Chair of communication systems department of applied sciences university of freiburg 2006
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Chair of Communication Systems Department of Applied Sciences University of Freiburg 2006. Communication systems 17 th lecture (last). 1 | 57. Communication systems administrational stuff. Last lecture for this semester

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Chair of communication systems department of applied sciences university of freiburg 2006

Chair of Communication Systems

Department of Applied Sciences

University of Freiburg

2006

Communication systems17th lecture (last)

1 | 57


Communication systems administrational stuff
Communication systemsadministrational stuff

  • Last lecture for this semester

  • Friday is written exam starting at 11am sharp, Room 03-026 in this building (attic, end of stairs)

    • We gave some hints in last practical course on Tuesday

    • Please bring a fountain/ballpin pen with you (seats, tables, writing paper are provided by us)

  • Grades in oral or written exams will be sent to the examinations office (an will be available there beginning of winter term)

    • If you need a special printed paper – please tell us/send an email, so we could prepare it – it will be available at the secretaries of the computing department

2 | 57


Communication systems administrational stuff seminar next semester
Communication systemsadministrational stuff – seminar next semester

  • Professorship will held a block seminar on “Security, trust and law in the Internet” next winter in cooperation with MPICC (dept. of Prof. Sieber)

    • Unfortunately the faculty was not able to held the central infrormation block on available seminars soon enough

    • We expect written seminar papers for the end of October, the three seminar dates are on Friday/Saturday end of November, beginning of December

    • Seminar could be taken for the field of specialization #6

    • Topics like SPAM, cracking, phishing, etc. will be covered

    • Seminar is in german only!

    • More information on the several topics could be found on the homepage

3 | 57


4 | 57


5 | 57


Communication systems last lecture sip and h323
Communication systemsLast lecture – SIP and H323

  • We talked on and demonstrated (in the practical course) SIP – session initialization protocol and H.323 (both might be part of the written exam questions)

    • Telephony over IP networks

    • Only session setup

    • compression, packet transport left to other services like RTP and RTCP

    • the latter define container and control protocols for multimedia data streams

    • H.323 – standard developed by Telcos - ITU

    • SIP – internet standard, thus they differ definitely in their designs

6 | 57


Communication systems this lecture security in computer networks
Communication systemsthis lecture – security in computer networks

  • We leave the area of telephony and talk of a complete different field again

  • The topic of this lecture will NOT asked in exam questions :-)

  • After some overview on the several network layers

    • IP v4 and v6 on the third OSI layer (network)

    • TCP, UDP on the fourth OSI layer (transport)

    • and several protocols for the underlying first and second layer (physical and data link layer)

  • “security” is a very broad topic not only connected to networks but many other aspects of computers

7 | 57


Communication systems this lecture security in computer networks1
Communication systemsthis lecture – security in computer networks

  • This lecture – short introduction into problems of open networks, types and points of possible attacks

    • more than introduction is not possible

    • whole lectures may be held on that topic

  • Security measures do not focus on a single network layer

  • Different measures try to solve different problems that might occur

  • There is no single measure, which will solve all security issues at once

  • There will evolve new types of attacks and new types of counter measures

8 | 57


Communication systems network insecurity simple packet snapshot pract course
Communication systemsnetwork insecurity – simple packet snapshot (pract. course)

9 | 57


Communication systems network insecurity
Communication systemsnetwork insecurity

  • IP packets are easily readable (if provided with the proper tools)

  • e.g. ethereal can provide the user/network administrator

    • with a graphical userinterface for interpreting packets

    • can grab all packets visible to a machine (promiscous mode in LANs like ethernets)

    • can sort out TCP streams (check which packets are part of a certain communication)

    • can interpret most of protocol packets

  • You should be familiar with this tool (and others like tcpdump) from the several practical courses

10 | 57


Communication systems network insecurity1
Communication systemsnetwork insecurity

  • why packets are as easily readable?

  • all communication has to follow standards – otherwise no communication would be possible (think of people talk in different languages with each other)

  • even not open protocols, like certain implementations of windows network service are interpretable – such the samba service is developed through trial-and-error and reverse engineering

  • such: no security by obscurity!!

