Java Glossary : DNS

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DNS

Domain Name Service. The name lookup the Internet uses to convert from alphabetic names such as oberon.ark.com to 32-bit binary IPv4 or 128-bit IPv6 addresses used on packet routing. There are computers spread all over the Internet called DNS (Domain Name Service) servers. They will automatically look up a name for you and give you the equivalent numeric IP. The process is analogous to looking up somone's name in an electronic phone book to find out their phone number. This happens completely automatically whenever you key a website name into your browser or FTP client.

IP addresses

IPv4 addresses are displayed for humans as a dotted quad with four numbers 0..255, e.g. as 204.50.21.34. In Windows 95/98/ME/NT/2000/XP there is an auxiliary DNS helper file called HOSTS. that lists pairs of domain names and dotted quads. You can use this file for commonly looked up names, or for local names, e.g. names of workstations on the LAN. This is considerably quicker than getting one of the DNS servers to do those lookups for you.

Explicit DNS Lookup

In Java, you can invoke DNS lookup to convert the name to the dotted quad IP address by:

String dottedQuad = InetAddress.getByName( "mindprod.com" ).getHostAddress();

Discovering your Host Name

The InetAddress class also lets you work backwards from the dotted quad to get the host name.

String hostname = InetAddress.getAllByName( "199.93.4.15" ) .getHostName();

Discovering your DNS Server

There is no official way in Java to find out the IP or name of your default DNS server, e.g. the one that DHCP assigned you. DNSjava is a pure Java set of classes to add a DNS server to your application. That is a rather large hammer to deal with the problem. In Java 1.3 you need the name of a DNS server to use JNDI to get at the other DNS information. In Java 1.4.1 there is reputedly a way around that, but I have not figured it out yet.

For a LAN, you can assign IP addresses such as 192.168.0.1, 192.168.0.2 to each station since these are never seen on the Internet. In Windows, you can use the utility ipconfig /all to display information such as DNS Server IP, host name, physical ethernet 48-bit MAC address, IP, subnet mask, gateway IP and DHCP server IP. In older versions of Windows there is a similar utility called winipconfig.

How DNS Works

Here is my current best understanding about how the DNS lookup works. Don't quote me yet.

There is a master database of all DNS names and their corresponding IP names. It it not stored all in one computer, but to simplify the explanation, we will pretend that it is. Some DNS names have associated several IPs of server farms that are assigned on a round robin basis to share the workload.

www.mindprod.com, mail.mindprod.com and mindprod.com may all translate to different IPs or to the same IP. All three are tracked in the master database. Ordinary humans aren't allowed to talk to this master database. Instead you talk to a DNS server run by your local ISP. It does not have the entire database in it, just the entries most popular with the clients of your ISP. If you ask your ISP DNS server for a name it does not know, it asks a higher level server. If that higher level server does not know, it asks still a higher level server, working all the way back to the master DNS database server.

Does this mean you pester the master database every time you ask for a non-existent DNS name? Yes, but since the master database is actually entirely replicated in many computers, this is not the bottleneck you might expect.

How are changes propagated? Roughly once a day each DNS server asks the next higher level server if each of the entries it has cached in its local database is still valid. If not, it drops each invalid entry from its local database. If somebody later asks for that DNS name, the server will asks the next higher level server.

The master database server, through a tree of helpers, periodically asks every station on the net if its DNS name to IP translations are still valid, and if not, what the new information is. The information percolates up so that the master database is then completely up-to-date. From the master database, the information gradually percolates back down to all the ISP servers, through the ordinary process of asking the next higher level server when a server does not have a name in its cache. Information does not percolate down until it is needed. It generally takes about two days for out-of-date information to be totally flushed out of the system. How long it takes depends on a configurable freshness date for each name.

This way your computer only has to tell the Internet once about a DNS change. It does not have to directly tell every DNS server on the Internet.

What goes wrong is sometimes servers refuse to delete invalid cached entries. Then the only way to clear the out-of-date information is to reboot them and clear the entire cache. NT DNS servers have this problem. When a server fails to invalidate its cache, all downstream servers will be badly (i.e. weeks) out-of-date as well. The problem is not cleared until that server is rebooted.

DNS can do a number of other things besides convert a domain name to an IP number. It can go the other way, starting with an IP, figure out the DNS name. It can find out the latitutude and longitude of the server hosting the domain. It can find out what kind of CPU the server is. It can find out who owns the domain. It can distribute digital certificates. It can tell you the name of the associated mail servers. In Java you access this additional information with JNDI.

DHCP ¤ DNSJava a tool for DNS lookup written in pure Java ¤ domain names ¤ DynDNS: three free services of interest to people who host webservers on their home machines ¤ hosts ¤ IP ¤ IPv6 ¤ JNDI ¤ Kimble essay on how DNS works ¤ localhost ¤ RFC 1876: how latitude and longitude are stored in a DNS record ¤ TCP/IP

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