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Debian 4.0 has been released. It is recommended that you upgrade the system to latest version. Upgrading remote Debian server is a piece of cake
Currently many of our boxes are powered by Debian 3.1 Sarge. For example typical web server may have following packages only:
=> Apache
=> PHP
=> Postfix and other mail server software
=> Iptables and backup scripts
=> MySQL 5.x etc
Procedure

Following are essential steps to upgrade your system:
1. Verify current system
2. Update package list
3. Update distribution
4. Update /etc/apt/sources.list file
5. Reboot system
6. Test everything is working
Backup your system

Before upgrading your Debian systems make sure you have backup (I’m assuming that you make backup copies of all important data/files everyday:

1. User data / files / Emails (/home, /var/www etc)
2. Important system files and configuration file stored in /etc
3. MySQL and other database backup
4. Backup Installed package list [Get list of installed software for reinstallation / restore software]

Step # 1: Verify current system

File /etc/debian_version stores current Debian version number :

Code:

$ cat /etc/debian_version

Output:

3.1 Find out kernel version

Code:

$ uname -mrs

Output:

Linux 2.6.8-3-386 i686
Step #2: Update package list

Use apt-get command:

Code:

# apt-get update

Step #3 : Update distribution

Pass dist-upgrade option to apt-get command. This will upgrade Sarge to Etch. dist-upgrade' in addition to performing the function of upgrade, also intelligently handles changing dependencies with new versions of packages; apt-get has a "smart" conflict resolution system, and it will attempt to upgrade the most important packages at the expense of less important ones if necessary.

Code:

# apt-get dist-upgrade

This upgrade procedure takes time. Depend upon installed softwares and other factors such as network-speed you may need to wait from 10 minutes to 1+ hour(s).
Step #4 : Update /etc/apt/sources.list file

There seems to be a small bug in upgrade procedure. You need to manually update Debian security source line. You will see an error as follows:
W: Conflicting distribution: Debian -- Security Information stable/updates Release (expected stable but got sarge)
W: You may want to run apt-get update to correct these problems
Just open /etc/apt/sources.list file:

Code:

# vi /etc/apt/sources.list

Find line that read as follows:

Code:

deb Debian -- Security Information stable/updates main contrib

Replace with :

Code:

deb Debian -- Security Information etch/updates main contrib non-free

Save and close the file. Now type the following command:

Code:

# apt-get update

Step #5: Reboot system

You are done. Just reboot the system:

Code:

# reboot

Step #6: Make sure everything is working...

See Debian distro version:

Code:

$ cat /etc/debian_version

Output:

4.0 Make sure all services are running, just go thought all log files once.

Code:

# netstat -tulpn
# tail -f /var/log/log-file-name
# less /var/log/dmesg
# top
....
...
....

Use apt-key command to manage the list of keys used by apt to authenticate packages. Packages which have been authenticated using these keys will be considered trusted. Make sure you see package etch related keys:

Code:

# apt-key  list

Code:

/etc/apt/trusted.gpg
--------------------
pub   1024D/2D230C5F 2006-01-03 [expired: 2007-02-07]
uid                  Debian Archive Automatic Signing Key (2006) ftpmaster@debian.org

pub   1024D/6070D3A1 2006-11-20 [expires: 2009-07-01]
uid                  Debian Archive Automatic Signing Key (4.0/etch) 

pub   1024D/ADB11277 2006-09-17
uid                  Etch Stable Release Key 
 If there is a problem use following command to update the local keyring with the keyring of Debian archive keys and removes from the keyring the archive keys which are no longer valid.

Code:

# apt-key update
# apt-key list

Finally just see if any new updates/security updates are available:

Code:

# apt-get update
# apt-get upgrade

Further readings

• Above instructions is server specific only. I’ve tested them on 3 different production web servers. The release notes for Debian GNU/Linux 4.0 ("etch"), Intel x86 has additional information about Upgrades from previous releases including special information about Debian Linux desktop system and other troubleshooting hints.