initial commit

README.md

@@ -0,0 +1,4 @@
+# nlup
+
+nlup is a small domain name lookup utility that interacts
+directly with the nameserver.

dns.go

@@ -0,0 +1,143 @@
+package main
+
+import (
+	"bufio"
+	"bytes"
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"net"
+	"os"
+	"strings"
+	"time"
+)
+
+const (
+	ARecord = 0x0001
+	INClass = 0x0001
+)
+
+const DNSPort = 53
+const Timeout = 2
+
+type dnsConn struct {
+	net.Conn
+}
+
+type header struct {
+	Id, Flags uint16
+	Qdcount   uint16
+	Ancount   uint16
+	Nscount   uint16
+	Arcount   uint16
+}
+
+type answer struct {
+	Ntype   byte
+	Ptr     uint16
+	Qtype   uint16
+	Class   uint16
+	Ttl     uint32
+	Addrlen uint16
+	Addr    uint32
+}
+
+func findNsAddr() (string, error) {
+	conf := []string{
+		"/etc/resolv.conf",
+	}
+
+	for _, i := range conf {
+		f, err := os.Open(i)
+
+		if err != nil {
+			return "", err
+		}
+
+		defer f.Close()
+		scanner := bufio.NewScanner(f)
+
+		for scanner.Scan() {
+			s := scanner.Text()
+
+			if s[0] == '#' {
+				continue
+			}
+
+			args := strings.Split(s, " ")
+			if args[0] == "nameserver" && len(args) > 1 {
+				return args[1], nil
+			}
+		}
+	}
+
+	return "", errors.New("nameserver not found")
+}
+
+func read(buf *bytes.Buffer, data any) error {
+	return binary.Read(buf, binary.BigEndian, data)
+}
+
+func write(buf *bytes.Buffer, data any) error {
+	return binary.Write(buf, binary.BigEndian, data)
+}
+
+func writeName(buf *bytes.Buffer, name string) (n int) {
+	for _, i := range strings.Split(name, ".") {
+		buf.WriteByte(byte(len(i)))
+		n++
+		buf.WriteString(i)
+		n += len(i)
+	}
+
+	buf.WriteByte(0)
+	return
+}
+
+func dial(server string) (dnsConn, error) {
+	c, err := net.Dial("udp", fmt.Sprintf("%s:%d", server, DNSPort))
+
+	if err == nil {
+		c.SetDeadline(time.Now().Add(Timeout * time.Second))
+	}
+
+	return dnsConn{c}, err
+}
+
+func (c dnsConn) sendQuery(name string) (answer, error) {
+	buf := &bytes.Buffer{}
+	ah := &header{}
+	a := &answer{}
+
+	h := header{
+		0x4c3f,
+		1 << 8, // recursion
+		0x0001, // single question
+		0x0000,
+		0x0000,
+		0x0000,
+	}
+
+	write(buf, h)
+	nc := writeName(buf, name)
+	write(buf, []uint16{ARecord, INClass})
+	if _, err := c.Write(buf.Bytes()); err != nil {
+		return *a, err
+	}
+
+	b := make([]byte, 64)
+	if _, err := c.Read(b); err != nil {
+		return *a, err
+	}
+
+	abuf := bytes.NewBuffer(b)
+	read(abuf, ah)
+	abuf.Next(nc + 4)
+	read(abuf, a)
+
+	if a.Addrlen != 4 {
+		return *a, errors.New("invalid name")
+	}
+
+	return *a, nil
+}

go.mod

@@ -0,0 +1,3 @@
+module github.com/rtcall/nlup
+
+go 1.20

main.go

@@ -0,0 +1,49 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+)
+
+func main() {
+	ns := flag.String("n", "", "nameserver to use")
+	v := flag.Bool("v", false, "verbose output")
+	flag.Parse()
+
+	if len(flag.Args()) == 0 {
+		fmt.Printf("usage: %s [-n nameserver] [-v] name\n", os.Args[0])
+		os.Exit(1)
+	}
+
+	if *ns == "" {
+		if s, err := findNsAddr(); err != nil {
+			fmt.Printf("error: %s\n", err)
+			os.Exit(1)
+		} else {
+			*ns = s
+		}
+	}
+
+	name := flag.Arg(0)
+	c, err := dial(*ns)
+	if err != nil {
+		fmt.Printf("error: %s\n", err)
+		os.Exit(1)
+	}
+
+	ans, err := c.sendQuery(name)
+	if err != nil {
+		fmt.Printf("error: %s\n", err)
+		os.Exit(1)
+	}
+
+	if *v {
+		fmt.Printf("nameserver: %s\ntype: %d\nttl: %d\n",
+			*ns, ans.Ntype, ans.Ttl)
+	}
+
+	fmt.Printf("%s: %d.%d.%d.%d\n", name,
+		(ans.Addr>>24)&0xff, (ans.Addr>>16)&0xff, (ans.Addr>>8)&0xff,
+		ans.Addr&0xff)
+}