boost daytime7 server example doesn't respond to netcat client - c++

Question

I am trying to get the daytime6 server example (Asynchronous UDP daytime server) in boost working. I compile the below program using
g++ -std=c++11 -g -Wall -pedantic udp_server.cpp -o udp_server -lboost_system
I start the udp_server. I can see port number 13 (UDP) being opened using netstat command.
However If I trying to client to the server using netcat
nc -u localhost 13
it doesn't seem to give any reply.However I can get the asynchronous TCP daytime server to work fine.
#include <ctime>
#include <iostream>
#include <string>
#include <boost/array.hpp>
#include <boost/bind.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/asio.hpp>
using boost::asio::ip::udp;
std::string make_daytime_string()
{
using namespace std; // For time_t, time and ctime;
time_t now = time(0);
return ctime(&now);
}
class udp_server
{
public:
udp_server(boost::asio::io_service& io_service)
: socket_(io_service, udp::endpoint(udp::v4(), 13))
{
start_receive();
}
private:
void start_receive()
{
socket_.async_receive_from(
boost::asio::buffer(recv_buffer_), remote_endpoint_,
boost::bind(&udp_server::handle_receive, this,
boost::asio::placeholders::error,
boost::asio::placeholders::bytes_transferred));
}
void handle_receive(const boost::system::error_code& error,
std::size_t /*bytes_transferred*/)
{
if (!error || error == boost::asio::error::message_size)
{
boost::shared_ptr<std::string> message(
new std::string(make_daytime_string()));
socket_.async_send_to(boost::asio::buffer(*message), remote_endpoint_,
boost::bind(&udp_server::handle_send, this, message,
boost::asio::placeholders::error,
boost::asio::placeholders::bytes_transferred));
start_receive();
}
}
void handle_send(boost::shared_ptr<std::string> message,
const boost::system::error_code& error,
std::size_t bytes_transferred)
{
}
udp::socket socket_;
udp::endpoint remote_endpoint_;
boost::array<char, 1> recv_buffer_;
};
int main()
{
try
{
boost::asio::io_service io_service;
udp_server server(io_service);
io_service.run();
}
catch (std::exception& e)
{
std::cerr << e.what() << std::endl;
}
return 0;
}

Answer 1

The following command does not send a message:
$ nc -u localhost 13
Instead, netcat will wait, reading from stdin until end-of-file. Upon receiving end-of-file, it will send the message it reads to localhost on port 13 using UDP.
On the other hand, the following command:
$ echo 'msg' | nc -u localhost 13
writes "msg" and end-of-file to netcat's stdin, resulting in netcat sending a UDP datagram containing "msg" to localhost on port 13.
The asynchronous UDP daytime server example responds to any message it receives with the current date and time:
class udp_server
{
public:
udp_server(...)
{
start_receive();
}
private:
void start_receive()
{
socket_.async_receive_from(...,
boost::bind(&udp_server::handle_receive, ...));
}
void handle_receive(...)
{
message = make_daytime_string();
socket_.async_send_to(boost::asio::buffer(message), ...);
}
};
As the first command does not write a message, the udp_server never receives a message to which it can respond. The latter command causes a message to be written, and the udp_server responds with the date and time.
The asynchronous TCP daytime server writes a message upon accepting a connection, then closes the connection. When using TCP, netcat will attempt to connect to the destination immediately. In the case of the tcp_server, netcat will establish a TCP connection, receive the date and time, detect that the remote peer has closed the connection and exit.