Subjects -> Computer Networks -> Lectures -> Lecture #20

Lecture 20: Network Security

The Nature Of Attacks

Computer systems (hosts) connected to the Internet are subject to an almost constant barrage of security attacks. Most attacks have at least some of the following goals:

To obtain unauthorised access to private, or secret, information stored on the compromised system -- eg lists of credit card numbers stored on commerce servers. Such access can be obtained by a wide variety of means. This is probably the most important type of attack.
To use the compromised system as a start point for attacks on other, more interesting, systems whilst "covering their tracks" -- the (so-called) Springboard Attack.
Denial of service (DOS) attacks attempt to use up system resources to inconvenience legitimate users. A classic example is to send vast megabytes of (anonymous) electronic mail to a target host in an attempt to exhaust its disk space. The (relatively) recently developed Distributed Denial of Service (DDOS) attacks are a particularly nasty form.
A variation of the "unauthorised access" attack is where a Bad Guy actually attempts to cause damage to the compromised system, for example, by removing important files, changing configurations, etc. Recent "Web site defacements" come under this category.
etc, etc...

Internet Security

Most interest in security issues is in connecting to the Internet in a secure way. Recall:

An Internet-connected system uses the TCP/IP protocol suite. The TCP/IP model of communications is based on server processes waiting (at well-known ports) for remote sites to establish connections.
A server process has very little information -- usually only an IP address and a reverse DNS lookup -- about the identity of a remote host which is attempting to connect. Even reverse lookups can be compromised by manipulation of DNS tables or by IP Spoofing, where a remote host generates IP packets that look as though they came from some other system.
Servers which generally accept all connections without any (initial) security checks (such as finger and sendmail) can be vulnerable to various forms of attack, such as attempted buffer over-running.^[1]
A major problem is the whether the sysadmin can trust the server programs to be bug free...An issue here is whether the vendor provides access to source code for scrutiny.

^[1] A successful buffer overrun attack requires extremely detailed knowledge of the vulnerable target system. However, many such exploits have been observed over the years. The famous "Morris Internet Worm" of 1988 was based (among other things) on a buffer overrun attack on sendmail, the Unix standard package for email delivery.

Firewall Security

A Firewall is usually implemented in the organisation's "gateway" router, situated at the interface between the Internet and an organisation's private network, thus:

There are three classes of Firewall:

Packet filtering firewalls
Application gateways
Circuit level gateways

Of these, the packet filtering type is the most common.

Because the firewall is simply an enhanced router (and not a general-purpose computer system/host), it does not itself run any vulnerable server processes.

Packet Filtering Firewalls

These only permit selected traffic to pass between the "inside" and "outside" networks.

The decision to forward a packet or to discard it is made by looking into its protocol headers, usually at either the IP source or destination address. The TCP or UDP port numbers in the packet can also be used.

TCP (and UDP) level filtering is more complex. and requires the firewall router to keep much more "state" information. For example, a particular site may allow outgoing TCP connections for some services (ie, port numbers), but prohibit most incoming TCP connections. This can be achieved by examining the ACK bit in the TCP header: it is cleared in the first connection request segment, and is set in all subsequent segments.

For example, the La Trobe University "gateway" router is configured to block outgoing port 80 (HTTP) connections, thus forcing Web users within the University to use the caching proxy server. At Nilai College, where this unit is also offered, outgoing HTTP (port 80) connections are permitted and most other services are blocked at the gateway router. Go figure :-)

Packet Filtering Configuration

As an example of how access based on packet filtering can be configured, consider the approach taken in Cisco(tm) routers.

An access list is used to define permisible packet flows through the router. The general form of an access list entry is:

{ permit | deny } address [mask]

The entries have meaning thus:

permit | deny: indicates whether this entry allows or blocks traffic from the specified address.
address: gives the source IP address.
mask: this (optional) field is interpreted as a "bitmap" which defines the significance of each bit in the preceding address field. Every bit in the mask which has a "1" value indicates a "don't care" bit in the address, whereas every "0" bit indicates a bit in the address which must match exactly.

