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I' using go1.11 net/http and want to decect if a domain is ipv6-only.

What did you do?

I create my own DialContext because want I to detect if a domain is ipv6-only. code below

package main
import (
    "errors"
    "fmt"
    "net"
    "net/http"
    "syscall"
    "time"
)
func ModifiedTransport() {
    var MyTransport = &http.Transport{
        DialContext: (&net.Dialer{
            Timeout:   30 * time.Second,
            KeepAlive: 30 * time.Second,
            DualStack: false,
            Control: func(network, address string, c syscall.RawConn) error {
                if network == "ipv4" {
                    // I want to  cancel connection here client.Get("http://myexample.com") return a non-nil err.
                    return errors.New("you should not use ipv4")
                }
                return nil
            },
        }).DialContext,
        MaxIdleConns:          100,
        IdleConnTimeout:       90 * time.Second,
        TLSHandshakeTimeout:   10 * time.Second,
        ExpectContinueTimeout: 1 * time.Second,
    }
    var myClient = http.Client{Transport: MyTransport}
    resp, myerr := myClient.Get("http://www.github.com")
    if myerr != nil {
        fmt.Println("request error")
        return 
    }
    var buffer = make([]byte, 1000)
    resp.Body.Read(buffer)
    fmt.Println(string(buffer))
}
func main(){
    ModifiedTransport();
}

I do not now how to close the request even when I can get into network == "ipv4".

addition

Python can solve the question via Force requests to use IPv4 / IPv6. I do not know how to do it in golang. Could someone one help me? Thanks so much!

fang jinxu
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3 Answers3

1

The network passed to the Control function is either tcp4 for an IPv4 connection or tcp6 for an IPv6 connection, if you are making an outgoing TCP connection.

From the comments on type Dialer:

        // Network and address parameters passed to Control method are not
        // necessarily the ones passed to Dial. For example, passing "tcp" to Dial
        // will cause the Control function to be called with "tcp4" or "tcp6".

(In case of non-TCP connection, other strings are possible.)

Known networks are "tcp", "tcp4" (IPv4-only), "tcp6" (IPv6-only), "udp", "udp4" (IPv4-only), "udp6" (IPv6-only), "ip", "ip4" (IPv4-only), "ip6" (IPv6-only), "unix", "unixgram" and "unixpacket".

Michael Hampton
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  • Thanks for your reply. But Do you know how to configure http Transport/DialContext to make the Control only receive a ipv6 connection? – fang jinxu Oct 31 '18 at 05:37
1

Oh. I fixed the problem myself.

We could not configure force ipv6 connection because it is hard coded

...
if cm.scheme() == "https" && t.DialTLS != nil {
        var err error
        pconn.conn, err = t.DialTLS("tcp", cm.addr())
        if err != nil {
            return nil, wrapErr(err)
        }
...

( code here.)

I add a ipv6only flag for transport.go, a getTcpString() and it works.

(diff here)

fang jinxu
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0

I think even when the server is listening on "tcp6", client running net.Dial using "tcp" should work fine - in dual stack setups.

When I ran into this issue, the culprit was /etc/hosts. It had an entry for the host which was mapped to an ipv4 address instead of ipv6 address. This lead to connection refusal.

My test example as follows:

Client Code:

package main

import "net"
import "fmt"

func main() {
    conn, err := net.Dial("tcp", "hostname:9876")
    if err != nil {
        fmt.Println("Error in net.Dial", err)
        return
    }

    conn.Close()
    fmt.Println("Successful")
}

Server Code:

package main

import "net"
import "fmt"

func main() {
    lis, err := net.Listen("tcp6", ":9876")
    if err != nil {
        fmt.Println("Error in listen", err)
        return
    }

    for {
        _, err := lis.Accept()
        if err != nil {
            fmt.Println("Error in Accept", err)
            return
        }
    }
}

This code should work fine with clean /etc/hosts file on dual stack setups.

Thanks.

Amit Kulkarni
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