  • in the beginning of "The Internet”

    • very few participants in networks

    • very few computers connected to each other

    • very few people with deep understanding of networking

    • not many network analyzation tools available (for free)

11 | 57


Communication systems network insecurity2
Communication systemsnetwork insecurity

  • restricted computing power of connected machines

    • protocols should be very simple and should not impose high loads on the machine

    • encryption technologies were not common knowledge / restricted for export ("strategic technology”)

  • and: simplicity of TCP/IP protocol suite helped the rapid growth of the Internet and fast adaptation for the different operating systems

  • by now: the Internet is one of base technologies for information exchange and communication

  • wide range of businesses directly depend on this network (online shops, auctions, b2b, games, advertisements, porn sites, ... :-))

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Communication systems network insecurity3
Communication systemsnetwork insecurity

  • inner and intra firm communication moves from the classic communication media telephone and fax over to mail and similar technologies

    • sending and reception of a wide range of digital objects

    • e.g. with the “melissa” virus you could observe employees entering their offices at eight and leaving them at half past nine (no mail and online communication was available – most MS operated networks)

    • production and development heavily depend on networks – most information between firms is directly interchanged between databases over the net

    • in the future: move of telecommunications into IP networks to avoid duplicated infrastructure and cut communication costs

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Communication systems network insecurity4
Communication systemsnetwork insecurity

  • networks could be attacked on all layers

  • layer 1 and 2

    • e.g. ARP spoofing in broadcast networks for man-in-the-middle attack, redirection of default gateway traffic over the attackers host (fifth lecture)

    • “dialer” programs – redirection of internet traffic over costly dial-in lines (attack is of course induced via web applications, trojan horses, ...)

  • layer 3

    • IP spoofing – forging of IP addresses for good or malicious reasons (explained later) for motivation of IPsec

    • attacking router protocols, e.g. RIP (II) for redirecting traffic in LANs

14 | 57


Communication systems network insecurity5
Communication systemsnetwork insecurity

  • networks could be attacked on all layers

  • layer 1 and 2

    • rather simple within WLANs (unguided media with no distinct boundaries):

      • spamming with corrupt packets or simply noise (microwave oven) – frequency band is rendered unusable

      • breaking the weak WEP algorithm

    • e.g. ARP spoofing in broadcast networks for man-in-the-middle attack, redirection of default gateway traffic over the attackers host (earlier lecture)

    • “dialer” programs – redirection of internet traffic over costly dial-in lines (attack is of often initiated via web applications, trojan horses, ...)

15 | 57


Communication systems network insecurity6
Communication systemsnetwork insecurity

  • layer 4

    • very simple to send unsolicited UDP packets – connectionless service (such spoof protocols like SNMP, DHCP, DNS, ...)

    • take over open TCP connections – grab an open telnet, mail, http session to use an authenticated session to a remote host

    • TCP syn attacs (open as many TCP connections as possible from different hosts and leave them in open state without further communication – type of distributed denial of service DdoS)

    • dynamic routing protocols (drop in replacement for TCP or UDP) have their weaknesses too ...

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Communication systems network insecurity7
Communication systemsnetwork insecurity

  • application layers (layer 5 – 7)

    • SPAM attack on productivity in every organization, network / overload mail boxes to stop reception of further email

    • redirection of users/traffic through modification of DNS replies, DNS caches

    • crack passwords to gain access to accounts, databases ...

    • by now: so called “bot-nets”

      • groups of computers corrupted by some worm or system / service weakness

      • waiting for special incoming packets for distributed denial of service (DDoS) attacks, SPAM relaying, file exchange, ...

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Communication systems network security measures
Communication systemsnetwork security measures

  • different security measures for different network layers and protocols

    • application layers: e.g. PGP for mail – end-to-end mail encryption - advantages:

      • PGP/GnuPG available for many OS / mail clients

      • independent of admin permissions of the underlying OS

      • key ring could be put to USB stick (or similar) and deployed on more than one machine

    • disadvantages:

      • available for mail / filesystem encryption only

      • mail header (and all protocols below), end-to-end communication visible to every one along the route

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Communication systems network security measures1
Communication systemsnetwork security measures

  • Transport layer as an extension to service protocols put between TCP and higher level protocol

    • Secure socket layer (SSL: initially developed by Netscape to secure http connections to allow secure applications prerequisite for online shopping, homebanking, ...)

