server - Upgrading Ubuntu from local mirror

I need to upgrade from Ubuntu Server 10.04 LTS to 12.04 LTS using local repository (approx); Internet access is granted by a Web Proxy.

The problem is:

• if I don't set proxy (so server can't reach Internet), it says:

# do-release-upgrade 
Checking for a new Ubuntu release
Failed Upgrade tool signature
Failed Upgrade tool
Done downloading            

Failed to fetch
Fetching the upgrade failed. There may be a network problem.

• If I set proxy, it can't reach local repositories:

# export https_proxy="http://user:password@proxy.localdomain:8080"

# export http_proxy="http://user:password@proxy.localdomain:8080"
...
# do-release-upgrade 
...
Err http://apt.localdomain precise/main Packages
  503  Service Unavailable
Err http://apt.localdomain precise/restricted Packages
  503  Service Unavailable
Err http://apt.localdomain precise/universe Packages
  503  Service Unavailable

Err http://apt.localdomain precise/multiverse Packages
  503  Service Unavailable
Err http://apt.localdomain precise/main Sources
  503  Service Unavailable
Err http://apt.localdomain precise/restricted Sources
  503  Service Unavailable
Err http://apt.localdomain precise/universe Sources
  503  Service Unavailable
Err http://apt.localdomain precise/multiverse Sources
  503  Service Unavailable

Err http://apt.localdomain precise-updates/main Packages
  503  Service Unavailable
...

Any ideas?

I answer myself:

• Setting no_proxy solve the problem:

export no_proxy="192.168.0.0/16,localdomain"

but warning is displayed:

WARNING: Failed to read mirror file

No valid mirror found 

While scanning your repository information no mirror entry for the 
upgrade was found. This can happen if you run a internal mirror or if 
the mirror information is out of date. 

Do you want to rewrite your 'sources.list' file anyway? If you choose 
'Yes' here it will update all 'lucid' to 'precise' entries. 

If you select 'No' the upgrade will cancel.