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NOTE: CentOS Enterprise Linux is built from the Red Hat Enterprise Linux source code. Other than logo and name changes CentOS Enterprise Linux is compatible with the equivalent Red Hat version. This document applies equally to both Red Hat and CentOS Enterprise Linux.

9.3. Network-based IDS

Network-based intrusion detection systems operate differently from host-based IDSes. The design philosophy of a network-based IDS is to scan network packets at the router or host-level, auditing packet information, and logging any suspicious packets into a special log file with extended information. Based on these suspicious packets, a network-based IDS can scan its own database of known network attack signatures and assign a severity level for each packet. If severity levels are high enough, a warning email or cellular pager is placed to security team members so they can further investigate the nature of the anomaly.

Network-based IDSes have become popular as the Internet grows in size and traffic. IDSes that can scan the voluminous amounts of network activity and successfully tag suspect transmissions are well-received within the security industry. Due to the inherent insecurity of the TCP/IP protocols, it has become imperative to develop scanners, sniffers, and other network auditing and detection tools to prevent security breaches due to such malicious network activity as:

  • IP Spoofing

  • denial-of-service attacks

  • arp cache poisoning

  • DNS name corruption

  • man-in-the-middle attacks

Most network-based IDSes require that the host system network device be set to promiscuous mode, which allows the device to capture every packet passed on the network. Promiscuous mode can be set through the ifconfig command, such as the following:

ifconfig eth0 promisc

Running ifconfig with no options reveals that eth0 is now in promiscuous (PROMISC) mode.

eth0      Link encap:Ethernet  HWaddr 00:00:D0:0D:00:01  
          inet addr:  Bcast:  Mask:
          RX packets:6222015 errors:0 dropped:0 overruns:138 frame:0
          TX packets:5370458 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:100 
          RX bytes:2505498554 (2389.4 Mb)  TX bytes:1521375170 (1450.8 Mb)
          Interrupt:9 Base address:0xec80 

lo        Link encap:Local Loopback  
          inet addr:  Mask:
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:21621 errors:0 dropped:0 overruns:0 frame:0
          TX packets:21621 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:1070918 (1.0 Mb)  TX bytes:1070918 (1.0 Mb)

Using a tool such as tcpdump (included with Red Hat Enterprise Linux), we can see the large amounts of traffic flowing throughout a network:

tcpdump: listening on eth0
02:05:53.702142 > \  udp 92 (DF)
02:05:53.702294 > \  udp 32 (DF)
02:05:53.702360 > \
 PTR? (45) (DF)
02:05:53.702706 > \
 6077 NXDomain* 0/1/0 (103) (DF)
02:05:53.886395 > \ NBT UDP PACKET(137): QUERY; BROADCAST
02:05:54.103355 802.1d config c000.00:05:74:8c:a1:2b.8043 root \
 0001.00:d0:01:23:a5:2b pathcost 3004 age 1 max 20 hello 2 fdelay 15 
02:05:54.636436 >\
02:05:56.323715 >\
 udp 56 (DF)
02:05:56.323882 >\
 udp 28 (DF)

Notice that packets that were not intended for our machine ( are still being scanned and logged by tcpdump.

9.3.1. Snort

While tcpdump is a useful auditing tool, it is not considered a true IDS because it does not analyze and flag packets for anomalies. Instead, tcpdump prints all packet information to the screen or to a log file without any analysis. A proper IDS analyzes the packets, tags potentially malicious packet transmissions, and stores them in a formatted log.

Snort is an IDS designed to be comprehensive and accurate in successfully logging malicious network activity and notifying administrators when potential breaches occur. Snort uses the standard libcap library and tcpdump as a packet logging backend.

The most prized feature of Snort, in addition to its functionality, is its flexible attack signature subsystem. Snort has a constantly updated database of attacks that can be added to and updated via the Internet. Users can create signatures based on new network attacks and submit them to the Snort signature mailing lists (located at so that all Snort users can benefit. This community ethic of sharing has developed Snort into one of the most up-to-date and robust network-based IDSes available.

Note Note

Snort is not included with Red Hat Enterprise Linux and is not supported. It has been included in this document as a reference to users who may be interested in evaluating it.

For more information about using Snort, refer to the official website at

  Published under the terms of the GNU General Public License Design by Interspire