Using UDP between remote server and client - sockets

Question

I am currently working on a project that needs a UDP communication between a server Amazon Frankfurt) and several hosts (Arduino). I need to be able to send JSON data from server to a host over UDP and it needs to work in real time (time is a real constraint here). Also, periodically a host will speak to server to inform server about its status.
The problem I'm facing is that although I can ping the server, when I create the UDP socket with a port, I get this error (for any port):
errno99: cannot assign requested address
I also believe that since the hosts are in different countries, I can only get router's IP. Hosts will be given to users therefore it is impossible to alter user's router by hand. I need to use something that can handle these by code.
I have been searching on Hole Punching and other techniques. I cannot decide what suits more to my scenario here. Sorry for my lack of network information, any help or guidance as "read or search these techniques" are appreciated.
This code runs on server:
import socket
import time
#Host's public IP
UDP_IP_ADDRESS = "178.243.98.86"
UDP_PORT_NO = 13000
Destination = (UDP_IP_ADDRESS,UDP_PORT_NO)
Message = "Hello, Server"
senderSock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
while True:
senderSock.sendto(Message, Destination)
time.sleep(1);
And this code runs on my machine:
import socket
#Server's public IP
UDP_IP_ADDRESS = "172.31.46.111"
UDP_PORT_NO = 13000
recieverSock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
recieverSock.bind((UDP_IP_ADDRESS, UDP_PORT_NO))
while True:
data, addr = recieverSock.recvfrom(1024)
print "Message: ", data

Answer 1

Server
# Host's public IP
UDP_IP_ADDRESS = "178.243.98.86"
You cannot rely on 178.243.98.86. Your arduinos are behind one or several NATs. With NAT, when a packet leaves a network, your packets source and/or destination addresses (and most likely ports) get rewritten. When this conversion happens, a device responsible for NAT remembers this conversion, but only for a limited period of time. The period usually defaults to 24h.
In order to overcome this issue, it's up to clients to initiate conversations with the server. Introduce a new message type to your protocol saying something like hey it's me, client #34. The server should be associating source IP and port of the received message with client #34 for some period of time, like hours.
In this case the server binds a socket to an address and port as well as a client. And beware that hey it's me, client #34 sent over UDP is not encrypted and is extremely vulnerable to record and replay attacks. I believe an emergency system should be encrypted.
Client
# Server's public IP
UDP_IP_ADDRESS = "172.31.46.111"
172.31.46.111 cannot be server's public IP, because it is an address from an IPv4 private space. You probably meant to say "host's private address". It makes sense to bind to such an address. You get an error most likely because you have incorrectly determined the IP address of your host.
There are several ways to get the correct IP. If you use DHCP (you use it if you've never though about what IPs to use in your SOHO), log into your nearest router's admin interface and check the DCHP Pool. Alternatively, Linux and Windows have terminal commands to do the same. For Arduino, you have probably assigned it a static address. If so, bind to it.

Answer 2

You also seem to open a tcp socket on the server, and connect to it from client via udp socket. Is this intentional?
gsn