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Network Configuration

Network Configuration. An Introductory Tutorial. Network Parameters. The following are common parameters that need to be set in a networked computer: DNS Servers (e.g. 137.207.32.32) The logical (e.g. IP) address of the DNS servers to be used to resolve domain names

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Network Configuration

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  1. Network Configuration An Introductory Tutorial

  2. Network Parameters • The following are common parameters that need to be set in a networked computer: • DNS Servers (e.g. 137.207.32.32) • The logical (e.g. IP) address of the DNS servers to be used to resolve domain names • These can be local (e.g. frankie.csfac8.uwindsor.ca) or remote (e.g. www.google.ca) • Host and domain information • This includes the computer’s name (e.g. elvis) and the domain on which it resides (e.g. csfac8.uwindsor.ca)

  3. Network Parameters • Logical address (e.g. IP address: 137.207.132.21) • This address is required to participate on a WAN, such as the Internet • Subnet mask (e.g. 255.255.0.0) • This is the binary mask that should be used to extract the network and subnet information from the host’s address • Default gateway (e.g. 137.207.32.2) • This is the logical (e.g. IP) address of the router which should receive the host’s Internet-bound packets • All of these values would be given to a computer’s user by a network administrator

  4. DNS Servers • DNS servers use the DNS protocol to determine the logical (e.g. IP) address of a host, given its domain name • e.g. www.google.ca: 64.233.167.99 • Many servers are given (in a specific order) so that local DNS servers are first • This is because the main DNS servers (Internic) would not know local domain names • e.g. An Internic server would know about uwindsor.ca, but not frankie.csfac8.uwindsor.ca) • These server addresses would usually be well-known

  5. Host and Domain Info • Host name: • This is the name of the host on the network • e.g. elvis • Domain name: • This is the name of the domain on which the host resides • A domain is a named group of computers, which may correspond to a subnet of addresses • e.g. csfac8.uwindsor.ca refers to the LAN on the 8th floor of Lambton Tower • Given these two pieces of information, the host’s DNS name would be elvis.csfac8.uwindsor.ca • Depending on the operating system, the domain may require that you log on first • The network admin will create one or more domains, but each host’s user can determine the host name, provided it is unique

  6. Logical Address • This is typically an IP address • This address is necessary for all WAN (including Internet) communication • Routing uses the logical address for finding the path for packets • These addresses are normally assigned automatically by a DHCP/BOOTP server • They can be random addresses on the network, or sequentially assigned

  7. Subnet Mask • The subnet mask that it used by the host is the same subnet mask used by the nearest routers • e.g. If using 2-level subnetting of 2 and 3 bits, this mask would be the last subnet mask used (137.207.248.0) • A network admin would know the subnet masks he/she assigned to each network

  8. Default Gateway • The default gateway (router) is the router to which all Internet-bound traffic should be sent • Typically, a LAN will have exactly one router, and this will be its address • If there is more than one router, and only one leads to the Internet, the Internet-connected router’s address is used here • If there is more than one router that connects to the Internet, either address could be used • The network admin would know the address of the routers

  9. Troubleshooting Networks How to Determine Network Configuration or Hardware Problems

  10. Troubleshooting • A network admin’s job is not just to set up a network, but maintain it • Thus, if anything goes wrong, the admin should track down the problem somehow • The problem may be a configuration error, which the admin should fix • The problem may be a hardware failure, which can only be fixed by repairing or replacing the hardware • Nevertheless, the admin must determine which hardware has failed, so this can happen

  11. A Typical Scenario • A user calls up the network admin, saying their network connection is down • For most non-technical users, even this clear of a statement of the problem might not be possible • A network admin will eventually figure at least this much out from the user’s comments

  12. Step 1: Test Local Connectivity • First, an admin will try to connect to a local computer cmd% ping frankie.csfac8.uwindsor.ca ping: unknown host frankie.csfac8.uwindsor.ca • This could mean DNS servers are unavailable • The admin should check the DNS server addresses, and the servers • Next, the admin will make sure the IP address works cmd% ping 137.207.140.93 Destination host unreachable. Destination host unreachable. Destination host unreachable. • This could mean no connectivity • The admin should check the configuration settings (ipconfig, netconfig, network-admin) • However, if the ping command responds from a router that a host is unreachable, the node may actually be unreachable, or the address may not be bound • Obviously, we have connectivity since the router sent a message to us

  13. How Ping Works • Ping works by sending an ICMP Echo Reqest to the destination • The destination sends back an ICMP Echo Reply to the host • The host records the round trip time, and repeats the process a few times

  14. A Problem With Ping • ICMP Echo Requests are often disabled in some networks • This is due to denial of service attacks that use ICMP Echo Request/Reply • In these networks, ping will not work

