Connection to internet while rosmaking

edit

I was trying to rosmake eigen cause it occured as a error package while compiling turtlebot, while i was not connected to internet each time i tried errors came. but suddenely when i got connected the netbook to internet I got it worked fine and even it took only some minutes. The error was :

'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''

[rosmake-1] Starting >>> eigen [ make ]                                         
[ rosmake ] All 7 linesroscpp: 1.4 sec ] [ eigen:... [ 2 Active 21/71 Complete ]
{-------------------------------------------------------------------------------
  mkdir -p build
  if [ ! -f eigen-3.0prebeta3.tar.bz2.md5sum ]; then echo "Error: Couldn't find md5sum file eigen-3.0prebeta3.tar.bz2.md5sum" && false; fi
  `rospack find rosbuild`/bin/download_checkmd5.py https://code.ros.org/svn/release/download/thirdparty/eigen/eigen-3.0prebeta3.tar.bz2 build/eigen-3.0prebeta3.tar.bz2 `awk {'print $1'} eigen-3.0prebeta3.tar.bz2.md5sum`
  [rosbuild] Checking md5sum on build/eigen-3.0prebeta3.tar.bz2
  [rosbuild] WARNING: md5sum mismatch (5010c6d093a4c6b59ed4804a0472bf39 != 7e048a2c20f2757a10aa8267d80b1392); re-downloading file build/eigen-3.0prebeta3.tar.bz2
  [rosbuild] ERROR: md5sum mismatch (5010c6d093a4c6b59ed4804a0472bf39 != 7e048a2c20f2757a10aa8267d80b1392) on build/eigen-3.0prebeta3.tar.bz2; aborting
-------------------------------------------------------------------------------}
[ rosmake ] Output from build of package eigen written to:
[ rosmake ]    /home/sunny/.ros/rosmake/rosmake_output-20110706-225013/eigen/build_output.log
[rosmake-1] Finished <<< eigen [FAIL] [ 0.64 seconds ]  



''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''

Do we really need to be connected while rosmaking? what is the role of internet in compiling?