Firewalls and intrusion detection systems

Bencsáth Boldizsár

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Outline

Firewalls Intrusion detection systems (IDS)

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Introduction

– A firewall is a system or group of systems that enforces an access control policy between networks

– Mostly the goal is to protect TCP/IP networks– Other possible firewalls: between applications on a

windows environment, java card firewalls, etc.

– Functions:• Blocking traffic• Permitting traffic• Enabling secure remote connections (VPN)• Logging traffic• Content filtering (blocking): viruses, attacks• Network management purposes (screening the traffic etc.)

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Main goals

The main goal of firewalling is – to control unnecessary services, traffic– to hide our internal network topology and services– to protect against protocol errors (e.g. invalid SMTP commands can be

filtered)– to enable logging– to control the activity of internal users– every accessible point is a possible security hole: With firewalling we

minimize the accessible points and we are making it more difficult to deploy an attack

– we can make it more difficult to exploit the vulnerability: E.g. with tftp denied it is more difficult to send files to the internet after an attack

– we can separate the network to subnetworks: an intrusion will not compromise our whole system, just a subnetwork/server

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A firewall is not good for…

- Stopping information flow/leakage:Data can be leaked out even through DNS applications or e.g. HTTP

tunnels. It is very hard to protect against covert channels.

- Complete protection against intrusions:A single open port can be used to gain privileged access

An application proxy might not stop attacking through badly formed parameters, etc.

An industry spy can use the telefax to transport secrets…

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Packet filtering – disable access to unwanted services

Port State Service9/tcp open discard 13/tcp open daytime 21/tcp open ftp 22/tcp open ssh 23/tcp open telnet 25/tcp open smtp 37/tcp open time 79/tcp open finger 80/tcp open http 109/tcp open pop-2 110/tcp open pop-3 139/tcp open netbios-ssn 143/tcp open imap2 515/tcp open printer 587/tcp open submission

1723/tcp open pptp 3128/tcp open squid-http

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Port State Service21/tcp open ftp 22/tcp open ssh 25/tcp open smtp 80/tcp open http 110/tcp open pop-3 143/tcp open imap2

3128/tcp filtered squid-http

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Packet filtering

Filtering based on network layer of the IP stack Filtering rules described in rule base Default permit / default deny design Most routers have packet filtering capabilities A good packet filter…-Permits connections to really-needed services

-Also filters internal access – Most of the intrusions come from employees

-Detects anomalies – TCP packet without SYN handshake etc.

-Filters out all the services what we do not use currently (not only those we don’t want to show)

-Hides internal network elements and architecture (NAT)

-Filters services available to internal hosts (e.g. filter out streaming)

Main problem:

Stateless? Stateful? How?

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Packet filtering

Packet filtering rules mostly based on: IP protocol (UDP, TCP, …) Source IP address Destination IP address Source/Destination port (socket) Connection state (TCP: SYN, RST, established,… or

e.g. FTP states) (rate control) (filter rules based on time schedule – no streaming

before 8 p.m.) incoming/outgoing interface etc.

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Application gateway

Proxies rebuild the whole protocol (application layer gateway) Needs to know the exact specification of the protocol we use Can investigate the content of the flow Can protect against protocol errors More vulnerable to DoS Can be more complicated to (internal) users (e.g. telnet proxy) Lower performance Higher security

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Packet filter vs. Application gateway

– Packet filtering without states is insecure– Stateful packet filtering is fast– Stateful packet filtering might not protect against some

protocol errors– Application gateways are more sophisticated– Application gateways are (mostly) not transparent, so an

internal webserver beyond an application gateway can not log who is downloading the page

– Modern solutions mix the two methods.

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Rule sets - example– Linux iptables:

/sbin/iptables -A INPUT -j ACCEPT -p tcp -d 0/0 -v --dport 23

/sbin/iptables -A INPUT -j LOG -p tcp -d 0/0 -v --dport 110 --tcp-flags SYN,ACK,FIN,RST SYN

/sbin/iptables -A INPUT -j ACCEPT -p tcp -d 0/0 -v --dport 110

/sbin/iptables -A INPUT -j DROP -p tcp -d 0/0 -v --dport 3128

– Checkpoint Firewall – text: :rule-base ("##Standard" :rule (

:src ( : Any )

:dst ( : Any )

:services ( : Any )

:action ( : (accept

:type (accept)

:color ("Dark green")

:macro (RECORD_CONN)

:icon-name (icon-accept)

:text-rid (61463)

