net: resolver add unexpected bytes to udp packet when wrapping the net.Conn #49750

ChenTanyi · 2021-11-23T11:30:29Z

What version of Go are you using (`go version`)?

$ go version
go version go1.17.3 linux/amd64

Does this issue reproduce with the latest release?

Yes, I tested in latest golang docker.

What operating system and processor architecture are you using (`go env`)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOENV="/root/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.17.3"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/root/test/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build2826491904=/tmp/go-build -gno-record-gcc-switches"
GOROOT/bin/go version: go version go1.17.3 linux/amd64
GOROOT/bin/go tool compile -V: compile version go1.17.3
uname -sr: Linux 5.4.0-1062-azure
/lib/x86_64-linux-gnu/libc.so.6: GNU C Library (Debian GLIBC 2.31-13+deb11u2) stable release version 2.31.

What did you do?

1. Start a simple udp echo server in localhost

#!/usr/bin/env python3
import socket

s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.bind(('', 8080))

while True:
    msg, addr = s.recvfrom(1024)
    print(msg)
    print(msg.hex(' '))

2. Run a dns resolver client, but redirect the dns udp packets to the echo server

package main

import (
	"context"
	"net"
	"time"
)

func main() {
	c := &net.Dialer{
		Timeout: 10 * time.Second,
	}
	r := &net.Resolver{
		PreferGo: true,
		Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
			return c.Dial("udp", "127.0.0.1:8080")
		},
	}
	ip, err := r.LookupHost(context.Background(), "www.google.com")
	if err != nil {
		panic(err)
	}
	println(ip[0])
}

3. Wrap the net.Conn and run again.

package main

import (
	"context"
	"net"
	"time"
)

type duplicateConn struct {
	conn net.Conn
}

func (d *duplicateConn) Read(b []byte) (n int, err error) {
	n, err = d.conn.Read(b)
	// if n > 0 {
	// 	fmt.Printf("Read %d: %x\n", n, b[:n])
	// }
	// if err != nil {
	// 	fmt.Printf("Read Error: %v\n", err)
	// }
	return
}

func (d *duplicateConn) Write(b []byte) (n int, err error) {
	n, err = d.conn.Write(b)
	// if n > 0 {
	// 	fmt.Printf("Write %d: %x\n", n, b[:n])
	// }
	// if err != nil {
	// 	fmt.Printf("Write Error: %v\n", err)
	// }
	return
}

func (d *duplicateConn) Close() error {
	return d.conn.Close()
}

func (d *duplicateConn) LocalAddr() net.Addr {
	return d.conn.LocalAddr()
}

func (d *duplicateConn) RemoteAddr() net.Addr {
	return d.conn.RemoteAddr()
}

func (d *duplicateConn) SetDeadline(t time.Time) error {
	return d.conn.SetDeadline(t)
}

func (d *duplicateConn) SetReadDeadline(t time.Time) error {
	return d.conn.SetReadDeadline(t)
}

func (d *duplicateConn) SetWriteDeadline(t time.Time) error {
	return d.conn.SetWriteDeadline(t)
}

func main() {
	c := &net.Dialer{
		Timeout: 10 * time.Second,
	}
	r := &net.Resolver{
		PreferGo: true,
		Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
			conn, err := c.Dial("udp", "127.0.0.1:8080")
			return &duplicateConn{conn: conn}, err
		},
	}
	ip, err := r.LookupHost(context.Background(), "www.google.com")
	if err != nil {
		panic(err)
	}
	println(ip[0])
}

