4

https://en.wikipedia.org/wiki/Virtual_hosting says

Name-based virtual hosts use multiple host names for the same IP address.

Does a "host name" correspond to an IP address or a pair of IP address and a port?

When specifying a "host name", do we still need to specify a port?

If that matters, consider only in virtual hosting.

Thanks.

21

Host names do not correspond to an {ipaddress,port} tuple. A host name is only the name of a server, which should be resolvable to one or more IP addresses. Ports have nothing to do with host names at all.

  • Thanks. (1) Can you tell me how to make a host name that can be resolve to more than one IP addresses? (2) When specifying a "host name", do we still need to specify a port? Is a host name specified with different ports resolved to the same IP address? Can a host name specified with different ports invoke different servers? – Tim Feb 11 at 16:37
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    @Tim, if you have more questions feel free to create new question(s) in to the platform pressing "Ask question" – Romeo Ninov Feb 11 at 16:56
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    @Tim: Resolving of a host name is entirely, fully, completely independent of a port number. As such, there is neither a need nor a possibility to specify a port when resolving a name. (aka "name lookup"). – MSalters Feb 11 at 17:39
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    @MSalters almost... SRV records usually specify the port of the service as well as the IP. One record can resolve to multiple ports on the same IP. – viraptor Feb 12 at 0:09
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A "virtual host" is simply a feature of a piece of software which takes advantage of extra context in a request to act differently.

An important thing to note is that TCP/IP itself does not know anything about host names; their main purpose is as a way to find IP addresses.

The classic example is an HTTP Server using name-based virtual hosting, which works like this:

  • The user requests a URL. The domain name is looked up in DNS, to find an IP address.
  • A TCP connection is opened to a particular IP address and port. (For HTTP, this defaults to port 80; for HTTPS, port 443).
  • The client sends a request on that connection which includes whatever information is specified by the protocol being used.
    • In HTTP 1.1 this includes the "Host" header, which is the domain name the user looked up to find the IP address.
    • For a secure connection, the TLS handshake can include a "Server Name Indication" field, so that this is available before certificates are agreed.
  • The server software listening on the IP address and port now has all three pieces of information: IP address, port number, and host name; it can use these to determine which configuration to apply to the request. This configuration is the "virtual host".
1

Host names are handled by DNS (or other name resolution like a hostfile). Webservers listen on IPs/Sockets, but when running virtual hosts they also look at the request header for what FQDN was used to request the page.

When a web server running virtual hosts responds to a request, it looks at the request header to see if the request is from a host it knows about, then serves up the correct page. i.e. if i have a server with virtual host for initech.xyz, DNS will point it to the IP of my web server, which is listening on the default http/s ports (80/443). Based on the configuration you can turn on/off different ports that each virtual host responds to, but from an IP/port perspective, if the port is enabled, it's open.

Also DNS can have multiple IPs resolving to the same name, and vice versa.

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