Wireless Local Area Networks

Transcription

1 Wireless Local Area Networks Experiments for Bachelor Students of Mobile Communication Networks Course Dr.-Ing. Florian Evers & Prof. Dr.-Ing. habil. Andreas Mitschele-Thiel Integrated Communication Systems Faculty of Computer Science and Automation Ilmenau University of Technology

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3 Contents 1. Introduction Test Set-up Gentoo Linux Network Topology IP-Addresses Netmasks Homework: Preparation 9 4. Experiments Building the Network Router Backbone Router Server Subnet Router Access Points Access Point Access Point Sniffing and Traffic Monitoring Traffic Capturing Traffic Filtering Display Filters Capture Filters Configure Cisco Aironet Web Interface Home Network Associations Setup Configuration IP and ESSID Default Gateway Radio Channel and Transmit Power WEP Abbreviations 29 Contact 33 A. Handouts 35 A.1. Handout: Network topology

4 4 A.2. Handout: IP addresses and netmasks A.3. Handout: Linux Commands

5 1. Introduction Wireless technologies are available since decades ago. One of the popular networks is the Wireless Local Area Network (WLAN). Main reasons for the popularity of WLAN are the always reduced cost of hardware and the ability of WLAN to be quick installed and configured in addition to the services that can be offered using such networks. The fact that WLAN is only suitable for indoor environments has resulted in deploying these networks in companies, airports, hospitals, etc. Thus, it is useful to provide students with knowledge on how to build and configure a WLAN, which is the purpose of these experiments. The rest of this document is structured as follows: chapter 2 provides the network topology of the experiment. In chapter 3 some tasks are given, which should be done before the experiment. Chapter 4 goes through the experiments that should be done step by step. How to configure the access points, used in these experiments, is provided in chapter Test Set-up The test set-up consists of one mobile node, two access points and four computers connected to a Keyboard Video Mouse (KVM) switch. With the help of the KVM switch, you can use one keyboard, mouse and monitor for all four computers. All network cables required for the experiment are already connected. Table 1.1.: KVM layout Port Computer 1 Server 2 Backbone Router 3 Router 4 Subnet Router 1.2. Gentoo Linux The operating system on all computers and the mobile node is Gentoo Linux. To configure the network you will work on the Unix-Shell. Each entered command will be saved in a command history and can be recalled with the help of the arrow up and down keys on the keyboard. If you want to abort a running command, press CTRL-C. 5

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7 2. Network Topology Figure 2.1 shows the network topology of the experiments. The network is hierarchically structured and can be assumed as a simple structure of a part of the Internet. As the figure shows, the network consists of a server, a backbone router, a router, a subnet router and two access points. The Internet Protocol (IP) addresses and netmasks of the network nodes according the figure. On all computers the network interface eth0 is used for the Internet connection, but is not used in the experiment. Server eth1: /30 Backbone Router eth1: /30 eth2: /30 Router eth1: /30 eth2: /16 eth3: /30 Subnet Router eth1: /30 eth2: /24 Access Point 1 IP: /24 Access Point 2 IP: /16 Mobile Node IP 1: /24 IP 2: DHCP Backbone Router eth1 eth / /12 Server Router eth1 eth1 eth3 eth2 Subnet Router / /16 Access Point 2 eth1 Access Point 1 eth /24 Mobile Node Figure 2.1.: Network topology of the experiments Access point 1 works on channel 5, transmits with 20mW power and has Extended Service Set Identifier (ESSID) set to IKS WLAN AP1, while access point 2 operates on channel 11, sends with 5mW and has ESSID set to IKS WLAN AP2. The target is to configure the network provided in the figure 2.1, so that the mobile node can communicate with the server, which represents the Internet as it runs a web server. 7

