You need to verify that that downloaded Linux DVD or ISO images are intact before you burn and use them. This is security feature. But how do you verify ISO images?
Answer is quite simple use md5sum command to compute and check MD5 message digest.
Why verify ISO images?
a) To protect yourself
b) Verify that a file has not changed as a result of file transfer, disk error, cracker attacks, etc.
How does it work?
When you run md5sum command on ISO file, you get checksum (or hash) on screen. You need to compare this checksum with original. It works as a compact digital fingerprint of a file. You can then compare the MD5 hash of your download ISO file, to the known good hash of the file you are checking. If the two values match, you are safe and use the ISO image.
Let us download FreeBSD ISO image:
$ cd /tmp
$ wget ftp://ftp.freebsd.org/pub/FreeBSD/releases/i386/ISO-IMAGES/6.2/6.2-RELEASE-i386-bootonly.iso
Next download MD5 checksum:
$ wget ftp://ftp.freebsd.org/pub/FreeBSD/releases/i386/ISO-IMAGES/6.2/CHECKSUM.MD5
Generate MD5 hash for ISO image
$ md5sum 6.2-RELEASE-i386-bootonly.iso
Verify ISO image
See the known good hash of the file (6.2-RELEASE-i386-bootonly.iso):
$ md5sum -c CHECKSUM.MD5
You can also use following command for the same purpose:
A note for Windows XP / Vista users
If your server crashes regularly it could be a buggy kernel, a driver, power supply or any other hardware part. Memory (RAM) is one of the critical server parts. Bad memory can cause various problems such as random Linux server restart or program segfaults.
Generally, I recommend using memtester command. It is an effective userspace tester for stress-testing the memory subsystem. It is very effective at finding intermittent and non deterministic faults under Linux.
Recently Rahul shah email me another interesting method for testing memory. His idea is based upon md5 checksum and dd command.
First find out memory site using free command.
total used free shared buffers cached
Mem: 768304 555616 212688 0 22012 270996
-/+ buffers/cache: 262608 505696
Swap: 979956 0 979956
In above example my server has 768304K memory. Now use dd command as follows:
$ dd if=/dev/urandom bs=768304 of=/tmp/memtest count=1050
$ md5sum /tmp/memtest; md5sum /tmp/memtest; md5sum /tmp/memtest
According to him if the checksums do not match, you have faulty memory guaranteed. Read dd command man page to understand all options. dd will create /tmp/memtest file. It will cache data in memory by filling up all memory during read operation. Using md5sum command you are reading same data from memory (as it was cached).
Look like a good hack to me. However I still recommend using memtester userland program. Another option is to use memtest86 program ISO. Download ISO, burn the same on a CD, reboot your system with it test it (it may take more time). From project home page:
Memtest86 is thorough, stand alone memory test for x86 architecture computers. BIOS based memory tests are a quick, cursory check and often miss many of the failures that are detected by Memtest86.
md5sum command is use to check or print MD5 (128-bit) checksums. For example purpose download your favorite Linux distribution from Linux distribution web site / project site. Now you will need to to check md5sum on a Linux ISO file.
1) Download Debian linux # 1 ISO please note down md5sum listed next to each ISO file with the help of wget command:
$ wget http://someproject.org/path/to/isofile.iso
2) Verify integrity of a Linux iso:
$ md5sum isofile.iso
Compare output (a0b162e26281ef097ee8b39b8690a8c2) with key listed online at linuxiso.org's site.
You can read MD5 sums from the FILEs and check them:
$ md5sum -c xcache-1.2.2.tar.gz.md5.txt