Connect to ftp through proxy from golang

php editor Zimo introduces you a method to connect to the Golang code of the FTP server by using a proxy. In some cases, we may need to use a proxy server when accessing the FTP server in order to increase security or resolve network restrictions. Golang is a powerful programming language with easy-to-use network libraries, so we can implement this feature easily. Next, we will use a simple sample code to demonstrate how to use Golang to connect to an FTP server through a proxy.

Question content

I need to connect to ftp server through proxy and download files from Golang code. I have implemented simple client:

<code>package main

import (
    "bufio"
    "fmt"
    "github.com/jlaffaye/ftp"
    "io"
    "log"
    "net"
    "net/http"
    "os"
    "strings"
    "time"
)

var targetAddr = "ftp.dlptest.com"
var proxyAddr = "146.19.106.109:3128"
var user = "dlpuser"
var pass = "rNrKYTX9g7z3RgJRmxWuGHbeu"
var path = "/xxx.txt"

func connectViaProxy(network, address string) (net.Conn, error) {

    parts := strings.Split(address, ":")
    addr := fmt.Sprintf("%s:%s", targetAddr, parts[1])

    proxyConn, err := net.Dial(network, proxyAddr)
    if err != nil {
        return nil, err
    }

    req, err := http.NewRequest(http.MethodConnect, "http://"+addr, nil)
    if err != nil {
        proxyConn.Close()
        return nil, err
    }

    req.Header.Set("Accept", "*/*")
    req.Header.Set("User-Agent", "curl/8.1.2")
    req.Header.Set("Host", targetAddr+":21")

    err = req.Write(proxyConn)
    if err != nil {
        return nil, err
    }

    resp, err := http.ReadResponse(bufio.NewReader(proxyConn), req)
    if err != nil {
        proxyConn.Close()
        return nil, err
    }
    if resp.StatusCode != 200 {

        x, _ := io.ReadAll(resp.Body)
        fmt.Println(string(x))

        proxyConn.Close()
        return nil, fmt.Errorf("non-200 status code received from proxy: %v", resp.Status)
    }

    return proxyConn, nil
}

func main() {
    c, err := ftp.Dial(targetAddr+":21",
        ftp.DialWithTimeout(5*time.Second),
        ftp.DialWithDebugOutput(os.Stdin),
        ftp.DialWithDialFunc(connectViaProxy),
    )
    if err != nil {
        log.Fatal(err)
    }

    err = c.Login(user, pass)
    if err != nil {
        log.Fatal(err)
    }

    if r, err := c.Retr(path); err != nil {
        fmt.Println(err, ":<")
    } else {
        if reader, err := io.ReadAll(r); err != nil {
            fmt.Println(err, ":(")
        } else {
            fmt.Println(string(reader))
        }
    }

    if err = c.Quit(); err != nil {
        log.Fatal(err)
    }
}

</code>

Unfortunately, I receive a 403 Forbidden when trying to connect. Normally I would assume this is a server limitation, but it works perfectly fine via curl:

curl -vvv -x 146.19.106.109:3128 -u dlpuser:rNrKYTX9g7z3RgJRmxWuGHbeu ftp://ftp.dlptest.com/xxx.txt

Does anyone know why this happens? As far as I know, curl also uses http CONNECT under the hood, so I'm wondering what I'm missing.

PS Don't worry about the credentials in the code - these are exposed for network testing

Workaround

FTP over the HTTP proxy is not done with a CONNECT request. Instead, the proxy is instructed to obtain the ftp://... URL using a normal GET request. This is also what curl is doing. In Go, it can be done like this (error handling omitted for simplicity):

// create the TCP connection to the proxy
conn, err := net.Dial("tcp", "146.19.106.109:3128")

// create the request with the ftp:// URL
req, err := http.NewRequest(http.MethodGet, "ftp://ftp.dlptest.com/xxx.txt", nil)
req.SetBasicAuth("dlpuser","rNrKYTX9g7z3RgJRmxWuGHbeu")

// don't use req.Write but req.WriteProxy, since the request line must
// contain the full URL and not only the path
err = req.WriteProxy(conn)

// read response
resp, err := http.ReadResponse(bufio.NewReader(conn), req)
body, _ := io.ReadAll(resp.Body)

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