shared/netutils/dhcpserver: Reply on correct netif.

The DHCP server broadcasts messages. This are being sent via the
default netif which might be completely the wrong network. We want to
send messages to the netif we got the original message from.

Original author: [Peter Harper](https://github.com/peterharperuk)
Source: raspberrypi/pico-examples#392

Signed-off-by: Samveen <samveen@samveen.in>

shared/netutils/dhcpserver.c

@@ -118,7 +118,7 @@ static int dhcp_socket_bind(struct udp_pcb **udp, uint32_t ip, uint16_t port) {
     return udp_bind(*udp, &addr, port);
 }
 
-static int dhcp_socket_sendto(struct udp_pcb **udp, const void *buf, size_t len, uint32_t ip, uint16_t port) {
+static int dhcp_socket_sendto(struct udp_pcb **udp, struct netif *nif, const void *buf, size_t len, uint32_t ip, uint16_t port) {
     if (len > 0xffff) {
         len = 0xffff;
     }
@@ -132,7 +132,12 @@ static int dhcp_socket_sendto(struct udp_pcb **udp, const void *buf, size_t len,
 
     ip_addr_t dest;
     IP4_ADDR(&dest, ip >> 24 & 0xff, ip >> 16 & 0xff, ip >> 8 & 0xff, ip & 0xff);
-    err_t err = udp_sendto(*udp, p, &dest, port);
+    err_t err;
+    if (nif != NULL) {
+        err = udp_sendto_if(*udp, p, &dest, port, nif);
+    } else {
+        err = udp_sendto(*udp, p, &dest, port);
+    }
 
     pbuf_free(p);
 
@@ -153,7 +158,7 @@ static uint8_t *opt_find(uint8_t *opt, uint8_t cmd) {
     return NULL;
 }
 
-static void opt_write_n(uint8_t **opt, uint8_t cmd, size_t n, void *data) {
+static void opt_write_n(uint8_t **opt, uint8_t cmd, size_t n, const void *data) {
     uint8_t *o = *opt;
     *o++ = cmd;
     *o++ = n;
@@ -281,7 +286,8 @@ static void dhcp_server_process(void *arg, struct udp_pcb *upcb, struct pbuf *p,
     opt_write_u32(&opt, DHCP_OPT_DNS, DEFAULT_DNS); // can have multiple addresses
     opt_write_u32(&opt, DHCP_OPT_IP_LEASE_TIME, DEFAULT_LEASE_TIME_S);
     *opt++ = DHCP_OPT_END;
-    dhcp_socket_sendto(&d->udp, &dhcp_msg, opt - (uint8_t *)&dhcp_msg, 0xffffffff, PORT_DHCP_CLIENT);
+    struct netif *nif = ip_current_input_netif();
+    dhcp_socket_sendto(&d->udp, nif, &dhcp_msg, opt - (uint8_t *)&dhcp_msg, 0xffffffff, PORT_DHCP_CLIENT);
 
 ignore_request:
     pbuf_free(p);