:windows-color (green) ) )

– Graphical tools / ruleset generators help

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Architecture / BasicA

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Internet

router

firewall

Internal network

„filters the traffic”

it can be a dual-homed gateway ora simple packet filter –

screening router

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ElementsA

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Internal network

Dual-homed gatewaysingle-homed gateway

gateway: application level border element, e.g.: proxy server

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Packet filter only – screening routerA

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Internet

router

packet filter

Internal network

Can be a singlescreening router

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Packet filter with bastion hostA

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Internet

router

packet filter

Internal network

Bastion host, here:a dual-homed gateway

== proxy server== application level firewall

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Packet filter with bastion hostA

rchi

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ure

Internet

router

packet filter

Internal network

Bastion host, here: a proxy firewall

internal packet filter

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Packet filter with bastion host, DMZ, internal pfA

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Internet

router

packet filter

Internal network

Bastion host, here: a proxy firewall

internal packet filter

Mail server Web server

DMZDe-Militarized Zone

?

many differenttopologies can be

considered

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Platform and other parameters

- Windows, Linux, Solaris, propriaty OS- Hardened operating system- With or without hardware

- Working method (stateful inspection, application proxy)- No. of interfaces- Authentication methods - Maximum traffic- Ease of use- Price

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Commercial & free products

– Iptables, netfilter packet filtering– Windows internal port filtering ( and IPSEC (policies), network sharing,

routing, etc. )– Checkpoint Firewall (FW-1) Linux, Windows, Nokia– Cisco PIX– Symantec Enterprise Firewall ( <- Raptor )– Secure Computing Sidewinder (incl. Gauntlet)– Zorp (Balabit)– NAI Firewall (Gauntlet->Secure Co)

– Evaluation: ICSA, Common Criteria, …

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Personal firewalls

– Every single host on the Internet is a target– Most users do not use tight security (no updates, bad passwords, no

security settings)– Attacked clients might become zombies for a DoS attack or a relay for

spams and other attacks– They need some protection– Personal firewalls are mostly simple packet filters– Drop incoming service requests (my windows pc is not a file server)– Alert on (anomalous) outgoing requests– Can protect against trojans / information leakage / privacy problems too– Can be integrated with virus protection

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Intrusion detection systems

– Intrusion detection:

detecting inappropriate, incorrect, or anomalous activity

misuse detection != intrusion detection

Host-based: operates on a (single) host

Network-based: operates on network data flows (e.g. promiscuous mode network card on a dedicated host)

IDS

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IDS Categories

In-Kernel vs. Userspace Distributed vs. Atomic Host-based vs. Network-based Statistical vs. Signature Detection Active vs. Passive Proactive vs. Retroactive Flat vs. Hierarchial(Justin Lundy)

IDS

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Host-based IDS Checking log files for traces of attacks Checking the condition of processes Looking for anomalies of the authentication system ( Why is X

logging in from Thailand? Why is Z logging in during the weekend?)

Checking the fingerprints of the installed binaries (Operating system integrity)

Checking for malicious user code – possible hacker tools, rootkits

Version (and critical security hole) checking Checking for invalid www request URLs in web server’s log

files Personal firewall? …

IDS

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Network based IDS

On a single network element (near the firewall) or can be distributed: more agents are distributed on the network and a central server makes the decision

Problem: Encrypted traffic cannot be analyzed (traffic analysis, timing only)

Signature filters: looks for various signatures. Usual attacks possess some kind of signature that identifies them– problem: large number of possible signatures – high traffic rate (~GBps

lines) – large number of dropped packets – less accurate result– problem: signatures has to be known. Regular updates needed and

much work to generate “good” signatures– problem: polymorphic attack: One might change the attack scenario so

that the signature will not match

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Anomaly detection

Mostly on statistical basis Detects statistically exceptional events Learning: Watching activity during ‘normal’ state and storing

patterns (who logs in, what is the origin, when, etc.) Experience shows that 90% of attacks can be considered as

protocol usage anomalies. Does not require signatures (except what it learns) We should carefully add knowledge about “normal” activity,

such as RFC compilant state machines, it needs much work. A non-RFC compilant client is not always an attacker – we

need flexibility

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CIDF

– Model: Common Intrusion Detection Framework

intrusion detection components can be reused in other systems

interface & communication protocols– Architecture

• Event generators (colloquially "E-boxes") • Event analyzers ("A-boxes") • Event databases ("D-boxes") • Response units ("R-boxes")

IDS