You, or someone using your computer, may have hacked the system to redirect queries to certain servers. The file modified is /etc/hosts.
The easiest way to fix the hosts file is to restore it from a backup that predates the modification, or to copy the unmodified file from another Mac. If you can't do that, then do as below.
Back up all data before proceeding.
If you have more than one user account, you must be logged in as an administrator.
Triple-click anywhere in the line below on this page to select it:
open -e /etc/hosts
Copy the selected text to the Clipboard by pressing the key combination command-C.
Launch the built-in Terminal application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad and start typing the name.
Paste into the Terminal window by pressing command-V. I've tested these instructions only with the Safari web browser. If you use another browser, you may have to press the return key after pasting. A TextEdit window should open. At the top of the window, you should see this:
##
# Host Database
#
# localhost is used to configure the loopback interface
# when the system is booting. Do not change this entry.
##
127.0.0.1 localhost
255.255.255.255 broadcasthost
::1 localhost
Below that, you may see some other lines. The first 9 lines should be exactly as above, apart from differences in the blank space within lines. Otherwise you can't use this procedure—STOP and ask for guidance.
If the contents of the TextEdit window are as described, and there are extra lines in the file, close the window, then enter the following command in the Terminal window in the same way as before (by copy and paste):
sudo sed -i~ '10,$d' /etc/hosts
This time you'll be prompted for your login password, which won't be displayed when you type it. Type carefully and then press return. If you don’t have a login password, you’ll need to set one before you can run the command. You may get a one-time warning to be careful. Confirm. Quit Terminal.
If you see a message that your username "is not in the sudoers file," then you're not logged in as an administrator. Log in as one and start over.
That will fix the hosts file. There is now a copy of the old hosts file with the name "hosts~" in the same folder as "hosts". You can delete the copy if you wish. Don't delete the file named "hosts".
