Databases, Systems & Networks

Source CipherDyne

psad is a collection of three lightweight system daemons (two main daemons and one helper daemon) that run on Linux machines and analyze iptables log messages to detect port scans and other suspicious traffic. A typical deployment is to run psad on the iptables firewall where it has the fastest access to log data.

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psad incorporates many signatures from the Snort intrusion detection system to detect probes for various backdoor programs (e.g. EvilFTP, GirlFriend, SubSeven), DDoS tools (mstream, shaft), and advanced port scans (FIN, NULL, XMAS) which are easily leveraged against a machine via nmap. When combined with fwsnort and the Netfilter string match extension, psad is capable of detecting many attacks described in the Snort rule set that involve application layer data. In addition, psad makes use of various packet header fields associated with TCP SYN packets to passively fingerprint remote operating systems (in a manner similar to p0f) from which scans originate. Further, psad can be integrated with Logstash, and also offers support for UFW firewalls [1]. For more information, see the complete list of features offered by psad.

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Recently, the anti-spam organization Spamhaus has come under yet another distributed denial-of-service attack. With some help from our good friends at myNetWatchman we were able to obtain a sample of the malware used in the attack. This one is particularly nasty, starting up 1500 threads to send randomized HTTP requests to Spamhaus’ webserver in a loop. This attack tool doesn’t have a command-and-control mechanism, so it was likely force-installed on all the infected systems of an existing botnet.

This kind of attack is particularly troublesome to deal with. While simplistic packet-based attacks can be more easily mitigated upstream, with an HTTP-based attack it is often difficult to distinguish attack traffic from legitimate HTTP requests. And because the attack consumes resources from the webserver, not just the system TCP/IP stack, it can quickly bring even a well-tuned webserver to its knees unless the target has better-than-average resources at its disposal to help weather the storm (like Spamhaus does).

Fortunately, most HTTP-based DoS attacks we have seen have a particular weakness – they are vulnerable to a technique known as « tarpitting ». This idea was first proposed by Tom Liston many years ago when the first scanning worms hit the Internet, and was implemented in his program LaBrea (which he no longer offers for download, due to legal concerns – however, the source code can still be found elsewhere and should work on Linux or BSD-based operating systems). The quickest way to implement tarpitting (if your webserver runs on Linux) is in the Linux netfilter source code. If the tarpit module is compiled for your Linux kernel, the operation becomes as simple as « iptables -A INPUT -s x.x.x.x -p tcp -j TARPIT« .

Tarpitting works by taking advantage of TCP/IP’s idea of window size and state. In a tarpitted session, we respond to the connection initiation as normal, but we immediately set our TCP window size to just a few bytes in size. By design, the connecting system’s TCP/IP stack must not send any more data than will fit in our TCP window before waiting for us to ACK the packets sent. This is to allow connections to deal with packet loss that might occur in a normal session. If the sending system doesn’t get an ACK to a packet sent, it will resend the packet at increasingly longer intervals. In our tarpitted session, we simply don’t ack any of the post-handshake packets at all, forcing the remote TCP/IP stack to keep trying to send us those same few bytes, but waiting longer each time. With this, bandwidth usage falls off quickly to almost nothing.

It might seem reasonable to simply use iptables -j DROP to not respond to connections at all – this will certainly prevent the webserver from seeing the connection, but because the connection may have a timeout set, it is likely we’ll keep seeing connection attempts at a fairly regular rate. Essentially a DROP rule turns our HTTP flood into a SYN flood.

Below is a chart made by running the Spamhaus DDoS malware on a single host in a sandnet, and comparing the bandwidth usage of each iptables mitigation technique with no mitigation. Here we extrapolated the bandwidth usage of the full non-mitigated attack (the blue line in the chart) from just the first 30 seconds of the attack, rather than let it fully DDoS the server the entire two hours. Under load, our untuned Apache server probably would have slowed down, forcing the attack bandwidth to decrease with it as it became unresponsive. In this simulation we are assuming the server can withstand a single attacker with no slowdown.

We see that although with tarpitting, there are momentary spikes as the sessions finally time out and try again, overall the average bandwidth usage is substantially lower with tarpitting as opposed to simply dropping the attacker’s packets.

There seems to be an added side-effect to mitigation by tarpit. When the attacked host mitigates with an iptables DROP (no response), the attacker’s CPU load is fairly minimal and the system is responsive. However, as demonstated by the graphic below, under tarpitting the CPU load in our test system quickly rose to 100% as the attacking system’s kernel tried to maintain a large number of open connections in a retry state.

