Comment 1 for bug 1693819

This problem is because isc-dhcp strips the interface alias when it enumerates the system interfaces, i.e. in common/discover.c function next_iface():

                /* interface aliases look like "eth0:1" or "wlan1:3" */
                s = strchr(info->name, ':');
                if (s != NULL) {
                        *s = '\0';
                }

However when later searching the previously-enumerated list for the matching interface for the cmdline interface parameter name, it does not strip the interface alias from the cmdline interface name, i.e. in common/discover.c function discover_interfaces():

        /* Cycle through the list of interfaces looking for IP addresses. */
        while (next_iface(&info, &err, &ifaces)) {

                /* See if we've seen an interface that matches this one. */
                for (tmp = interfaces; tmp; tmp = tmp->next) {
                        if (!strcmp(tmp->name, info.name))
                                break;
                }

The 'info' interface will always have its info.name set to the actual interface name, without any alias extension, while the 'tmp' interface (from the cmdline) will have its tmp->name set to the interface including alias extension; the strcmp then never matches, so isc-dhcp is not able to correctly match the requested interface with the actual system interface.