以编程方式检索IPv4和IPv6域名服务器 [英] Retrieve IPv4 and IPv6 nameservers programmatically

问题描述

我正在尝试使用libresolv读取/etc/resolv.conf文件中的IPv4和IPv6名称服务器:

I'm trying to use libresolv to read both the IPv4 and IPv6 nameservers in my /etc/resolv.conf file:

# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
#     DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
# 127.0.0.53 is the systemd-resolved stub resolver.
# run "systemd-resolve --status" to see details about the actual nameservers.

nameserver 127.0.0.53
nameserver 2001:4860:4860:0:0:0:0:8888

这是我的C程序:

#include <resolv.h>
#include <stdlib.h>

int main(int argc, char** argv)
{
    res_state res = malloc(sizeof(struct __res_state));
    res_ninit(res);

    printf("IPv4 nscount:  %d\n", res->nscount);
    printf("IPv6 nscount6: %d\n", res->_u._ext.nscount6);

    return 0;
}

哪个产生以下输出:

IPv4 nscount:  2
IPv6 nscount6: 0

这让我感到惊讶.为什么将IPv6地址计为IPv4地址?

Which surprises me. Why is it counting the IPv6 address as an IPv4 address?

GDB显示第二个地址被清零:

GDB shows that the second address is zeroed out:

(gdb) display res.nsaddr_list[0]
5: res.nsaddr_list[0] = {sin_family = 2, sin_port = 13568, sin_addr = {s_addr = 889192575}, sin_zero = "\000\000\000\000\000\000\000"}
(gdb) display res.nsaddr_list[1]
6: res.nsaddr_list[1] = {sin_family = 0, sin_port = 0, sin_addr = {s_addr = 0}, sin_zero = "\000\000\000\000\000\000\000"}

有人可以帮助我了解这种行为吗?

Can anyone help me understand this behavior?

推荐答案

您确实不应该访问解析器状态的_u._ext部分，它们是内部实现的详细信息. nscount6成员当前未使用，并且始终为零.必须保留它以避免由于结构偏移/大小更改而导致更改ABI.

You really should not access the _u._ext parts of the resolver state, they are an internal implementation detail. The nscount6 member is currently unused and always zero. It had to be kept to avoid changing the ABI as the result of struct offset/size changes.

如果需要名称服务器列表，则应自己解析/etc/resolv.conf.请注意，glibc最终还将支持三个以上的名称服务器，并且这些额外的解析器将不会反映在公共解析器状态中.

If you need the nameserver list, you should parse /etc/resolv.conf yourself. Note that eventually, glibc will also support more than three name servers, and those extra resolvers will not be reflected in the public resolver state.

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