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)
        }
}

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).