psad: Linux Detect And Block Port Scan Attacks In Real Time

by on August 6, 2008 · 33 comments· LAST UPDATED August 6, 2008

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Q. How do I detect port scan attacks by analyzing Debian Linux firewall log files and block port scans in real time? How do I detect suspicious network traffic under Linux?

A. A port scanner (such as nmap) is a piece of software designed to search a network host for open ports. Cracker can use nmap to scan your network before starting attack. You can always see scan patterns by visiting /var/log/messages. But, I recommend the automated tool called psad - the port scan attack detector under Linux which is a collection of lightweight system daemons that run on Linux machines and analyze iptables log messages to detect port scans and other suspicious traffic.

psad makes use of Netfilter log messages to detect, alert, and (optionally) block port scans and other suspect traffic. For tcp scans psad analyzes tcp flags to determine the scan type (syn, fin, xmas, etc.) and corresponding command line options that could be supplied to nmap to generate such a scan. In addition, psad makes use of many tcp, udp, and icmp signatures contained within the Snort intrusion detection system.

Install psad under Debian / Ubuntu Linux

Type the following command to install psad, enter:
$ sudo apt-get update
$ sudo apt-get install psad

Configure psad

Open /etc/syslog.conf file, enter:
# vi /etc/syslog.conf
Append following code       |/var/lib/psad/psadfifo

Alternatively, you can type the following command to update syslog.conf:
echo -e ’\t|/var/lib/psad/psadfifo’ >> /etc/syslog.conf
psad Syslog needs to be configured to write all messages to a named pipe /var/lib/psad/psadfifo. Close and save the file. Restart syslog:
# /etc/init.d/sysklogd restart
# /etc/init.d/klogd

The default psad file is located at /etc/psad/psad.conf:
# vi /etc/psad/psad.conf
You need to setup correct email ID to get port scan detections messages and other settings as follows:


Set machine hostname (FQDN):

HOSTNAME          ;

If you have only one interface on box (such as colo web server or mail server), sent HOME_NET to none:

HOME_NET                NOT_USED;  ### only one interface on box

You may also need to adjust danger levels as per your setup. You can also define a set of ports to ignore, for example to have psad ignore udp ports 53 and 5000, use:

IGNORE_PORTS                udp/53, udp/5000;

You can also enable real time iptables blocking, by setting following two variables:

ENABLE_AUTO_IDS             Y;

psad has many more options, please read man pages for further information. Save and close the file. Restart psad:
# /etc/init.d/psad restart

Update iptables rules

psad need following two rules with logging enabled:

iptables -A INPUT -j LOG
iptables -A FORWARD -j LOG 

Here is my sample Debian Linux desktop firewall script with logging enabled at the end:

echo "Starting IPv4 Wall..."
$IPT -t nat -F
$IPT -t nat -X
$IPT -t mangle -F
$IPT -t mangle -X
modprobe ip_conntrack
BADIPS=$(egrep -v -E "^#|^$" /root/scripts/blocked.fw)
# DROP all incomming traffic
# block all bad ips
for ip in $BADIPS
    $IPT -A INPUT -s $ip -j DROP
    $IPT -A OUTPUT -d $ip -j DROP
# sync
$IPT -A INPUT -i ${PUB_IF} -p tcp ! --syn -m state --state NEW  -m limit --limit 5/m --limit-burst 7 -j LOG --log-level 4 --log-prefix "Drop Syn"
$IPT -A INPUT -i ${PUB_IF} -p tcp ! --syn -m state --state NEW -j DROP
# Fragments
$IPT -A INPUT -i ${PUB_IF} -f  -m limit --limit 5/m --limit-burst 7 -j LOG --log-level 4 --log-prefix "Fragments Packets"
$IPT -A INPUT -i ${PUB_IF} -f -j DROP
# block bad stuff
$IPT  -A INPUT -i ${PUB_IF} -p tcp --tcp-flags ALL FIN,URG,PSH -j DROP
$IPT  -A INPUT -i ${PUB_IF} -p tcp --tcp-flags ALL ALL -j DROP
$IPT  -A INPUT -i ${PUB_IF} -p tcp --tcp-flags ALL NONE -m limit --limit 5/m --limit-burst 7 -j LOG --log-level 4 --log-prefix "NULL Packets"
$IPT  -A INPUT -i ${PUB_IF} -p tcp --tcp-flags ALL NONE -j DROP # NULL packets
$IPT  -A INPUT -i ${PUB_IF} -p tcp --tcp-flags SYN,RST SYN,RST -j DROP
$IPT  -A INPUT -i ${PUB_IF} -p tcp --tcp-flags SYN,FIN SYN,FIN -m limit --limit 5/m --limit-burst 7 -j LOG --log-level 4 --log-prefix "XMAS Packets"
$IPT  -A INPUT -i ${PUB_IF} -p tcp --tcp-flags SYN,FIN SYN,FIN -j DROP #XMAS
$IPT  -A INPUT -i ${PUB_IF} -p tcp --tcp-flags FIN,ACK FIN -m limit --limit 5/m --limit-burst 7 -j LOG --log-level 4 --log-prefix "Fin Packets Scan"
$IPT  -A INPUT -i ${PUB_IF} -p tcp --tcp-flags FIN,ACK FIN -j DROP # FIN packet scans
$IPT  -A INPUT -i ${PUB_IF} -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG -j DROP
# Allow full outgoing connection but no incomming stuff
$IPT -A INPUT -i eth0 -m state --state ESTABLISHED,RELATED -j ACCEPT
# allow ssh only
$IPT -A INPUT -p tcp --destination-port 22 -j ACCEPT
$IPT -A OUTPUT -p tcp --sport 22 -j ACCEPT
# allow incoming ICMP ping pong stuff
$IPT -A INPUT -p icmp --icmp-type 8 -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
$IPT -A OUTPUT -p icmp --icmp-type 0 -m state --state ESTABLISHED,RELATED -j ACCEPT
# No smb/windows sharing packets - too much logging
$IPT -A INPUT -p tcp -i eth0 --dport 137:139 -j REJECT
$IPT -A INPUT -p udp -i eth0 --dport 137:139 -j REJECT
# Log everything else
# *** Required for psad ****
# Start ipv6 firewall
# echo "Starting IPv6 Wall..."
exit 0

How do I view port scan report?

Simply type the following command:
# psad -S
Sample output (some of the sensitive / personally identified parts have been removed):

[+] psadwatchd (pid: 2540)  %CPU: 0.0  %MEM: 0.0
    Running since: Sun Jul 27 07:14:56 2008
[+] kmsgsd (pid: 2528)  %CPU: 0.0  %MEM: 0.0
    Running since: Sun Jul 27 07:14:55 2008
[+] psad (pid: 2524)  %CPU: 0.0  %MEM: 0.8
    Running since: Sun Jul 27 07:14:55 2008
    Command line arguments: -c /etc/psad/psad.conf
    Alert email address(es):
    src:            dst:            chain:  intf:  tcp:  udp:  icmp:  dl:  alerts:  os_guess:  xx.22.zz.121    INPUT   eth0   1     0     0      2    2        - xx.22.zz.121    INPUT   eth0   1     0     0      2    2        - xx.22.zz.121    INPUT   eth0   1     0     0      2    2        -   xx.22.zz.121    INPUT   eth0   1     0     0      2    2        -
    122.167.xx.11   xx.22.zz.121    INPUT   eth0   4642  0     0      4    50       -
    122.167.xx.80   xx.22.zz.121    INPUT   eth0   0     11    0      1    2        -
    123.134.xx.34   xx.22.zz.121    INPUT   eth0   20    0     0      2    9        -
    125.161.xx.3    xx.22.zz.121    INPUT   eth0   0     9     0      1    4        -
    125.67.xx.7     xx.22.zz.121    INPUT   eth0   1     0     0      2    2        - xx.22.zz.121    INPUT   eth0   0     9     0      1    3        - xx.22.zz.121    INPUT   eth0   0     10    0      1    2        -
    202.xx.23x.196  xx.22.zz.121    INPUT   eth0   0     13    0      1    10       -
    202.xx.2x8.197  xx.22.zz.121    INPUT   eth0   0     20    0      2    17       -  xx.22.zz.121    INPUT   eth0   0     17    0      2    12       -  xx.22.zz.121    INPUT   eth0   0     18    0      2    15       -  xx.22.zz.121    INPUT   eth0   0     17    0      2    14       -  xx.22.zz.121    INPUT   eth0   0     15    0      2    12       -  xx.22.zz.121    INPUT   eth0   0     21    0      2    16       -  xx.22.zz.121    INPUT   eth0   12    0     0      2    6        Windows XP/2000
    211.90.xx.14    xx.22.zz.121    INPUT   eth0   1     0     0      2    2        -   xx.22.zz.121    INPUT   eth0   0     0     1      2    2        - xx.22.zz.121    INPUT   eth0   0     35    0      2    31       -  xx.22.zz.121    INPUT   eth0   0     33    0      2    21       -  xx.22.zz.121    INPUT   eth0   0     33    0      2    27       -  xx.22.zz.121    INPUT   eth0   0     39    0      2    26       -  xx.22.zz.121    INPUT   eth0   0     33    0      2    19       -  xx.22.zz.121    INPUT   eth0   0     40    0      2    33       -  xx.22.zz.121    INPUT   eth0   0     14    0      1    11       -  xx.22.zz.121    INPUT   eth0   0     18    0      2    15       -
    Netfilter prefix counters:
        "SPAM DROP Block": 161519
        "Drop Syn Attacks": 136
    Total scan sources: 95
    Total scan destinations: 1
    Total packet counters:
        tcp:  5868
        udp:  164012
        icmp: 2