  15. Check Configuration Settings • In Windows (2000/XP): C:\> ipconfig (success) Windows IP Configuration Ethernet adapter Wireless Network Connection: Connection-specific DNS Suffix . : csfac8.uwindsor.ca IP Address. . . . . . . . . . . . : 192.168.137.71 Subnet Mask . . . . . . . . . . . : 255.255.255.0 Default Gateway . . . . . . . . . : 192.168.137.2 (failure) Windows IP Configuration Ethernet adapter Wireless Network Connection: Media State . . . . . . . . . . . : Media disconnected

  16. Edit Configuration Settings • In Windows (2000/XP), editing your network configuration settings can be done from TCP/IP properties • Open control panel, select Network Connections • Select your connection, click Properties • Select Internet Protocol (TCP/IP), click Properties • From here you can edit your configuration settings manually, or specify that a DHCP should provide the values for you

  17. Check/Edit Configuration Settings • In Linux: Linux% network-admin • In Solaris: Solaris% netconfig • These bring up a graphical dialog where configuration can be viewed and edited

  18. Step 2: Test Remote Connectivity • First, an admin will try to connect to a local computer cmd% ping www.google.ca ping: unknown host www.google.ca • This could be a routing problem • Most likely the routing problem is within the organization • The network administrator should check the route using the traceroute (traceroute, tracert) command

  19. Testing Routes • The traceroute command can give the logical addresses of each router in a route • It may also tell you if there is a problem with the route • In Windows (2000/XP): tracert www.google.ca • In Linux/Solaris: traceroute www.google.ca

  20. Windows Tracert Output C:\>tracert www.google.ca Tracing route to www.l.google.com [66.102.7.104] over a maximum of 30 hops: 1 1 ms 1 ms 1 ms 192.168.1.1 2 23 ms 16 ms 20 ms 10.71.96.1 3 11 ms 13 ms 13 ms d226-4-1.home.cgocable.net [24.226.4.1] 4 30 ms 15 ms 18 ms cgowave-0-157.cgocable.net [24.226.0.157] 5 30 ms 17 ms 17 ms cgowave-0-101.cgocable.net [24.226.0.101] 6 29 ms 18 ms 17 ms gw-google.torontointernetxchange.net [198.32.245.6] 7 19 ms 37 ms 17 ms 66.249.94.75 8 44 ms 23 ms 31 ms 64.233.175.96 9 53 ms 40 ms * 64.233.175.96 10 98 ms 92 ms 86 ms 66.249.95.215 11 98 ms 89 ms 94 ms 216.239.49.142 12 88 ms 95 ms 90 ms 216.239.49.142 13 104 ms 98 ms 89 ms 216.239.49.142 14 99 ms 94 ms 88 ms 66.102.7.104 Trace complete.

  21. Windows Tracert Output • The output contains lines like the following: 5 30 ms 17 ms 17 ms cgowave-0-101.cgocable.net [24.226.0.101] 5: The 5th hop 30,17,17ms: Round trip times to/from the router cgowave-0-101.cgocable.net [24.226.0.101]: The domain name and address of the router

  22. Linux traceroute Output rfortier@elvis:~ $ traceroute www.google.ca traceroute: Warning: www.google.ca has multiple addresses; using 64.233.167.147 traceroute to www.l.google.com (64.233.167.147), 30 hops max, 38 byte packets 1 ssr68 (137.207.140.65) 0.395 ms 0.302 ms 0.250 ms 2 ssr (137.207.140.66) 0.597 ms 0.508 ms 0.472 ms 3 cs-ssr-main.newcs.uwindsor.ca (137.207.76.254) 1.038 ms 0.901 ms 0.692 ms 4 passport1.uwindsor.ca (137.207.32.1) 1.701 ms 1.245 ms 1.225 ms 5 internet.uwindsor.ca (137.207.92.14) 1.434 ms 1.136 ms 0.967 ms 6 acs7204.uwindsor.ca (137.207.232.3) 2.494 ms 1.563 ms 1.852 ms 7 f0-1.na03.b011027-0.yyz01.atlas.cogentco.com (38.112.19.97) 10.358 ms 10.811 ms 9.377 ms 8 g9-1.core01.yyz02.atlas.cogentco.com (66.28.66.53) 10.628 ms 9.289 ms 10.427 ms 9 p15-0.core01.yyz01.atlas.cogentco.com (154.54.2.221) 10.265 ms 15.687 ms10.139 ms 10 p13-0.core02.ord01.atlas.cogentco.com (66.28.4.213) 24.619 ms 24.274 ms 25.259 ms 11 p15-0.core01.ord01.atlas.cogentco.com (66.28.4.61) 24.502 ms 23.292 ms 24.221 ms 12 p6-0.core01.ord03.atlas.cogentco.com (154.54.2.238) 24.316 ms 24.378 ms 24.401 ms 13 google.ord03.atlas.cogentco.com (154.54.11.186) 22.021 ms 22.070 ms 21.698 ms 14 66.249.94.193 (66.249.94.193) 22.524 ms 22.307 ms 66.249.94.195 (66.249.94.195) 24.050 ms 15 64.233.175.38 (64.233.175.38) 26.037 ms 64.233.175.42 (64.233.175.42) 64.185 ms 64.233.175.38 (64.233.175.38) 25.547 ms 16 64.233.167.147 (64.233.167.147) 23.303 ms 24.924 ms 22.002 ms

