Difference between revisions of "NIDS"

Revision as of 07:18, 29 November 2011

Network layer Intrusion Detection Systems

NIDS are an IDS that monitor all traffic to and from all network devices. It will read incoming packets looking for suspicious patterns. Popular NIDs such as snort and Cisco IDS are used often-times in conjunction with a firewall to form an IPS. Also commonly used to send information to a SIM or SIMS

Snort

Snort is a packet sniffer and intrusion detection system that can be used to add a layer of surveillance to your system. In this way, it is similar to tcpdump, but with some more advanced rule-setting capabilities for detecting certain forms of packets. Snort is one of the most widely used IDSes under Linux, and a port exists for the Windows Operating System.

Basic Packet Sniffing Utilities

Once installed, snort can be set up with basic packet sniffing functionality simply by typing "snort" into a terminal or command prompt. It will run in the default mode using the default interface - for me, this is eth0, the wired connection. A new interface can be specified with the -i option. For example, to run snort on the wireless interace:

By default, the packets will be displayed to STDOUT as packets are capture, which isn't particularly useful. In order to log to a tcpdump-syntax file, you can use the -l option, for example:

In order to read the logs, use a program capable of reading tcpdump-style logs - with tcpdump, use the -r option to read - you may wish to pipe it through less for readability:

Some other useful options:

• -v: show only packet headers
• -vd: show bothc packet headers and data

Rules

Under its default configuration, Snort really isn't much more useful than a packet sniffer - its power as an intrusion detection system comes from its power to process received packets and raise a flag based on a file containing directives on how to treat malformed & potentially malicious packets, which is known as a rule file.

Enable processing a rule file in snort with the -c option, which takes the path to a rule file as its argument.

A snort rule is composed of two parts: the rule header and the rule options. The rule header contains the action of the rule, its protocol, source and destination IP addresses and netmasks, and information regarding the source and target ports. The option section contains the details of the alert messages and information concerning which parts of the packet should be examined.[1]

Rule Headers

Rule Actions

There are available actions to a snort rule:

• alert: generates an alert flag of some kind when the rule is matched
• log: quietly logs the packet
• pass: forget about the packet

Protocols

There are three protocol options snort will look for:

• tcp
• udp
• icmp

These are pretty self-explanatory.

IP Addresses

Source and destination IP addresses to apply the rule to can be specified next, in the format 'source IP -> destination IP'. It is possible to use CIDR notation - for example, to indicate anything from the internl network, you can use 192.168.1.0/24. You can also prepend an IP address with the ! operator to indicate any packet NOT from the stated IP address. Depending on the direction of the arrow (-> or <-), you can also specify whether the rule applies to incoming our outgoing traffic.

Port Number

The final part of the header for a rule is the port reference, which can be in several formats. If a number is given by itself, the rule will apply only to that port. A range of ports can also be specified with the colon.

Based on this, here is a sample rule without the option section:

This particular rule flags an alert on any host from outside of the internal network trying to make a TCP connection on port 31337.

Rule Option Section

The rule options dictate how a flag is handled, and what procedure is taken when a flag appears. They are encapsulated in parentheses after the rule header, and each option is seperated by a semicolon.

Rule Options:

msg

Gives a message to print along with the packet dump.

logto

log all packets with this flag to a special log file, specified.

minfrag

Sets the minimum size (bytes) of a fragmented packet. This is useful, as some forms of invasive port scanning use fragmented packets to evade firewalls and intrusion detection systems.

ID

Searches for a match in the IP header. Good for combatting some programs that set these to specific values.

Content

A very important option: searches for content in packets received. If the content is matched exactly, the option string continues - otherwise, it is terminated and the packet does not flag. Binary search strings can be represented as hexadecimal bytecode and enclosed with the pipe character '|'. Note that this is computationally expensive to run on each packet.

Depth

A content option modifier that specifies how much of the packet to search for the string match before giving up. Useful for making searches more efficient, as it is often unnecessary to search further than the first 20 bytes.

Flags

A very important feature for detecting scans. Most port scanners (including nmap) use strategically placed flags in a packet to gain information on the operating system. This option allows you to set certain flags to "watch out for".

Flag Options:

• F: FIN
• S: SYN
• R: RST
• P: PSH
• A: ACK
• U: URG
• 2: reserved 2-bit
• 1: reserved 1-bit