from dhcp.c:163
static u8_t dhcp_discover_request_options[] = {
DHCP_OPTION_SUBNET_MASK,
DHCP_OPTION_ROUTER,
DHCP_OPTION_BROADCAST
#if LWIP_DHCP_PROVIDE_DNS_SERVERS
, DHCP_OPTION_DNS_SERVER
#endif
#if LWIP_DHCP_GET_NTP_SRV
, DHCP_OPTION_NTP
#endif
};