Follow Techotopia on Twitter

On-line Guides
All Guides
eBook Store
iOS / Android
Linux for Beginners
Office Productivity
Linux Installation
Linux Security
Linux Utilities
Linux Virtualization
Linux Kernel
System/Network Admin
Scripting Languages
Development Tools
Web Development
GUI Toolkits/Desktop
Mail Systems
Eclipse Documentation

How To Guides
General System Admin
Linux Security
Linux Filesystems
Web Servers
Graphics & Desktop
PC Hardware
Problem Solutions




Linux Administrator's Security Guide
Prev Home Next

Linux Attack Detection

By Kurt Seifried [email protected]



No matter how good your security is you it can be compromised. That is why it is so important to have various forms of attack detection present, so that when an incident happens you are alerted to it as soon as possible (and not when you start getting complaints from other sites).


One major oversight made by a lot of people when securing their machines is that they forget to create a baseline of the system, that is a profile of the system, its usage of resources, and so on in normal operation. For example something as simple as a "netstat -a -n > netstat-output" can give you a reference to latter check against and see if any ports are open that should not be. Memory usage and disk usage are also good things to keep an eye on. A sudden surge in memory usage could result in the system being starved of resources. Likewise for disk usage. It might be a user accident, a malicious user, or a worm program that has compromised your system and is now scanning other systems. Various tools exist to measure memory and disk usage: vmstat, free, df, du, all of which are covered by their respective man pages.

At the very minimum make a full system backup, and regularly backup config files and log files, this can also help you pinpoint when an intrusion occurred (user account "rewt" was added before the April 4th backup, but isn't in the March 20th backup). Once a system is compromised typically a "rootkit" is installed, these consist of trojaned binaries, and are near impossible to remove safely, you are better of formatting the disk and starting from scratch. There is of course a notable exception to this rule, if you were diligent and used file/directory integrity tools such as L5 you will be able to pinpoint the affected files easily and deal with them.

There are also a variety of tools that do not quite fit under the headings here, but are aimed at attack detection. One is the Linux Intrusion Detection System (LIDS) project.

File system monitoring

So you've secured your machines, and done all the things that needed to be done. So how do you make sure it's actually doing what it is supposed to do, or prove to someone that it is as secure as you say it is? Well you conduct an audit. This can be as simple as reviewing the installed software, configuration files and other settings, or as complex as putting together or hiring a tiger team (or ethical hackers, or whatever buzzword(s) you prefer) to actively try and penetrate your security. If they can't then you did your job well (or they suck), and if they do get in, you know what needs to be fixed (this is also a good method to show the CIO that security is not a one shot affair, it is a constant battle). One thing almost all attackers do is modify system files, once you detect a break in, how do you know which files are ok and which are not? Short of a complete reinstall the only way to be sure (and even then it's not always 100%) is to use software to create signatures of files that cannot be forged so you can compare them later on.


Tripwire is no longer a open source tool. I have absolutely NO problems with commercial software. However, when you expect me to rely on a program to provide security, when I (nor anyone else really) can not view the source (it is available under some special license agreement, probably an NDA) I must decline. Tripwire costs approximately $70 for Linux, and is only available as an RPM package aimed at Red Hat Linux (tripwire is $500 for other operating systems). I feel this is rather on the high side for a piece of software that can easily be replaced with alternatives such as L5 or Gog&Magog. Tripwire is available There is also the "classic" version available


AIDE is a tripwire replacement that attempts to be better then tripwire. It is GPL licensed which makes it somewhat more desirable then tripwire from a trust point of view. It supports several hashing algorithms, and you can download and


ViperDB checks setuid/setgid programs and folders and can notify you (via syslog) of any changes or reset their permissions and ownership to what they should be. ViperDB creates a series of databases (flat text files actually) in the directory root, i.e.: /etc/.ViperDB might contain:


