notes/docs/lectures/security/12_firewalls.md

Firewalls

• A hardware and/or software system
• Prevents unauthorised access of packets from one network to another
• All data leaving any subnet must pass through it

Firewall Functions

• Implements single point security measures
• Security event monitoring through packet analysis and logging
• Network-based access control through implementation of a rules set

Network Firewalls - placed between a subnet and the internet

Host-based Firewalls - placed on individual machines

• A standard home router is a good example of a network firewall

DMZ

• A demilitarised zone is a small subnet that separates externally facing services from the internal network

• Imagine we have a web and email server running, these servers need different firewall rules to personal machines on the network

Basic Function

• Defends a network against parties accessing internal services
• Can also restrict access from inside to outside services
• Network Address Translation
  • Hides the internal machines with private addresses

Firewalls are not enough

• Cannot protect against attacks that bypass the firewall
  • e.g. tunnelling
• Cannot protect against internal threats or insiders
  • Might help a bit by egress filtering
• Network firewalls cannot always protect against the transfer of virus-infected programs or files

Packet Filters

• Specify which packets are allowed or dropped
• Rules based on:
  • Source / destination IP
  • TCP / UDP port numbers
• Possible for both inbound and outbound traffic
• Can be implemented in a router by only examining packet headers (IP / TCP)

Packet Filter Rules

• Rule execution depends on implementation
  • IPTABLES: First rule to match is applied
  • PF: All rules are examined, last match is applied
• Rules are organised in chains, which are logical subgroups of rules
• Depending on the packet, different chains are activated

IPTABLES

• An application that provides access to the Linux firewall rule tables
  • Not actually a firewall, but configures the firewall
  • The firewall is mostly implemented as netfilter modules

Tables and Chains

• IPTABLES uses tables to store chains
  • Default is the filtering table
• Chains are ordered in lists of rules
  • Rules match, or they don’t
• Matches result in a jump, else we check the next rule.

Default policy on this chain is DROP

• There can be multiple chains per table
  • e.g. a TCP handling chain
• Jumps can go to ACCEPT, DROP, LOG or another chain
• Complex behaviour can be built up

Defaults

• There are four built-in tables in IPTABLES
  • Filter
  • NAT
  • Mangle - packet alteration
  • Raw - skips connection tracking
• The default table is the filtering table, including input, output and forward chains

Rules Examples

• Using the command line, we add rules onto the end of chains

$ iptables -A INPUT  -i eht0 -p tcp --dport 80 -j ACCEPT
$ iptables -A OUTPUT -i eht0 -p tcp --sport 80 -j ACCEPT

• Remember http requests are not sent from the client’s port 80; they are sent from a random high-numbered port
  • This is how clients can have multiple web requests open at the same time

Policies

• Permissive - allow everything by default except dangerous services
  • Make a black list
  • Easy to make a mistake or forget something

iptables -p INPUT ACCEPT
iptables -p FORWARD ACCEPT
iptables -p OUTPUT ACCEPT

iptables -A INPUT -s X.X.X.X -j DROP
iptables -A OUTPUT -p tcp --dport ssh -j DROP

• Restrictive - block everything except designated useful services
  • Make a white list
  • More secure by default

iptables -p INPUT DROP
iptables -p FORWARD DROP
iptables -p OUTPUT DROP

iptables -A INPUT -p tcp --dport ssh -j ACCEPT
iptables -A OUTPUT -s 192.168.0.2 -j ACCEPT

Packet Filter Issues

• Packet filters are simple, low-level and have high assurance
• However:
  • They cannot prevent attacks that employ application-specific vulnerabilities
  • Do not support higher-level authentication schemes
  • Easy to accidentally allow or deny packets incorrectly

Stateful Packet Filters

• Understand requests and replies (ACK/SYN)
• Dynamically generate rules
  • Based on what it sees from TCP handshakes (can be FTP or SSH etc)
• Can support policies for a wider range of protocols
• IPTABLES has a module for stateful packet filtering
• Allow incoming / outgoing SSH connections

Connection Tables

• ACK packets are used to keep track of the session - the connection is ongoing
• Packets without the ACK are the connection establishment messages

Application-level Gateways

• Packet filters have limited criteria that allow data in and out
• An application gateway considers the application-layer protocol that is in use
  • For example if someone sends an HTTP request to port 22, it is blocked

Proxy Server

• Proxy servers initiate a connection on our behalf
• They can block certain access, and scan for malicious files or web pages

Issues:

• Large overhead per connection
• More expensive than packet filtering
• Configuration is complex
• A separate server is required for each service

Network Address Translation

The shortage of IP addresses means that most routers now perform NAT automatically

• The implicit advantage in NAT is that your machine is almost totally hidden from the internet
• Only established connections are forwarded to your internal machine
  • Or, specific port forwarding rules
• This prevents any unsolicited attacks on random ports, but no other types of attack