Protection From DDOS ATTACk - 28 January 2011 - Rahul Tyagi Official Blog | Information Security
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Main » 2011 » January » 28 » Protection From DDOS ATTACk
10:39 AM
Protection From DDOS ATTACk
To provide protection from DoS or DDoS attacks, basic security measures are mandatory. If a running system is hacked into, no more network attacks are necessary, since local attacks (like processes consuming lots of memory or CPU time, or simply shutting down the system) are far more effective. A set of firewalls should be used to separate the interior net (and probably a demilitarized zone) from the Internet. Intrusion Detection Systems should be used to notify the system administrators of unusual activities.

The firewall rules should include some sanity checks for source and destination addresses: Packets arriving from the Internet must not have a source address originating from the interior net, and vice versa. By rejecting packets from the interior net with a non-local source address, packet spoofing becomes impossible. This technique is known as ingress and egress filtering . Even if a host is invaded by a hacker, these rules make it impossible to use that host as a platform for further attacks requiring spoofed packets.

Linux Kernel

We have carefully chosen Linux Kernel Version 2.2.16 as base for all our systems as this is known to be immune to most poisoned traffic attacks like teardrop or TARGA. The backlog queue of the system defaults to 128 entries and tcp_syn_cookies is enabled. This makes the system very robust against SYN flood attacks.

Linux Virtual Server

The load balancer we use is the Linux Virtual Server (LVS) [35]. LVS inserts itself directly into the kernel which provides a maximum performance again stabilizing the system against overload attacks. LVS has two load balancing algorithms: round robin and least connection. We are using 'least connection' as this provides generally a fairer load distribution between the webservers. There are three different modes to access the webservers.

Network address translation (NAT) transcripts every incoming packet and changes the destination IP from the load balancer's to the web server's IP. All outgoing traffic is transcripted alike. As all in- and outgoing traffic has to pass the load balancer, this is not an ideal solution for our purposes, as the load balancer may easily become a bottle neck.

ipchains Firewall

All systems protect themselves from unauthorized access by filtering incoming packets according to a number of security rules. In brief the rules state that only port 80 is reachable directly and only ICMP host unreachable messages are accepted. Another set of rules allows communication between the load balancer and the webservers as well as access to some important local services (DNS server etc.). This configuration may later be modified dynamically by the Traffic Shaping Monitor to totally block all traffic from attacking hosts. All these measures provide a pretty stable environment which should block all common attacks to the systems and leave only the web server reachable.

Class Based Queuing and the Traffic Monitor

Class Based Queuing (CBQ) is a function of the Linux kernel. It allows the setup of different traffic queues and of rules that determine what packets to put in what queue. Furthermore you can assign a certain amount of the available bandwidth to each of the queues. If a queue is full packets get discarded. There are different queuing disciplines from which we have chosen Stochastic Fairness Queuing (SFQ) because it consumes only few memory and computing power. On the other side it is not fully deterministic in what packets end up in what queues. For our purposes this is no problem. Other available disciplines include Token Bucket Filtering or Random Early Detect.
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