Unfortunately ViperDB doesn't seem to handle sub directories, so you will have to add them to the viperdb.ini file with something like:

find /etc/ -type d >> /usr/local/etc/viperdb.ini has 3 options, -init (creates a set of databases), -check (checks files against databases, sends any messages to syslog, and then recreates the databases) and checkstrict (checks files against databases, resets permissions if necessary, sends any messages to syslog, and then recreates the databases). What this means is if you use check, you will get a warning that say /etc/passwd is now world writeable, and since it recreates the databases the next time you run viperdb you will NOT get a warning. I would advise running viperdb is checkstrict mode only, and make sure you run viperdb with the init option after manipulating any file / folder permissions in protected directories. ViperDB is available


Pikt is an extremely interesting tool, it is actually more of a scripting language aimed at system administration then a simple program. Pikt allows you to do things such as killing off idle user processes, enforcing mail quotas, monitor the system for suspicious usage patterns (off hours, etc), and much more. About the only problem with Pikt will be a steep learning tools, as it uses it's own scripting language, but ultimately I think mastering this language will pay off if you have many systems to administer (especially since Pikt runs on Solaris, Linux and FreeBSD currently). Pikt is available


Something people forget about, but you can compare the current files to old backups, many backup formats (Tape, floppy, CDR, etc.) can be made read only, so a backup of a newly installed system provides a good benchmark to compare things to. The utility "diff" and "cmp" can be used to compare files against each other.

Network monitoring / attack detection

If the last section has you worried you should be. There are however many defenses, active and passive against those types of attacks. The best ways to combat network scans are keep software up to date, only run what is needed, and heavily restrict the rest through the use of firewalls and other mechanisms.

Luckily in Linux these tools are free and easily available, again I will only cover open source tools, since the idea of a proprietary firewall/etc is rather worrying. The first line of defense should be a robust firewall, followed by packet filters on all Internet accessible machines, liberal use of TCP-WRAPPERS, logging and more importantly automated software to examine the logs for you (it is unfeasible for an administrator to read log files nowadays).


The Deception ToolKit is a set of programs that emulate well known services in order to provide a false set of readings to attackers. The hope is to confuse and slow down attackers by leading them to false conclusions, you can download DTK from

Psionic TriSentry - PortSentry, HostSentry and LogSentry

Psionic TriSentry consists of three components, PortSentry, HostSentry and LogSentry. PortSentry detects and logs port scans, including stealthy scans (basically anything nmap can do it should be able to detect). Psionic PortSentry can be configured to block the offending machine (in my opinion a bad idea as it could be used for a denial of service attack on legitimate hosts), making completion of a port scan difficult. As this tool is in beta I would recommend against using it, however with some age it should mature into a solid and useful tool. HostSentry spots local anomolous behaviour in user accounts, and reports situations that fall outside of normal parameters (i.e. Bob from accounting logging in at 2 AM on a Sunday). The last component is LogSentry, essentially a log file monitoring applications which will alert you if it spots strange problems. TriSentry is available


scanlogd monitors network packets and if a threshold is exceeded it logs the packets. You can get it at:


Most firewalls support logging of data, and ipfwadm/ipchains are no exception, using the -l switch you get a syslog entry for each packet, using automated filters (Perl is good for this) you can detect trends/hostile attempts and so on. Since most firewalls (UNIX based, and Cisco in any case) log via the syslog facility, you can easily centralize all your firewall packet logging on a single host (with a lot of harddrive space hopefully).


