LAB 1 HOW THE WEB WORKS

Transcription

1 LAB 1 HOW THE WEB WORKS What You Will Learn About IP addresses and Domain Names About some web browser plugins to help with development Examine headers sent by the HTTP protocol How to trace the route that your packets take Approximate Time The walkthroughs in this lab should take approximately 30 minutes to complete. Fundamentals of Web Development with PHP Randy Connolly and Ricardo Hoar Textbook by Pearson

2 2 Lab 1 HOW THE WEB WORKS Exercise 1.1 Y OUR IP ADDRESS 1 Depending on what OS you are using, there are different tools you must use to get your IP address. We will use the most fundamental techniques possible, which are least affected by OS upgrades and user interfaces. I N W INDOWS Open up the command window by either clicking a shortcut, or typing cmd in the run window. O N MAC OR L INUX Open up a terminal window, by going into applications and choosing a terminal. Either way a small text window should appear that will take your commands. It may show your computer name or current location depending on the version and configuration. This command window is an essential tools for the web developer. Become familiar with it. 2 In the newly opened command window you will type a simple command and get back information about your IP address. Because most computers have multiple adapters you will get IP information for each installed and configured device. This could be a wired connection, and a wireless connection. Since only 1 is being used at a time, we will look for the information about that at adapter. This output may seem daunting at first, but provides you with esstenial information. I N W INDOWS Into the cmd window type ipconfig The output will include output showing your IP address similar to: Wireless LAN adapter Wireless Network Connection: Connection specific DNS Suffix. : Link local IPv6 Address..... : fe80::fc13:750:ffdd:1337%14 IPv4 Address : Subnet Mask : Default Gateway :

3 Fundamentals of Web Development 3 WEB DEVELOPMENT O N MAC OR L INUX Into the terminal type: /sbin/ifconfig The output will include output similar to the following. en1: flags=8863<up,broadcast,smart,running,simplex,multicast> mtu 1500 ether c8:bc:c8:c2:74:91 inet6 fe80::cabc:c8ff:fec2:7491%en1 prefixlen 64 scopeid 0x5 inet netmask 0xffffff00 broadcast media: autoselect status: active 3 Now you very likley have an internal IP address, unless you connected directly to the internet. IP addresses in the ranges are internal If so, we will have to determine what IP address wea re sharing, since that is the IP we will be known as outside our internal network. The easiest technique is to go to a web server on the internet that is designed to echo your IP address. Surf to and you will see your external IP address. It's important to remember that every time you visit a web page you transmit this external IP address in order to respond to your request with data. Similarly, every visitor to your site sends you their IP. Copyright Randy Connolly and Ricardo Hoar

4 4 Lab 1 Exercise 1.2 A NALYZING COMPONENTS OF A WEB PAGE 1 To do this next walkthorugh you must have firebug installed in your firefox web browser. To check, or install firebug 1. Go to tools > add ons in the firefox menu 2. Click on the Extensions tab and see if if firebug is listed. If not 3. Type firebug into the search window 4. Click install next to firebug and restart your browser 2 With firebug installed we can now do some powerful analysis of web page load times as described in Chapter 1. The first time you use firebug you will have to enable it. Surf to the webpage from Exercise 1 (to and enable firebug by clicking on the firebug icon. 3 You will now see the firebug tool opoened below the web page. The tool has many tabs as shown below. Select the Net tab, and you may have to click Enable, and then reload 4 With the Net tab selected, you will now see all the requests that this web page makes to load itself completely. Trneds can be seen such as how many seperate domains resources are requested from, which resources take the most time, and the total load time.

5 Fundamentals of Web Development 5 WEB DEVELOPMENT Exercise 1.3 N AME SERVERS 1 There are tools built into linux and mac that allow one to do some internet queries regarding websites, their nameservers and name resolution. In particular tools named nslookup and dig are used in this walkthrough. We will again be using the cmd or terminal window to run these commands. Open such a window 2 To determine the ip address for a domain name type: nslookup google.com or nslookup type=a google.com 3 As output you will see that google.com actually has several answers. That is many ip addresses are ready to answer queries to google.com. Server: Address: #53 Non authoritative answer: Address: Address: Address: Address: Address: Address: Address: Address: Copyright Randy Connolly and Ricardo Hoar

