All Questions(server)

This is a Canonical Question about DoS and DDoS mitigation.

I found a massive traffic spike on a website that I host today; I am getting thousands of connections a second and I see I'm using all 100Mbps of my available bandwidth. Nobody can access my site because all the requests time out, and I can't even log into the server because SSH times out too! This has happened a couple times before, and each time it's lasted a couple hours and gone away on its own.

Occasionally, my website has another distinct but related problem: my server's load average (which is usually around .25) rockets up to 20 or more and nobody can access my site just the same as the other case. It also goes away after a few hours.

Restarting my server doesn't help; what can I do to make my site accessible again, and what is happening?

Relatedly, I found once that for a day or two, every time I started my service, it got a connection from a particular IP address and then crashed. As soon as I started it up again, this happened again and it crashed again. How is that similar, and what can I do about it?

I have some experience using linux but none using nginx. I have been tasked with researching load-balancing options for an application server.

I have used apt-get to install nginx and all seems fine.

I have a couple of questions.

What is the difference between the sites-available folder and the conf.d folder. Both of those folders were INCLUDED in the default configuration setup for nginx. Tutorials use both. What are they for and what is the best practice?

What is the sites-enabled folder used for? How do I use it?

The default configuration references a www-data user? Do I have to create that user? How do I give that user optimal permissions for running nginx?

With the servers that mount Infiniband cards, when I use the ifconfig command, I get this warning:

Ifconfig uses the ioctl access method to get the full address 
information, which limits hardware addresses to 8 bytes.
Because Infiniband address has 20 bytes, only the first 8 bytes 
are displayed correctly.
Ifconfig is obsolete! For replacement check ip.

Should I quit using ifconfig? Is it deprecated in favor of the ip command? Or will it be updated in the near future?

Note: This question and answers are in regards to GNU/Linux's "major" distributions. It should not be assumed that the information applies to all distributions, and especially not other OSes.

How to pause execution for a while in a Windows batch file between a command and the next one?

For a more comprehensive list of monitoring tools and their features, check out this Wikipedia page.

As the question states, what are the most commonly used tools used for this task and what are their strengths and weaknesses?