Issue

I need to connect to ftp server through proxy and download a file from Golang code. I've implemented simple client:

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)
    }
}

Unfortunately, I'm receiving 403 Forbidden while trying to connect. Normally I would assume that this is server restriction but it works completely fine through curl:

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

Does anyone have some idea why is that happening? As far as I saw, curl is also using http CONNECT under the hood, so I'm wondering what I am missing.

P.S. don't worry about credentials in code - these are publicly available for testing in the web

Solution

FTP over a HTTP proxy is not done with a CONNECT request. Instead the proxy is instructed to fetch a ftp://... URL with a normal GET request. This is also what curl is doing. Within Go this can be done like this (omitting error handling 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)

Answered By - Steffen Ullrich

Answer Checked By - David Marino (WPSolving Volunteer)

Sunday, January 28, 2024

[SOLVED] Connecting to the ftp by proxy from golang

Issue

Solution