easy:SetOpt_Interface(interface)
interface name to use as
outgoing network interface. The name can be an interface name, an IP address,
or a host name.
If the parameter starts with "if!" then it is treated as only as interface
name and no attempt will ever be named to do treat it as an IP address or to
do name resolution on it. If the parameter starts with "host!" it is
treated as either an IP address or a hostname. Hostnames are resolved
synchronously. Using the if! format is highly recommended when using the
multi interfaces to avoid allowing the code to block. If "if!" is specified
but the parameter does not match an existing interface, #CURLE_INTERFACE_FAILED
is returned from the libcurl function used to perform the transfer.
libcurl does not support using network interface names for this option on Windows.