sftp: add --sftp-http-proxy to connect via HTTP CONNECT proxy

lib/proxy/http.go

@@ -0,0 +1,75 @@
+package proxy
+
+import (
+	"bufio"
+	"crypto/tls"
+	"fmt"
+	"net"
+	"net/http"
+	"net/url"
+	"strings"
+
+	"golang.org/x/net/proxy"
+)
+
+// HTTPConnectDial connects using HTTP CONNECT via proxyDialer
+//
+// It will read the HTTP proxy address from the environment in the
+// standard way.
+//
+// It optionally takes a proxyDialer to dial the HTTP proxy server.
+// If nil is passed, it will use the default net.Dialer.
+func HTTPConnectDial(network, addr string, proxyURL *url.URL, proxyDialer proxy.Dialer) (net.Conn, error) {
+	if proxyDialer == nil {
+		proxyDialer = &net.Dialer{}
+	}
+	if proxyURL == nil {
+		return proxyDialer.Dial(network, addr)
+	}
+
+	// prepare proxy host with default ports
+	host := proxyURL.Host
+	if !strings.Contains(host, ":") {
+		if strings.EqualFold(proxyURL.Scheme, "https") {
+			host += ":443"
+		} else {
+			host += ":80"
+		}
+	}
+
+	// connect to proxy
+	conn, err := proxyDialer.Dial(network, host)
+	if err != nil {
+		return nil, fmt.Errorf("HTTP CONNECT proxy failed to Dial: %q", err)
+	}
+
+	// wrap TLS if HTTPS proxy
+	if strings.EqualFold(proxyURL.Scheme, "https") {
+		tlsConfig := &tls.Config{ServerName: proxyURL.Hostname()}
+		tlsConn := tls.Client(conn, tlsConfig)
+		if err := tlsConn.Handshake(); err != nil {
+			_ = conn.Close()
+			return nil, fmt.Errorf("HTTP CONNECT proxy failed to make TLS connection: %q", err)
+		}
+		conn = tlsConn
+	}
+
+	// send CONNECT
+	_, err = fmt.Fprintf(conn, "CONNECT %s HTTP/1.1\r\nHost: %s\r\n\r\n", addr, addr)
+	if err != nil {
+		_ = conn.Close()
+		return nil, fmt.Errorf("HTTP CONNECT proxy failed to send CONNECT: %q", err)
+	}
+	br := bufio.NewReader(conn)
+	req := &http.Request{URL: &url.URL{Scheme: "http", Host: addr}}
+	resp, err := http.ReadResponse(br, req)
+	if err != nil {
+		_ = conn.Close()
+		return nil, fmt.Errorf("HTTP CONNECT proxy failed to read response: %q", err)
+	}
+	if resp.StatusCode != http.StatusOK {
+		_ = conn.Close()
+		return nil, fmt.Errorf("HTTP CONNECT proxy failed: %s", resp.Status)
+	}
+	return conn, nil
+}