synaptic package manager can't install security items

I wish to delete all the local/obsolete items

To clean-up maybe try command line instead of GUI.
Close all GUIs, e.g. Synaptic, Software-Center, Update-Manager.
Open a terminal: ctrl+alt+t
sudo apt-get autoremove
sudo apt-get autoclean
sudo apt-get clean

Update package list and system.
sudo apt-get update
sudo apt-get upgrade

https://help.ubuntu.com/community/AptGet/Howto

> Run 'uuntu-bug' shows apport-gtk. But can't continue.

You need to assign a package, otherwise apport doesn't know on what to proceed.
ubuntu-bug krb5

https://help.ubuntu.com/community/ReportingBugs

Without using Internet I tried the 4 steps except 'sudo apt-get upgrade'. Do I need to proceed using internet? .... The output is ..

-desktop:~$ sudo apt-get autoremove
[sudo] password for
Reading package lists... Done
Building dependency tree
Reading state information... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
-desktop:~$ sudo apt-get autoclean
Reading package lists... Done
Building dependency tree
Reading state information... Done
-desktop:~$ sudo apt-get clean
-desktop:~$ sudo apt-get update
Err http://in.archive.ubuntu.com natty InRelease

Err http://security.ubuntu.com natty-security InRelease

Err http://in.archive.ubuntu.com natty Release.gpg
  Could not resolve 'in.archive.ubuntu.com'
Err http://security.ubuntu.com natty-security Release.gpg
  Could not resolve 'security.ubuntu.com'
Reading package lists... Done
W: Failed to fetch http://in.archive.ubuntu.com/ubuntu/dists/natty/InRelease

W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/natty-security/InRelease

W: Failed to fetch http://in.archive.ubuntu.com/ubuntu/dists/natty/Release.gpg Could not resolve 'in.archive.ubuntu.com'

W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/natty-security/Release.gpg Could not resolve 'security.ubuntu.com'

W: Some index files failed to download. They have been ignored, or old ones used instead.
-desktop:~$

Unless there is a locale repo available, update and upgrade need an internet connection.
Update tries to fetch newest package list and upgrade shows installation candidates.

https://help.ubuntu.com/community/Kerberos

Without installing any additional item I face the problem mentioned above. Just before reinstallation I had faced some problems. I had 90 items in security packages and among them 2 'abrowsers', so many 'chromium' packages and 'php' items...all these in security.ubuntu.com. My doubt is, which one will come first--archives or security?.. No problem in the order mentioned above ( # 4 ) ?... That is why I didn't use the internet.

Example of Natty sources.list. (The commented '#' or '##' lines do nothing)
http://paste.ubuntu.com/609653/

Try with 'main server' enabled.
https://help.ubuntu.com/community/Repositories/Ubuntu#Download%20Server

Usually the order starts with 'main'.
Another example can be generated and tested there, it'll display repos in one line and without comments what they mean.
http://repogen.simplylinux.ch/

Today I see some changes in Synaptic Package manager. Security.ubuntu.com disappeared. In its place I can see natty-security/main(archive.ubuntu.com) & natty/main. In natty-security 93 packages seen listed 4 marked in green ( those items I mentioned earlier ) and 10 items are ready to install. So I wish to remove unwanted packages first. Not possible? What should I do with the paste.ubuntu.com? Need I use it Terminal? The last link is not suitable now, I think ( Actually I didn't understand what to next after clicking 'Generate' )

I think today's change is due to this clicking. I didn't do anything more than that.

> wish to remove unwanted packages
https://help.ubuntu.com/community/AptGet/Howto#Removal%20commands

Please post your sources.list, open a terminal: ctrl+alt+t
gedit /etc/apt/sources.list

# deb cdrom:[Ubuntu 11.04 _Natty Narwhal_ - Release i386 (20110427.1)]/ natty main restricted

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://archive.ubuntu.com/ubuntu natty main

## Major bug fix updates produced after the final release of the
## distribution.

## 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.

## 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.

## Uncomment the following two lines to add software from the 'backports'
## repository.
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
# deb http://in.archive.ubuntu.com/ubuntu/ natty-backports main restricted universe multiverse
# deb-src http://in.archive.ubuntu.com/ubuntu/ natty-backports main restricted universe multiverse

deb http://archive.ubuntu.com/ubuntu natty-security main

## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu natty partner
# deb-src http://archive.canonical.com/ubuntu natty partner

## This software is not part of Ubuntu, but is offered by third-party
## developers who want to ship their latest software.

