How do I restore the sources.list file?

I accidentally removed my /etc/apt/sources.list file. Is it hosted by Canonical somewhere so I can re-add it to the system? It's a bit of an emergency, so I hope someone can provide a quick answer.

You can boot into a Live CD (a virtual machine will suffice) and copy /etc/apt/sources.list from it. Alternatively, have a Live CD image ready and follow these instructions:

1. Mount the CD. If you've an .iso file, mount it using:

   
   
   
   

   sudo mount -o loop ubuntu-11.04-desktop-amd64.iso /media/cdrom
   




2. Mount the filesystem file from the CD:

   
   
   

   sudo mount -o loop /media/cdrom/casper/filesystem.squashfs /mnt
   




3. Copy the sources.list file over:

   
   
   

   sudo cp {/mnt,}/etc/apt/sources.list
   
   




4. Change the file permissions if necessary:

   
   
   

   sudo chmod 644 /etc/apt/sources.list
   

From my Ubuntu 11.04 Live CD:

deb http://archive.ubuntu.com/ubuntu natty main restricted

deb-src http://archive.ubuntu.com/ubuntu natty main restricted

deb http://security.ubuntu.com/ubuntu natty-security main restricted
deb-src http://security.ubuntu.com/ubuntu natty-security main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://archive.ubuntu.com/ubuntu natty-updates main restricted
deb-src http://archive.ubuntu.com/ubuntu natty-updates main restricted


## Uncomment the following two lines to add software from the 'universe'
## repository.
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
# deb http://archive.ubuntu.com/ubuntu natty universe
# deb-src http://archive.ubuntu.com/ubuntu natty universe
# deb http://archive.ubuntu.com/ubuntu natty-updates universe
# deb-src http://archive.ubuntu.com/ubuntu natty-updates universe
# deb http://security.ubuntu.com/ubuntu natty-security universe

# deb-src http://security.ubuntu.com/ubuntu natty-security universe

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
# deb http://archive.ubuntu.com/ubuntu natty multiverse
# deb-src http://archive.ubuntu.com/ubuntu natty multiverse
# deb http://archive.ubuntu.com/ubuntu natty-updates multiverse

# deb-src http://archive.ubuntu.com/ubuntu natty-updates multiverse
# deb http://security.ubuntu.com/ubuntu natty-security multiverse
# deb-src http://security.ubuntu.com/ubuntu natty-security multiverse

For the best download speed, you need to re-select a mirror near you:

1. Open the Ubuntu Software Center


2. Open Edit -> Software Sources...



3. Click the select box next to Download from:


4. Select a server at your choice

You might want to enable some repositories as well, like Community-maintained Open Source software (universe) and Software restricted by copyright and legal issues (multiverse)