i was following the intallation instructions given in
http://www.nsnam.com/2011/11/ns2-ns-...untu-1110.html
the first two steps were just fine.
on the third step
sudo apt-get update

i got messages like

sh: getcwd() failed: No such file or directory
Err http://extras.ubuntu.com oneiric InRelease

Err http://extras.ubuntu.com oneiric Release.gpg
Unable to connect to extras.ubuntu.com:http:
Err http://archive.canonical.com oneiric InRelease

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

please help

Welcome to LQ.

Ubuntu 11.10 : The release version of 'apt' works OK.
? May be you updated apt to an unstable version ?
Apt can also fail when you run it from a directory that doesn't exist.

Solutions : May be the package list is already updated.
Then continue to the next step, like :
sudo apt-get install \
g++ make autoconf automake make libtool libxmu-dev patch xgraph


If it's not working either, then install the few packages with
the 'Muon Package Manager' or 'Muon Software Center'.
Muon* can also be used to install 'synaptic'.
The 'synaptic' package manager has a 'Refresh' button,
which does the "package list update" like 'sudo apt-get update'.

.