'Third party sources' was enabled in the Update manager when I saw it first. But not now. Since the source.list is like this, which is the first thing I should do?

It's obviously missing lots of repos. Replace content and try this one.
http://paste.ubuntu.com/610765/

During the procedure make sure Synaptic and other package management GUIs are closed, after saving the file update via terminal.

The downloaded item didn't appear anywhere in my Home Folder ( I had set it to save downloads there ). Used copy..paste. Tried removing # or ## a few lines appeared are these. .. (The 3rd authentication key as default was -- 3E5C1192 2010-09-20) ... The output follows..(giving only a few lines only)

-desktop:~$ cdrom:[Ubuntu 11.04 _Natty Narwhal_ - Release i386 (20110427.1)]/ natty main restricted
bash: syntax error near unexpected token `('
-desktop:~$
-desktop:~$ http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
bash: http://help.ubuntu.com/community/UpgradeNotes: No such file or directory
-desktop:~$ newer versions of the distribution.
newer: command not found
-desktop:~$
-desktop:~$ http://archive.ubuntu.com/ubuntu natty main restricted
bash: http://archive.ubuntu.com/ubuntu: No such file or directory

Try to put only the rows below into your /etc/apt/sources.list file...

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

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

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://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://archive.ubuntu.com/ubuntu/ natty-backports main restricted universe multiverse
# deb-src http://archive.ubuntu.com/ubuntu/ natty-backports main restricted universe multiverse

# deb http://archive.canonical.com/ubuntu natty partner
# deb-src http://archive.canonical.com/ubuntu natty partner

deb http://extras.ubuntu.com/ubuntu natty main
#deb-src http://extras.ubuntu.com/ubuntu natty main

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

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

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

I didn't install any new item. Altogether I have 22 items in my File System, 220 items in /etc/ and 10 items in /apt/. But there are nothing in preferences.d, sources.list.d, and trusted.gpg.d.
When I clicked sources.list the window appeared was of 'Software Sources.' So nothing could do with it. [ AFTER I clicked 'Yes,sign me in', just before the next window appeard, in the fast changing part at left bottom corner, the last one appeared was ssl.google-analytics.com ...( happened several times ) Don't know any problem here ]

> When I clicked sources.list the window appeared was of 'Software Sources.'

Yep, software sources is assigned as default app.
You need to open sources.list via terminal, remove your content and replace it as suggested above. No need to download, just copy and paste the content as is from either pastebin or answers webpage.
Open a terminal: ctrl+alt+t
gksudo gedit /etc/apt/sources.list

Save the file and close it.
Afterwards update package list.
sudo apt-get update

apt.conf.d, preferences.d, sources.list.d, trusted.gpg.d, apt.conf, secring.gpg, sources.list, sources.list.save, trustdb.gpg, & trusted.gpg are the 10 items in 'sources.list.save' in 'apt'. And the 'sources.list' mentioned above is in 'sources.list.save'. Since just one Updating can make so many changes, before updating I wish to know whether I should delete the blank file (sources.list) and rename 'sources.list.save' as 'sources.list' and proceed as you said.

Actually sources.list was not blank (it was a mistake in my last message).

Don't delete sources.list, just replace the content as suggested above.

Somehow I could remove it from the window of sources.list appeared. It is a Read-only file. I couldn't paste anything. But the sources.list is there in 'sources.list.save' (in apt). After that no sources.list appeared when I used Terminal.

In the last versions of Ubuntu, the Terminal window itself had provisions for opening Terminal, saving Files etc. But in the present version Desktop's name is the only item in the title bar. For all other things I have to go to the Top panel of the Desktop. In this panel the Terminal version shows (About)--"GNOME Terminal 2.32.1

Another sources.list in usr/share/doc/apt/examples shows "install software additionally or only from these sources?" In it altogether 6 sources--( 2 items in) /security.ubuntu.com/ubuntu hardy-security and 4 items in /us.archives.ubuntu.com/ubuntu hardy. So many 'addremove-sources page' in usr/share/gnome/help/gnome-help/ & 'gnome-terminal' items in ~/help/gnome-terminal/ are seen link(broken) on 15 May 2011

Reinstalled Ubuntu. Now I have only 9 items in /apt/ . Now I can't see 'apt.conf'. I had 10 items there (see #18). Now I have more than 7500 items listed in Natty/main

Just after installation Synaptic Package Manger showed 4 items installed in security.ubuntu.com. Then saw changes in it. I wound up this question to avoid complications. Thank you Sam. Thank you Marco Braida.

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