This is also affecting trusty flashplugin-installer=11.2.202.332ubuntu1

==========================================
Workaround: Set a local file in debconf

$ sudo apt-get install debconf-utils

$ cd /path/to/plugin/folder/ ; wget -c http://archive.canonical.com/pool/partner/a/adobe-flashplugin/adobe-flashplugin_11.2.202.332.orig.tar.gz

$ sudo -s
# echo flashplugin-installer   flashplugin-installer/local string /path/to/plugin/folder/ |  debconf-set-selections
# apt-get install --reinstall flashplugin-installer
# exit

==========================================
Diagnostic output:

$ grep proxy /etc/apt/apt.conf
Acquire::http::Proxy "http://user:pass@proxy1:3128";

$ grep proxy /etc/environment 
http_proxy=http://user:pass@proxy1:3128
https_proxy=http://user:pass@proxy1:3128
export http_proxy https_proxy

$ grep proxy /etc/wgetrc 
http_proxy = http://user:pass@proxy1:3128/
https_proxy = http://user:pass@proxy1:3128/
# If you do not want to use proxy at all, set this to off.
use_proxy = on

$ sudo grep proxy -B1 /etc/sudoers
Defaults        env_reset
Defaults        env_keep="http_proxy"

$ sudo echo $http_proxy
http://user:pass@proxy1:3128


$ wget -c http://archive.canonical.com/pool/partner/a/adobe-flashplugin/adobe-flashplugin_11.2.202.332.orig.tar.gz
--2013-12-16 14:53:37--  http://archive.canonical.com/pool/partner/a/adobe-flashplugin/adobe-flashplugin_11.2.202.332.orig.tar.gz
Resolving proxy1 (proxy1)... 10.10.3.137
Connecting to proxy1 (proxy1)|10.10.3.137|:3128... connected.
Proxy request sent, awaiting response... 200 OK
Length: 14136904 (13M) [application/x-gzip]
Saving to: 'adobe-flashplugin_11.2.202.332.orig.tar.gz’

100%[======================================================================>] 14,136,904   504KB/s   in 20s    

2013-12-16 14:53:58 (700 KB/s) - 'adobe-flashplugin_11.2.202.332.orig.tar.gz’ saved [14136904/14136904]

$ sudo apt-get install --reinstall flashplugin-nonfree
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Note, selecting 'flashplugin-installer' instead of 'flashplugin-nonfree'
0 upgraded, 0 newly installed, 1 reinstalled, 0 to remove and 0 not upgraded.
Need to get 0 B/6,832 B of archives.
After this operation, 0 B of additional disk space will be used.
Preconfiguring packages ...
(Reading database ... 241937 files and directories currently installed.)
Preparing to replace flashplugin-installer 11.2.202.332ubuntu1 (using .../flashplugin-installer_11.2.202.332ubuntu1_amd64.deb) ...
Unpacking replacement flashplugin-installer ...
Processing triggers for update-notifier-common ...
flashplugin-installer: downloading http://archive.canonical.com/pool/partner/a/adobe-flashplugin/adobe-flashplugin_11.2.202.332.orig.tar.gz
Traceback (most recent call last):
  File "/usr/lib/update-notifier/package-data-downloader", line 235, in process_download_requests
    dest_file = urllib.urlretrieve(files[i])[0]
  File "/usr/lib/python2.7/urllib.py", line 94, in urlretrieve
    return _urlopener.retrieve(url, filename, reporthook, data)
  File "/usr/lib/python2.7/urllib.py", line 240, in retrieve
    fp = self.open(url, data)
  File "/usr/lib/python2.7/urllib.py", line 208, in open
    return getattr(self, name)(url)
  File "/usr/lib/python2.7/urllib.py", line 345, in open_http
    h.endheaders(data)
  File "/usr/lib/python2.7/httplib.py", line 969, in endheaders
    self._send_output(message_body)
  File "/usr/lib/python2.7/httplib.py", line 829, in _send_output
    self.send(msg)
  File "/usr/lib/python2.7/httplib.py", line 791, in send
    self.connect()
  File "/usr/lib/python2.7/httplib.py", line 772, in connect
    self.timeout, self.source_address)
  File "/usr/lib/python2.7/socket.py", line 553, in create_connection
    for res in getaddrinfo(host, port, 0, SOCK_STREAM):
IOError: [Errno socket error] [Errno -2] Name or service not known
Setting up flashplugin-installer (11.2.202.332ubuntu1) ...