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)

242
votes
12answers
741k views

What's the command-line utility in Windows to do a reverse DNS look-up?

Is there a built-in command line tool that will do reverse DNS look-ups in Windows? I.e., something like <toolname> w.x.y.z => mycomputername I've tried: nslookup: seems to be forward look-...
172
votes
7answers
244k views

Setting the hostname: FQDN or short name?

I've noticed that the "preferred" method of setting the system hostname is fundamentally different between Red Hat/CentOS and Debian/Ubuntu systems. CentOS documentation and the RHEL deployment guide ...
170
votes
2answers
241k views

How to include multiple domains in an spf TXT Record

I am looking to setup a TXT spf record that has 2 included domains... individually: v=spf1 include:_spf.google.com ~all and v=spf1 include:otherdomain.com ~all What is the proper way of combining ...
169
votes
16answers
147k views

Why is DNS failover not recommended?

From reading, it seems like DNS failover is not recommended just because DNS wasn't designed for it. But if you have two webservers on different subnets hosting redundant content, what other methods ...
165
votes
8answers
317k views

List all DNS records in a domain using dig?

My company runs an internal DNS for mycompany.example There is a machine on the network that I need to find, but I’ve forgotten its name. If I could see a list, it would probably jog my memory. ...
153
votes
3answers
153k views

Linux command to inspect TXT records of a domain

Is there a linux shell command that I can use to inspect the TXT records of a domain?
140
votes
4answers
92k views

What is a glue record?

This is a Canonical Question about DNS glue records. What exactly (but briefly) is a DNS glue record? Why are they needed and how do they work?
116
votes
3answers
58k views

Why can't a CNAME record be used at the apex (aka root) of a domain?

This is a Canonical Question about CNAMEs at the apices (or roots) of zones It's relatively common knowledge that CNAME records at the apex of a domain are a taboo practice. Example: example.com. IN ...
114
votes
7answers
98k views

Top level domain/domain suffix for private network?

At our office, we have a local area network with a purely internal DNS setup, on which clients all named as whatever.lan. I also have a VMware environment, and on the virtual-machine-only network, I ...
113
votes
4answers
178k views

How can I see Time-To-Live (TTL) for a DNS record?

I would like to see the Time-To-Live (TTL) value for a CNAME record. I have access to dig (on Apple Mac OS X), which gives me an answer like this: % dig host.example.gov <*SNIP*> ;; ANSWER ...
110
votes
4answers
244k views

DNS - NSLOOKUP what is the meaning of the non-authoritative answer?

For some domains nslookup gives me a Non-authoritative answer section. What does this mean? Got answer: HEADER: opcode = QUERY, id = 3, rcode = NXDOMAIN header flags: response, ...
97
votes
3answers
88k views

What's the meaning of '@' in a DNS zone file?

I have the following data in my DNS zone file for my domain: $ORIGIN mydomain.com. @ IN A 208.X.Y.Z mail IN A 208.X.Y.Z ... etc.. What does the @ line mean? I know ...
92
votes
21answers
43k views

Should we host our own nameservers?

This is a Canonical Question about whether to outsource DNS resolution for ones own domains I currently have my ISP providing DNS for my domain, but they impose limitations on adding records. ...
88
votes
6answers
45k views

Why can't MX records point to an IP address?

I understand you should not point a MX record at an IP address directly, but should instead point it to an A record, which, in turns, points to the IP address of your mail server. But, in principle, ...
84
votes
4answers
93k views

Force dig to resolve without using cache

I'm wondering if there is a way to query a DNS server and bypass caching (with dig). Often I change a zone on the DNS server and I want to check if it resolves correctly from my workstation. But since ...
84
votes
8answers
116k views

Is a wildcard CNAME DNS record valid?

I know it's valid to have a DNS A record that's a wildcard (e.g. *.mysite.com). Is it possible/valid/advised to have a wildcard CNAME record?
83
votes
2answers
65k views

Should CNAME Be Used For Subdomains?