4. Test wrapping net.Conn only, which means the problem is not come from the struct wrapping.

package main

import (
	"net"
	"time"
)

type duplicateConn struct {
	conn net.Conn
}

func (d *duplicateConn) Read(b []byte) (n int, err error) {
	n, err = d.conn.Read(b)
	// if n > 0 {
	// 	fmt.Printf("Read %d: %x\n", n, b[:n])
	// }
	// if err != nil {
	// 	fmt.Printf("Read Error: %v\n", err)
	// }
	return
}

func (d *duplicateConn) Write(b []byte) (n int, err error) {
	n, err = d.conn.Write(b)
	// if n > 0 {
	// 	fmt.Printf("Write %d: %s\n", n, b[:n])
	// }
	// if err != nil {
	// 	fmt.Printf("Write Error: %v\n", err)
	// }
	return
}

func (d *duplicateConn) Close() error {
	return d.conn.Close()
}

func (d *duplicateConn) LocalAddr() net.Addr {
	return d.conn.LocalAddr()
}

func (d *duplicateConn) RemoteAddr() net.Addr {
	return d.conn.RemoteAddr()
}

func (d *duplicateConn) SetDeadline(t time.Time) error {
	return d.conn.SetDeadline(t)
}

func (d *duplicateConn) SetReadDeadline(t time.Time) error {
	return d.conn.SetReadDeadline(t)
}

func (d *duplicateConn) SetWriteDeadline(t time.Time) error {
	return d.conn.SetWriteDeadline(t)
}

func main() {
	c := &net.Dialer{
		Timeout: 10 * time.Second,
	}

	conn, err := c.Dial("udp", "127.0.0.1:8080")
	if err != nil {
		panic(err)
	}
	d := &duplicateConn{conn: conn}
	d.Write([]byte("test"))
	d.Write([]byte{1, 240, 192})
}

What did you expect to see?

Both 2 and 3 send valid dns packet to echo server or dns server.

For 4, echo server receives same data as I sent, which is expected.

What did you see instead?

The data sent by 2 is a valid dns packet. If I change destination to 8.8.8.8:53, I could get the dns result.
The data sent by 3 is not a valid dns packet. If I change destination to 8.8.8.8:53, I can't get the dns result.

The following is the data received from echo server, I found the invalid data would always have two more bytes in the front of the packet. These two bytes seems to be the length of the following bytes whenever I change the hostname.

b'\x8ax\x01\x00\x00\x01\x00\x00\x00\x00\x00\x00\x03www\x06google\x03com\x07default\x03svc\x07cluster\x05local\x00\x00\x1c\x00\x01'
8a 78 01 00 00 01 00 00 00 00 00 00 03 77 77 77 06 67 6f 6f 67 6c 65 03 63 6f 6d 07 64 65 66 61 75 6c 74 03 73 76 63 07 63 6c 75 73 74 65 72 05 6c 6f 63 61 6c 00 00 1c 00 01
b'\\y\x01\x00\x00\x01\x00\x00\x00\x00\x00\x00\x03www\x06google\x03com\x07default\x03svc\x07cluster\x05local\x00\x00\x01\x00\x01'
5c 79 01 00 00 01 00 00 00 00 00 00 03 77 77 77 06 67 6f 6f 67 6c 65 03 63 6f 6d 07 64 65 66 61 75 6c 74 03 73 76 63 07 63 6c 75 73 74 65 72 05 6c 6f 63 61 6c 00 00 01 00 01
b'\x00:\x86\xf9\x01\x00\x00\x01\x00\x00\x00\x00\x00\x00\x03www\x06google\x03com\x07default\x03svc\x07cluster\x05local\x00\x00\x1c\x00\x01'
00 3a 86 f9 01 00 00 01 00 00 00 00 00 00 03 77 77 77 06 67 6f 6f 67 6c 65 03 63 6f 6d 07 64 65 66 61 75 6c 74 03 73 76 63 07 63 6c 75 73 74 65 72 05 6c 6f 63 61 6c 00 00 1c 00 01
b'\x00:K_\x01\x00\x00\x01\x00\x00\x00\x00\x00\x00\x03www\x06google\x03com\x07default\x03svc\x07cluster\x05local\x00\x00\x01\x00\x01'
00 3a 4b 5f 01 00 00 01 00 00 00 00 00 00 03 77 77 77 06 67 6f 6f 67 6c 65 03 63 6f 6d 07 64 65 66 61 75 6c 74 03 73 76 63 07 63 6c 75 73 74 65 72 05 6c 6f 63 61 6c 00 00 01 00 01

The first 4 lines is valid dns packet, length 58.
The last 4 line is invalid dns packet, length 60, while 003a is 58 in OCT.