Wietse's TCP-WRAPPERS allow you to restrict connections to various services based on IP address and so forth, but even more importantly it allows you to configure a response, you can have it email you, finger the offending machine, and so on (use with caution however). TCP_WRAPPERS comes standard with most distributions and is available

Intrusion Detection Papers

FAQ: Network Intrusion Detection Systems, an excellent FAQ that covers all the major (and many minor) issues with IDS systems. Available

Dealing with attacks

So you've done your homework, you installed tripwire, DTK, and so on. Now what do you do when your pager starts going off at 3am and tells you that someone just made changes on the primary NIS server? Dealing with an attack depends on several factors, is the attack in progress? Did you discover your company plan being sent out by the mail server to a Hotmail address? Did you get called in to find a cluster of dead servers? What are your priorities? Restoring service? Ensuring confidential data is safe? Prosecuting the attacker(s)? Several things to keep in mind:

  • Response from the admin will depend heavily on the environment they are in. The attacker may have compromised the administrative accounts, so sending email may not work.
  • Most sites usually don't want to report attacks (successful or not) due to the potential embarrassment and related public relations problems.
  • Most quick attacks, denial of service attacks and the like are spoofed. Tracking down the real attacker is very difficult and resource intensive.
  • Even if all goes well there is a chance law enforcement will seize your equipment as evidence, and hold it, not something to be taken lightly.
  • Do you know how the attacker got in (i.e. NFR recorded it), if so you might just want to plug the holes and go on.
  • Try not to ignore attacks, but at the same time there are many people running garbage attacks in an effort to waste administrators time and energy (and possibly distract them from more subtle attacks).

Also before you deal with an attack, you should consult your company policy. If you don't have one consult your manager, the legal department, etc. It's also a good idea to have a game plan to deal with attacks (i.e., the mail server is first priority, checking fileservers is number two, who do you notify, etc) this will prevent a lot of problems when it happens (be prepared). The O'Reilly book "Practical Unix and Internet Security" covers this topic in great detail so I'm not going to rehash it. Go buy the book. There is also "Incident response" from O'Reilly, you should probably read it, it's quite good.

Packet sniffers

Packet sniffing is the practice of capturing network data not destined for your machine, typically for the purpose of viewing confidential/sensitive traffic such as telnet sessions or people reading their email. Unfortunately there is no real reliable way to detect a packet sniffer since it is mostly a passive activity, however by utilizing network switches and fiber optic backbones (which are very difficult to tap) you can minimize the threat. There is also a tool called AntiSniff, that probes network devices and sees if their response indicates an interface in promiscuous mode. These tools are also invaluable if your network is under attack and you want to see what is going on. There is an excellent FAQ on sniffing


Snort is the king of packet sniffers now. It can also be used to detect various attacks. It can watch for activity such as Queso TCP-IP fingerprinting scans, Nmap scans, and the like. Snort is available


The granddaddy of packet sniffers for Linux, this tool has existed as long as I can remember, and is of primary use for debugging network problems. It is not very configurable and lacks advanced features of newer packet sniffers, but it can be useful. Most distributions ships with tcpdump.


A nice looking network protocol analyzer (a.k.a., a souped up sniffer) with an interface very similar to NT's network monitor. It allows easy viewing of data payloads for most network protocols (tftp, http, Netbios, etc). It is based on GTK, thus meaning you will probably have to be running gnome to use it. I haven't tested it yet (but intend to). It is available Ethereal can also be used to open trace files from other software.


SPY is an advanced multi protocol sniffer that runs on various platforms. You can get it

Other sniffers

There are a variety of packet sniffers for Linux, based on the libpcap library among others, here is a short list:

Numerous other packet sniffers are available;q=packet+sniff&section=projects.

Packet sniffer detection

In theory most operating systems leave tell tale signs when packet sniffing (that is to say their network interfaces respond in certain, non standard ways to network traffic). If the attacker is not to savvy, or is using a compromised machine then chances are you can detect them. On the other hand if they are using a specially built cable, or induction ring there is no chance of detecting them unless you trace every physical piece of network cable and check what is plugged into it.


As mentioned before AntiSniff i a tool that probes network devices to try and see if they are running in promiscuous mode, as opposed to normal modes of operation. It is supposedly effective, and will work against most sniffers. You can get it AntiSniff has not been maintained in over a year.

Linux Administrator's Security Guide
Prev Home Next

  Copyright Kurt Seifried 2001 [email protected]. Published under the terms of the Open Content License Design by Interspire