1. If it's a cPanel server check to ensure it's not reachable through a different internet connection (such as 3g/4g on your mobile phone). If it is it means you have simply blocked yourself in the firewall (most likely by trying to login accidentally using incorrect credentials). To rectify this see: https://www.vpsblocks.com.au/support/Knowledgebase/Article/View/73/0/locked-out-of-cpanel-vps

2. If you serve websites check to see if they are reachable from elsewhere, a good website to do that is: http://isup.me 

3. If you are in China, most likely the China traffic block is active. This blocks all incoming requests from China and is on by default. To turn off this block see: https://www.vpsblocks.com.au/support/Knowledgebase/Article/View/188/0/disable-china-block

4. Check the console to see what your VPS is actually doing. Even if your VPS is offline or unreachable you should be able to view the console to see what has actually happened. This is available in our portal system, see: https://www.vpsblocks.com.au/support/Knowledgebase/Article/View/28/0/how-can-i-get-console-access-to-my-vps

5. If you've accessed the console and the VPS is not responding or still not functioning properly, you can easily power off and back on your VPS. This should bring your VPS back online right away, you after powering it back on you can also view the console to watch it booting. See: https://www.vpsblocks.com.au/support/Knowledgebase/Article/View/34/0/how-do-i-stop--start--restart-my-vps

6. If your VPS is still not online after a reboot please open a ticket with support.

Why did it go down?

If your VPS was unresponsive and required a reboot, generally the reason is a lack of available memory or very heavy server load.

Linux

Linux VPSs can behave normally but once they are under stress if your usable RAM runs out the operating system will attempt to kill various services, if it runs out very quickly it may hang completely.

1. Firstly make sure you have a file based swap partition, see: https://www.vpsblocks.com.au/support/Knowledgebase/Article/View/131/0/creating-a-file-based-swap-partition

2. Add some more physical RAM to your machine, see: https://www.vpsblocks.com.au/support/Knowledgebase/Article/View/86/0/how-to-upgrade-your-memory--ram

3. Setup a monitor so that if it reoccurs your VPS can be automatically restarted upon detection of failures (we recommend at least 3 failures before you set it to restart however). See: https://www.vpsblocks.com.au/support/Knowledgebase/Article/View/244/0/setup-a-vps-monitor

4. Add CPU cores, if your server comes under heavy load at times adding CPU cores can help, see: https://www.vpsblocks.com.au/support/Knowledgebase/Article/View/87/0/how-to-upgrade-your-processor--cpu-cores

5. If cPanel limit your apache configuration in WHM -> Service Configuration -> Apache Configuration. Lower your server limit, max clients, max requests per child and possible timeout values. This should help reduce the amount of RAM apache processes would use if the server comes under heavy load.

6. Change your PHP configuration from suPHP to DSO or CGI. suPHP is a little more secure, however, it uses more RAM than both DSO and CGI and also affects performance.

Windows

Windows VPSs rarely go offline. They can become unresponsive however if you have used all your RAM, this is particularly the case for servers with only 1GB of RAM.

1. Add more RAM, see: https://www.vpsblocks.com.au/support/Knowledgebase/Article/View/86/0/how-to-upgrade-your-memory--ram

2. Monitor your RAM usage in the task manager, when you are logged in open the task manager. Anything more than 80% usage and you will very quickly run into memory issues.

3. If you are using the Windows machine to browse the internet, make sure you have all other applications closed, this includes server management which loads automatically upon login.

1. You are blocked by ConfigServer Security & Firewall.

This is a plugin product that is installed by default on cPanel servers we provision. This is the most likely cause of losing access to your cPanel server. You can do a quick test to ensure that your server is online and it's only you who is blocked by testing it at http://downforeveryoneorjustme.com/ 

If the server is indeed online, and it's only you who are blocked, then most likely you are blocked by the firewall.

Getting access again

If you are blocked by ConfigServer Security & Firewall, the quickest thing to do is reset your ADSL modem and get a new IP address. This will allow you access again.

Alternatively you'll need to access WHM using another internet connection (for example through 3G or 4G on your phone), go to WHM -> Plugins -> ConfigServer Security & Firewall and choose 'Flush Blocks'. This will remove all current blocks and allow you regular access again.

Whitelisting your IP

You can now login to WHM -> Plugins -> ConfigServer Security & Firewall. Find the 'Quick Allow' button, and hit a little cog icon next to it. This will auto fill the field with your IP address, then click 'Quick Allow'. This will whitelist your current IP address.

Find out why you were blocked

ConfigServer Security & Firewall will block you if you are NOT on the whitelist, and if you try to login using incorrect credentials to any service on the server (this includes mail, cPanel, WHM, SSH, website authentication and FTP).

If you were blocked and want to see exactly why you need to:

• Go to WHM -> ConfigServer Security & Firewall
• Click on 'Search System Logs'
• Type your IP address and click the 'Search'  button.
• This will show you any entries with regards to that IP and a reason it was blocked.

2. You are blocked by the cPHulk (cPanel brute force protection).

The easiest way to fix this is reset your ADSL modem and get a new IP address. You can then access WHM and find 'cPHulk Brute Force Protection'. You can whitelist your IP address in there. You can also flush all blocks in there.

Alternatively (if you cannot change your IP address or access WHM via another internet connection to flush any blocks), to fix this, you will need to restart your VPS (this can be done through the 'My Account' -> 'Manage VPS' on our website).

Set a constant ping to your VPS IP address while it's restarting by opening a command prompt and typing:

ping x.x.x.x -t

As soon as it starts to ping, you need to login through SSH (we recommend Putty)

Once you are in, type the following:

• Type mysql at console to access MySQL client.
• At MySQL client prompt, enter the following commands (preceding with mysql>)one after one, pressing Enter each time:

  mysql> use cphulkd;

  Expected result: Database changed.

  mysql> DELETE FROM `brutes`;
  mysql> DELETE FROM `logins`;

  Above commands will remove all blocked IP addresses and locked accounts from the system, enabling full access again. If you’re familiar with SQL statements, it’s possible to use WHERE clause to specify logins or IP address that you want to remove only.

  mysql> quit;

  Exit MySQL client; (do not close your SSH connection)

Try connecting now through WHM. If it still fails type:

rm -f /var/cpanel/hulkd/enabled

/etc/rc.d/init.d/cpanel restart

That will disable cpHulk which is the cause of the lockout, and restart cPanel. Now try logging into WHM again and you should be fine.

To avoid future blockage or lock out, it’s recommended to add your own IP address to the whitelist in cPHulk Brute Force Protection. To do so, go to WHM -> Security -> Security Center -> cPHulk Brute Force Protection.

Symptoms:

1. A lot of entries in the system log looking like this:
   
   kernel: [xxxx.yyyy] eth0: IPv6 duplicate address aaaa:bbbb:cccc:d:eee:fff:aaaa:aaaa detected!
   
   Where last several numbers in IPv6 address are matching the network interface MAC address.
   
   
2. Public ETH interface shows "dadfailed" when you run "ip a":
   
   inet6 fe80::aaaa:bbbb:cccc:dddd/64 scope link dadfailed

Fix:

Edit the file /etc/sysctl.conf and insert following lines into it:

net.ipv6.conf.eth0.accept_dad = 0
net.ipv6.conf.default.accept_dad = 0
net.ipv6.conf.eth0.dad_transmits = 0
net.ipv6.conf.default.dad_transmits = 0

Where "eth0" is your Ethernet interface name.

Then run: 

sysctl -p

And restart networking:

service network restart

OR

service networking restart

Or just reboot the VPS.