ares_set_local_dev - Man Page

#include <ares.h>

void ares_set_local_dev(ares_channel_t *channel, const char* local_dev_name)

The ares_set_local_dev function causes all future sockets to be bound to this device with SO_BINDTODEVICE.  This forces communications to go over a certain interface, which can be useful on multi-homed machines. This option is only supported on Linux, the interface must not be already bound to the  socket or the current effective user must have CAP_NET_RAW capability in the current network namespace for the option to work.  If SO_BINDTODEVICE is not supported or the setsockopt call fails (probably because of permissions), the error is silently ignored.

ares_set_local_ip4(3) ares_set_local_ip6(3) network_namespaces(7)

This function was added in c-ares 1.7.4