在文件中的特定位置之后插入字符串

Question

I have this file:

upstream frontends {
    server service-example1.example.com;
    server service-example2.example.com;
}

server {
  listen 80;

  location / {
      proxy_pass http://frontends;
  }
}

What I want to do is to remove and add lines in the upstream frontends section. Removing lines is quite easy:

package main

import (
    "bytes"
    "fmt"
    "io/ioutil"
)

func main() {
    content, err := ioutil.ReadFile("nginx.conf")
    if err != nil {
        panic(err)
    }

    lines := bytes.Replace(content, []byte("server service-example1.example.com;"), []byte(""), 1)
    fmt.Println(string(lines))
    err = ioutil.WriteFile("nginx.conf", lines, 0644)
    if err != nil {
        panic(err)
    }
}

However, I find it a lot more difficult to add the same line that I removed again. My scenario is that I want to remove the line containing "server service-example2.example.com;", reload nginx conf. Later I want to add the same line at it's previous position and reload nginx again.

I find it difficult to get the exact byte position in the file where I want to add the line again with *(f File) WriteAt. I've been looking at this example: https://play.golang.org/p/eaWYAkxyLI, but cannot figure out a good way how to do it.

Anyone got any idea how I can solve this?