In the case of our test system, the user interface became nearly unresponsive. Of course, this may depend on the overal system CPU speed and RAM (our test was done in a VMware environment which also may be a factor), but if the system becomes unacceptibly slow, it serves as an impetus for the unknowing owner of the attacking system to have the computer cleaned of its botnet infection.

Unfortunately, the TARPIT iptables module is not part of every Linux distribution. It is possible to obtain it by getting the netfilter Patch-o-Matic-NG source and compiling it for your kernel (don’t forget the iptables source must also be patched to support the TARPIT option).

At some point, the scalability of having thousands of iptables rules may come into play. At the University of Florida, Chuck Logan and Jordan Wiens were able to successfully mitigate a DDoS attack from the Netsky virus by writing customized software to detect the attacking hosts and create intelligent chains of iptables rules that were more scalable. Details and source code can be found here.

No matter how you implement it, tarpitting isn’t going to completely stop an attack – it can only slow it down. However, it could be an effective stop-gap measure until upstream providers can effectively null-route an attack on your website, or until the attack stops on its own.

Source: Dell SecureWorks – Author: Joe Stewart

Source: blackmoreops.com

In computing, a denial-of-service (DoS) or distributed denial-of-service (DDoS) attack is an attempt to make a machine or network resource unavailable to its intended users. Although the means to carry out, the motives for, and targets of a DoS attack vary, it generally consists of efforts to temporarily or indefinitely interrupt or suspend services of a host connected to the Internet. In this article I will show how to carry out a Denial-of-service Attack or DoS using hping3 with spoofed IP in Kali Linux.

As clarification, distributed denial-of-service attacks are sent by two or more persons, or bots, and denial-of-service attacks are sent by one person or system. As of 2014, the frequency of recognized DDoS attacks had reached an average rate of 28 per hour.

Perpetrators of DoS attacks typically target sites or services hosted on high-profile web servers such as banks, credit card payment gateways, and even root nameservers.

Denial-of-service threats are also common in business, and are sometimes responsible for website attacks.

This technique has now seen extensive use in certain games, used by server owners, or disgruntled competitors on games, such as popular Minecraft servers. Increasingly, DoS attacks have also been used as a form of resistance. Richard Stallman has stated that DoS is a form of ‘Internet Street Protests’. The term is generally used relating to computer networks, but is not limited to this field; for example, it is also used in reference to CPU resource management.

One common method of attack involves saturating the target machine with external communications requests, so much so that it cannot respond to legitimate traffic, or responds so slowly as to be rendered essentially unavailable. Such attacks usually lead to a server overload. In general terms, DoS attacks are implemented by either forcing the targeted computer(s) to reset, or consuming its resources so that it can no longer provide its intended service or obstructing the communication media between the intended users and the victim so that they can no longer communicate adequately.

Denial-of-service attacks are considered violations of the Internet Architecture Board’s Internet proper use policy, and also violate the acceptable use policies of virtually all Internet service providers. They also commonly constitute violations of the laws of individual nations.

hping3 works well if you have other DoS tools such as GoldenEye running (using multiple tools that attacks same site/server/service increases the chances of success). There are agencies and corporations to runs DoS attack map in Realtime. that shows worldwide DDoS attacks almost in realtime.

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Dans un réseau ethernet relié par un concentrateur (ou hub), chaque machines reçoit tous les paquets qui circulent sur le réseau. En fonctionnement normal, les cartes réseau ne réceptionnent que les paquets qui leur sont destinés, mais on peut faire en sorte qu’elles transmettent tous les paquets au système et les inspecter avec tcpdump.

Les hubs sont de moins en moins utilisés. Ils sont généralement remplacés par des commutateurs (ou switch) qui savent déterminer (en fonction de l’adresses MAC) sur quel câble il faut envoyer un paquet. Les machines ne reçoivent donc généralement que les paquets qui leur sont destinés.

L’utilitaire tcpdump permet d’inspecter les paquets qui sont reçus et transmis par une carte réseau.

Filtrage

Il est possible de sélectionner les paquets à « écouter » en fonction d’expressions. Ainsi, ne seront affichées / traitées que les informations pour lesquelles le résultat de l’expression est vérifié. Une expression est composée de primitives et d’opérateurs logiques.

Une primitive est un identifiant précédé de mots clés qui indiquent le type de l’identifiant. Par exemple la primitive src port 21 contient les éléments suivants :

• le mot clé src qui indique que l’identifiant ne porte que sur la source du paquet
• le mot clé port qui indique que l’identifiant est le port du paquet
• l’identifiant 21

La primitive correspond donc au port source 21.

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