I'm using boost::beast for TCP connections, with an async flow. I'm wondering what the most appropriate way is to specify the source IP/port to bind to.
I illustrate one attempt below,
template <typename Stream, typename Endpoints, typename Handler>
static void connect(Stream& stream, const Endpoints& endpoints, Handler handler) {
auto& tcpStream = beast::get_lowest_layer(stream);
tcpStream.expires_after(std::chrono::seconds(10));
// one attempt.
tcpStream.socket().open(boost::asio::ip::tcp::v4());
tcpStream.socket().bind( boost::asio::ip::tcp::endpoint(boost::asio::ip::address::from_string("127.0.0.1"), 0));
tcpStream.async_connect(
endpoints,
[&stream, handler](const auto ec, const auto&){ onConnect(stream, ec, handler); }
);
}
This appears to work fine for one end-point (but is perhaps non-idiomatic?), but presumably the socket gets opened and closed repeatedly on failure in case of an end-point sequence as in https://www.boost.org/doc/libs/develop/libs/beast/doc/html/beast/ref/boost__beast__basic_stream/async_connect/overload2.html - and then this wouldn't behave correctly.
Is there a more idiomatic approach? Thank you.
but presumably the socket gets opened and closed repeatedly on failure
I was skeptical. The documentation specifically states:
It seems fair to expect that if the socket was already open, it will also not be returned to the closed state.
However, you're right to smell the opportunity for a problem here. And I went to check.
Indeed, the bind doesn't appear to hold, even when there's a single endpoint, but it's using the endpoint-sequence overload. Here's a comprehensive demonstration:
#include <boost/asio.hpp>
#include <boost/beast/core/tcp_stream.hpp>
#include <iostream>
namespace asio = boost::asio;
using asio::ip::tcp;
using error_code = boost::system::error_code;
auto fake_dns_result() {
tcp::endpoint eps[]{
{{}, 7878}, // not available
{{}, 6767}, // running netcat
};
return tcp::resolver::results_type::create( //
std::begin(eps), std::end(eps), "localhost", "dummysvc");
}
void test(tcp::endpoint local_ep) {
asio::io_context ioc;
auto endpoints = fake_dns_result();
boost::beast::tcp_stream s(ioc);
s.socket().open(tcp::v4());
s.socket().bind(local_ep);
auto log = [&s, local_ep](error_code ec, tcp::endpoint const& next) {
std::cout << ec.message() << " next:" << next << " "
<< (s.socket().is_open() ? "open" : "closed")
<< " bound:" << s.socket().local_endpoint()
<< std::endl;
return true;
};
s.async_connect(endpoints, log, [&s](auto ec, const auto& /*ep*/) {
std::cout << " --> Final " << ec.message() << " local "
<< s.socket().local_endpoint() << " to "
<< s.socket().remote_endpoint() << "\n\n";
});
ioc.run();
}
int main() {
using A = asio::ip::address_v4;
test({}); // 0.0.0.0:0
test({A::loopback(), 0}); // 127.0.0.1:0
test({A{{127, 0, 0, 42}}, 0}); // 127.0.0.1:0
test({A{{192, 168, 50, 225}}, 0}); // 192.168.50.225:0
}
See it Live On Coliru:
Success next:0.0.0.0:7878 open bound:0.0.0.0:36700
Connection refused next:0.0.0.0:6767 open bound:127.0.0.1:35126
--> Final Success local 127.0.0.1:59672 to 127.0.0.1:6767
Success next:0.0.0.0:7878 open bound:127.0.0.1:51845
Connection refused next:0.0.0.0:6767 open bound:127.0.0.1:35130
--> Final Success local 127.0.0.1:59676 to 127.0.0.1:6767
Success next:0.0.0.0:7878 open bound:127.0.0.42:37227
Connection refused next:0.0.0.0:6767 open bound:127.0.0.1:35134
--> Final Success local 127.0.0.1:59680 to 127.0.0.1:6767
Success next:0.0.0.0:7878 open bound:173.203.57.63:49499
Connection refused next:0.0.0.0:6767 open bound:127.0.0.1:35138
--> Final Success local 127.0.0.1:59684 to 127.0.0.1:6767
As you suggested, the bound endpoint is not honoured. Even moving the bind into the connection condition doesn't help:
s.socket().open(tcp::v4());
auto log = [&s, local_ep](error_code ec, tcp::endpoint const& next) {
std::cout << ec.message() << " next:" << next << " "
<< (s.socket().is_open() ? "open" : "closed")
<< " bound:" << s.socket().local_endpoint()
<< std::endl;
s.socket().bind(local_ep);
return true;
};
Still prints the same
Success next:0.0.0.0:7878 open bound:173.203.57.63:49499
Connection refused next:0.0.0.0:6767 open bound:127.0.0.1:35138
--> Final Success local 127.0.0.1:59684 to 127.0.0.1:6767
Reviewing the implementation, it looks like asio::[async_]connect should have the exact same behaviour, because Beast only adds the timeout logic. Let's reduce: Coliru.
As expected, same output. On to reviewing the Asio implementation.
Just to simplify the review, let's also check the synchronous connect behaves the same:
error_code ec;
/*auto ep =*/ asio::connect(s, endpoints, log, ec);
std::cout << " --> Final " << ec.message() << " local "
<< s.local_endpoint() << " to " << s.remote_endpoint() << "\n\n";
Still the same: Coliru
Indeed, the implementation of the range-connect in Asio looks like this:
for (Iterator iter = begin; iter != end; ++iter)
{
iter = (detail::call_connect_condition(connect_condition, ec, iter, end));
if (iter != end)
{
s.close(ec);
s.connect(*iter, ec);
if (!ec)
return iter;
}
else
break;
}
It follows that it will be impossible to get the required behavior on range-connect, short of