The text was updated successfully, but these errors were encountered:

heschi · 2021-11-24T19:26:33Z

cc @ianlancetaylor @neild

davecheney · 2021-11-24T20:02:55Z

In 3 the dialer will always use udp regardless of the network requested. This may cause a problem if the dns resolver was trying to to use tcp because the udp response was truncated

ChenTanyi · 2021-11-25T04:41:30Z

@davecheney Thanks for advice! I would use network directly in dailer for further usage.
As the test in this issue, I've printed the network and address before c.Dail. They are always "udp", so I simplify it.

mateusz834 · 2022-10-17T19:05:49Z

You must implement net.PacketConn to send udp requests.

go/src/net/dnsclient_unix.go

Lines 176 to 188 in b65e259

    
           c, err := r.dial(ctx, network, server) 
        
           if err != nil { 
        
           	return dnsmessage.Parser{}, dnsmessage.Header{}, err 
        
           } 
        
           if d, ok := ctx.Deadline(); ok && !d.IsZero() { 
        
           	c.SetDeadline(d) 
        
           } 
        
           var p dnsmessage.Parser 
        
           var h dnsmessage.Header 
        
           if _, ok := c.(PacketConn); ok { 
        
           	p, h, err = dnsPacketRoundTrip(c, id, q, udpReq) 
        
           } else { 
        
           	p, h, err = dnsStreamRoundTrip(c, id, q, tcpReq)

mateusz834 · 2022-10-17T19:22:03Z

Do we want to change the logic and use the network (variable from the loop) instead of checking if it implements net.PacketConn (simple if network == "udp")?

go/src/net/dnsclient_unix.go

Lines 166 to 172 in b65e259

    
           var networks []string 
        
           if useTCP { 
        
           	networks = []string{"tcp"} 
        
           } else { 
        
           	networks = []string{"udp", "tcp"} 
        
           } 
        
           for _, network := range networks {

@ianlancetaylor @neild

ChenTanyi · 2022-10-18T14:06:36Z

Thanks! @mateusz834

I test again with the net.PacketConn implementation and it works! As I hardly used UDP, I didn't realize about net.PacketConn in the testing before.
Maybe the implementation could be more clear like return error for "udp" if the net.Conn is not net.PacketConn.

Anyway, I'd close this issue as it is resolved. Thanks again!

heschi added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Nov 24, 2021

heschi added this to the Backlog milestone Nov 24, 2021

ChenTanyi closed this as completed Oct 18, 2022

golang locked and limited conversation to collaborators Oct 18, 2023

gopherbot added the FrozenDueToAge label Oct 18, 2023

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net: resolver add unexpected bytes to udp packet when wrapping the net.Conn #49750

net: resolver add unexpected bytes to udp packet when wrapping the net.Conn #49750

ChenTanyi commented Nov 23, 2021

heschi commented Nov 24, 2021

davecheney commented Nov 24, 2021

ChenTanyi commented Nov 25, 2021

mateusz834 commented Oct 17, 2022 •

edited

mateusz834 commented Oct 17, 2022 •

edited

ChenTanyi commented Oct 18, 2022

net: resolver add unexpected bytes to udp packet when wrapping the net.Conn #49750

net: resolver add unexpected bytes to udp packet when wrapping the net.Conn #49750

Comments

ChenTanyi commented Nov 23, 2021

What version of Go are you using (go version)?

Does this issue reproduce with the latest release?

What operating system and processor architecture are you using (go env)?

What did you do?

What did you expect to see?

What did you see instead?

heschi commented Nov 24, 2021

davecheney commented Nov 24, 2021

ChenTanyi commented Nov 25, 2021

mateusz834 commented Oct 17, 2022 • edited

mateusz834 commented Oct 17, 2022 • edited

ChenTanyi commented Oct 18, 2022

What version of Go are you using (`go version`)?

What operating system and processor architecture are you using (`go env`)?

mateusz834 commented Oct 17, 2022 •

edited

mateusz834 commented Oct 17, 2022 •

edited