DDOS, or Distributed Denial of Service is an advanced version of DOS(Denial of Service) attack. Like DOS, DDOS also tries to deny important services running on a server by broadcasting packets to the destination server in a way that the Destination server cannot handle it. The speciality of the DDOS is that, it relays attacks not from a single network/host like DOS. The DDOS attack will be launched from different dynamic networks which has already been compromised.
Normally, DDOS consists of 3 parts . The Master, the slave and at
last the victim. The master is the attack launcher, i.e the person/machine
behind all this. The slave is the network that is being compromised
by the Master and Victim is the target site/server. Master informs
the compromised machines, so called slaves to launch attack on the
victim's site/machine. Hence its also called co-ordinated attack.
Here is how I see it. Master is the Master Brain, Slave is said to
be the launch pad for the attack and Victim is the target.
DDOS is done in 2 phases. In the first phase they try to compromise
weak machines in different networks around the world. This phase is
called Intrusion Phase. Its in the next phase, that they install DDOS
tools and start attacking the victims machines/site. This Phase is
called Distributed DoS attack phase.
What Allowed them to do it? Simple.
- Vulnerable softwares/Applications running on a machine or network.
- Open network setup.
- Network/ machine setup without taking security into account.
- No monitoring or Data Analysis are being conducted.
- No regular Audit / Software upgrades being conducted.
What should we do if we are under DDOS attack?
Check if your machines load is high and you have large number of
HTTP process running.
To find the load just use the command w or uptime -
#w 12:00:36 up 1 day, 20:27, 5 users, load average: 0.70, 0.70, 0.57
USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT
To find if there is large number of HTTP process running use the command
" ps -aux|grep HTTP|wc -l "
# ps -aux|grep HTTP|wc -l
23
In a heavy server , the number of connections will go above 100. But
during DDOS attack, the number will go even higher and thats when
we need to find out from which all networks are these attacks coming.
In DDOS the host machine doesn't have much importance. Its the network
which is of importance here because, an attacker will use any machine
on the compromised network or even will use all the machines in the
network. Hence network address is of importance while fighting with
the attack.
If you have high load (say 5 or more ) and you have large number
of HTTP process then i would request you to do the following
At command prompt execute the below command
bash#netstat -lpn|grep :80 |awk '{print $5}'|sort
Check each block of ips. Lets assume you have more than 30 connection
from a single ip. Under normal cases there is no need for that many
number of connection requests from a single IP. Try to identify such
ips/networks from the list you get
If more than 5 host/ip connects from the same network then its a clear
sign of DDOS .
Block that ips/networks using iptables /Apf
iptables -A INPUT -s <Source IP> -j DROP
If you have APF, then just add the IPs which you want to block in
the file /etc/apf/deny_hosts.rules
Continue this process untill the attack on the machine gets reduced.
There is no complete or perfect solution to DDOS . The logic is simple,
NO software or measures could handle attacks from multiple servers
say from 50 - 100 servers all at a time. All that can be done is to
take preventive measures .
How can we prevent or defend ourselves from these attacks?
Prevention is better than cure. Its very much true in the case of
DDOS . In my Introduction, I had mentioned that DDOS happens because
of vulnerable softwares/applications running on a machines in a particular
network. Attackers use those security holes to compromise the servers
in different network and install the DDOS tools (eg trinoo -DDOS tool).
To prevent DDOS in future, follow these steps.
Setup machine / network keeping security in mind (Implement Good
Security policy)
Setup a firewall which does Ingress and Egress Filtering at Gateway
Steps to Install APF
bash# wget http://www.rfxnetworks.com/downloads/apf-current.tar.gz
bash# tar -zxf apf-current.tar.gz
bash# cd apf-<version number>
bash# ./install.sh
Notes: Go through the Document in the Apf and configure it for your
needs. All configuration is set at conf.apf which is normally located
at /etc/apf/conf.apf
Enable Anit-DOS mode in Apf (ie in conf.apf) . Also make sure that
your root's cron has an entry like the one below
*/8 * * * * root /etc/apf/ad/antidos -a >>
/dev/null 2>&1
Install IDS on your gateway/hosts to alert you when someone tries
to sniff In.
(a) Wget ftp://ftp.cs.tut.fi/pub/src/gnu/aide-0.7.tar.gz
(b) Untar it
tar -zxvf aide-0.7.tar.gz
cd aide-0.7
(d) Then execute
./configure -with-gnu-regexp
(e) Final steps to install
make;make install
(f) Now the main step. To configure AIDE. AIDE stores all its rule
sets in the file called aide.conf. To get more details on how to configure
from 'man aide.conf'
(g) Here is a sample short aide.conf:
Rule = p+i+u+g+n+s+md5
/etc p+i+u+g
/sbin Rule
/usr/local/apache/conf Rule
/var Rule
!/var/spool/.*
!/var/log/.*
In this configuration, a rule called "Rule" is set
to check permissions (p), inode (i), user (u), group (g), number of
links (n), size (s), and md5 checksum (md5). This rules are applied
to all files in /bin, /sbin, /var, and /usr/local/apache/conf because
they should rarely if ever change. Files in /etc are checked for changes
in only permissions, inode, user, and group because their size may
change, but other things shouldn't. Files and directories in /var/spool
and /var/log are not checked because those are folders where maximum
updation takes place.