8 2.1. IP-Addresses The IP addresses used in this experiment are free to use for private networks. The Internet Assigned Numbers Authority (IANA) defined 3 address ranges for private networks: Table 2.1.: IP addresses for private networks [ipa11] address range CIDR # addresses to / = to / = to / = In this practical course, IP addresses of the range /8 are used Netmasks A netmask is a bitmask, for the network protcol IPv4, that describes how many bits from the beginning of the IP address are used for the netprefix. A netmask has a length of 32 bit. All bits of the network part are set to 1, and all remaining bits of the host part are set to 0. A short notation of the netmask is the Classless Inter-Domain Routing (CIDR) notation, where the number of continious 1 bits from the left is counted. Table 2.2.: netmask examples [net11] CIDR netmask binary netmask # addresses / / / / / /

9 3. Homework: Preparation eth1 eth1 eth2 eth / /8 Computer Router Server Figure 3.1.: Network topology Complete the following table with the help of figure 3.1. Some fields may not be necessary. System: Computer interface eth1: /24 route default route IP forwarding System: Router interface route eth1: eth2: default route IP forwarding System: Server interface eth1: /30 route /8 gateway default route IP forwarding no 9

10 Table 3.1.: Sample routing table # Destination Gateway Genmask Interface eth eth eth eth eth1 Table 3.1 shows the routing table of a computer. The computer receives packets for another computer. Which entry of the routing table will be used, if the packet should be forwarded to: , , , , ? Inform yourself about the structure of Internet Control Message Protocol (ICMP), Network Time Protocol (NTP) and Hypertext Transfer Protocol (HTTP). How ping packets differ from other ICMP packets? Which transport protocol and port is used by NTP? Which transport protocol and port is used by HTTP? 10

11 4. Experiments The interfaces in all network nodes are not configured yet. First you have to configure the interfaces and assign the correct IP adresses. Afterwards you have to construct suitable routing tables in the network nodes, so that each node can reach all nodes connected directly to it. The next step is to activate IP forwarding, so each node can reach all other nodes in the network. IP forwarding enables the packet transfer between all network interfaces of one computer. The following steps help you. Remember, all steps are just explained once, the procedure for the other computers is analog Building the Network Turn on all computers and the mobile node. username: praktikum password: praktikum Router i. Type ifconfig -a. How many interfaces can you see? Which interfaces are already active? ii. The network interface eth1 is not configured. Use ifconfig eth /30 to configure the interface for the transit network. iii. The interface eth2 is already configured, because the router runs a Dynamic Host Configuration Protocol (DHCP) server on it. Configure interface eth3 according to eth1. iv. Type ifconfig. What do you see now? v. Type route add default gw This command adds interface eth2 of the backbone router as default gateway for the router. If the router does not know where to forward a packet, it will send the packet to the defined gateway. vi. Type route add -net /16 gw , to add a route from eth1 to the interface of the subnet router. Notice that the server is directly connected to the router. vii. If you want to check, whether the routing table is built correctly, type route -n. Interpret what you see. 11

12 viii. Type nano /proc/sys/net/ipv4/ip forward. To enable IP forwarding, replace 0 with 1, save (CTRL-O) the file and exit (CTRL-X) nano Backbone Router i. Configure the network interfaces to the following addresses: /30 (eth1) and /30 (eth2). ii. Add a route from the backbone router to the server and the router. iii. If a default gateway is necessary, add one. iv. Check, whether the routing table is built correctly. v. Type ping c 4. If everything is configured correct, you should obtain four replies from the interface. vi. Don t forget to enable IP forwarding Server i. The network interface eth1 is not yet configured. Configure it to /30. ii. Add a route, from eth1 to the backbone router. iii. Add the backbone router as default gateway for the server. iv. Is there a need of step ii and step iii, or is one of them enough? In case one is enough, which one would you choose? Discuss by looking at the routing table. v. Ping the backbone router ( ). If everything is configured correctly, you should obtain a reply from the interface. vi. Is IP forwarding necessary for the server? Why or why not? Subnet Router i. Configure the network interfaces and create corresponding routing tables. ii. If a error message appears, explain it. How to fix the error? iii. Test the network with the help of the ping command. ( ). iv. Enable IP forwarding if necessary. Especially ping the server 12

