Some notes we've thrown together.

Here are a few articles we've prepared on technical topics which other people may find helpful.

Configuring IP Addresses for a Plesk 7.x.x Hosting Server under FreeBSD UNIX :

Below are some considerations to keep in mind when configuring a Plesk 7.x.x hosting server. Some of this is a combination of information learned from forum postings, other administrators, and personal experience. Much of this isn't really documented anywhere (although it may seem obvious to some). These details appear to be relevant for both FreeBSD v4.x and v5.x systems, and in general seem to apply to both 7.1.x and 7.5.x versions of Plesk.

How many IP addresses does a Plesk 7.x.x server need ?

I would recommend at least three IP addresses to start. The first IP address would be used as the main address of the server for the majority of your virtual hosting needs, and the remaining two addresses would commonly be spares used for temporary assignment to sites for file uploads.

In what situations would I need additional IP addresses beyond the primary address of my server?

  • Spares for temporary use (uploading site contents before DNS changeovers), as mentioned above
  • Hosted websites which require SSL certificates to be used (one per site)

Don't I need one IP address for every website/domain I am hosting ?

While I have seen some hosting providers do this in the past, it is entirely unnecessary. Following the above IP address guidelines, it is entirely possible to run a 100- or 300-domain hosting server using only the three IP addresses described, assuming none of them use SSL certificates on their webhosting.

Which IP addresses get used by Plesk behind the scenes ?

The primary IP address of your server will be used for outbound communications. Examples of these can include :

  • Sending all outbound email messages using qmail
  • Outbound network connections (for example, by the CURL module when called from inside PHP scripts)
It may not be obvious which IP address this primary address actually is, especially if you have a more complex network configuration, or multiple interfaces with many aliased IP addresses assigned to each. It isn't actually set anywhere in Plesk or in FreeBSD, but rather is decided based on your specific configuration. Below is a rough guide as to how the system determines which IP address gets used :
  • Typically it will try and use the IP address used to register the Plesk license key during installation
  • Alternately, it will use the IP address bound natively (non-aliased) in FreeBSD to the highest network card polled by the system. For example, if you have two network interfaces configured in your system, and the second interface found by the system during bootup has one IP address configured, with additional ones configured as aliases, the system will use the first (non-aliased) IP address on that second interface

Why should I care which IP address actually gets used for outbound communications ?

  • Customer-provided scripts and applications running on the hosting server may need to communicate with outside systems on the internet which authenticate you based on IP address. Some services may ask for the IP address in order to manually restrict access to their systems, and the customer may be puzzled because the address actually used is different from the one you have assigned to them for their hosting.
  • More importantly, in order to send emails to many mailservers on the internet, the IP address used by your Plesk mailservers will need to be accessible by reverse DNS lookups. This is critical to have in place, since some of these external mailservers will reject all emails from your system outright until this is resolved. As such, you will either need to put PTR records in place for that primary IP address (if your upstream network provider delegates reverse DNS responsibility to you), or else ask your upstream provider to provide a valid domain name for reverse DNS lookups on that specific IP address (if they handle reverse DNS responses for you).

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