  23. How To Interpret the Output • The round trip times can tell you if there is a problem • A round trip time of * means a timeout has occurred, which is a definite problem • A round trip time > 500 ms usually means there is a problem with that router • The router could be dropping packets (due to overloaded buffer or routing loops typically)

  24. How Does Traceroute Work? • Traceroute sends either a UDP or ICMP Echo packet to the destination • Initially, the hop limit is set to 1, which causes the first router to drop the packet • Routers, however, will respond with an ICMP Time Exceeded error message, which contains the router’s IP address • The process is repeated a few times to get a few round trip times • The domain name of the IP address is found (for convenience of the network admin) • Next, the process is repeated with a hop limit of 2 • Then 3, 4, etc. until the destination host is reached

  25. A Problem with Traceroute • One problem with traceroute is that it could read a false path • If the routers reconfigure (e.g. due to congestion) during its execution, the readings could be inaccurate A1 A2 A2 S D B2 B2

  26. A Problem with Traceroute • One problem with traceroute is that it could read a false path • If the routers reconfigure (e.g. due to congestion) during its execution, the readings could be inaccurate A1 A2 A2 S D B2 B2

  27. A Problem with Traceroute • One problem with traceroute is that it could read a false path • If the routers reconfigure (e.g. due to congestion) during its execution, the readings could be inaccurate A1 A2 A2 S D B2 B2

  28. A Problem with Traceroute • One problem with traceroute is that it could read a false path • If the routers reconfigure (e.g. due to congestion) during its execution, the readings could be inaccurate A1 A2 A2 S D B2 B2

  29. A Problem with Traceroute • For networks with ICMP Echo Request/Reply disabled, the UDP format must be used • Some traceroute commands do not support the UDP message format, however

  30. LAN Setup 10BaseT Ethernet 802.11g Wireless

  31. 10BaseT Ethernet Setup • To connect a 10BaseT Ethernet network: • Install a network interface card (NIC) into each computer (host) • Connect each NIC to a hub or switch, using unshielded twisted pair (UTP) cabling • Hub: • Every frame received by a hub is reproduced on all of it’s ports • Switch (or Cable/DSL Router): • Every frame received by a switch is examined, and the correct port is determined for the frame • A Cable/DSL also provides network address translation (NAT)

  32. 802.11g Wireless Setup • To setup a 802.11g wireless network: • Install an 802.11g (or 802.11b) NIC into each host • Plug a wireless router or base station into your Internet connection (e.g. cable modem) • Configure the SSID and security protocols on the wireless router and each of the NICs • e.g. WPA encryption, and specify the key

  33. Ad Hoc Wireless Networks • If you have only a few hosts (2-3), you can create an ad hoc network • This type of network does not have a base station (or wireless router) • The hosts communicate directly with one another

  34. Wireless Networks Comment • A wireless network connection is often not useful for a desktop PC • For this reason, wireless routers typically contain a few Ethernet ports • It is typical to have desktop PCs wired, and notebooks and PDAs will have wireless • e.g. Notebooks with 802.11g and PDAs with 802.11b • Wireless networks will be discussed in depth later

  35. Packet Sniffers • Packet sniffers are a useful tool for exploration of networking • They can also be used by network programmers to diagnose packet format errors • They typically examine many protocols • This includes the header information • e.g. Wireless, Ethernet, TCP/IP, HTTP • Some common packet sniffers are: • Ethereal (Linux/Mac/Windows, multi-protocol) • TCPDump (Linux/Windows, TCP/IP) • Kismet (Linux, wireless)

  36. Ethereal Tutorial • Here is how to sniff packets using Ethereal: • Start Ethereal • Select the correct network interface • Start capturing packets, stop when done • Select a packet from the list • Examine the packet hierarchy and header information, such as: • Ethernet or wireless frame headers • IP datagram headers • TCP datagram headers • SMTP message

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