    • Transport layer security (TLS, or SSL v3) – modern version of SSL

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Communication systems network security measures2
Communication systemsnetwork security measures

  • by now implemented to a wide range of TCP applications

    • Web: https – port 443

    • Mailboxes: imap4 – imaps, port 993

    • Hierarchical database: ldap – ldaps, port 636

  • OpenSSL – open source implementation of the SSL library

  • SSL requires certificate authorities (CA) to really know how the communication partner is

    • hierarchical structures of trust are rather costly

    • information of CA has to be put into application, e.g. Web browser

  • Rather strong requirement in the rather “unregulated” Internet

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Communication systems network security measures3
Communication systemsnetwork security measures

  • Advantages of SSL/TLS:

    • Library functions which could be relatively easily applied to every TCP application

    • Freely available for all common OS

    • Relatively wide spread through use with HTTP communication

    • Relatively mature (some security flaws where detected and fixed)

    • For not SSL enabled / rather old applications or protocols secure tunnels via SSH (secure shell) could be established

    • Some certificate authorities are available

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Communication systems network security measures4
Communication systemsnetwork security measures

  • Disadvantages of SSL/TLS:

    • Not available for applications using UDP (or more difficult to apply), no SSH tunnels possible

    • Incompatibilities with/of older versions of SSL

    • CA are rather expensive and not really compatible with each other

      • e.g. University of Freiburg uses some CA but would pay extra money to enable every virtual web / mail host to use authorized certificate (e.g. examine the certificate of the mail server ...)

      • Every CA has to be known to the web browsers and protocols using SSL

22 | 57


Communication systems network security measures5
Communication systemsnetwork security measures

  • By now many universities and scientific organizations use the services of DFN CA

    • This CA is available free of charge to the members of that network

    • The Root certificate is integrated into the popular open source browsers (of course not into IE – M$ will most probably charge for that :-))

  • There is a more “general” solution to link encryption and authentication than SSL/TLS

23 | 57


Communication systems network security measures6
Communication systemsnetwork security measures

  • Network layer: IP sec protocol

  • Mostly in parallel to the SSL development need for secured IP connections was stated

  • IETF created work group which should backport IP v6 security features to IP v4 networks

    • Many participants in that workgroup

    • Long processes

    • Many incompatibilities between different vendors

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Communication systems network security measures7
Communication systemsnetwork security measures

  • Data link layer: PPTP or L2TP

  • PPTP (point-to-point-tunneling-protocol) is a Microsoft development for security enhancements to the PPP

  • PPP allows to transport more than one network layer protocol (e.g. IPX) beside IP

  • PPTP was cracked some years ago – some security issues not solved ...

  • PPTP is available to other operating systems too

  • L2TP (layer-2-tunneling-protocol) is prepared for adding security features too – but some issues not solved

  • For layer 2 tunneling OpenVPN (open source project available for OS with tun/tap network device)

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Communication systems network security measures8
Communication systemsnetwork security measures

  • OpenVPN uses the SSL library to encrypt traffic, could be used for securing layer 2 and IP connections

  • Uses UDP packets for easy crossing of masquerading routers

  • Could deploy TCP connections, connections over HTTP proxies too

  • Disadvantages: only point-to-point connections by now

    • need to setup of several connection endpoints on a server with the older 1.N versions

    • multipoint connections to the same server port would be available with the 2.0 version

  • Not an officially standardized protocol, but in broad use in many setups

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Communication systems network security measures summary
Communication systemsnetwork security measures – summary

27 | 57


Communication systems network insecurity address spoofing
Communication systemsnetwork insecurity – address spoofing

  • Talked on ARP and ARP spoofing earlier this lecture / practical course

    • Without authentication it is impossible to say which communication partner generated a certain packet

    • Same problem on higher layers too

  • Same problems with WEP (lecture on Wireless LAN), layer 2 security measures ...

  • IP spoofing is creation of IP packets using some other IP address as source

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Communication systems ip insecurity ip spoofing
Communication systemsIP insecurity – IP spoofing

  • IP source and destination addresses could be easily modified (you have only to recompute the headers checksum after it)– e.g. useful for IP masquerading (hide whole networks behind a masq. router – common technique for home LANs)

  • Tools to do so: iptables (Linux firewall package - example given in one of the practical courses), wincap, sendpacket, raw socket, ...

