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Questions tagged [domain-name-system]

The Domain Name System, usually referred to by the acronym DNS, is a hierarchical, distributed database where the keys are domain names. Questions involving publicly accessible domains should include the real, Fully Qualified Domain Name (FQDN)

DNS is the Domain Name System, a hierarchical, distributed database where the keys are domain names.

The primary references are:

  • RFC 1034 - Domain Names - Concepts and Facilities
  • RFC 1035 - Domain Names - Implementation and Specification

These documents collectively form STD 13. RFC 2181 is a frequently cited reference for STD 13 that addresses some of the more confusing areas of operation.

The most common record types found in the DNS are:

  • "A" records - the mapping from a domain name to an IPv4 address
  • "AAAA" records - the mapping from a domain name to an IPv6 address
  • "MX" records - the mapping from a domain name to the host name of an SMTP server
  • "NS" records - used to delegate a portion of the hierarchy to specific DNS servers
  • "PTR" records - typically used (via in-addr.arpa.) to map an IPv4 address back to a domain name
  • "CNAME" records - used to alias a domain name to its canonical version

DNS packets are conventionally transported over UDP and TCP port 53. UDP is more commonly used, but larger DNS responses and zone transfers require TCP. See RFC 5966.

This Wikipedia article provides an approachable introduction to DNS.

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Why can't I reach my Amazon EC2 instance via its Elastic IP address?

The server works fine via the Amazon assigned DNS entry, but I cannot reach it (using a browser) via the Elastic IP address Amazon assigned the box. Ping does not work either. I am trying to confirm it is reachable before I add the IP address to my…
Guido Anselmi
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DNS using CNAMEs breaks MX records?

We are trying to move all our websites we host to CNAMES as we are planning on moving servers in the new year and would like the ability to move some clients to one server and other clients somewhere else. We were planning on giving clients a unique…
johnwards
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Can different AWS accounts manage different subdomains?

I have two AWS accounts. The master account with example.com as a Hosted Zone, this then has a number of record sets (i.e. api.example.com and kibana.example.com). A second account will be managing testing.example.com as a Hosted Zone, with the…
mlk
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Find DKIM and DMARC Records?

Is there a method to find a domain's DKIM and DMARC records using dig or nslookup? I have attempted to do the following: dig somedomain.org any returns many records, but not the known DKIM and DMARC text records. nslookup -type=txt…
Evil Genius
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Is the hostname case sensitive?

Is the hostname case sensitive? Is ping MYHOST equal to ping myhost Does it depend on the DNS used? Are there differences between Win/Mac/Unix systems?
michelemarcon
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Phishing site uses subdomain that I never registered

I recently received the following message from Google Webmaster Tools: Dear site owner or webmaster of http://gotgenes.com/, [...] Below are one or more example URLs on your site which may be part of a phishing…
gotgenes
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EC2: is an instance's public DNS stable? Can I rely on it not changing?

I've launched my first instance, and am using it as a web server. I see that it has a public DNS (a public URL), e.g.: ec2-123-45-6-789.compute-1.amazonaws.com I can successfully go to this server in my browser, hit it via cURL, etc. I want to use…
Aseem Kishore
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Run antivirus software on linux DNS servers. Does it make sense?

During a recent audit we were requested to install antivirus software on our DNS servers that are running linux (bind9). The servers were not compromised during the penetration testing but this was one of the recommendations given. Usually linux…
John Dimitriou
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Why does DNS work the way it does?

This is a Canonical Question about DNS (Domain Name Service). If my understanding of the DNS system is correct, the .com registry holds a table that maps domains (www.example.com) to DNS servers. What is the advantage? Why not map directly to an…
sabof
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How does the DNS protocol switch from UDP to TCP?

Before anyone asks: I've seen When do DNS queries use TCP instead of UDP? and it doesn't answer my question. All I keep hearing is "if the answer is too long, DNS will use TCP". This does not explain how it happens though. So here's the situation:…
StanTastic
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Why multiple PTR records in DNS is not recommended?

I often read that using multiple PTR records in a DNS configuration is not recommended. However, the reasons are often vague, or not so obvious, naming: "it can cause problems", "can trigger bugs in programs expecting a single answer": it's the…
Totor
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Can you lookup a domains past nameserver & mx record history?

If is possible to find out which nameservers a domain was previously pointing too? I need to try and find where a domain was previously hosted so that I can try and gain access to an old hosted email account. Any ideas?
Luke McCallum
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Using CNAME to point to another domain to save IP addresses

I have one server which I'm hosting a handful of sites on. Currently, each site has it's domain hosted by an independent provider and each has an A record pointing to the server's IP address. But if I want to change the server in the future, I will…
wows
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Why can host and nslookup resolve a name but dig cannot?

Can anyone tell me why this is happening? I can resolve a hostname using host and/or nslookup but forward lookups do not work with dig; reverse lookups do: musashixxx@box:~$ host someserver someserver.somenet.internal has address…
musashiXXX
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Point multiple IP addresses to a single host name

In Windows System, there is this file at C:\WINDOWS\system32\drivers\etc\hosts. This file allows us to default a specific IP address to a host name. The issue now is whether I can set multiple IP addresses to a host name. For example, can I do…
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