I’ve run into the problem with disconnecting vpnc (version 0.5.3) from Cisco VPN Concentrator 3000. The connection was dropped and vpnc process was dying with message:

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Oct 12 20:04:52 server vpnc[26143]: connection terminated by dead peer detection
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The solution is very simple and took me a while to Google it.

To prevent VPN tunnel from disconnection one must add

DPD idle timeout (our side) 0

to vpn configuration file e.g /etc/vpnc/example.conf

or run vpnc client with

vpnc –dpd-idle 0 example.conf