29 | 57


Communication systems ip insecurity ip spoofing1
Communication systemsIP insecurity – IP spoofing

  • forging source IP address causes responses to be misdirected, meaning that no normal network connection might be created

  • originates in packet switched type of IP networks

    • IP routing is done on a hop by hop basis

    • delivery route is determined by the routers that participate in the delivery process

    • routers use the “destination IP” address in order to forward packets through the Internet, but “ignore” the source address field – point of attack for IP spoofing

  • or asymmetric routing – packet is sent out on one interface and received over another

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Communication systems ip insecurity ip address spoofing in special scenarios
Communication systemsIP insecurity – IP address spoofing in special scenarios

  • prerequisite for some type of SAT connections (incoming via SAT, outgoing via Modem / ISDN)

    • user makes request using return channel

    • ISP receives data from Internet and sends it out through satellite

    • user receives data through satellite receiver (card)

31 | 57


Communication systems ip sec ip v4 insecurity
Communication systemsIP sec – IP v4 insecurity

  • IP v4 does not implement any security (easy IP spoofing, easy rewriting of packets, no encryption)

  • As we will see firewalls does not secure outgoing or inbound traffic but shields the internal LAN

  • For secure communication over an insecure network (not because of lost packets or connections - but special agencies listening on routers and wires) encryption will be needed

  • If hosts in an secured internetwork should interoperate as easily as in the classical Internet a standard for secure communication is needed

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Communication systems ip sec ip v4 insecurity1
Communication systemsIP sec – IP v4 insecurity

  • IP and transportation headers must be easily readable for routers and network engines

  • But packet payload is easily readable too, if the proper tools for analysis are applied (i.e. Ethereal)

  • Example of HTTP post packet (login to a wellknown free mail provider: ID and password could be identified without problem)

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Communication systems ip sec overview
Communication systemsIP sec - overview

  • IP level security -> IPsec

  • IPSEC is Internet Protocol SECurity

  • The level above the network layer is the place where IPsec was put - No alteration to the IP was needed, simply the transportation protocol was interchanged (or and additional security header introduced)

  • It uses strong cryptography to provide both authentication and encryption services

    • Authentication ensures that packets are from the right sender and have not been altered in transit

    • Encryption prevents unauthorized reading of packet contents

  • Topic covered in other lectures: Telematics/Internet-Working

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Communication systems ip sec vpns
Communication systemsIP sec – VPNs

  • It allows multiple access for e.g. teleworkers to the company LAN

  • Without VPN

    • costly separate infrastructure would be needed

    • often inflexible

  • Construction of a VPN

    • connection of all participating parties to the internet

    • VPN client asks for secure connection from the server

    • authentication via username/password, shared secret, key cards ...

    • after validation tunnel is set up with special IP routes

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Communication systems ip sec vpn problems
Communication systemsIP sec – VPN problems

  • Problems with VPN gateways

    • gateway machines reachable over the public internet

    • could be attacked for break-in, denial of service

    • security could be increased through combination of authentication methods

  • Security at tunnel end point

    • split tunnel – unencrypted interface to the internet needed (transport medium for encrypted traffic)

    • user machine is not secured against attacks from the internet

    • “hardened tunnel” - no connection/routing to the local LAN is allowed, user end point machine obtains a private IP from the internal network

36 | 57


Communication systems network security other directions to look
Communication systems network security – other directions to look

  • By now we discussed encryption and authentication measures put to different protocol layers to improve security

  • We ensure this way, that nobody can read/alter the packets of a communication during transit

  • We do not secure a machine that way – vulnerability to attacks, DoS have to be abated some other way

  • Completely other path of thought

    • not to protect own traffic from sniffing ...

    • but allow or block traffic at gateway, router, end system ...

  • Traffic / packet filtering on different levels is another concept to increase security – parts of it will be discussed next part of lecture ...

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Communication systems network security the magic device firewall
Communication systemsnetwork security – “the magic device”: firewall

  • Take a completely new track now ...

  • Firewalls are traffic / packet filters that operate on different layers of our OSI protocol stack

  • Try for a definition: “A Firewall is a network security device designed to restrict access to resources (information or services) according to a security policy”

  • Important remark is to be made here:

    • Firewalls are not a “magic solution” to network security problems, nor are they a complete solution for remote attacks or unauthorized access to data!!