I manage multiple websites that currently have the following DNS configuration: example.com - A Record - Production Server IP test.example.com - A Record - Test Server IP www.example.com - ...
81
votes
2answers
137k views

How to configure a Windows machine to allow file sharing with a DNS alias

What process is necessary to configure a Windows environment to allow me to use DNS CNAME to reference servers? I want to do this so that I can name my servers something like SRV001, but still have \\...
78
votes
11answers
23k views

Multiple data centers and HTTP traffic: DNS Round Robin is the ONLY way to assure instant fail-over?

Multiple A records pointing to the same domain seem to be used almost exclusively to implement DNS Round Robin as a cheap load balancing technique. The usual warning against DNS RR is that it is not ...
77
votes
2answers
50k views

Vagrant / VirtualBox DNS 10.0.2.3 not working

I am running a fresh install of Linux Mint Nadia (14). I am following the instructions on Vagrant Getting Started but have gotten stuck on the Provisioning. It seems the Vagrant box cannot connect ...
70
votes
22answers
11k views

How the heck is http://to./ a valid domain name?

Apparently it's a URL shortener. It resolves just fine in Chrome and Firefox. How is this a valid top-level domain? Update: for the people saying it's browser shenanigans, why is it that: http://com./...
67
votes
1answer
61k views

How long does it take for DNS records to propagate?

This is a Canonical Question about DNS Propagation How long does it take for an the various types of records to propagate? Do some propagate faster than others? Why does it take time for DNS records ...
66
votes
7answers
8k views

DNS failing to propagate worldwide

I haven't changed anything related to the DNS entry for serverfault.com, but some users were reporting today that the serverfault.com DNS fails to resolve for them. I ran a justping query and I can ...
66
votes
13answers
72k views

What's the reverse DNS command line utility?

What's the command to find the name of a computer given its IP address? I always forget what this command is, but I know it exists in Windows and I assume it exists on the *nix command-line.
66
votes
3answers
32k views

Is a CNAME to CNAME chain allowed?

Is it allowed in DNS to have a CNAME record that points to another CNAME record? The reason we need this is that we have a hostname that we want to be looked up to the IP address of our web server ...
65
votes
15answers
13k views

Are IP addresses “trivial to forge”?

I was reading through some of the notes on Google's new public DNS service: Performance Benefits Security Benefits I noticed under the security section this paragraph: Until a standard system-...
65
votes
4answers
106k views

What type of DNS record is needed to make a subdomain?

I'm making a website, and I need a sub-domain. I need to add the new part to my website, but I don't know which type of DNS record to add in the DNS console to point to this new site. Is it A or ...
64
votes
18answers
61k views

Is Round-Robin DNS “good enough” for load balancing static content?

We have a set of shared, static content that we serve up between our websites at http://sstatic.net. Unfortunately, this content is not currently load balanced at all -- it's served from a single ...
64
votes
2answers
20k views

Why does Heroku warn against “naked” domain names?

I ran across this page in the Heroku docs... Naked domains, also called bare or apex domains, are configured in DNS via A-records and have serious availability implications when used in highly ...
60
votes
3answers
176k views

How does ServerName and ServerAlias work?

It's the following part of a virtual host config that I need further clarification on: <VirtualHost *:80> # Admin email, Server Name (domain name), and any aliases ServerAdmin example@...
59
votes
10answers
71k views

Private IP address in public DNS

We have an SMTP only mail server behind a firewall which will have a public A record of mail.. The only way to access this mail server is from another server behind the same firewall. We do not run ...
53
votes
11answers
39k views

Why should I use Amazon Route 53 over my registrar's DNS servers?

I am building a site that I anticipate will have high usage. Currently, my registrar (GoDaddy) is handling DNS. However, Amazon's Route 53 looks interesting. They promise high speed and offer globally ...
52
votes
3answers
158k views

What is the difference between a hostname and a fully qualified domain name?

