Linux and firewall

Learning Outcomes

On successful completion of this course, students will be able to:

•Identify various types of firewalls and their functions, including which firewalls operate at which OSI protocol layer, and the basic variations of firewall architectures

•Describe risk mitigation techniques to varying threats with the use of different firewall architectures

•Demonstrate the ability to design and deploy policies on a firewall

Basic Types of Firewalls

• Packet filtering firewalls• State full packet inspection

firewalls• Application proxies• Hybrids

Packet filterA packet filter firewall is the simplest type of firewall. Dealing with each individual packet, the firewall applies its rule set to determine which packet to allow or disallow. The firewall examines each packet based on the following criteria:

• Source IP address • Destination IP address • TCP/UDP source port • TCP/UDP destination port

Packet Filter - Pros•They are fast because they operate on IP addresses and TCP/UDP port numbers alone, ignoring the data contents (payload) of packets.

•Due to the fact that packet payload is ignored, application independence exists.

•Least expensive of the three types of firewalls. •Packet filtering rules are relatively easy to configure.

•There are no configuration changes necessary to the protected workstations.

Packet filters - Cons•Allow a direct connection between endpoints through the firewall. This leaves the potential for a vulnerability to be exploited.

•There is no screening of packet payload available. It is impossible to block users from visiting web sites deemed off limits, for example. •Logging of network traffic includes only IP addresses and TCP/UDP port numbers, no packet payload information is available.

Packet filters – Cons

• Complex firewall policies are difficult to implement using filtering rules alone.

• There is a reliance on the IP address for authentication rather than user authentication.

• Dynamic IP addressing schemes such as DHCP may complicate filtering rules involving IP addresses.

Application proxies

An application proxy is a program running on the firewall that emulates both ends of a network connection. One can think of it as a sort of "translator" in-between the two computers communicating.

Application proxies - Pros

• Firewall does not let end points communicate directly with one another. Thus a vulnerability in a protocol which could slip by a packet filter or stateful packet inspection firewall could be overcome by the proxy program.

• Has the best content filtering capability.

• Can hide private systems.

• Robust user authentication.

• Offers the best logging of activities.

• Policy rules are usually easier than packet filtering rules.

Application proxies - Cons• Performance problems; much slower than the other

two • Must have a proxy for every protocol. Failure to have a

proxy may prevent a protocol from being handled correctly by the firewall.

• TCP is the preferred transport. UDP may not be supported.

• Limited transparency, clients may need to be modified. Setting up the proxy server in a browser, for example.

• No protection from all protocol weaknesses.

OSI – Open System Interconnect

TCP/IP Protocol Architecture

Three way TCP handshake

Common Ports and Services• Windows:

%windir%\System32\drivers\etc\services

• Linux:/etc/services

• Examples: SMTP = port 25 HTTP = port 80 POP3 = port 110 PPTP = port 1723

The STRIDE Threat Model• Spoofing identity

– Attacker obtains something that enables authentication

• Tampering with data– Unauthorized change made to stored or in-transit

information

• Repudiation– Performing an illegal operation in a system that lacks the

ability to trace such operations

• Information disclosure– Exposing critical information to unauthorized individuals

• Denial of Service (DoS)– Denies service to others

• Elevation of privileges– Attacker exploits a weakness to gain greater privileges on a

system than were intended

How to Respond to Threats

1.Do nothing.2.Inform the user of the

threat.3.Remove the problem.4.Fix the problem.

Building a firewall port matrix

• Determine trust zones• Determine ports that need opening• Determine packet type (tcp/udp)• Determines direction of packet flow• Determine any limitations you can set on

src/dst

Introduction to iptables

• 3rd generation firewall on Linux• Supports basic packet filtering as well as connection

state tracking

• For our needs for this course, we will use simple/basic packet filtering

• # Sample firewall – incomplete… do not use. For discussion only• IPTABLES=/sbin/iptables• ANY=“0.0.0.0/0”• ETHIP=“10.10.1.1”• ADMINNOC=“10.10.1.250”

• # Flush chains• $IPTABLES --flush

• # Set default policies• $IPTABLES -P INPUT ACCEPT• $IPTABLES -P OUTPUT ACCEPT• $IPTABLES -P FORWARD ACCEPT

• # Allow SSH from admin NOC• $IPTABLES -A INPUT -p tcp -s $ADMINNOC --sport 1024:65534 --dport 22 -j

ACCEPT• $IPTABLES -A OUTPUT -p tcp -d $ADMINNOC -sport 22 --dport 1024:65534 -j

ACCEPT

• # Allow Web access• $IPTABLES -A INPUT -p tcp --dport 80 -j ACCEPT

• # Allows secure web access• $IPTABLES -A INPUT -p tcp --dport 443 -j ACCEPT

• $IPTABLES -A INPUT -j DROP

Introduction to NMAP• Can scan networks to find active (online) hosts

• Can scan hosts to find open ports

• Can send crafted packets to fingerprint the operating system

• Can be used defensively to identify weaknesses that need to be corrected, or offensively by an attacker to probe for vulnerabilities to exploit.

Interesting NMAP options

• -v = Verbose logging• -O = OS fingerprinting• -sS = SYN stealth scan• -P0 = Scan without ping probes

• nmap –v –O –sS your.host.com

Reference

Dana Epp, January 2005 dana@scorpionsoft.com

http://silverstr.ufies.org/blog/