Slowloris is a form of denial of carrier assault tool invented with the aid of Robert “RSnake” Hansen which lets in a single system to take down some other the gadget’s net server with minimum bandwidth and side effects on unrelated services and ports.

Slowloris attempts to hold many connections to the goal net server open and keep them open as long as possible. It accomplishes this by using commencing connections to the target internet server and sending a partial request. Periodically, it’s going to send the next HTTP headers, including to—however by no means completing—the request. Affected servers will hold these connections open, filling their most concurrent connection pool, subsequently denying extra connection tries from clients.

How does a Slowloris attack work?

Slowloris is an application layer attack that operates via making use of partial HTTP requests. The attack capabilities by commencing connections to a focused Web server and then maintaining the one’s connections open as long because it can.

Slowloris is not a class of attack but is rather a specific attack device designed to permit a single system to take down a server without the usage of lots of bandwidth. Unlike bandwidth-eating reflection-based DDoS attacks inclusive of NTP amplification, this type of assault uses a low quantity of bandwidth, and as an alternative aims to use up server assets with requests that seem slower than normal however in any other case mimic everyday traffic. It falls in the class of assaults that are recognized as “low and gradual” attacks.

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The centered server will handiest have so many threads to be had to handle concurrent connections. Each server thread will strive to live alive while expecting the sluggish request to complete, which never occurs. When the server’s most possible connections were exceeded, every additional connection will now not be responded and denial-of-provider will occur.

A Slowloris assault occurs in 4 steps:

The attacker first opens multiple connections to the focused server by way of sending a couple of partial HTTP request headers.

The goal opens a thread for every incoming request, with the rationale of final the thread once the connection is completed. To be efficient, if a connection takes too long, the server will timeout the enormously lengthy connection, freeing the thread up for the next request.

To save you the target from timing out the connections, the attacker periodically sends partial request headers to the goal to hold the request alive. In essence saying, “I’m still here! I’m just gradually, please look forward to me.”

The centered server is never capable of releasing any of the open partial connections whilst waiting for the termination of the request. Once all to be had threads are in use, the server will be not able to respond to additional requests made from regular traffic, ensuing in denial-of-carrier.

The key at the back of a Slowloris is its capability to cause a lot of hassle with very little bandwidth consumption.

How to save you a slow loris assault?

Use Hardware Load Balancers that accept the most effective complete HTTP connections.

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Using hardware load balancers with an HTTP profile configured may be the excellent method to prevent such an assault.

Because the burden balancer will look at the packet’s and could forward only the ones HTTP requests to the web server which might be complete.

If you are using an F5 based BIG-IP Load Balancer I recommend studying the below link for mitigating slowloris attacks.

Another Load balancer’s like the Citrix NetScaler and Cisco CSS can be configured with an HTTP profile to mitigate such an assault.

Protect your internet server with the aid of the usage of IPtables with the aid of limiting connections from a selected host

You can without a doubt limit the quantity of connections with the assistance of tables to port 80. For example, if assume I want to block

iptables -A INPUT -p tcp –syn –port 80 -m connlimit –connlimit-above 30 -j DROP

Configure the timeout directive in apache

Although this isn’t always at all a great solution, you could still increase the rate with which your web server will obtain inactive connections.

You can in reality adjust the timeout directive in /etc/httpd/conf/httpd.Conf file.

Reducing it to a lower cost will at the least make the attack difficult(but still the assault can take down the server, by means of growing the variety of requests).