Using Kernel 2.6.39 and later, it may occur that vpnc spits out the following error message
Error: either "to" is duplicate, or "ipid" is a garbage.
This isn’t really an error in this case, because vpnc uses some regex and sed to extract the output of ip route get – but that has changed.
In order to fix this (source), replace line 119.
# vim /etc/vpnc/vpnc-script :119 #sed 's/cache//;s/metric ?[0-9]+ [0-9]+//g;s/hoplimit [0-9]+//g' sed 's/cache//;s/metric ?[0-9]+ [0-9]+//g;s/hoplimit[0-9]+//g;s/ipid 0x....//g'