InternetPeriscope enables you to look up the Start of Authority (SOA) record for a DNS zone.
The SOA Record gives you the following information:
- The Primary Nameserver of the domain. When a DNS makes changes to a domain's zone file, they usually perform the changes on this name server. The "secondary" name servers later receive the zone's changes from the primary nameserver.
- The email address of the person responsible for this domain (the DNS Administrator).
- The Serial number of the zone. Every time the administrator changes a record in the domain's zone file, she increases this number. A secondary nameserver can use this number to determine if the primary nameserver has an updated zone file. If so, the secondary nameserver can request the new zone file from the primary nameserver.
- The refresh interval, in seconds. This is how often the secondary nameservers check with the primary nameserver to see if any changes have been made to the domain's zone file.
- The retry interval, in seconds. If a secondary nameserver is unable to contact the primary nameserver (perhaps it has crashed, or the connection between them is saturated), then it will retry the connection after this many seconds.
- The Expire interval, in seconds. If a secondary nameserver is unable to contact the primary nameserver for this many seconds, then it will "expire" the zone. That is, it will no longer answer questions about this domain.
- The Minimum TTL (time to live), in seconds. When you type in a URL like http://www.microsoft.com, your computer asks its DNS server for the IP address for www.microsoft.com. (This DNS server is probably located at your ISP). This DNS server then asks Microsoft's DNS server for the IP address of www.microsoft.com. Microsoft's DNS server returns an answer to your DNS server, which then caches this result for TTL seconds. By caching this record, it reduces the load on Microsoft's DNS server, and allows your web browser to more quickly connect to the website, as it doesn't have to spend as much time looking up the IP address, since it already exists in its cache.
For this example, we will look up the SOA Record for bind.com
To bring up the DNS Tool, click on the "Tools" menu, move your mouse over the "DNS" menu item, and click on "DNS Query...", as shown in the figure.
The user enters "bind.com" into the "Query String" text box. She clicks on the "SOA" radio button, and then clicks on "Send Query", as shown in the figure.
After a few moments, the DNS Results dialog appears (see figure above), giving us the following information:
- The primary nameserver for bind.com is nameserv1.bind.com. When the DNS Administrator for bind.com makes a change to the DNS for the bind.com zone, she makes the change on this server.
- The email address of the DNS administrator is firstname.lastname@example.org.
- The Serial number of this zone is 20000627. Different administrators use different schemes for their serial numbers. This administrator is probably using the "date format", and this number probably indicates that the last DNS change was made to this zone on June 27, 2000.
- Every hour, the secondary nameserver, nameserv2.bind.com, checks the Serial number on the primary nameserver, nameserv1.bind.com, to see if the administrator has made any changes to the bind.com zone. If so, the secondary nameserver gets the new zone file from the primary nameserver.
- If the secondary nameserver is unable to contact the primary nameserver (to see if the Serial number has been updated), then it will retry the primary nameserver every 15 minutes.
- If the secondary nameserver (nameserv2.bind.com) is unable to contact the primary nameserver (nameserv1.bind.com) for 5 weeks, 6 days, and 16 hours, then it will delete the bind.com zone from its database, and it will no longer answer any DNS queries for the zone bind.com.
- When the user clicked on the "Send Query" button, her DNS server, 184.108.40.206 "talked" to nameserv1.bind.com. Her nameserver asked nameserv1.bind.com to send it the SOA Record. nameserv1.bind.com told her nameserver the answer, and told it that it should only cache (save) this information for one hour. If the user presses the "Send Query" button again, her nameserver will return the cached record. If she waits an hour and again presses the "Send Query" button, the record will have expired, and her nameserver will have to "ask" the nameserv1.bind.com server for the SOA record again.