Why do ' i/o timeout' occur when I use codeberg.org/miekg/dns?

Question

I had problems using codeberg.org/miekg/dns. When the server starts and the client requests a DNS message, the client will output an i/o timeout. I have been investigating for a long time, but still haven't found a solution. The AI analysis indicates that the windows firewall has blocked tcp. This is the server code:

package main

import (
    "context"
    "fmt"
    "log"
    "net"

    "codeberg.org/miekg/dns"
    "codeberg.org/miekg/dns/dnsutil"
)

func main() {
    handleTCP("tcp")
    select {}
}
func handleTCP(net string) {
    go func() {
        ds := dns.NewServer()
        ds.Addr = ":6553"
        ds.Net = net
        ds.Handler = dns.HandlerFunc(handleRequest)
        if err := ds.ListenAndServe(); err != nil {
            log.Fatalf("Failed to start UDP server: %v", err)
        }
    }()
    log.Println("DNS server started on TCP :53")
}

func handleRequest(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) {
    m := new(dns.Msg)
    // dnsutil.SetReply(m, r)
    fmt.Printf("%#v\n", r.Question[0])
    name, t := dnsutil.Question(r)
    if len(r.Question) > 0 && t == dns.TypeA {
        mx := &dns.A{Hdr: dns.Header{Name: name, Class: dns.ClassINET}, A: net.IPv4(192, 168, 1, 1)}
        m.Answer = append(m.Answer, mx)
    }
    if err := m.Pack(); err != nil {
        log.Fatalf("Failed to pack DNS response: %v", err)
    }
    code, err := w.Write(m.Data)
    if err != nil {
        log.Fatalf("Failed to write DNS response: %v", err)
    }
    log.Println("write bytes : ", code)
}

This is the client code:

package main

import (
    "context"
    "fmt"

    "codeberg.org/miekg/dns"
    "codeberg.org/miekg/dns/dnsutil"
)

func main() {
    ctx := context.Background()
    m := new(dns.Msg)
    dnsutil.SetQuestion(m, "miek.nl.", dns.TypeA)
    m.Pack()

    c := new(dns.Client)
    resp, rtt, err := c.Exchange(ctx, m, "tcp", "127.0.0.1:6553")
    if err != nil {
        fmt.Println(err) // !!! writeto tcp 127.0.0.1:50714->127.0.0.1:6553: read tcp 127.0.0.1:50714->127.0.0.1:6553: i/o timeout
        return
    }
    fmt.Println(rtt)
    fmt.Println(resp.Answer)
}

Mr_Pink · Accepted Answer · 2025-11-25 15:57:56Z

You're not returning a complete TCP DNS response, so the client is waiting for more data. You need to set the full response headers, which you can do with the commented out dnsutil.SetReply. The response should then be written with WriteTo method, because there is more to a TCP response than simply returning the packed message bytes.

func handleRequest(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) {
    m := new(dns.Msg)
    dnsutil.SetReply(m, r)
    log.Printf("%#v\n", r.Question[0])
    name, t := dnsutil.Question(r)
    if len(r.Question) > 0 && t == dns.TypeA {
        mx := &dns.A{Hdr: dns.Header{Name: name, Class: dns.ClassINET}, A: net.IPv4(192, 168, 1, 1)}
        m.Answer = append(m.Answer, mx)
    }
    code, err := m.WriteTo(w)
    if err != nil {
        log.Fatalf("Failed to write DNS response: %v", err)
    }
    log.Println("write bytes : ", code)
}

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