Lab Objectives Domain Name System (DNS) Acquiring skills related to the Domain Name System (DNS) functions Practical studying of DNS protocol in the process of its functioning Background Information DNS functions Domain Name System is a distributed client-server system of storing and presenting domain names as well as mapping those names to IP addresses, translating host name into the host IP address and vice versa. The process of translation of the name into IP address is called name resolution. This process is important since IP datagram uses the IP address to specify destination host. On the other hand, a network user can easily memorize and address the computers by their names rather than IP addresses. Every time, when making a request to a TCP/IP computer by its DNS name in the web browser, (e.g. www.microsoft.com) prior to sending data to destination server, a name resolution process takes place. To do this, the client computer sends a resolution request to the DNS server in the TCP/IP client s configuration. The DNS server resolves IP address and sends it to the client. Then the client handles the original request with the resolved IP address. The DNS client is called an interpreter. In the early stage of Internet, a mapping of the IP addresses and host names were stored in the HOSTS file (C:\WINDOWS\system32\drivers\etc\hosts). A HOSTS file still exists. For the most frequently used names of the computers, it makes sense to store their names in the HOSTS file locally stored on the same PC, for faster communication and traffic reduction. With the growth of Internet, the HOSTS file usage became unsustainable. DNS name architecture The domain system of naming the nodes of the network allows the administrators to assign network computers names with a hierarchic structure. The main DNS element is the hierarchical namespace. Computers are identified by names, consisting of three or more words, separated by dots. The DNS name consists of the host name and domain name. The domain name, consisting of two or more words, identifies the network or organization. The host name identifies a specific computer within this network or organization. For example, in www.microsoft.com name www is the computer name in the microsoft.com domain. The Internet domain names should be registered in the organization, responsible for preventing the domain names duplication. DNS names are read from the right to the left; the rightmost
word is relevant to the top level in the domain hierarchy. For example, соm in www.microsoft.com is called top-level domain. In the initial DNS standard there were 8 top-level domains defined: com, edu, gov, mil, net, org, int and аrра. Aгра domain is used for the reverse reflection, i.e. for translating the address into a name. Currently, the number of top-level domains is much more. The host is identified by the leftmost word in its full DNS name, and the rest is the domain name. Thus, the owner of microsoft.com domain can create domains of the next levels, e.g. msn.microsoft.com, and arbitrary number of hosts in each of these domains, e.g. www.msn.microsoft.com. The fact that the DNS namespace is distributed supposes that none of the servers has a complete list of Internet domain names. Each DNS-server knows a number of IP addresses of the root-name servers. These servers are responsible for maintaining the list of the first level domains and IP addresses of authoritative servers for these domains. The authoritative server is the computer, storing entire information on this domain. In fact, root servers are authoritative for almost all eight initial top-level domains. Domain name resolution process Below is the illustration of the DNS-name resolution process on the example of www.microsoft.com Upon receiving the request for www.microsoft.com name resolution, the DNS-server sends to the root server a request on the authoritative server address for com domain. Since the same root server is an authoritative server for соm domain, it skips this step and immediately sends the DNS-server the address of the authoritative server for the microsoft.com domain. For other domains the root server would choose the address of the relevant server of the first-level domain, where the DNS server would send a new query about the address of the authoritative server for the second-level domain. After obtaining the address of the authoritative server for microsoft.com domain, the DNS-server sends a query about the IP address of the www host. Microsoft.com server returns the requested IP address, and the DNS server forwards it to the client. Thus, any DNS-server in the Internet can find the IP-address of the computer in any domain. However, in many cases the process of name resolution is substantially simplified, as DNSservers usually cash the information about the resolved names. The cash would likely contain addresses of the popular domains such as www.microsoft.com, is quite high. If so, the DNSserver immediately sends it to the client, without requesting to other servers. Reverse name resolution For the reverse name resolution, a special domain in-addr.arpa is included into the DNS namespace. In that special domain the reverse transformation (of the address into the name) is performed with the help of splitting into domains, where IP addresses are used as names.
