Bencsáth Boldizsár

 Firewalls
 Intrusion detection systems (IDS)
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– 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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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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- 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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Port State Service
9/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
Port State Service
21/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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 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 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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 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 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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– 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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Internet router
Internal network
„filters the traffic” firewall it can be a dual-homed gateway or a simple packet filter – screening router
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Dual-homed gateway single-homed gateway gateway: application level border element, e.g.: proxy server
Internal network
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router
Internet packet filter
Can be a single screening router
Internal network
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Internet router packet filter
Bastion host, here: a dual-homed gateway
== proxy server
== application level firewall
Internal network
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Internet router packet filter
Bastion host, here: a proxy firewall internal packet filter
Internal network
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Internet router packet filter many different topologies can be considered
Bastion host, here: a proxy firewall
?
internal packet filter
Mail server Web server
DMZ
De-Militarized Zone
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Internal network

- 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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– 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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– 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: 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)
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 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)
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 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?
 …
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 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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 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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– 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")
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