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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 port 87. How can I accomplish this using DNS only?

Example 2

I'm running a service on my server on a non-standard port. How can I get clients to connect to this non-standard port automatically? Can I use DNS? Is there some application specific support where DNS could indicate the IP and Port?

Example 3

Do some application protocols specifically support hostname awareness, and allow special actions to be taken based on this information? Are there other questions on Server Fault that cover some of these?

Commandeering: This question was originally asking about running IIS and Apache on the same server, but the same concepts can be applied to any server software receiving connections from clients. The Answers below describe the technical problems and solutions of using DNS and application protocol support to assign a port number for a client to connect.

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6 Answers

Technically you can use SRV records on DNS servers as defined in RFC 2782 to tell browsers which servers handle http on which ports for a (sub-)domain:

_http._tcp.www.example.com.  IN      SRV 0    5      80   www.example.com.
_http._tcp.www2.example.com. IN      SRV 0    5      87   www.example.com.

This works well for many protocols/services, especially where the usage of SRV records is already defined in the protocol specification.

However as this "Hall of Shame" states, most of the web browsers/clients do not support this (for HTTP). Also see why-do-browsers-not-use-srv-records.

The deal is basically, that SRV is not included in the http protocol as a must so every browser that implements it resolves URLs differently than browsers that don't.

So you should only use this as some optional load balancing, where it is not relevant which server is chose in terms of content. "Optional" because it won't balance much of the load if only few clients implement this.

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In order to use any (TBT) service on non-standard port and don't write port in URI, everybody can use SRV records, defined in RFC 2782.

_http._tcp.www.example.com. IN      SRV 0    5      87   www.example.com.

All other http-hosts in zone still will be served on default port 80

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+1 for pointing out that several of the statements below are incorrect, as far as the spec goes - DNS absolutely can indicate a service port via SRV records. But it also assumes the client knows to ask for the SRV record first. –  mcauth Sep 14 '12 at 3:11
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You cannot use the DNS to point to a port. You will have to put some other method in place to do this. Typically you would use a front end webserver (or a dedicated proxy server) to proxy the connection from port 80 to port 100 based on the name of the server being requested.

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I'm afraid domain names can only be associated with an IP address and not a port.

Most web servers e.g. (Apache, IIS etc.) do allow you to have two domains hosted on the same IP address by using the fact that web requests contain a host-header field that identifies the domain in the request itself.

If you say what the web server is that you are using I'm sure people can point you to the relevant documentation to set up your server as you wish

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That's the point. I'm using two different web servers. –  Omar Abid Jan 26 '12 at 14:21
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When you type http://www.domain.com into your browser, it is understood that the HTTP port is on 80. Therefore, there is no direct way to point www.domain.com to port 87 if you already have a service running on that port in IIS.

That being said, there are a few "workarounds".

  • Just use http://www.domain.com:87/ - this will connect to port 87 (apache) on your server.
  • You can set up a redirect, so that http://www.domain.com/apache will forward (or proxy, if you want to get fancy) to www.domain.com:87.
  • You can set up a "VirtualHost" so that www.domain2.com will still be on port 80, shared with www.domain.com. You can not set this up without modifying IIS.

Sam is right, DNS is agnostic when it comes to ports. Any sort of port redirection happens by the service that is running on that port. Therefore you would need to do something with IIS to make this happen, if you have no choice but to leave it on port 80.

I've also gotten around your situation by using mod_proxy on Apache, not sure if there is a way to do this with IIS.

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Ok, so how to set proxy on IIS ? –  tomaszs Oct 14 '09 at 13:47
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If it's IIS7, you can use Application Request Routing (ARR). –  Scott Forsyth - MVP Oct 14 '09 at 13:50
    
Scott, do you have a good link for documentation on ARR? –  Jacques Jul 19 '13 at 8:41
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DNS does not have the capability to redirect to a specific Port, all DNS cares about is the IP address resolution of a name, and vice versa.

Some services, such as Dynamic IP DNS providers, such as NO-IP provide a serivce that can help you do something similar to get round blocking of IP's on home DNS services.

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Some registrars (GoDaddy) offer domain forwarding through their parked servers. You could give that a shot, but it's a bit of a kludge. Alternately, you could write your own web browser that looks up SRV records, and then try to convince the world to use it :) –  Jason Antman Oct 14 '09 at 17:38
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