6 6 Lab 1 4 Notice that the first part of the output lists your Server, and likley this is your local DNS resolver. In our case is definitely a local address. While the answers given are likely correct, we want to investigate more deeply and see what google thinks it's IP addresses should be without relying on a cahed answer in our router. To determine the nameservers for google.com we type: nslookup type=ns google.com this returns Non authoritative answer: google.com nameserver = ns1.google.com. google.com nameserver = ns4.google.com. google.com nameserver = ns2.google.com. google.com nameserver = ns3.google.com. 5 Now we can query the official nameservers for google directly and ask what it thinks the IP address for google.com should be. Using the result from the previous query we can type: nslookup google.com ns1.google.com This forces a lookup directly from the authoritative google nameservers, which may be updated from our cache. When we ran the commands we did in fact get different results, but eventually our cache would be updated to reflect these changes: Address: Address: Address: Address: Address: Address: Address: Address: Address:

7 Fundamentals of Web Development 7 WEB DEVELOPMENT Address: Address: Now that you know how to query a nameserver for a IP address and nameserver, you may be interested in knowing that there are other types of DNS records you can query. Of particular interest to a web administrator would be the mail records. That answers where do I send mail to for this domain. For example to see where for google.com is hosted we could query nslookup type=mx google.com ns1.google.com and see the output of Server: ns1.google.com Address: #53 google.com mail exchanger = 20 alt1.aspmx.l.google.com. google.com mail exchanger = 40 alt3.aspmx.l.google.com. google.com mail exchanger = 10 aspmx.l.google.com. google.com mail exchanger = 50 alt4.aspmx.l.google.com. google.com mail exchanger = 30 alt2.aspmx.l.google.com. Copyright Randy Connolly and Ricardo Hoar

8 8 Lab 1 Exercise 1.4 S EEING HTTP HEADERS 1 To do this next walkthorugh you must have firebug installed and enabled in your firefox web browser. As shown in exercise 1. 2 Surf to and open firebug into the Net tab. (You may be asked to enable and refresh) 3 To simplify seeing HTML from the other resources, click the HTML filter in firebug so that only HTML resources are shown. 4 You should see but 1 resource, the main HTML page for the site. Click the small arrow to the left to open up the details from the request. There are then multiple tabs, but by defauilt the request and response headers are shown.

9 Fundamentals of Web Development 9 WEB DEVELOPMENT 5 From the Request Headers section of the image above you can see that the client machine I surfed from was a Mac, with OS What does your User Agent header reveal about you? Other things revealed are what type of encodings my browser can interpret. For example gzip. Does your browser accept compression? There is alot of information in the response headers as well. Can you tell what type of OS the server is running? What type of encoding is the page using? Copyright Randy Connolly and Ricardo Hoar

10 10 Lab 1 Exercise 1.5 T RACING A PACKET 1 This next walkthrough introduces a powerful tool that allows on to trace the route of a packet from your computer to a web host. We will use the linux command: traceroute 2 We will now trace the route from your computer to one of the biggest domains on the web, google.com, by typing in the command line: traceroute google.com 3 The output from the command will vary based on many factors, but you should see something similar to: traceroute to google.com ( ), 30 hops max, 60 byte packets 1 * * * 2 * * * 3 google.com.any2ix.coresite.com ( ) ms ms ms ( ) ms ms ms ( ) ms ms ms 6 lax02s01 in f9.1e100.net ( ) ms ms ms 4 What you are seeing in your output is the path your request took to get to the webserver at google.com. In our case six hops were required to get there, with the first two being hidden from us. Each numbered line of output is a hop on the way to the destination and tells you something about the path of the request. Note: You will definitely get different output than that shown here (unless you're on our server). 3 google.com.any2ix.coresite.com ( ) ms ms ms Tells us that the request went through a server hosted by google, and that the time it took at this router was between and 15.5 milliseconds. If a domain name is available it will list it. If not you will see an IP address. Some routers will simply show *** which indicates that there is either a problem, or firewall settings have been configured to prevent the router information from getting back to you. On busy routers, ICMP requests (which are what traceroute uses) will be dropped because they are low priority.

11 Fundamentals of Web Development 11 WEB DEVELOPMENT What is e100.net, you may be asking (which is the last stop before google)? Using a whois command we can inquire: whois 1e100.net The output tells us it s a domain owned and run by Google. Registrant: DNS Admin Google Inc Amphitheatre Parkway Mountain View CA Why is tracing the route important? Well, we can identify who could theoretically alter a packet along the way, or identify sources of delay. The entire issue of Network Neutrality becomes more meaningful when you realize that many providers play a part in getting your packet to Google and back. What are some interesting routers on your traceroute path to google.com, or Microsoft.com? Copyright Randy Connolly and Ricardo Hoar