Dynamic Domain Name System in Linux - Assignment Example

Dynamic DNS In Linux Domain System (DNS) makes it possible to refer to Internet Protocol (IP) based systems (hosts) by human-friendly s (domain names). Name Resolution is the act of determining the IP address (or addresses) of a given host name. Moreover the Domain names can be logical and easily remembered. Should the IP address for a host change, the domain name can still resolve transparently to the user or application. Dynamic DNS is a network service that provides the capability for a networked device using the Internet Protocol Suite, such as an IP router or computer system, to notify a domain name server to change, in real time, the active DNS configuration of its configured hostnames, addresses or other information stored in DNS. The Dynamic Host Configuration Protocol (DHCP) allowed enterprises and Internet service providers (ISPs) to assign addresses to computers on the fly as they powered up. The Domain Name System brought a method of distributing the address information automatically online through recursive queries to remote databases configured for each domain. In addition, this helped conserve the address space available, since not all devices might be actively used at all times and addresses could be assigned as needed. This feature required that DNS servers be kept current automatically as well. The first implementations of dynamic DNS fulfilled this purpose. The excessive use of the Internet by all even in people's homes brought a growing shortage of available IP addresses. DHCP became an important tool for ISPs to manage their address spaces for connecting home and small-business end-users with a single IP address each by connecting them through a Network Address Translation (NAT) router. Behind these routers (in the private network) it was possible to reuse address space set aside for these purposes . This, however, broke the end-to-end principle of Internet methods were required to allow private networks, masqueraded by frequently changing IP addresses, to discover their routable 'outside' address and insert it into the domain name system in order to participate in Internet communications more fully. Today, numerous providers, called Dynamic DNS service providers, offer such technology and services on the Internet. They provide a software client program that automates this function. The client program is executed on a computer in the private network. It connects to the service provider's systems and causes those systems to link the discovered public IP address of the home network with a hostname in the domain name system. Depending on the provider, the hostname is registered within a domain owned by the provider or the customer's own domain name. These services can function by a number of mechanisms. Often they use an HTTP service request since even restrictive environments usually allow HTTP service. This group of services is commonly also referred to by the term Dynamic DNS, although it is not the standards-based DNS Update method. However, the latter might be involved in the providers systems. Most home networking routers today have this feature already built into their firmware. One of the early routers to support Dynamic DNS was the UMAX UGate-3000 in 1999, which supported the TZO.COM dynamic DNS service. Dynamic DNS Preparation Unlike DSL, most cable modem providers may not allow you to host sites at home by blocking inbound HTTP (TCP port 80) and SMTP mail (TCP port 25) while allowing most other TCP traffic through. Many DDNS providers are aware of this and provide a redirect service to bypass the problem. Under the system, Web queries first hit their servers on the regular TCP ports and then these servers automatically redirect the Web clients to use the IP address of your server on a different TCP port. Registering DDNS Once you have decided to go ahead with DDNS you'll need to choose between the broad categories of Dynamic DNS service. Free Dynamic DNS: Your website name will be a sub domain of the DDNS provider's domain. For example if the DDNS provider's domain is isp.net, then your site will become my-site.my-isp.net. However this type of service may be undesirable for a company that wants to establish its own corporate identity. Paid Customized DNS: You can register the domain name of your choice and still host your website on a DHCP line. Install a DDNS Client On Your Server All DDNS service providers require that you use a DDNS client on your web server that will periodically update the IP address information in your provider's DDNS record. The very popular one DDclient, for example, now comes in a RPM format. Dynamic DNS And NAT Router/Firewalls To conserve the limited number of IP addresses available for Internet purposes, most home router/firewalls use network address translation (NAT) to map a single, public, DHCP-obtained IP address to the many private IP addresses within your network. NAT can fool the operation of some DDNS client software. In these cases, the software can report only the true IP address of the Linux box's NIC interface. If the Linux box is being protected behind a NAT router/firewall, then the NIC reports in its data stream to the DDNS provider a private IP address that no one can reach directly via the Internet. The reported value is therefore invalid. Some DDNS providers use more intelligent clients, such as DDclient, that can be configured to let the DDNS provider record the public IP address from which the data stream is originating. Once this is done, you'll have