tshttpproxy: support synology proxy configuration

Fixes #4395
Fixes #2605

Signed-off-by: James Tucker <james@tailscale.com>

net/tshttpproxy/tshttpproxy_synology.go

@@ -0,0 +1,132 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build linux
+// +build linux
+
+package tshttpproxy
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"net"
+	"net/http"
+	"net/url"
+	"os"
+	"strings"
+	"sync"
+	"time"
+
+	"tailscale.com/util/lineread"
+)
+
+// These vars are overridden for tests.
+var (
+	synologyProxyConfigPath = "/etc/proxy.conf"
+
+	openSynologyProxyConf = func() (io.ReadCloser, error) {
+		return os.Open(synologyProxyConfigPath)
+	}
+)
+
+var cache struct {
+	sync.Mutex
+	proxy   *url.URL
+	updated time.Time
+}
+
+func synologyProxyFromConfigCached(req *http.Request) (*url.URL, error) {
+	if req.URL == nil {
+		return nil, nil
+	}
+
+	cache.Lock()
+	defer cache.Unlock()
+
+	modtime := mtime(synologyProxyConfigPath)
+
+	if cache.updated == modtime {
+		return cache.proxy, nil
+	}
+
+	val, err := synologyProxyFromConfig(req)
+	cache.proxy = val
+
+	cache.updated = modtime
+
+	return val, err
+}
+
+func synologyProxyFromConfig(req *http.Request) (*url.URL, error) {
+	r, err := openSynologyProxyConf()
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, nil
+		}
+		return nil, err
+	}
+	defer r.Close()
+
+	return parseSynologyConfig(r)
+}
+
+func parseSynologyConfig(r io.Reader) (*url.URL, error) {
+	cfg := map[string]string{}
+
+	if err := lineread.Reader(r, func(line []byte) error {
+		// accept and skip over empty lines
+		line = bytes.TrimSpace(line)
+		if len(line) == 0 {
+			return nil
+		}
+
+		key, value, ok := strings.Cut(string(line), "=")
+		if !ok {
+			return fmt.Errorf("missing \"=\" in proxy.conf line: %q", line)
+		}
+		cfg[string(key)] = string(value)
+		return nil
+	}); err != nil {
+		return nil, err
+	}
+
+	if cfg["proxy_enabled"] != "yes" {
+		return nil, nil
+	}
+
+	proxyURL := new(url.URL)
+	if cfg["auth_enabled"] == "yes" {
+		proxyURL.User = url.UserPassword(cfg["proxy_user"], cfg["proxy_pwd"])
+	}
+
+	proxyURL.Scheme = "https"
+	host, port := cfg["https_host"], cfg["https_port"]
+	if host == "" {
+		proxyURL.Scheme = "http"
+		host, port = cfg["http_host"], cfg["http_port"]
+	}
+
+	if host == "" {
+		return nil, nil
+	}
+
+	if port != "" {
+		proxyURL.Host = net.JoinHostPort(host, port)
+	} else {
+		proxyURL.Host = host
+	}
+
+	return proxyURL, nil
+}
+
+// mtime stat's path and returns it's modification time. If path does not exist,
+// it returns the unix epoch.
+func mtime(path string) time.Time {
+	fi, err := os.Stat(path)
+	if err != nil {
+		return time.Unix(0, 0)
+	}
+	return fi.ModTime()
+}