原始套接字未收到icmp响应

Question

I'm trying to send an icmp message whose TTL is just 1, and expect to receive a time exceeded message. that message does come(I see it from wireshark), but my program blocks on syscall.Recvfrom. Anyone knows why?
icmp.go

package main

import (
    "bytes"
    "encoding/binary"
    "fmt"
    "net"
    "os"
    "syscall"
)

type ICMP struct {
    Type       uint8
    Code       uint8
    Checksum   uint16
    Identifier uint16
    SeqNo      uint16
}

func Checksum(data []byte) uint16 {
    var (
        sum    uint32
        length int = len(data)
        index  int
    )

    for length > 1 {
        sum += uint32(data[index])<<8 + uint32(data[index+1])
        index += 2
        length -= 2
    }

    if length > 0 {
        sum += uint32(data[index])
    }

    sum += (sum >> 16)

    return uint16(^sum)
}

func main() {
    h := Header{
        Version:  4,
        Len:      20,
        TotalLen: 20 + 8,
        TTL:      1,
        Protocol: 1,
        //  Dst:
    }

    argc := len(os.Args)
    if argc < 2 {
        fmt.Println("usage: program + host")
        return
    }

    ipAddr, _ := net.ResolveIPAddr("ip", os.Args[1])
    h.Dst = ipAddr.IP

    icmpReq := ICMP{
        Type:       8,
        Code:       0,
        Identifier: 0,
        SeqNo:      0,
    }

    out, err := h.Marshal()
    if err != nil {
        fmt.Println("ip header error", err)
        return
    }

    var icmpBuf bytes.Buffer
    binary.Write(&icmpBuf, binary.BigEndian, icmpReq)
    icmpReq.Checksum = Checksum(icmpBuf.Bytes())

    icmpBuf.Reset()
    binary.Write(&icmpBuf, binary.BigEndian, icmpReq)

    fd, _ := syscall.Socket(syscall.AF_INET, syscall.SOCK_RAW, syscall.IPPROTO_RAW)
    addr := syscall.SockaddrInet4{
        Port: 0,
    }

    copy(addr.Addr[:], ipAddr.IP[12:16])
    pkg := append(out, icmpBuf.Bytes()...)

    fmt.Println("ip length", len(pkg))

    if err := syscall.Sendto(fd, pkg, 0, &addr); err != nil {
        fmt.Println("Sendto err:", err)
    }

    var recvBuf []byte
    if nBytes, rAddr, err := syscall.Recvfrom(fd, recvBuf, 0); err == nil {
        fmt.Printf("recv %d bytes from %v
", nBytes, rAddr)
    }
}

additionally, I use header.go and helper.go from https://github.com/golang/net/tree/master/ipv4

Answer 1

As Andy pointed out, the raw(7) man page says:

An IPPROTO_RAW socket is send only. If you really want to receive all IP packets, use a packet(7) socket with the ETH_P_IP protocol. Note that packet sockets don't reassemble IP fragments, unlike raw sockets.

I know I can receive ICMP reply if I set IPPROTO_ICMP as the protocol when I create the socket, but I need to set TTL to 1 which must be done in IP layer. Therefore I send the ICMP request with IPPROTO_RAW socket, after that I use net.ListenIP to receive ICMP messages. Here is the code:

package main

import (
    "bytes"
    "encoding/binary"
    "log"
    "net"
    "os"
    "syscall"
)

const icmpID uint16 = 43565 // use a magic number for now

type ICMP struct {
    Type       uint8
    Code       uint8
    Checksum   uint16
    Identifier uint16
    SeqNo      uint16
}

func Checksum(data []byte) uint16 {
    var (
        sum    uint32
        length int = len(data)
        index  int
    )

    for length > 1 {
        sum += uint32(data[index])<<8 + uint32(data[index+1])
        index += 2
        length -= 2
    }

    if length > 0 {
        sum += uint32(data[index])
    }

    sum += (sum >> 16)

    return uint16(^sum)
}

func main() {
    h := Header{
        Version:  4,
        Len:      20,
        TotalLen: 20 + 8,
        TTL:      1,
        Protocol: 1,
    }

    argc := len(os.Args)
    if argc < 2 {
        log.Println("usage: program + host")
        return
    }

    ipAddr, _ := net.ResolveIPAddr("ip", os.Args[1])
    h.Dst = ipAddr.IP

    icmpReq := ICMP{
        Type:       8,
        Code:       0,
        Identifier: icmpID,
        SeqNo:      1,
    }

    out, err := h.Marshal()
    if err != nil {
        log.Println("ip header error", err)
        return
    }

    var icmpBuf bytes.Buffer
    binary.Write(&icmpBuf, binary.BigEndian, icmpReq)
    icmpReq.Checksum = Checksum(icmpBuf.Bytes())

    icmpBuf.Reset()
    binary.Write(&icmpBuf, binary.BigEndian, icmpReq)

    fd, _ := syscall.Socket(syscall.AF_INET, syscall.SOCK_RAW, syscall.IPPROTO_RAW)
    addr := syscall.SockaddrInet4{
        Port: 0,
    }

    copy(addr.Addr[:], ipAddr.IP[12:16])
    pkg := append(out, icmpBuf.Bytes()...)

    if err := syscall.Sendto(fd, pkg, 0, &addr); err != nil {
        log.Println("Sendto err:", err)
    }

    laddr, err := net.ResolveIPAddr("ip4:icmp", "0.0.0.0")
    if err != nil {
        log.Fatal(err)

    }

    c, err := net.ListenIP("ip4:icmp", laddr)
    if err != nil {
        log.Fatal(err)
    }

    for {
        buf := make([]byte, 2048)
        n, raddr, err := c.ReadFrom(buf)
        if err != nil {
            log.Println(err)
            continue
        }
        icmpType := buf[0]
        if icmpType == 11 {
            if n == 36 { // Time exceeded messages
                // A time exceeded message contain IP header(20 bytes) and first 64 bits of the original payload
                id := binary.BigEndian.Uint16(buf[32:34])
                log.Println("recv id", id)
                if id == icmpID {
                    log.Println("recv Time Exceeded from", raddr)
                }
            }
        }
    }
}

Actually, I am writing a traceroute in go, if anyone is interested about that, the whole code is in github.