13 Access Points The next step is to configure the access points. If you turn on the access points for the first time, the default settings are loaded. If no DHCP server is connected to the access point, the IP address is set to /24. On default ESSID is set to tsunami and the access point transmits on channel 7. Use the mobile node to configure the access points. Configure them one after another, to avoid confusion. Which access point is connected to a DHCP server? Access Point 1 i. Change the IP address of the interface wlan0 to the following address /24. ii. Take care that only one access point 1 is powered. iii. With the command iwconfig wlan0 essid tsunami you can connect the mobile node to the access point. iv. Type iwconfig and check if you are connected. v. Send ping packets to the access point. vi. Type konqueror, to open the webbrowser, and access vii. You have to configure the IP address, network mask, default gateway, ESSID, working channel and sending power of the access point. Reflect about the order of the changes. Maybe you have to reconnect to the accesspoint after some changes. Read chapter 5 at page 19 if you need some help for configuration. viii. Configure the interface wlan0 to the following IP address: /24 and add access point 1 as default gateway. ix. Connect the mobile node to the access point IKS WLAN AP1. x. If everything is configured correct, you should be able to ping the server Access Point 2 i. At first plug in access point 2. ii. Deactivate the interface wlan0 to delete the IP address and the default gateway with the help of ifconfig wlan0 down. iii. Type ifconfig wlan0 up to reactivate the interface. iv. Type iwlist scan wlan0 to scan for visible accesspoints and analyze the output. v. Connect the mobile node to the access point tsunami vi. Check if you are connected. vii. Type dhcpcd wlan0. Now you get your IP address, netmask and default gateway from the DHCP server. viii. Open Konqueror and access 13

14 ix. You have to configure ESSID, working channel and sending power of the access point. Maybe you have to reconnect to the accesspoint after some changes. x. Connect the mobile node to the access point IKS WLAN AP2. xi. Scan for visible accesspoints and analyze the output. What has changed? xii. If everything is configured correct, you should be able to ping the server. 14

15 4.2. Sniffing and Traffic Monitoring Packet sniffers are capable to capture network traffic, decoding packet contents and presenting them in readable way. Sniffers are used to monitor network traffic and debug errors in the network operation. This experiment provides an insight into how to use such sniffers to monitor, filter and analyze network traffic. The packet sniffer used in this experiment is wireshark ( Traffic Capturing Start wireshark on the mobile node by typing wireshark on the command line. Click on Capture Interface, to open the Capture Interfaces window. This window shows all interfaces that wireshark has detected. Click on the Start button, responsible to your wireless interface, to start capturing packets. i. Wait about one minute. Which protocols do you see? ii. Which IP addresses do you find in the current trace file? iii. Wireshark provides you with lots of informations about the captured packets. Which kind of informations can you obtain? iv. Send ping packets from the mobile node to the server. v. Monitor the ping packets in wireshark and notice their content and delay Traffic Filtering For effective traffic monitoring, traffic should be filtered, so only traffic matching the filter will be presented. The next steps should give you an insight into how to create and use traffic filters Display Filters i. Send ping packets from the mobile node to the server. ii. Create a filter to present only the packets sent by the server to the interface of your mobile node or reverse (Filter: ip.addr == ). What do you see? 15

16 iii. You can extend the filter, to show only traffic of a certain protocol. For example, ping packets are sent, using ICMP. If we are interested in this traffic only, we have to use an adequate filter (Filter: ip.addr == && icmp ). iv. Now delete the filter to show all packets. Notice the types of packets you receive. Write filters to show the traffic of each protocol for itself. v. Access in your browser and repeat step iv Capture Filters In contrast to display filters, which capture all packets and present them matching certain rules, capture filters affect the capturing of packets themselves. Only packets matching the given filter will be captured. The basic syntax of capture filters in wireshark is the following: [not] primitive [and or [not] primitive...] 16

