gists/projects/go-tools/goipgrep/internal/resolve/reverse.go

package resolve

import (
	"context"
	"net"
	"runtime"
	"sort"
	"strings"
	"sync"
	"time"
)

type ReverseResult struct {
	IP       string `json:"ip"`
	Hostname string `json:"hostname,omitempty"`
}

func ReverseLookupAll(ctx context.Context, ips []string, timeout time.Duration, jobs int) []ReverseResult {
	if jobs <= 0 {
		jobs = runtime.GOMAXPROCS(0)
	}

	in := make(chan string)
	out := make(chan ReverseResult)
	var wg sync.WaitGroup

	worker := func() {
		defer wg.Done()
		for ip := range in {
			r := ReverseResult{IP: ip}
			cctx, cancel := context.WithTimeout(ctx, timeout)
			names, err := net.DefaultResolver.LookupAddr(cctx, ip)
			cancel()
			if err == nil && len(names) > 0 {
				// trim trailing dot for readability
				r.Hostname = strings.TrimSuffix(names[0], ".")
			}
			select {
			case out <- r:
			case <-ctx.Done():
				return
			}
		}
	}

	wg.Add(jobs)
	for i := 0; i < jobs; i++ {
		go worker()
	}

	go func() {
		defer close(in)
		for _, ip := range ips {
			select {
			case in <- ip:
			case <-ctx.Done():
				return
			}
		}
	}()

	go func() {
		wg.Wait()
		close(out)
	}()

	var res []ReverseResult
	for r := range out {
		res = append(res, r)
	}

	sort.Slice(res, func(i, j int) bool { return res[i].IP < res[j].IP })
	return res
}