How do I remove automatically blocked ips?

Simply type the following command to remove any auto-generated firewall block
# psad -F

How do I view detailed log for each IP address?

Go to /var/log/psad/ip.address/ directory. For example, view log for IP address, enter:
# cd /var/log/psad/
# ls -l

Sample output:

-rw------- 1 root root 2623 2008-07-30 13:02 xx.22.zz.121_email_alert
-rw------- 1 root root   32 2008-07-30 13:02 xx.22.zz.121_packet_ctr
-rw------- 1 root root    0 2008-07-29 00:27 xx.22.zz.121_signatures
-rw------- 1 root root   11 2008-07-30 13:02 xx.22.zz.121_start_time
-rw------- 1 root root    2 2008-07-30 13:02 danger_level
-rw------- 1 root root    2 2008-07-30 13:02 email_count
-rw------- 1 root root 1798 2008-07-29 00:27 whois

Use cat / more or less command to view rest of the information.

Further readings:

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{ 33 comments… read them below or add one }

1 Diya August 6, 2008 at 2:28 pm

I was not aware of psad. Thanks for writing out tutorial.


2 tachiiNiiJinx August 7, 2008 at 7:35 pm

I append the following code ( |/var/lib/psad/psadfifo) to /etc/syslog.conf. Which will save just fine, but I enter the following at the command line with or without sudo, echo -e ’\t|/var/lib/psad/psadfifo’ >> /etc/syslog.conf. I am getting am Permission Denied error. Do I need to use chmod to set the permission’s to the User, Group, or Other?


3 John Allen August 13, 2008 at 8:17 am

You must be the real root user for the >> to work.

When using sudo you will execute the echo command as root, but the >> redirect is executed as the current user.


4 somename November 9, 2011 at 5:57 am

that’s what `sudo su` is for :p


5 Noah August 18, 2008 at 7:45 pm

PSAD has been only an annoyance to me as an administrator. Often I use nmap to do perfectly legitimate scans of a clients machine for debugging purposes. I setup tools for automating data feeds between my servers and client servers. Data feeds can go over HTTP, SSH, various direct database sockets, FTP, etc. Often there are firewalls in the way or a client might not have a required service active and running or they might have configured a service on a non-standard port. I’m sure there are lots of other reasons that I can’t even remember now.

Clients that use PSAD hinder debugging. All of my servers are under constant automated attack by bots. This is simply the nature of the internet. None of these bots do port scanning. Some of them do scan a range of IP addresses looking for specific ports with running services, so I can see the value of a system could be to detect when someone may be scanning a range of IP address. But systems that detect port scans on an individual IP address seem overkill.


6 Ryan January 6, 2009 at 7:09 am

Nice howto. Thank you.


7 Asaduzzaman Shuvo February 18, 2009 at 7:59 am

How to observe deny web site Ip address or port in Linux Redhat squid server?


8 Linuxnoob March 31, 2009 at 4:07 pm

Anyone know if I could some how run this in the firmware DD-WRT. Like in a SSH session? or can I just save thos IPtables to the firewall.


9 Munch June 23, 2009 at 12:41 pm

What version of psad should I use for centOS?
Is installation procedure of psad for centOS same as above?


