    HTTP-01[17] challenge type. Thus, it can be used with any
    certificate controller to obtain SSL certificates on the fly.

    Assuming your certificate controller is configured to store
    challenges in directory /var/lib/pound/acme, all you need to do is
    add the ACME statement to the ListenHTTP block, for example:

        ListenHTTP
        	ACME "/var/lib/pound/acme"
        	.
        	.
        	.
        End

    Now, each request whose URL ends in
    /.well-known/acme-challenge/NAME will be served by directly by
    pound: it will send the content of the file
    /var/lib/pound/acme/NAME as a reply.

                             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"

    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.

                             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[18].

    When reporting failed tests, please make an archive of the
    tests/testsuite.dir subdirectory and attach it to your report.

                               References
                               ==========

    [0] https://web.archive.org/web/20221202094441/https://apsis.ch/
    [1] https://www.gnu.org.ua/software/pound/pound.html
    [2] https://www.openssl.org/
    [3] https://www.pcre.org/
    [4] https://www.lua.org
    [5] http://www.gnu.org/software/automake
    [6] http://www.gnu.org/software/autoconf
    [7] http://www.gnu.org/software/libtool
    [8] https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.71/html_node/Running-configure-Scripts.html
    [9] http://www.gnu.org/software/adns
    [10] https://www.perl.org
    [11] https://metacpan.org/pod/JSON
    [12] https://metacpan.org/pod/IO::FDPass
    [13] https://www.gnu.org.ua/software/pound/manual/Special-Backends.html#index-SendFile
    [14] https://www.gnu.org.ua/software/pound/manual/Management-interface.html
    [15] https://www.gnu.org.ua/software/pound/manual/Cookie-injection.html
    [16] https://www.gnu.org.ua/software/pound/manual/Logging.html#log-format
    [17] https://letsencrypt.org/docs/challenge-types/#http-01-challenge
    [18] https://github.com/graygnuorg/pound/issues

