使用 boost asio 时出现“解决：找不到主机（权威）”异常答案

【问题标题】：Getting "resolve: Host not found (authoritative)" exception when using boost asio使用 boost asio 时出现“解决：找不到主机（权威）”异常
【发布时间】：2020-09-11 14:52:40
【问题描述】：

我正在尝试创建一个简单的代理服务器。我想在port_ 连接到远程host_（可能是google.com），并转发请求req_buf_to_send。以下是我的代码：

boost::asio::io_service io_service;
tcp::resolver resolver(io_service);
tcp::resolver::query query(host_, port_string, boost::asio::ip::resolver_query_base::numeric_service);
tcp::resolver::iterator endpoint_iterator = resolver.resolve(query);
tcp::socket socket(io_service);
tcp::endpoint connectionEndpoint(endpoint_iterator->endpoint().address(), port_);
boost::system::error_code ec;
socket.connect(connectionEndpoint, ec);
boost::asio::write(socket, req_buf_to_send);

当我运行上述代码并向其发送需要转发的请求时，代码失败并引发异常：

Exception: resolve: Host not found (authoritative)

我也对上述代码进行了单元测试，但也失败了，说：

C++ exception with description "resolve: Service not found" thrown in the test body.

所以，我的解析器逻辑似乎有问题。谁能帮帮我？

谢谢。

【问题讨论】：

标签： c++ sockets boost tcp boost-asio

【解决方案1】：

主机或服务可能未知或为空。以下是解析器如何根据输入失败的一些演示：

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

int main() {
    boost::asio::io_context io;
    boost::asio::ip::tcp::resolver r(io);

    for (auto q : {
            boost::asio::ip::tcp::resolver::query 
            // these are good:
            {"google.com", ""},              // port 0
            {"google.com", "ftp"},           // port 21
            {"google.com", "80"},            // port 80
            // these ar bad:
            {"google.ggg", ""},              // host not found
            {"google.com", "bogus_service"}, // service not found
            {"google.com", "-1"},            // service not found
            {"", ""},                        // host not found
        })
    {
        try
        {
            for (auto ep : boost::make_iterator_range(r.resolve(q), {}))
                std::cout << std::quoted(q.host_name()) << " -> " << ep.endpoint() << "\n";
        } catch(std::exception const& e) {
            std::cout << std::quoted(q.host_name()) << " " << e.what() << "\n";
        }
    }
}

打印：

"google.com" -> 172.217.168.206:0
"google.com" -> [2a00:1450:400e:80c::200e]:0
"google.com" -> 172.217.168.206:21
"google.com" -> [2a00:1450:400e:80c::200e]:21
"google.com" -> 172.217.168.206:80
"google.com" -> [2a00:1450:400e:80c::200e]:80
"google.ggg" resolve: Host not found (authoritative)
"google.com" resolve: Service not found
"google.com" resolve: Service not found
"" resolve: Host not found (authoritative)

现在，如果您的系统不知道服务名称，可能是因为您在一些精简的极简安装（docker 容器或嵌入式系统？）上运行。解决方案是安装标准服务数据库（例如，将 IANA 列表放入 /etc/services。

在 Debian 上，这将是 sudo apt-get install netbase 的问题

【讨论】：