    • Firewalls could be circumvented in several ways and may increase the complexity of network and this way decrease the level of security!

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Communication systems network security firewalls
Communication systems network security – firewalls

  • A Firewall is a often a network security device, but can be or simply is implemented directly into the end systems

  • It serves to connect two parts of a network a control the traffic (data) which is allowed to flow between them

  • Often installed between an entire organization's network and the Internet

  • A Firewall is always the single path of communication between protected and unprotected networks

    • Of course there are special cases of multiple Firewalls, redundant connections, fault-tolerant failover etc.

    • A Firewall can only filter traffic which passes through it

    • If traffic can get to a network by other means, the Firewall cannot block it

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Communication systems network security firewalls1
Communication systemsnetwork security – firewalls

  • Types of firewalling concepts:

    • (MAC / ethernet frame filter)

    • Packet filter

    • Circuit-level proxy

    • Stateful packet filter

    • Application-level proxy

  • Filtering on data link layer

    • ethernet packets contain source and destination addresses: MAC

    • allow only frames to be delivered from known sources, block frames with unknown MACs

40 | 57


Communication systems network security firewalls2
Communication systems network security – firewalls

  • Filtering on network layer

    • Source & destination IP addresses

      • Source address

      • Destination address

        • Both are numerical – it is not easy for a Firewall to deal with machine or domain names

        • e.g. www.hotmail.com

      • Request: client = source, server = destination

      • Response: server = source, client = destination

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Communication systems network security firewalls3
Communication systems network security – firewalls

  • Filtering on transport level

    • This is where we deal with (mostly) TCP and UDP port numbers

      • e.g.: 25 SMTP – sending email (TCP)

      • 110 POP3 – collecting email (TCP)

      • 143 IMAP – collecting email (TCP)

      • 389 LDAP – directory service (TCP)

      • 636 LDAPS – TLS secured directory service (TCP)

      • 80 HTTP – web pages (TCP)

      • 443 HTTPS – secure web pages (TCP)

      • 53 DNS – name lookups (UDP)

      • 68, 69 DHCP – dynamic end system IP config (UDP)

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Communication systems network security firewalls4
Communication systemsnetwork security – firewalls

  • Most Firewalls and their administrators assume that the port number defines the service – not necessarily

    • who could stop me from sending or receiving mail over the HTTP port

    • who could stop users from tunneling all their IP traffic over an open port (AOL left UDP 53 completely open for DNS traffic some year ago :-))

  • Here we get major problem: If users are blocked from using a service and try to avoid the blocking firewall they might find a way through – the admin still thinks all is fine with the network, but the situation might be even worse than without firewall at all ...

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Communication systems network security firewalls5
Communication systems network security – firewalls

  • Layer 7 – Application

    • There is where we find all the 'interesting' stuff ...

      • Web requests

      • Images

      • Executable files

      • Viruses

      • Email addresses

      • Email contents

      • Usernames

      • Passwords

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Communication systems network security firewalls6
Communication systems network security – firewalls

  • packet filter – a special router that have the ability to throw packets away independently of network congestion

  • Examines TCP/IP headers of every packet going through the Firewall, in either direction

  • Choice of whether to allow or block packet based on:

    • (MAC source & destination)

    • IP source & destination addresses (layer 3)

    • TCP / UDP source & destination ports (layer 4)

  • Stateful filter

    • Same as a packet filter, except initial packets in one direction are remembered, and replies are automatically allowed fo

    • Simpler rules than simple port based packet filter

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Communication systems network security firewalls7
Communication systems network security – firewalls

  • Packet filter use rules specify which packets are allowed through the Firewall, and which are dropped

    • Rules must allow for packets in both directions

    • Rules may specify source / destination IP addresses, and source / destination TCP / UDP port numbers

    • Certain (common) protocols are very difficult to support securely (e.g. FTP, IRC, SIP, ...)

    • Low level of security

  • Stateful packet filter

    • Packet filter which understands requests and replies (e.g.: for TCP: SYN, SYN-ACK, ACK)

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Communication systems network security firewalls8
Communication systems network security – firewalls

  • Stateful packet filter

    • Rules need only specify packets in one direction (from client to server – the direction of the first packet in a connection)

    • Replies and further packets in the communication are automatically processed

    • Supports wider range of protocols than simple packet filter (eg: FTP, IRC, H323)

    • Medium-high level of security

47 | 57


Communication systems network security firewalls9
Communication systems network security – firewalls

  • Layer-7 proxy server – application level proxy

    • Client and server in one box

    • For every supported application protocol

      • SMTP, POP3, HTTP, SSH, FTP, NNTP, Q3A, ...

