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hans
Joined: Eons Ago
# Posts: 165
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Posted: 2004-Nov-02 16:29
Sorry if this is a stupid question/request, but.
Could someone explain what a DNS and Reverse DNS lookup is.
Many thanks in advance...
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g1smd
Staff
Joined: Jul 28, 2002
# Posts: 10418
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Posted: 2004-Nov-02 16:43
DNS
You type in jimworld.com in the browser and the DNS system looks that up and says the IP is 80.74.35.96 or whatever it is. Your computer than connects to that IP and asks for the index file.
Reverse DNS
You know that a resource sits at 45.22.123.75 and so Reverse DNS lookup, allows you to find out what domain names point to that IP address.
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hans
Joined: Eons Ago
# Posts: 165
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Posted: 2004-Nov-02 16:52
g1smd thanks for the reply,
what happens if there are more than 1 domains pointed to that IP, and there is a reverse DNS, or is that impossible?
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g1smd
Staff
Joined: Jul 28, 2002
# Posts: 10418
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Posted: 2004-Nov-02 16:55
There can be hundreds of domains hosted on one IP address, which is where name-based serving comes in.
As I understand it, DNS always translates the domain name to an IP address, however when there are multiple sites on a server, when you actually arrive at the server it asks you again "exactly which site on here did you want?" before letting you have it.
How the reverse DNS works in that case, I'm not quite sure. Have you Googled for a tutorial? There must be many already out there.
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unreviewed
Joined: Dec 07, 2000
# Posts: 6776
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Posted: 2004-Nov-02 17:30
You can have many domain names pointed to one IP. This is possible because modern browsers, send header information that your server can parse and separate the traffic using that information. However, this is not the case with reverse DNS, you can have only one reverse DNS record per IP, and it can only be set by the owner of the IP, usually your host if you are dealing directly with a large telecom co.
Although most IP's do not have a reverse DNS record, it has become common for most of the major email handlers, such as AOL or MSN, to block email sent via a server operated on an IP that doesn't have a reverse DNS record. The domain it resolves doesn't currently need to match your domain, for example your email server can be yourdomian.com, and the reverse DNS look up can be largetelecom.com. It doesn't matter what it is, as long as it is a valid domain.
Currently, running an email server, is really the only time you need to worry about reverse DNS.
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langard
Joined: Dec 15, 1999
# Posts: 339
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Posted: 2004-Nov-03 09:34
Hmmm,
"You can have many domain names pointed to one IP. This is possible because modern browsers, send header information that your server can parse and separate the traffic using that information."
- Actually, all browsers just send HTTP requests. Internet Servers are the ones that parse and separate the information that is sent back to each browser according to the request for content from a specific URL, regardless of the header info. Old browsers get the same information back that new ones do, they just can't read it.
"However, this is not the case with reverse DNS, you can have only one reverse DNS record per IP, and it can only be set by the owner of the IP, usually your host if you are dealing directly with a large telecom co."
- If you have control over your own Internet Protocol Address and host multiple domains, you can set CNAME (canonical), MX and other top level records and adjust them to fit different needs for each, individual domain and avoid reverse lookup problems.
"Although most IP's do not have a reverse DNS record,..."
- All IPs have a DNS record that I'm aware of, but you're right that not all respond to reverse lookups, or pings or traceroutes... If they're on the Net at all they're in the BIND system. Think of it this way: A simple DNS lookup is a WHOIS from the bottom up. It goes URL => IP. A reverse lookup is IP => URL. The difference is that while a Uniform Resource Locator is just a domain name, an Internet Protocal Address is something like abc.def.ghi.jkl - which translates to roughly REALM.BLOCK.NETWORK.MACHINE, the last being the server that "serves" your pages. In other works a reverse lookup is seeking first who owns the whole Realm, then the Block of IP addresses they have, then the Primary and Secondary Name Servers they're using, then the specific Machine that's assigned to you...
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