The access list for each interface can have many entries of the above form.

In addition, an access list can be specified to restrict either incoming or outgoing packets on the particular interface. Cisco's Extended ACLs are used for transport-layer (and other complex) filters.

More Complex Firewall Configurations

Most corporate firewall structures utilise a de-militarised zone (DMZ, also called a "stub network") between the internal LAN and the Internet, thus:

Both of the routers in this diagram are configured as packet filtering firewalls. Note that the DMZ is, of necessity, a separate subnet. Exercise: why?

There are many options for the level of filtering in the routers, and for the functionality of the bastion host. These are examined in the next slides.

Application Gateways

In this configuration, the only system which has access to the outside Internet is the bastion host. In turn, the systems on the internal LAN (nowadays often referred to as the "Intranet") can only establish TCP connections to the bastion host, or gateway. Both of these conditions are configured using TCP and IP address-based packet filtering in the routers. No TCP connection (or even packet flow) is permitted between the Intranet and the external Internet.

If users on the Intranet wish to use services provided on the outside Internet (for example, to ssh, or "shell" into an external Unix system) they must first connect to the gateway, and then use softare running on it to access the exterior machine.

An example of such a structure is provided by an electronic mail gateway which receives messages from the Internet. Typically, a mail gateway in the DMZ receives messages addressed to an alias address, such as:

P.Scott@latrobe.edu.au

and delivers (SMTP) the message to an appropriate second (department-level) server within the Intranet. MX records in the DNS support such mail gateway systems.

Circuit Level Gateways

These operate at the level of the TCP connection (or circuit). In effect, the application gateway transfers a copy of application-level data between the internal network and the outside Internet instead of the IP packets themselves.

Such a structure might be used where an internal host wishes to, for example, fetch a WWW page from a server in the external Internet. The client web browser establishes a connection to a server which is running on the gateway host. The gateway server in turn establishes a second connection to the actual WWW server on the external Internet, and passes the incoming HTML document back to the internal browser process. Such a server (running on the gateway, or bastion, host) is called a proxy.

One of the advantages of a proxy server is the caching of external web pages, thereby potentially reducing traffic on the connection to the outside Internet.

Real World Firewall Systems

Most installations combine both circuit level and application gateways, using the DMZ model of the previous slides. As mentioned, this implies the use of routers which can be configured to do packet filtering on both IP addresses and TCP connections.

The bastion host normally runs, for example, the organisation's WWW server, its FTP server (if applicable) and any other public information services. It also usually acts as an email gateway. The WWW server usually is configured as a proxy for the hosts connected to the internal LANs (the Intranet). It may also perform a similar function for other services such as FTP.

This type of structure can be very effective, combining many of the best characteristics of all of the other types. However, the gateway system must usually be a general-purpose timeshared (and therefore Unix) system, which raises doubts about the safety and reliability of the proxy software processes. This is not a solved problem, for obvious reasons.

Although... Highly secure versions of Unix for just such applications are now said to be available.

Other Issues

Important undiscussed (and firewall related) issues in network security include:

The importance of logging system activity in tracking break in attempts, and the use of log files in subsequent court actions.
Legal and ethical issues, both from the perspective of the system manager and in relation to the crooks.
Passwords -- their general applicability, the tendency of people to share them around and the sacrosanct nature of /etc/passwd
Security within Unix (and other server environments), and the use of tools such as cops and crack.
Network security analysis tools, including nmap and (earlier) satan.
The entirely unresolved philosophical and ethical question as to whether it is nobler to discuss issues like all of these in a full and frank way, or to hide them and hope the students and other potential bad guys don't hear about them... See, for example, Charles Tomlinson's 1853 "Rudimentary Treatise on the Construction of Locks" (available on lots of Web sites) for an excellent viewpoint on this.

If you are interested, there are various Firewall-related sites on the Web with much more information than can possibly be provided here.
The tutorial for this lecture is Tutorial #20.
La Trobe Uni Logo

[Previous Lecture] [Lecture Index] [Next Lecture]