Parity Check

How they work: parity checks in computer memory

Computers have internal checks to ensure they are working correctly. This article covers checks using a scheme called parity bits to ensure that the computer’s memory is accurate. 

It is important for the computer to know of memory failures, to avoid operating on bad data. An undetected memory failure could be catastrophic. Memory is hardware and does fail from time to time. This article is the second in the series on memory. 

 

Parity’s purpose

Memory parity is what helps the computer system detect that its memory has failed. If there is a problem with memory and the value of memory changes, parity can detect the problem and stop the computer. 

 

How parity works

Recall that memory is organized in bytes, which is eight bits. I When memory is manufactured, an additional bit is added to each byte. This is called the parity bit. The actual size of each memory location is therefore nine bits, but the computer only uses eight of the nine bits for data. The parity bit is checked and never sent to the rest of the computer. 

 

Writing parity

Every time a memory address is written, the parity circuit checks each bit of the byte. It counts all 1 bits in the byte. The sum of the 1 bits results either in an even or odd number. 

 

To continue, we must know whether the designers of the memory used even or odd parity. The circuit will work either way. Let’s assume they used even parity. Stay with me; we’re almost there. 

 

The parity bit is then set to a 0 or a 1 depending on the following. 

 

·         If the sum of the 1 bits is an even number, the parity bit will be written as a 0. This keeps the parity even, or keeps the sum of all 1 bits an even number. 

·         If the sum of the 1 bits is an odd number, the parity bit will be written as a 1. This keeps the parity even, or keeps the sum of all 1 bits an even number. 

 

Reading parity

If you haven’t seen the purpose yet, keep reading. After parity is written, the sum of all the bits, including the parity bit, will be even. Then when the given address is read later, its parity is checked by summing all 1 bits including the parity bit. The result better be an even number, or there is a problem. 

 

Examples

Here are a couple of examples of memory values and their parity bits. 

 

 

Notice that the total sum of all bits for each memory value is always an even number.
 

DNSChanger Malware

DNSChanger Malware

DNS (Domain Name System) is an Internet service that converts user-friendly domain 

names into the numerical Internet protocol (IP) addresses that computers use to talk to 

each other. When you enter a domain name, such as www.fbi.gov, in your web browser 

address bar, your computer contacts DNS servers to determine the IP address for the 

website. Your computer then uses this IP address to locate and connect to the website. DNS 

servers are operated by your Internet service provider (ISP) and are included in your 

computer’s network configuration. DNS and DNS Servers are a critical component of your

computer’s operating environment—without them, you would not be able to access 

websites, send e-mail, or use any other Internet services.

Criminals have learned that if they can control a user’s DNS servers, they can control what 

sites the user connects to on the Internet. By controlling DNS, a criminal can get an 

unsuspecting user to connect to a fraudulent website or to interfere with that user’s online 

web browsing. One way criminals do this is by infecting computers with a class of malicious 

software (malware) called DNSChanger. In this scenario, the criminal uses the malware to 

change the user’s DNS server settings to replace the ISP’s good DNS servers with bad DNS 

servers operated by the criminal. A bad DNS server operated by a criminal is referred to as 

a rogue DNS server.

The FBI has uncovered a network of rogue DNS servers and has taken steps to disable it. 

The FBI is also undertaking an effort to identify and notify victims who have been impacted 

by the DNSChanger malware. One consequence of disabling the rogue DNS network is that 

victims who rely on the rogue DNS network for DNS service could lose access to DNS 

services. To address this, the FBI has worked with private sector technical experts to 

develop a plan for a private-sector, non-government entity to operate and maintain clean

DNS servers for the infected victims. The FBI has also provided information to ISPs that can 

be used to redirect their users from the rogue DNS servers to the ISPs’ own legitimate 

servers. The FBI will support the operation of the clean DNS servers for four months,

allowing time for users, businesses, and other entities to identify and fix infected 

computers. At no time will the FBI have access to any data concerning the Internet activity 

of the victims.

It is quite possible that computers infected with this malware may also be infected with 

other malware. The establishment of these clean DNS servers does not guarantee that the 

computers are safe from other malware. The main intent is to ensure users do not lose DNS 

services.What Does DNSChanger Do to My Computer?

DNSChanger malware causes a computer to use rogue DNS servers in one of two ways. 

First, it changes the computer’s DNS server settings to replace the ISP’s good DNS servers 

with rogue DNS servers operated by the criminal. Second, it attempts to access devices on 

the victim’s small office/home office (SOHO) network that run a dynamic host configuration 

protocol (DHCP) server (eg. a router or home gateway). The malware attempts to access 

these devices using common default usernames and passwords and, if successful, changes 

the DNS servers these devices use from the ISP’s good DNS servers to rogue DNS servers 

operated by the criminals. This is a change that may impact all computers on the SOHO

network, even if those computers are not infected with the malware.  

Am I Infected?