10 glas October 22, 2009 at 8:18 pm

apt-get install Thank you very much.
Nice tutorial.


11 bonkhi November 3, 2009 at 10:15 am

Had no ideal of psad……………….. thanks


12 cybernet November 16, 2009 at 10:28 am

what i do with this ?

echo “Starting IPv4 Wall…”
$IPT -t nat -F
$IPT -t nat -X
$IPT -t mangle -F
$IPT -t mangle -X
modprobe ip_conntrack


13 deni December 8, 2009 at 2:05 pm

any commands how to detect the ddos from where attacking my servers pls.?


14 tunmsk December 22, 2009 at 5:17 pm

do psad can be configured with rsyslog on a debian lenny?


15 Vlado March 24, 2010 at 6:04 pm

One thing to have in mind is the huge hdd space required for psad. My /var/log/ grew up with around 1Gb for like 20mins!


16 Istvan C March 27, 2015 at 1:21 pm

Use logrotate to shrink your logs


17 emcgfx June 16, 2010 at 10:25 am

This option bellow:
BADIPS=$(egrep -v -E “^#|^$” /home/tux/blocked.fw)

Needs to be this in Ubuntu 10.04:
BADIPS=$(egrep -v -e “^#|^$” /home/tux/blocked.fw)

NOTES: Simply use lower case “e” instead of capital one ;-)

Works like a charm, thanks CyberCiti Authors.


18 rokin June 22, 2010 at 8:58 pm

Hello all, thank for the tuto.

But psad “don’t work” with Debian Lenny and rsyslog (default) :(
cf :
I have test modifications, after, psad launch good but the psadfifo are empty and no detections :(

sorry for my bad english.
can you have a solution or a similar software ?

thank you very much !


19 cviniciusm September 3, 2010 at 2:41 pm

PSAD is broken on the Ubuntu 10.04 (Lucid Lynx) and on the new beta 10.10 (Maverick).

And nice job.



20 skullboxx September 21, 2010 at 1:19 pm

Can’t confirm that, PSAD is working fine on my Ubuntu 10.04.1 LTS Box.



21 sniper December 10, 2010 at 7:56 am

Hi all
How could I whitliste IPs? PSAD is everytime blocking my resolver in my network and the lo interface… :-(



22 sniper December 10, 2010 at 7:31 pm

Hi All
On Ubuntu Server 10.10 it works fine.
On Debian Lenny psad does not work. The counters be ever 0.
What could I do on the Debian Lenny Server, to become psad to work?


23 Gargonzo Bastardo van Rothschildt January 30, 2011 at 12:50 am

This is a stupid configuration, because it will write Gigabytes of Data in your log directory – you will literally DOS yourself. Are you using this in real life anywhere? I assume, that you are not a sysadmin anymore then?
PSAD documentation explains that you should redirect the iptables info into the fifo file – and if your harddisk is filled up with iptables logs you will understand why.


24 Raul March 12, 2011 at 7:53 am

sniper psad on Debian Lenny works well.If your not, that’s your mistake.You have to pay attention to configure psad.conf file.
Best regards,


25 cviniciusm March 12, 2011 at 12:55 pm


There is a bug on the 10.04 package, I filed a bug on Ubuntu Launchpad.

The original psad there is not the bug.

I solved the bug disabling the line that sends e-mail to .



26 Prakash May 13, 2011 at 4:13 am


Please let me know the steps for installation of above for centos.

Awaiting for your reply.



27 Alex October 9, 2011 at 8:26 am

Thank you!
It works perfect!


28 Yonatan Ryabinski November 15, 2011 at 4:58 am

Thank you very much!


29 fix January 3, 2012 at 8:07 pm

what kind of an asshole would want to block outgoing nmap scans???


30 joshlinx January 14, 2012 at 7:31 pm

Note the authors other software as well excellent security software. fwsnort to use snort rules with firewall and fwknop for single packet authentication for port access. I have also bought the book and recommend reading it, very useful security software.


31 sanchit January 19, 2013 at 7:56 pm

Can you post a psad tutorial for centos?


32 Silvio March 4, 2014 at 8:58 pm

Thanks for howto, on Gentoo run perfect. Only one problem is, the logfile will be full is there a way to limited?


33 Nix Craft March 5, 2014 at 6:03 am

Configure logrotate.


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