What is resolv.conf and why VPN client's hack it?

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http://unix.stackexchange.com – As stated in a previous question I want to connect to 2 VPN servers at the same time and for each connection specify the IPs of computers I want to reach with it. One of this VPN connection is done with vpnc and a default.conf file, and the other one is done with a Cisco client (I am not able, for now at least to connect with vpnc because I don't have the IPSecrete element required to connect). I am ok with the connection done through vpnc: I am able to connect configure the targeted IPs to use the created interface as following: #!/bin/sh #Get default gateway DEFGW=`ip route list | grep d (HowTos)