ASIO: What is the difference between a loopback address and a "local service binding"?

user_19264919 0
Viewed 46

I am trying to get into ASIO and I have been reading the docs and came across this:

https://think-async.com/Asio/asio-1.24.0/doc/asio/reference/ip__basic_resolver/resolve/overload5.html

If an empty string and the passive flag has been specified, the resolved endpoints are suitable for local service binding. If an empty string and passive is not specified, the resolved endpoints will use the loopback address.

I am not well versed with networking stuff, so I tried to find out what the difference between these two is (loopback and local service binding), but none of the sources I found really gave a good explanation to what these two things are.

Both of the options are just going to use ports on the local machine as far as I know, so what's the difference? Which one should I use if I intend to use ASIO to just establish connections between C++ programs that all run on the same machine?

networking boost-asio c++
1 Answers

The wording seems curious. However we can "just try" it:

Live On Coliru

#include <boost/asio.hpp>
#include <iomanip>
#include <iostream>

int main() {
    using namespace boost::asio;
    io_context ioc;
    ip::tcp::resolver r(ioc);

    auto report = [](auto results) {
        std::cout << "---\n";
        for (auto el : results)
            std::cout << std::quoted(el.host_name()) << "+"
                      << std::quoted(el.service_name()) << " -> "
                      << el.endpoint() << "\n";
    };

    report(r.resolve("", "finger", ip::resolver_base::flags::passive));
    report(r.resolve("", "finger"));
}

Printing

---
""+"finger" -> 0.0.0.0:79
""+"finger" -> [::]:79
---
""+"finger" -> [::1]:79
""+"finger" -> 127.0.0.1:79

Clearly, specifying passive leads to "unbound" interface in the endpoint, whereas the default result binds to the explicit address of the loopback adapter.

I think the main difference here would be when you bind a listening port to such an endpoint.

user_853710
Related