(h) After configuring AIDE should be initiated with all these rules.
For that execute
aide -init
Conduct regular Audits on each host on the network to find installation
of DDOS tools / Vulnerable applications.
Use tools like RKDET(vancouver-webpages.com/rkdet), RKHUNTER(www.rootkit.nl)
and CHKROOTKIT(www.chkrootkit.org) to find if any rootkit has been
already installed and to locate the effected binaries in the machine,
if any.
This is a simple Audit check List to check for.
- Software Vulnerabilities.
- Kernel Upgrades and vulnerabilities.
- Check for any Trojans.
- Run chkrootkit.
- Check ports.
- Check for any hidden processes.
- Use audit tools to check system.
- Check logs.
- Check binaries and RPMS.
- Check for open email relays.
- Check for malicious cron entries.
- Check /dev /tmp /var directories.
- Check whether backups are maintained.
- Check for unwanted users, groups, etc. on the system.
- Check for and disable any unneeded services.
- Locate malicious scripts.
- Querylog in DNS.
- Check for the suid scripts and nouser scripts.
- Check valid scripts in /tmp.
- Use intrusion detection tools.
- Check the system performance.
- Check memory performance (run memtest).
Enforce and Implement Security Measures on all hosts in the network.
Machines new or old should only be allowed to run on your network,
if your Security Admin or DSE (Dedicated Security Expert) member approves
it with status ``OK-to go live'' after auditing the box. All Host
in the network should be checked on a regular basis by your DSE team
to make sure that all hosts are up-to-date and can fight any attacks.
Audit network on a regular basis to see if your network is vulnerable
to attacks
Use Open Source Tools like NESSUS (www.nessus.org), NMAP(www.insecure.org/nmap),
SAINT( www.saintcorporation.com/products/saint_engine.html), SARA
(www-arc.com/sara/sara.html) for auditing a network to find its vulnerabilities.
Create a DSE (Dedicated Security Expert ) Team for your company.
Collect your network and hosts data. Analyse and study them
to see from where and what kind of attacks are coming into the network.
This step will help us to understand what kind of attacks we are facing
and will help us to strengthen the preventive measures. Let me tell
you this move is worth the money you spend, for sure.
Implement Sysctl protection against DDOS
bash# vi /etc/sysctl.conf
Add this code to this file:
# Enable IP spoofing protection, turn on Source Address Verification
net.ipv4.conf.all.rp_filter = 1
# Enable TCP SYN Cookie Protection
net.ipv4.tcp_syncookies = 1
Add the below code in /etc/rc.local and restart network
for f in /proc/sys/net/ipv4/conf/*/rp_filter;
do echo 1 > done
echo 1 > /proc/sys/net/ipv4/tcp_syncookies
Install Mod_dosevasive to your apache.
Mod_dosevasive is module for Apache to perform evasive action in
the event of an HTTP DDoS attack or brute force attack. Please find
the installation step of mod_dosevasive in DSO mode below
Eg: Install Mod_dosevasive
# wget http://www.nuclearelephant.com/projects/mod_evasive/mod_evasive_1.10.1.tar.gz
# tar -zxvf mod_evasive_1.10.1.tar.gz
# cd mod_evasive_1.10.1
# $APACHE_ROOT/bin/apxs -iac mod_evasive.c
``$APACHE_ROOT'' is a variable that stores the location of the
apache installation (eg $APACHE_ROOT =/usr/local/apache)
# vi /usr/loca/apache/conf/httpd.conf
After this add the below code in httpd.conf
<IfModule mod_dosevasive.c>
DOSHashTableSize 3097
DOSPageCount 2
DOSSiteCount 50
DOSPageInterval 1
DOSSiteInterval 1
DOSBlockingPeriod 10
</IfModule>
Then restart Apache
# /usr/local/apache/bin/apachectl restart
Install Mod_security
Since DDOS normally targets HTTP. Its always good to have a filtering
system for apache . So that the request gets analyzed before web server
handles it. Please find the installation step of mod_security in
DSO mode below
bash# http://www.modsecurity.org/download/modsecurity-apache-1.9.2.tar.gz
bash# tar -zxvf modsecurity-apache-1.9.2.tar.gz
bash# cd modsecurity-apache-1.9.2
bash# /usr/local/apache/bin/apxs -cia mod_security.c
Create a file named mod_security.conf under the folder /usr/local/apache/conf
bash# vi /usr/local/apache/conf/mod_security.conf
Create the rule with reference to the link http://www.modsecurity.org/documentation/quick-examples.html
and add it in the mod_security.conf file.
Add the location of mod_security.conf to httpd.conf
bash# vi /usr/local/apache/conf/httpd.conf
by adding the string below
Include /usr/local/apache/conf/mod_security.conf
Restart apache
# /usr/local/apache/bin/apachectl stop
# /usr/local/apache/bin/apachectl start
Best solution to fight DDOS to a certain extend will be to setup load balancer for your services.
Creating awareness on Security
This is the most important part. People, including users should be
Security conscious. Only then will they understand the importance
of Security measures . Server owner's and users should be made aware
of the issues which can rise due to bad security measures.
About the author: Blessen Cherian works in Bobcares. He is passionate about Server Security and looks forward to gain expertise in this area.
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