wen-long · July 15, 2023 08:16 · Jul 29, 2022
diff --git a/dns.go b/dns.go
@@ -0,0 +1,92 @@
+package main
+
+import (
+	"fmt"
+	"math/rand"
+	"os"
+
+	"github.com/miekg/dns"
+)
+
+var (
+	ROOT_NAMESERVERS = []string{
+		"198.41.0.4", "199.9.14.201", "192.33.4.12", "199.7.91.13", "192.203.230.10",
+		"192.5.5.241", "192.112.36.4", "198.97.190.53", "192.36.148.17", "192.58.128.30",
+		"193.0.14.129", "199.7.83.42", "202.12.27.33",
+	}
+)
+
+func resolve(name string) ([]dns.RR, error) {
+	nameserver := ROOT_NAMESERVERS[rand.Intn(len(ROOT_NAMESERVERS))]
+	c := new(dns.Client)
+
+	for {
+		// Prepare a message asking for an A record (an IP address) for `name`
+		m := new(dns.Msg)
+		m.SetQuestion(dns.Fqdn(name), dns.TypeA)
+
+		// Send the DNS request to the IP in `nameserver`
+		fmt.Printf("Asking %s about %s\n", nameserver, name)
+		resp, _, err := c.Exchange(m, fmt.Sprintf("%s:53", nameserver))
+		if err != nil {
+			return nil, err
+		}
+
+		if len(resp.Answer) > 0 {
+			// If an ANSWER SECTION exists and contains a CNAME, recurse
+			if cname, ok := resp.Answer[0].(*dns.CNAME); ok {
+				return resolve(cname.Target)
+			}
+
+			// If an ANSWER SECTION exists, we're done
+			return resp.Answer, nil
+		}
+
+		// If the ADDITIONAL SECTION is empty and the AUTHORITY SECTION is not, resolve
+		// one of the names in the AUTHORITY SECTION and have that be the nameserver
+		if len(resp.Extra) == 0 && len(resp.Ns) != 0 {
+			ns := resp.Ns[0].(*dns.NS)
+			nsIP, err := resolve(ns.Ns)
+			if err != nil {
+				return nil, fmt.Errorf("break in the chain")
+			}
+			nameserver = nsIP[0].(*dns.A).A.String()
+		} else {
+			// If an ADDITIONAL SECTION exists, look in it for an A record for the
+			// next-level nameserver. If one doesn't exist, we have to error out
+			found := false
+			for _, rr := range resp.Extra {
+				record, ok := rr.(*dns.A)
+				if ok {
+					nameserver = record.A.String()
+					found = true
+					break
+				}
+			}
+			if !found {
+				return nil, fmt.Errorf("break in the chain")
+			}
+		}
+
+		// ... and recurse!
+	}
+
+}
+
+func main() {
+	if len(os.Args) != 2 {
+		fmt.Fprintf(os.Stderr, "usage: dns <name>\n")
+		os.Exit(1)
+	}
+
+	name := os.Args[1]
+	answer, err := resolve(name)
+	if err == nil {
+		for _, record := range answer {
+			fmt.Println(record)
+		}
+	} else {
+		fmt.Fprintf(os.Stderr, "Failed to resolve %s\n", name)
+		os.Exit(1)
+	}
+}