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DNS records

The DNS records tool retrieves the domain name records for a specified domain.

Usage

Two versions of the tool are available. To use the basic tool, enter a domain name in the text box and select the "DNS Records" radio button, then click "Go."

Alternatively, click "Advanced Tool" next to the DNS Records radio button. This will bring up a page with more options:

  • Domain: The domain to look up.
  • Query type: The type of record requested. Leaving it as "ANY" will usually collect the most information. With some servers, you need to ask for a specific type of record to get it.
  • Server: The full domain name or IP address of the DNS server to query.
  • Query class: Leave this at "IN - Internet" unless you know what you're doing.
  • Port: Leave this at 53 unless the server expects a different port.
  • Timeout: The default is 5000 milliseconds. You can increase this if you're dealing with an unusually slow server.
  • No recursion: Normally a DNS server can query other servers if it doesn't have the requested information. Check this box if you want only the information which the server itself has.
  • Advanced output: Check this box to get a more detailed response.

After you're done, click "Go" to run the advanced tool.

When to use it

The DNS records tool helps to diagnose problems with a domain's name servers. If normal lookup of a domain isn't working, it can help to find the cause.

The advanced tool lets you choose the DNS server to query and the type of records to request. If the regular tool doesn't return a result, using the advanced tool with a different server may work better. It also lets you check if different servers are reporting consistent and up-to-date information.

What it does

The tool is equivalent to the nslookup command on Unix and Linux systems. It queries the default DNS server or (with the advanced tool) the specified one and retrieves the records for the domain. They provide the domain's IP addresses, as well as other information. The following record types are especially useful.

A: The IPv4 address for the domain.

AAAA: The IPv6 address.

CNAME (canonical name): An alias of one domain name to another. This lets a server use more than one domain name.

MX (mail exchange record): A list of message transfer agents for the domain. A message transfer agent handles email for the domain.

NS: An authoritative name server for the domain. There can be more than one NS record per domain. All other servers use a copy of information from the authoritative servers.

TXT: A record containing information for use outside the DNS. The content takes the form "name=value". Authentication schemes such as SPF and DKIM use TXT records.

A deeper look

The domain name system works by passing information from one server to another. Any domain first has its records stored on an "authoritative" server. This is usually one which belongs to the domain or its host. The records propagate to the root servers and to other servers around the world, but this process can take as much as a couple of days.

Many DNS servers will accept queries only for authoritative records, i.e., for their own domains. To check a domain where you don't know the authoritative server, you need to check a public server. You can choose from this list of free and public DNS servers.

Querying any DNS server that accepts your request will return the NS records for the domain's authoritative servers. You can then repeat the query using the authoritative server to see if the results are consistent.

There are thirteen root DNS servers. They hold authoritative information on top-level domains such as .com, as well as A, AAAA, and NS records for nearly all domains. Since their information is incomplete, they're used mostly by other DNS servers to discover a domain's authoritative servers. If information on a new or changed domain hasn't propagated to the root servers, most clients won't be able to locate the domain.

Propagation speed depends largely on the records' TTL (time to live) value. A lower value will get them updated more quickly, but they won't be cached as efficiently. Since domain records don't change often, the TTL needs to be reasonably high. Typical values are 3600 (1 hour) to 86400 (1 day) seconds. There may be an additional delay in making the records available on the authoritative server.

Interesting stuff

Host and domain names appeared very early in the history of the Internet, when people realized that remembering IP addresses was hard. The original idea, in RFC 811, was that there would be one NIC Internet Hostnames Server with a Host Table. This worked in 1982, but it soon became obvious it wouldn't scale up.

The Domain Name System appeared in 1987 with RFC 1034. It was designed to use multiple servers and caching. It still operates in substantially that form, and it's fragile. Occasionally false information has propagated through it. Denial of service attacks on a DNS server can damage large pieces of the Internet. Most of the time it works correctly, but it's useful to have diagnostic tools at hand if something appears to have gone wrong.