gnrc_sock/udp: add option to disable remote address check

If a server has e.g. multiple global IPv6 addresses, it might
respond with a different address that the one it received the request on.

This is against the CoAP spec, but that doesn't stop existing implementations
from doing it.

sys/net/gnrc/sock/include/gnrc_sock_internal.h

@@ -55,6 +55,14 @@ extern "C" {
  */
 #define GNRC_SOCK_DYN_PORTRANGE_ERR (0)
 
+/**
+ * @brief   Check if remote address of a UDP packet matches the address the socket
+ *          is bound to.
+ */
+#ifndef CONFIG_GNRC_SOCK_UDP_CHECK_REMOTE_ADDR
+#define CONFIG_GNRC_SOCK_UDP_CHECK_REMOTE_ADDR (1)
+#endif
+
 /**
  * @brief   Structure to retrieve auxiliary data from @ref gnrc_sock_recv
  *

sys/net/gnrc/sock/udp/gnrc_sock_udp.c

@@ -231,7 +231,9 @@ static bool _accept_remote(const sock_udp_t *sock, const udp_hdr_t *hdr,
               ipv6_addr_to_str(addr_str, (ipv6_addr_t *)&sock->remote.addr, sizeof(addr_str)));
         DEBUG(", source (%s) does not match\n",
               ipv6_addr_to_str(addr_str, (ipv6_addr_t *)&remote->addr, sizeof(addr_str)));
-        return false;
+        if (CONFIG_GNRC_SOCK_UDP_CHECK_REMOTE_ADDR) {
+            return false;
+        }
     }
 
     return true;