How to save the packages received by a network interface or some port in a file and resend the packages received when needed?

[king6cong at gmail.com]

hi,
  This is a question not specific to Python,but its related somehow,and I
believe I can get some help from your fellow:)
  I am doing my work on a server service program on Linux that processes the
packages sent to the socket it listens.Their is already a old such service
listening on the port doing its job,and
I can't stop the old server service, and I need to get the packages sent to
the old server and send them to my new server service to make sure it works
well .How can I get the package and resent them to my new service? Is there
such a tool or is there some functionality that tools such as tcpdump
already provides?
Thanks:)
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