ftp,sftp: add socks_proxy support for SOCKS5 proxies

Fixes #3558

lib/proxy/socks.go

@@ -0,0 +1,41 @@
+package proxy
+
+import (
+	"fmt"
+	"net"
+	"strings"
+
+	"golang.org/x/net/proxy"
+)
+
+// SOCKS5Dial dials a net.Conn using a SOCKS5 proxy server.
+// The socks5Proxy address can be in the form of [user:password@]host:port, [user@]host:port or just host:port if no auth is required.
+// It will optionally take a proxyDialer to dial the SOCKS5 proxy server. If nil is passed, it will use the default net.Dialer.
+func SOCKS5Dial(network, addr, socks5Proxy string, proxyDialer proxy.Dialer) (net.Conn, error) {
+
+	if proxyDialer == nil {
+		proxyDialer = &net.Dialer{}
+	}
+	var (
+		proxyAddress string
+		proxyAuth    *proxy.Auth
+	)
+	if credsAndHost := strings.SplitN(socks5Proxy, "@", 2); len(credsAndHost) == 2 {
+		proxyCreds := strings.SplitN(credsAndHost[0], ":", 2)
+		proxyAuth = &proxy.Auth{
+			User: proxyCreds[0],
+		}
+		if len(proxyCreds) == 2 {
+			proxyAuth.Password = proxyCreds[1]
+		}
+		proxyAddress = credsAndHost[1]
+	} else {
+		proxyAddress = credsAndHost[0]
+	}
+	proxyDialer, err := proxy.SOCKS5("tcp", proxyAddress, proxyAuth, proxyDialer)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create proxy dialer: %w", err)
+	}
+	return proxyDialer.Dial(network, addr)
+
+}