17 Table 4.1.: Some primitives available in wireshark [wir11] Command Task [src dest] host <ip address> This primitive allows you to filter on a host IP address or name. You can optionally precede the primitive with the keyword src dst to specify that you are only interested in source or destination addresses. ether [src dst] host <ehost> This primitive allows you to filter on Ethernet host addresses. You can optionally include the keyword src dst between the keywords ether and host to specify that you are only interested in source or destination addresses. gateway host <host> This primitive allows you to filter on packets that used host as a gateway. That is, where the Ethernet source or destination was host but neither the source nor destination IP address was host. [tcp udp] [src dst] port <port> This primitive allows you to filter on TCP and UDP port numbers. You can optionally precede this primitive with the keywords src dst and tcp udp which allow you to specify that you are only interested in source or destination ports and TCP or UDP packets respectively. The keywords tcp udp must appear before src dst. less greater <length> This primitive allows you to filter on packets whose length was less than or equal to the specified length, or greater than or equal to the specified length, respectively. ether ip broadcast multicast This primitive allows you to filter on either Ethernet or IP broadcasts or multicasts. Table 4.2.: Operators for primitives in wireshark [wir11] Operator Meaning!, not logical NOT-operation &&, and logical AND-operation, or logical OR-operation & bitwise AND-operation 17

18 You can access and check the content of packets with the help of the capture filters. The basic syntax is: protocol[<offset in bytes from the beginning of the header>:<number of bytes>] Table 4.3.: Examples for filter expressions checking the contents of packets [wir11] Filter Result ip[8] = 1 Filters IP packets, where the byte number 8 (TTL) equals 1. tcp[0:2] = 22 Filters TCP packets, where the value of the first two bytes of TCP header (port number) equals 22. Notice that the <number of bytes> in the basic syntax can be 1, 2 or 4. If you do not give any number, this value will be 1. If you want to use a capture filter, you have to stop capturing first. Next step is to click Capture Interface. Before you can restart capturing, you have to click Options responsible to your wireless interface. Enter your filter behind Capture Filter: and press Start. Now practice your knowledge about filtering. i. Access in Konqueror. ii. Write a filter that captures all packets with Time to Live (TTL) greater than 63. iii. Ping the server to test the filter. What can you see? iv. Write a filter that captures all HTTP traffic. v. Test the filter by accessing the server in Konqueror. vi. Write a filter that captures all NTP traffic sent from the mobile node. Reflect which protocol and port is used by NTP. vii. Write a filter that captures all ICMP packets, but no ping ones. Reflect the structure of the ICMP header. 18

19 5. Configure Cisco Aironet Web Interface Home Figure 5.1 shows the Home page, that displays the status of the access point. There you can see informations about the number of radio devices associated, the status of the Ethernet as well as the radio interface and a list of recent events. Also, this page shows information about IP and Media Access Control (MAC) addresses of both interfaces. Figure 5.1.: Summary Status 19

20 Network Figure 5.2 shows the Network page. It displays the status and statistics for the Ethernet and radio interfaces. Figure 5.2.: Network Ports 20

21 Associations The Associations page displays a list of all devices in your WLAN. It lists their system names, network roles, and parent-client relationships. Figure 5.3 presents a screenshot of the page. Figure 5.3.: Association Table 21

22 Setup The Setup page is shown in Figure 5.4. It contains links for configure associations, event log, services and network ports. Section 5.2 gives you a insight in some of the configurations. Figure 5.4.: Setup 22

23 5.2. Configuration IP and ESSID Click on the link Identification behind AP Radio: Internal on the Setup page. Here you can configure the IP address, network mask and ESSID of the radio interface. Figure 5.5.: AP Radio: Internal Identification 23

24 Figure 5.6 shows the Ethernet Identifaction page. Here you can configure the IP address and network mask of the Ethernet interface. You get to the page with the link Identification behind Ethernet on the Setup page. Figure 5.6.: Ethernet Identification 24

25 Default Gateway Click on the link Routing on the Setup page. Figure 5.7 shows the page that appears. Here you can define the default gateway for the access point. Figure 5.7.: Routing Setup 25

