Golang DNS test

gistfile1.txt

$ GODEBUG=netdns=cgo+1 go run main.go
go package net: using cgo DNS resolver
2020/08/26 11:15:12 #01 212.58.233.253:443
2020/08/26 11:15:12 #02 212.58.237.251:443
2020/08/26 11:15:13 #03 212.58.233.251:443
2020/08/26 11:15:13 #04 212.58.233.251:443
2020/08/26 11:15:13 #05 212.58.237.253:443
2020/08/26 11:15:13 #06 212.58.237.254:443
2020/08/26 11:15:14 #07 212.58.237.254:443
2020/08/26 11:15:14 #08 212.58.233.254:443
2020/08/26 11:15:14 #09 212.58.233.254:443
$ GODEBUG=netdns=go+1 go run main.go
go package net: GODEBUG setting forcing use of Go's resolver
2020/08/26 11:15:17 #01 212.58.233.254:443
2020/08/26 11:15:17 #02 212.58.233.253:443
2020/08/26 11:15:18 #03 212.58.233.251:443
2020/08/26 11:15:18 #04 212.58.233.254:443
2020/08/26 11:15:18 #05 212.58.233.253:443
2020/08/26 11:15:19 #06 212.58.233.254:443
2020/08/26 11:15:19 #07 212.58.233.253:443
2020/08/26 11:15:19 #08 212.58.233.253:443
2020/08/26 11:15:19 #09 212.58.233.253:443

main.go

package main

import (
        "context"
        "log"
        "net"
        "net/http"
)

func main() {
        var remoteAddr string
        client := &http.Client{
                Transport: &http.Transport{
                        DialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
                                conn, err := net.Dial(network, addr)
                                remoteAddr = conn.RemoteAddr().String()
                                return conn, err
                        },
                },
        }

        for i := 1; i < 10; i++ {
                resp, err := client.Get("https://www.bbc.co.uk/")
                if err != nil {
                        log.Fatal(err)
                }
                defer resp.Body.Close()
                log.Printf("#%02d %v %s", i, remoteAddr, resp.Request.RemoteAddr)
        }
}

Interestingly I get very different behavior on my system running a slightly modified version of the above Gist:

package main

import (
	"context"
	"log"
	"net"
	"net/http"
)

func main() {
	var remoteAddr string
	remoteAddrs := make(map[string]int)
	client := &http.Client{
		Transport: &http.Transport{
			DialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
				conn, err := net.Dial(network, addr)
				remoteAddr = conn.RemoteAddr().String()
				_, ok := remoteAddrs[remoteAddr]
				if !ok {
					remoteAddrs[remoteAddr] = 0
				}
				remoteAddrs[remoteAddr]++

				return conn, err
			},
		},
	}

	for i := 1; i < 100; i++ {
		resp, err := client.Get("https://www.bbc.co.uk/")
		if err != nil {
			log.Fatal(err)
		}
		defer resp.Body.Close()
		log.Printf("#%02d %v %s", i, remoteAddr, resp.Request.RemoteAddr)
	}

	log.Printf("%20s | %s", "Address", "Count")
	for addr, count := range remoteAddrs {
		log.Printf("%20s | %03d", addr, count)
	}
}

$ GODEBUG=netdns=cgo+1 go run main.go
...
2020/09/01 11:35:22              Address | Count
2020/09/01 11:35:22   212.58.237.253:443 | 099

$ GODEBUG=netdns=go+1 go run main.go
...
2020/09/01 11:36:05              Address | Count
2020/09/01 11:36:05   212.58.237.251:443 | 044
2020/09/01 11:36:05   212.58.237.253:443 | 027
2020/09/01 11:36:05   212.58.237.254:443 | 028

Running your code I get a fairly equal distribution with both DNS methods. I think you might have something caching locally (like a nscd daemon)

Gosh darn it, I looked for systemd-resolved because I thought that NixOS was pretty systemd heavy but it turns out that indeed I have nscd running. I thought most distributions moved away from nscd...

Good catch. It was indeed good evidence for local DNS caching (although weirdly the Go resolver seemed to bypass it anyway).