to also configure your router/firewall to do port forwarding to make all HTTP traffic destined for the IP address of the router/firewall to be exclusively address translated using NAT and forwarded to a single server on your home network. Many Web-based small office/home office (SOHO) firewalls have easy interfaces to configure port forwarding. DDNS Client Software - SOHO Router / Firewalls Most new SOHO router/firewalls have built in dynamic DNS clients for one or more of the major DDNS service providers. There is usually a Dynamic DNS web menu which will prompt for the name of the service provider and your DDNS username and password. With this support, there is no need to install software on your web server. DDNS Client Software - Linux DDclient One of the most commonly used clients is DDclient, which can overcome the NAT limitations of DDNS by actually logging into your SOHO firewall to determine the latest IP address information. Like most RedHat and Fedora Linux software products, DDclient is available in the RPM format. (If you need a refresher on RPMs. You can usually download the software from your DDNS provider, or you can find it at rpmfind.net. The RPM name usually starts with ddclient followed by a version number, as in ddclient-3.6.3-1.noarch.rpm. The /etc/ddclient.conf file The ddclient.conf file is usually installed completely commented out, but provides many configuration examples for the most popular DDNS providers. The important general parameters to configure are: Username: Your DDNS account's login name Password: Your DDNS account's password Use: The method used to determine the IP address to advertise to the DDNS server. Some important DDNS provider parameters are Server: The name of the DDNS provider's main DNS server Protocol: The methodology the DDNS client should use to communicate with the DDNS server Your domain: The domain to which your web server will belong You can use the ddclient command to determine the best use parameter to use in the ddclient.conf file. In this example, only the use=web option gives a valid Internet IP address and should be considered as a first option: [root@bigboy tmp]# ddclient -daemon=0 -query use=if, if=lo address is 127.0.0.1 use=if, if=wlan0 address is 192.168.1.100 use=web, web=dyndns address is 97.158.253.26 [root@bigboy tmp]# Take a look at a sample configuration. This example specifies a username of my-account-login-name and a password of my-account-password using the dyndns DDNS service provider's settings to track the Web site named mysite-example.dnsalias.com. # General Parameter Section login=my-account-login-name password=my-account-password # DDNS Provider Parameters Section server=members.dyndns.org, protocol=dyndns2 mysite-example.dnsalias.com You can add one of the following use lines to the General Parameter Section near the top of the file to define the method that will be used to determine the correct IP address: Query A Well Known Internet Server: The web method queries two well known servers run by DynDNS.org and DNSpark to determine the public Internet IP address of the web server running the DDclient software. This method is the simplest as it requires no further information and handles NAT correctly. use=web Use The IP Address Of A Specific Server NIC: You can also use this option which will query the IP address of the DDclient web server's NIC interface of your choice. This is probably most valuable for servers connected directly to the Internet, and not via NAT use=if, if=eth0 Login To Your SOHO Firewall For Information: The ddclient.conf file has a list of use statements for various vendor's firewalls. If your model isn't listed, you can create your own parameters as outlined in the ddclient README file. This option is good for NAT environments where the "use=web" option isn't considered a good alternative. After editing your configuration file you'll have to start ddclient as shown in the next section. How to Get DDclient Started You can configure DDclient to start at boot time using the chkconfig command: [root@bigboy tmp]# chkconfig ddclient on You can start, stop, and restart DDclient after boot time using the DDclient initialization script as in: [root@bigboy tmp]# service ddclient start [root@bigboy tmp]# service ddclient stop [root@bigboy tmp]# service ddclient restart Remember to restart the ddclient process every time you make a change to the ddclient.conf file for the changes to take effect on the running process. You can test whether the ddclient process is running with the pgrep command, you should get a response of plain old process ID numbers: [root@bigboy tmp]# pgrep ddclient Getting DDclient to Periodically Update For update use the cron sample file that ddclient provides. 1. Use the rpm command to get a list of installed DDclient files, one of which is the cron file . [root@bigboy tmp]# rpm -ql ddclient | grep cron /usr/share/doc/ddclient-3.7.1/sample-etc_cron.d_ddclient [root@bigboy tmp]# 2. Copy the file to the /etc/cron.d directory. [root@bigboy tmp]# cp /usr/share/doc/ddclient-3.7.1/sample-etc_cron.d_ddclient /etc/cron.d/ddclient.cron 3. Edit the file and uncomment the cron entries, [root@bigboy tmp]# vi /etc/cron.d/ddclient.cron 4. Restart cron. [root@bigboy tmp]# service crond restart References: 1. Dynamic DNS, "Wikipedia the free Encyclopedia", [accessed on 26/12/2008] 2. Quick How to ch:19 Dynamic DNS, "Linux Home Networking", 28 Oct. 2007, [Accessed on 27/12/2008] 3. Understanding Domain Name System (DNS), "All About Linux", September 4, 2005, [Accessed on 27/12/2008] Read More