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Lifecycle: Preventing, Detecting & Removing Bots

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The most effective means of guarding against botnets are preventing attackers from planting bots on your network and removing them once they're detected. Enterprises need to harden systems against botnet infiltration and restore compromised machines to trusted states to prevent further compromises.

Prevention
   Harden end hosts. Make sure your servers, desktops and mobile machines have up-to-date patches; harden your TCP/IP stack (e.g., using syncookies and maximizing TCP queue handling capacity); eliminate unnecessary services; partition required services as much as possible; and make use of backdoor networks for things like file services and DNS to limit externally exposed points of attack.
   Overprovision hosts and networks. Make sure your servers have more than enough RAM and the fastest hard drives, drive interfaces and interface cards (possibly using multiple interfaces to segregate front-end network services from back-end file services, and DNS from internal hosts); and tune/monitor system performance on a regular basis.
   Leverage IPSes/IDSes and firewalls. Restrict all externally exposed access to only those services that are absolutely necessary (e.g., only allow TCP ports 80 and 443 on Web servers, TCP/UDP ports 53 on DNS servers, etc.). Use your IDS/IPS to monitor access attempts on any open ports, and tune it to look for specific OS-version and patch-level vulnerabilities. Also, monitor what services are running--there's no need to check for Windows/x86-based IIS attacks aimed at a DNS server running BIND on Solaris/SPARC.

Detection
   Monitor and respond to incidents. Security managers should dedicate human and automated resources to check their IDS/IPS and other network monitoring devices for anomalous activity, such as spikes in traffic, unusual protocols, unauthorized connection attempts and large volumes of e-mail. Security managers should monitor ports and protocols commonly used by bots, such as TCP port 6667.
   Watch network traffic. Flow-level monitoring and logging, even for short periods--a few days or weeks--is critical for addressing multifaceted network attacks. Botnets are great at concealing the source of attacks, making host-based logging ineffective for diagnosis. In DDoS attacks, having a full picture of traffic to and from the victim host can often lead you closer to the attackers by noticing when they check to see if their attacks are succeeding.

Reaction/Remediation
   Filter the flood. In many cases, filters can drop incoming traffic from some or all of the attacking hosts in a DDoS attack. Attackers can, and usually do, vary their attack methods, so change your filters frequently. Bots can be blocked with enough precise information about command and control traffic patterns, ports, protocols, peers and servers. (Note: It's risky to do this with routers, as you may disrupt legitimate traffic. It's even riskier to use firewalls, since a failure will open your entire network to attack.)
   Remediate and recover. If you aren't already using integrity-checking software that fingerprints files and file system metadata, it can be extremely difficult to clean up bot-infested hosts. Effective cleanup requires detailed knowledge of the specific bots, how they're used and how their variants are altered or configured. Some antivirus/antispyware applications may be able to remove bots, but nastier variants require manual removal of both the software and registry keys. The most resilient bots and rootkits require wiping the hard drive and reinstalling the OS.
   Preserve the evidence. This is tricky; doing the "right thing" by preserving evidence is costly, while "wipe-and-reinstall" is cheap. Victims may soon not have a choice, since downstream liability cases and law enforcement efforts are compelling enterprises to preserve any and all evidence. Victims should attempt to get a hard drive image of at least one bot-compromised system to assist investigations.

But, there's no effective defense for a botnet attack--your only hope is survival. (See "Preventing, Detecting and Removing Bots".)

Anticipating Bot Attacks
Before you connect to the Internet, there are some things you can do to mitigate the effects of botnet attacks. The following are some prudent steps all enterprises should consider:

Define service requirements with ISPs. You should define for your ISP what your expectations and response requirements are in the event of a DDoS attack. This includes network address agility (or switching your address block), which makes it harder to target your network and can reveal attacker reconnaissance; topological changes to compartmentalize your network, protect high-value assets and preserve connectivity to specific network segments; and traffic capture and analysis for tracing attacks and--perhaps--prosecuting attackers. Traffic filtering by your ISP or upstream traffic sources (sometimes called traffic blocking or null routing) can also help.

Manage out-of-band network. When your primary (or secondary) network interfaces are flooded, you may lose all ability to communicate with your network devices. If your provider can establish an out-of-band control mechanism--be it a network connection through a peering point or a DSL line to a terminal server within your network perimeter--you can regain remote access and reroute critical traffic, such as e-mail, even if your main network paths are unavailable.

Coordinate with peers. Cases of DDoS attacks that involve source-address forgery and traffic reflection off widely distributed servers (e.g., DNS reflection, SYN-ACK reflection off routers and firewalls) may require manual traceback to determine the source. Getting the cooperation of peers using the same upstream provider to block traffic and perform traceback may be very difficult; persuading your upstream provider to commit to working with you, even if the problem is difficult, is the first step.

While not fully effective against botnet DDoS attacks, several open-source and commercial products can provide some measure of response capabilities. Each has some value, but is only one soldier in the information assurance/availability army.

Most defenses are directed at either the host or network level, but rarely both. Host-level defenses, including personal firewalls, antivirus and host-based IDSes, are designed to protect computers, OSes and applications, and to detect and possibly contain intrusions.

Commercial applications from Arbor Networks, Captus Networks, Cisco Systems, Lancope, Mazu Networks and Top Layer identify anomaly traffic and irregular volume flows to detect DDoS attacks. These same applications have had some success in filtering floods by dropping traffic based on source IP address and protocol. But, their success is limited by the size and scope of a botnet flood. DDoS attacks, especially those launched via massive botnets, have a numerical advantage that may overwhelm these tools.

Security solutions that maintain the trusted state of machines, such as those from Tripwire, can monitor deviations in configurations. When a machine falls out of compliance, it can be rapidly detected and restored to a trusted state.

Network-level defenses focus on large sets of computers on a network or routing infrastructure. They may monitor individual or aggregate traffic flows between computers looking for anomalous activity, filter suspicious traffic, manage device configurations or patch systems to prevent exploitation.

Being a Good 'Netizen
Botnets are marauders waiting at the edge of every network for the one vulnerable machine that will become their key through enterprise fortifications. And, eradicating botnets after an invasion is nearly impossible because their numbers and growth are too great to effectively eradicate.

So, what can be done to shield your network from botnets? Formulate a good defensive strategy by safeguarding and protecting your network and mobile computers, and preventing attacks before they happen. For those enterprises already under attack, systematically rooting out compromised servers and PCs is essential.

Only through vigilance and best practices will enterprises stay ahead of, or at least keep pace with, the botnet threat.

This was first published in March 2005

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