ipv6 header was stripped from the ip packets in packetConn read apis

[yusong0926]

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

$ go version
go version go1.14.1 darwin/amd64

Does this issue reproduce with the latest release?

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/songyu/Library/Caches/go-build"
GOENV="/Users/songyu/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/songyu/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/82/ywnjzhzn0bv_9w46jyyxp5kh0000gp/T/go-build674890034=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

tried all apis (ipConn, icmp packetConn), couldn't get ip header from the ip packets especially for ipv6 using read or readFrom api.

func main() {
conn, err := icmp.ListenPacket("ip6:ipv6-icmp", "::")
if err != nil {
log.Fatal(err)
}

for {
    var msg []byte
    length, sourceIP, err := conn.ReadFrom(msg)
   //msg was only the payload of the packets, ipv6 header was stripped from it. I want to get hopLimit from the header
    if err != nil {
        log.Println(err)
        continue
    }

    log.Printf("message = '%s', length = %d, source-ip = %s", string(msg), length, sourceIP)
}
_ = conn.Close()

}

What did you expect to see?

read ipv6 header and read hopLimit or ttl from ip header for icmp packets.

What did you see instead?

ipv6 header was stripped. Read or readfrom only receive ip payload.

[davecheney]

Can you please provide some sample code. Thank you.

[yusong0926]

@davecheney added

[acln0]

Go is only giving you what the operating system network stack is giving you. Here is the relevant code:

Here, package icmp selects the protocol for the ip6 transport. Here, it calls net.ListenPacket in order to create the socket. Through ListenConfig, it ends up calling listenIP on a *net.sysDialer. listenIP calls internetSocket, which is a thin wrapper around socket. Moving back up the stack, the netFD created in socket is wrapped in an *IPConn and returned as the net.PacketConn implementation from ListenPacket.

When you call ReadFrom on an *IPConn, the inner implementation is only a thin wrapper around (*poll.FD).ReadFrom, which, itself, is a thin wrapper around syscall.Recvfrom.

Based on this, it is my understanding that none of the Go code in your call stack strips IPv6 headers away. A C program which calls socket(), bind(), followed by recvfrom(), would also not return the IPv6 headers, as far as I can tell. Perhaps there is another way to tell the operating system to return this data to you, but I am not aware of it. Perhaps something connected with using recvmsg() and requesting it in the ancillary data section? I skimmed through the man pages for a little bit, but I couldn't find anything.

[davecheney]

@yusong0926 could you describe a little more what you mean by IPv6 header? Do you mean the ethernet frame data?

[seankhliao]

@yusong0926 L2/L3 headers are only accessible through raw sockets, you'll need to make syscalls to get them, as explained in https://css.bz/2016/12/08/go-raw-sockets.html

[davecheney]

Closing, answered. Unlike many projects, the Go project does not use GitHub Issues for general discussion or asking questions. GitHub Issues are used for tracking bugs and proposals only.

For asking questions, see:

• Stack Overflow with questions tagged "go"

• The Go Forum, a web-based forum

• Gophers Slack, use the invite app for access. The #general channel is a good starting point.

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