In other words, in-addr is the second-level domain in агра domain. It is split into 256 third-level domains, the names of which are numbers from 0 to 255, representing the first byte of the IP address. Each third-level domain is further split into 256 fourth-level domains, representing the second byte of the IP address. The fifth-level and sixth-level domains are made the same way. With their help it is possible to find the DNS-name for any possible IP address. For example, IP address 192.168.2.6 is associated with the name 6.2.168.192.in-addr.arpa. The resource record of this domain contains the DNS name of the computer with the assigned IP address. In the domain name the bytes of the IP address are concatenated in the reverse order, as in the IP address the most significant identifier goes first, while in the name of the first-level domain it goes last. DNS resource record The information storage unit on the DNS-server is the resource record, which in most cases is created manually by the administrator of the authoritative server. In response to the request of the interpreter or other server, the DNS server sends the message, containing the resource record for the resolved name. The most important resource records are listed below: Start of Authority (SOA): points out that this server of names is authoritative in this domain. Name Server (NS): identifies name servers in the domain Address (A): maps the name and address for the computer in the domain Canonical Name (CNAME): is used to create an alias of the computer, for which there is an Address (A) record. Pointer (PTR): maps the address and the in-addr.apra domain name for the computer in the domain. Mail Exchange (MX): points to the computer, responsible for processing the traffic of the e-mail, addressed to the domain. Lab Assignment In this work the DNS system is studied in the process of functioning, when the DNS server resolves the name, requested by the DNS client (interpreter). Requisite Equipment Personal computer (PC) with installed network adapter and Windows 7 Dual DHCP DNS Server, installed in the Windows 7 Mini2440 FriendlyARM single board computers (2pcs) NI ELVIS II workstation NETWORK TEST BENCH board (NTB) Network switch Mounted network UTP straight cable 5cat with RJ-45 connectors (3 pcs)
Preparations Make sure that required equipment is available With the help of the NI ELVIS II workstation make sure that the network cables are mounted properly Equipment should be laid out comfortably for work. Step-By-Step Instructions 1. With the help of the PC, two «mini2440» FriendlyARM single board computers and the network switch build the Star -topology LAN (See lab work 6). 2. Set the IP address 192.168.0.1 on the PC, the subnet mask 255.255.255.0, default gateway 192.168.0.1, preferential DNS address 192.168.0.1. Selection of these values is conditioned by configuration of Dual DHCP DNS Server (for details see Lab 12 and Point 8.). 3. For mini2440 single board computers dynamic IP addresses may be assigned (see Point 8.10). 4. Switch on the mini2440 single board computers. 5. Run Dual DHCP DNS Server on PC (Fig.0-1) Fig.0-1: Starting dialog box Dual DHCP DNS Server 6. Run the command line interpreter (CLI) on PC. 7. Enter nslookup command on the CLI (Pointer 1 in Fig. 2) and press Enter to run nslookup in the interactive mode (Fig.0-2)
4 1 2 3 Fig.0-2: Running nslookup (left) and Dual DHCP DNS Server (right) 8. Observe that: When nslookup is launched, it sends the request for the reverse resolution of the host s IP address (192.168.0.1), on which it works. Related to this request, the DNS server displays the message on the reverse resolution of the IP address (Pointer 4 in Fig.0-2). Upon receiving the response, Nslookup displays information from the DNS server (Pointers 2 and 3 in Fig.0-2). 9. Enter ls workgroup.com in the command line (Pointer 1 in Fig.0-3) to display the list of computers, provided from the DNS server. Observe that due to configuration settings of Dual DHCP DNS Server this request failed (pointers 2 and 3 in Fig.0-3).
2 1 3 Fig.0-3: Request nslookup for the list of computers in the domain failed 10. Open the configuration file Dual DHCP DNS Server on PC (Fig.0-4) and remove ; before the line ;workgroup.com=0.168.192.in-addr.arpa (pointer 2 in Fig.0-4). Save the changes and close the file. 1 2 Fig.0-4 Dual DHCP DNS Server configuration file 11. Restart DNS Server and again launch ls workgroup.com command (pointer 1 in Fig.0-3) 12. Observe that this time the request completed successfully. 13. Switch on one of the single board computers. Wait until the operation system is booted.
14. Note that the Dual DHCP DNS Server screen displays the message on the process of allotting a dynamic IP address to the single board computer (Fig.0-5). As it is known from Lab work 12, this process is associated with the DHCP server. Fig.0-5 Message on allotting a dynamic IP address to the single board computer 15. Launch ls workgroup.com command once again (pointer 1 in Fig.0-3) 16. Observe that the connected (switched on) single board computer appeared in the list of computers 17. Repeat points 13-16 for the second single board computer 18. Write down the results in the lab work report
Test Questions 1. What are the functions of the DNS system? 2. What is DNS system? 3. What is a resource record? Answers 1. Domain Name System is a distributed client-server system of storing and presenting domain names as well as mapping those names to IP addresses, translating host name into the host IP address and vice versa. 2. The domain name system is a computer distributed system of the client-server architecture with a hierarchic structure. 3. Resource record is the information storage unit on the DNS-server.