The best way to determine if your computer or SOHO router has been affected by

DNSChanger is to have them evaluated by a computer professional. However, the following 

steps can help you gather information before consulting a computer professional.

To determine if a computer is using rogue DNS servers, it is necessary to check the DNS 

server settings on the computer. If the computer is connected to a wireless access point or 

router, the settings on those devices should be checked as well.

Checking the Computer:

If you are using a Windows computer, open a command prompt. This can be done by 

selecting Run from the Start Menu and entering cmd.exe or starting the command prompt 

application, typically located in the Accessories folder within Programs on your Start Menu, 

as shown below:At the command prompt, enter:

ipconfig /all

Look for the entry that reads “DNS Servers……….”  

The numbers on this line and the line(s) below it are the IP addresses for your DNS servers. 

These numbers are in the format of nnn.nnn.nnn.nnn, where nnn is a number in the range 

of 0 to 255. Make note of the IP addresses for the DNS servers and compare them to the 

table of known rogue DNS servers listed later in this document. If the IP addresses of your 

DNS server appear in the table below, then the computer is using rogue DNS.   

You can also look for your DNS servers without using the command prompt.  

For windows XP machines, click on Start and select My Network Places. Then select Network 

Connections. In this example, the wireless connection is used.Click on the connection that is active. This will bring up the Network Connection Status screen. 

Click on Support and then Details. Check for the values that correspond to the DNS servers.

  

If you are using an Apple computer, click on the Apple in the top left corner and choose System 

Preferences. Then, from the Apple System Preferences window, choose Network. The Apple Network pane will show a number of possible connections on the left side. Choose 

the one that is active for you and click on the Advanced button in the right lower corner. 

Then choose DNS from the options to show the DNS servers you are using.

Compare whether your computer has DNS servers listed in the number ranges listed below.

Rogue DNS Servers

85.255.112.0 through 85.255.127.255 To make the comparison between the computer’s 

DNS servers and this table easier, start by 

comparing the first number before the first dot.

For example, if your DNS servers do not start 

with 85, 67, 93, 77, 213, or 64, you can move on 

to the next step. If your servers start with any of 

those numbers, continue the comparison.

67.210.0.0 through 67.210.15.255

93.188.160.0 through 93.188.167.255

77.67.83.0 through 77.67.83.255

213.109.64.0 through 213.109.79.255

64.28.176.0 through 64.28.191.255

If your computer is configured to use one or more of the rogue DNS servers, it may be 

infected with DNSChanger malware.

Home computers with high-speed Internet connections and office computers typically obtain 

their IP settings via DHCP from a device on the network. In these cases, the computers are 

provided with an IP address, default gateway, and DNS server settings. The IP addresses 

usually fall into one of three ranges of private addresses—192.168.0.0 to 192.168.255.255;

172.16.0.0 to 172.31.255.255; and 10.0.0.0 to 10.255.255.255. In most homes, computers 

are assigned an IP address in the range 192.168.1.2 to 192.168.1.254, and the default 

gateway and DNS servers are set to 192.168.1.1. To determine if your computer is utilizing 

the rogue DNS servers, read the next section, Checking the Router.If you are unable to locate your DNS server settings, obtain assistance from the Help 

program bundled with your operating system, reputable online sources, or a trusted 

professional.

Checking the Router

Small office/home office routers connect your network of computers and devices to your 

Internet service provider. The SOHO router may have been purchased and installed by you 

or installed by your ISP. Linksys, D-Link, Netgear, and Cisco are common SOHO router 

brands, but there are many others.  

The DNSChanger malware is capable of changing the DNS server settings within SOHO 

routers that have the default username and password provided by the manufacturer. If you 

did not change the default password at the time the SOHO router was installed, you must 

check the SOHO router settings. 

The procedure to access your SOHO router setting varies by manufacturer, so consult your 

product documentation. Once you have access to the SOHO router configuration, compare 

the DNS servers listed to those in the rogue DNS servers table above. If your SOHO router 

is configured to use one or more of the rogue DNS servers, a computer on your network 

may be infected with DNSChanger malware.

What Should I Do?

In addition to directing your computer to utilize rogue DNS servers, the DNSChanger 

malware may have prevented your computer from obtaining operating system and antimalware updates, both critical to protecting your computer from online threats. This 

behavior increases the likelihood of your computer being infected by additional malware. 

The criminals who conspired to infect computers with this malware utilized various methods 

to spread the infections. At this time, there is no single patch or fix that can be downloaded 

and installed to remove this malware. Individuals who believe their computer may be 

infected should consult a computer professional.  

Individuals who do not have a recent back-up of their important documents, photos, music, 

and other files should complete a back-up before attempting to clean the malware or utilize 

the restore procedures that may have been packaged with your computer.

Information regarding malicious software removal can be found at the website of the United 

States Computer Emergency Readiness Team: https://www.uscert.gov/reading_room/trojan-recovery.pdf.