    • Packets are received and processed by server

    • New packets generated by client

  • Prevents the need for direct network connection of clients, no client packet is directly routed into the Internet, no packet from Internet is directly handed to the client

  • Special proxy protocol supported by many applications which offers authentication: socks5

48 | 57


Communication systems network security firewalls10
Communication systems network security – firewalls

  • Complete server & client implementation in one box for every protocol which can be expected through it

  • Client connects to Firewall

  • Firewall validates request

  • Firewall connects to server

  • Response comes back through Firewall and is also processed through client/server

  • Large amount of processing per connection

  • High level of security

  • And: lot of funny stuff could be tried with filtering (SPAM, Ads, porno sites, ...)

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Communication systems network security firewall taxonomy
Communication systems network security – firewall taxonomy

  • Packet filters, circuit-level proxies and stateful packet filters are like telephone call-barring by number

    • block or allow mobile calls

    • block or allow international calls

    • block or allow 0190/0900 calls

    • from different internal extensions

  • Application level proxy is like telephone call monitoring by listening to the conversations

    • conversations may still be encoded, or in a foreign language !!

50 | 57


Communication systems network security personal firewalls
Communication systems network security – “personal” firewalls

  • Applications which run on Windows machines

    • commonest home PCs

    • often insecure

    • increasingly connected using ADSL etc.

  • Packet filter (sometimes stateful)

  • Learn which applications are permitted to make what type of connections outbound

  • Block inbound access except replies

  • But nobody nows exactly

    • how personal firewalls are bound to Windows network stack

    • how firewalls could be disabled by malicious applications

51 | 57


Communication systems firewalls conclusion
Communication systems firewalls - conclusion

  • Firewalls control network traffic to and from the protected network

  • Can allow / block access to services (both internal and external)

  • Can enforce authentication before allowing access to services

  • Can monitor traffic in/out of network

  • Firewalls typically defend a protected network against an attacker, who tries to access vulnerable services which should not be available from outside the network

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Communication systems firewalls conclusion1
Communication systems firewalls - conclusion

  • Firewalls are also used to restrict internal access to external services, for many different reasons:

    • security (don't want people downloading and installing unknown applications)

    • productivity (don't want people wasting time on non-work related websites etc)

    • cost (many Internet connections, e.g.: Dial-Up are charged by data transferred – ensure this is all necessary)

  • But firewalls could mislead to total control and monitoring

    • or distract admins from more important security issues ...

53 | 57


Communication systems conclusion of the lecture
Communication systemsconclusion of the lecture

  • Gave a broad overview on network related issues with focus on IP and digital telephony networks

  • Defined a model for network protocol layering

    • talked on network layer: IP v4 / v6

    • routing on this layer

    • DNS as a helper application for the convinience of the Internet users

    • physical and data link layer – several lower layer protocols and techniques for transportation of bitstreams

    • encoding digital data into analogous signals

54 | 57


Communication systems conclusion of the lecture osi layers and examples
Communication systemsconclusion of the lecture – OSI layers and examples

55 | 57


Communication systems conclusion of the lecture1
Communication systemsconclusion of the lecture

  • Many topics were not or rather short covered

  • Range of lectures which focus on

    • network security

    • network programming

    • dynamic networks and routing protocols

    • network applications

    • ...

  • Courses of the professorship next semester

    • Interdisz. seminar as introduced beginning of lecture ...

    • Special practical course on OpenSource PBX Asterisk (SIP, mobile telephony, ...) at Summercampus2006: 16th - 19th of August

56 | 57


Communication systems end for today and this semester
Communication systemsend for today and this semester!!

  • Thanks to our hiwis

    • Rui Zhou

    • Ahmad Abdul Majeed and Christoph Hanke

  • Helping preparation of practical courses

    • Discussing and defining excercises

    • Correcting excercises

    • Preparing services and tools

    • ...

  • Have nice summer holidays!!

  • See you tomorrow :-))

57 | 57


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