26 Radio Channel and Transmit Power Click on the link Hardware behind AP Radio: Internal on the Setup page. Next to many other configurations, you can set up the transmission power and the radio channel here. Figure 5.8.: AP Radio: Internal Hardware 26

27 WEP If you follow the link Radio Data Encryption (WEP) on the page shown on figure 5.8, you can define a Wired Equivalent Privacy (WEP) encryption key for the radio interface. Figure 5.9.: AP Radio: Internal Data Encryption 27

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29 Abbreviations CIDR DHCP ESSID HTTP IANA ICMP IP KVM MAC NTP TCP TTL UDP WEP WLAN Classless Inter-Domain Routing Dynamic Host Configuration Protocol Extended Service Set Identifier Hypertext Transfer Protocol Internet Assigned Numbers Authority Internet Control Message Protocol Internet Protocol Keyboard Video Mouse Media Access Control Network Time Protocol Transmission Control Protocol Time to Live User Datagram Protocol Wired Equivalent Privacy Wireless Local Area Network 29

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31 Bibliography [air11] Cisco Aironet 1200 Series Access Point Installation and Configuration Guide. ConfigurationGuide.pdf, last accessed 12. July 2011 [gen11] Gentoo Linux x86 Handbook. handbook-x86.xml, last accessed 12. July 2011 [htt11] Hypertext Transfer Protocol. last accessed 12. July 2011 [icm11] Internet Control Message Protocol. Control_Message_Protocol, last accessed 12. July 2011 [ipa11] Private IP address. last accessed 12. July 2011 [ipf11] IP forwarding. last accessed 12. July 2011 [net11] Netmask. last accessed 12. July 2011 [ntp11] Network Time Protocol. Protocol, last accessed 12. July 2011 [rou11a] Routing. last accessed 12. July 2011 [rou11b] Routing table. last accessed 12. July 2011 [wir11] Wireshark Documentation. last accessed 12. July

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35 A. Handouts A.1. Handout: Network topology Server eth1: /30 Backbone Router eth1: /30 eth2: /30 Router eth1: /30 eth2: /16 eth3: /30 Subnet Router eth1: /30 eth2: /24 Access Point 1 IP: /24 Access Point 2 IP: /16 Mobile Node IP 1: /24 IP 2: DHCP Backbone Router eth1 eth / /12 Server Router eth1 eth1 eth3 eth2 Subnet Router / /16 Access Point 2 eth1 Access Point 1 eth /24 Mobile Node Figure A.1.: Network topology of the experiments

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37 A.2. Handout: IP addresses and netmasks Table A.1.: IP addresses for private networks [ipa11] address range CIDR # addresses to / = to / = to / = Table A.2.: netmask examples [net11] CIDR netmask binary netmask # addresses / / / / / /

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39 A.3. Handout: Linux Commands dhcpcd wlan0 Receive IP address, netmask and default gateway for interface wlan0 from the DHCP server. ifconfig Display informations about all active network interfaces. ifconfig -a Display informations about all network interfaces, active or inactive. ifconfig eth /30 Assign IP address and netmask to interface eth0. ifconfig wlan0 down Deactivate interface wlan0 if it is not already inactive. ifconfig wlan0 up Activate interface wlan0 if it is not already active. iwconfig Display informations about all wireless network interfaces. iwconfig wlan0 essid tsunami Connect the interface wlan0 to the accesspoint with ESSID tsunmai. iwlist scan wlan0 Scan for visible access points seen by interface wlan0. konqueror Open the webbrowser konqueror. nano /proc/sys/net/ipv4/ip forward Open the file /proc/sys/net/ipv4/ip forward to edit it. ping c 4 Ping the host four times to see if it can be reached. route -n Show the routing table for all interfaces. route add -net /16 gw Add a route for the network /16 via gateway route add default gw Add the IP address as default gateway. route del -net /16 gw Delete the route for the network /16 via gateway route del default gw Delete the IP address as default gateway. wireshark Open the network analyzer wireshark.