I am new to the world of setting up servers and am baffled by the term hostname and fully qualified domain name (FQDN). For example, if I want to set up a server that hosts files on the local network ...
49
votes
6answers
60k views

Do SPF Records For Primary Domain apply to subdomains?

I have a quick question regarding SPF records: Do they need to be present for all subdomains? Lets say that I have a TXT record with SPF info for domain.com Let's also say that I have a seperate ...
48
votes
8answers
52k views

How to use DNS/Hostnames or Other ways to resolve to a specific IP:Port

This is a Canonical Question about DNS/Hostnames resolution to IPs/Ports Example 1 I'm running a web server on port 80 and another on port 87. I would like to use DNS so that www.example.com goes to ...
48
votes
2answers
18k views

What are SPF records, and how do I configure them?

This is a canonical question about setting up SPF records. I have an office with many computers that share a single external ip (I'm unsure if the address is static or dynamic). Each computer ...
46
votes
4answers
7k views

Can someone using the same DNS server as me hijack my domains?

When I register a new domain, I send it to my hosting provider by assigning it its domain name servers in the registar's settings. For example, with Digital Ocean, I input the following: ns1....
45
votes
9answers
73k views

Docker containers can't resolve DNS on Ubuntu 14.04 Desktop Host

I'm running into a problem with my Docker containers on Ubuntu 14.04 LTS. Docker worked fine for two days, and then suddenly I lost all network connectivity inside my containers. The error output ...
45
votes
3answers
34k views

CNAME for top of domain?

Is it possible to set a CNAME record at the top of a domain? (i.e. @ CNAME www, @ CNAME foobar.com., etc.) My ISP says that it's only possible to use CNAME's for subdomains but I've read somewhere ...
44
votes
6answers
67k views

Windows 7: “localhost name resolution is handled within DNS itself”. Why?

After 18 years of hosts files on Windows, I was surprised to see this in Windows 7 build 7100: # localhost name resolution is handled within DNS itself. # 127.0.0.1 localhost # ::1 localhost ...
43
votes
5answers
189k views

How to redirect domain A to domain B using A-Records and CNAME records only

I have 2 domains hosted with different hosts. I need to redirect Domain A to Domain B. Unfortunately I can't do a 301 redirect from Host A, but can only modify/add DNS entries (A-Records and CNAMEs) ...
42
votes
2answers
72k views

Can we have multiple CNAMES for a single Name?

I need this for load balancing. For example, I've two azure storage accounts (say a and b) and the blob addresses for those are a.blob.core.windows.net and b.blob.core.windows.net. Both of them store ...
42
votes
3answers
93k views

Recommended DNS SOA record TTL default?

We currently have our DNS SOA record set to the following for stackoverflow.com: primary name server = ns1.p19.dynect.net serial = 2009090909 refresh = 3600 (1 hour) retry = 600 (...
42
votes
2answers
105k views

When do DNS queries use TCP instead of UDP? [duplicate]

Possible Duplicate: Is it true that a nameserver have to answer queries over TCP? I know DNS uses UDP for most of its queries, but in what circumstances will it use TCP instead?
41
votes
4answers
105k views

Can a CNAME DNS record point to a subdirectory

Can a DNS record point to an address like my.domain.com/subdir1
41
votes
1answer
19k views

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 ...
41
votes
2answers
25k views

How long does negative DNS caching typically last?

If a DNS server looks up a record and it's missing, it will often "negatively cache" the fact that this record is missing, and not try to look it up again for a while. I don't see anything in the RFC ...
40
votes
4answers
5k views

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. ...
40
votes
6answers
4k views

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 ...
40
votes
3answers
107k views

How to ensure OpenVPN connection uses specific DNS?

I'm using OpenVPN through Tunnelblick on MacOS X Lion. I need to set specific DNS (with local IP, which works only when VPN is up) for the duration of this VPN session only. I do not have access to ...