The vpnc client expects to be configured using the uci interface.

To setup a VPN connection, add the following to /etc/config/network:

config interface 'MYVPN'
        option proto 'vpnc'
        option interface 'wan'
        option server 'vpn.example.com'
        option username 'test'
        option password 'secret' # or:
        option hexpasswd 'AE7FF6A0426F0A0CD0A02EB9EC3C5066FAEB0B25'
        option authgroup 'DEFAULT'
        option passgroup 'groupsecret' # or:
        option hexpassgroup '52B0BEAF6605C3CE9BE20A0DC0A0F6240A6FF7EA'
        option domain 'WORKGROUP'
        option vendor 'cisco' # or 'netscreen'
        option natt_mode 'natt' # or 'none' or 'force-natt' or 'cisco-udp'
        option dh_group 'dh2' # or 'dh1' or 'dh5'
        option pfs 'server' # or 'nopfs' or 'dh1' or 'dh2' or 'dh5'
        option enable_single_des '0'
        option enable_no_enc '0' # '1' to enable unencrypted VPN
        option mtu '0'
        option local_addr '0.0.0.0'
        option local_port '500' # '0' to use a random port
        option udp_port '10000' # '0' to use a random port
        option dpd_idle '300'
        option auth_mode 'psk' # or 'hybrid'
        option target_network '0.0.0.0/0.0.0.0' # network/netmask or CIDR

The additional file(s) are also used:
/etc/vpnc/ca-vpn-MYVPN.pem: The server's CA certificate (for auth_mode 'hybrid')

After these are setup you can initiate the VPN using "ifup MYVPN", and
deinitialize it using ifdown. You may also use the luci web interface
(Network -> Interfaces -> MYVPN Connect).

Note that you need to configure the firewall to allow communication between
the MYVPN interface and lan.