Skip to content

Commit

Permalink
tls: Add distributed_stek module
Browse files Browse the repository at this point in the history
This migrates a feature that was previously reserved for enterprise
users, according to #2786.

TLS session ticket keys are sensitive, so they should be rotated on a
regular basis. Only Caddy does this by default. However, a cluster of
servers that rotate keys without synchronization will lose the benefits
of having sessions in the first place if the client is routed to a
different backend. This module coordinates STEK rotation in a fleet so
the same keys are used, and rotated, across the whole cluster. No other
server does this, but Twitter wrote about how they hacked together a
solution a few years ago:
https://blog.twitter.com/engineering/en_us/a/2013/forward-secrecy-at-twitter.html
  • Loading branch information
mholt committed Oct 10, 2019
1 parent 20fe9cf commit dedcfd4
Showing 1 changed file with 228 additions and 0 deletions.
228 changes: 228 additions & 0 deletions modules/caddytls/distributedstek/distributedstek.go
Original file line number Diff line number Diff line change
@@ -0,0 +1,228 @@
// Copyright 2015 Matthew Holt and The Caddy Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Package distributedstek provides TLS session ticket ephemeral
// keys (STEKs) in a distributed fashion by utilizing configured
// storage for locking and key sharing. This allows a cluster of
// machines to optimally resume TLS sessions in a load-balanced
// environment without any hassle. This is similar to what
// Twitter does, but without needing to rely on SSH, as it is
// built into the web server this way:
// https://blog.twitter.com/engineering/en_us/a/2013/forward-secrecy-at-twitter.html
package distributedstek

import (
"bytes"
"encoding/gob"
"encoding/json"
"fmt"
"log"
"time"

"github.com/caddyserver/caddy/v2"
"github.com/caddyserver/caddy/v2/modules/caddytls"
"github.com/mholt/certmagic"
)

func init() {
caddy.RegisterModule(Provider{})
}

// Provider implements a distributed STEK provider.
type Provider struct {
Storage json.RawMessage `json:"storage,omitempty"`

storage certmagic.Storage
stekConfig *caddytls.SessionTicketService
timer *time.Timer
}

// CaddyModule returns the Caddy module information.
func (Provider) CaddyModule() caddy.ModuleInfo {
return caddy.ModuleInfo{
Name: "tls.stek.distributed",
New: func() caddy.Module { return new(Provider) },
}
}

// Provision provisions s.
func (s *Provider) Provision(ctx caddy.Context) error {
// unpack the storage module to use, if different from the default
if s.Storage != nil {
val, err := ctx.LoadModuleInline("module", "caddy.storage", s.Storage)
if err != nil {
return fmt.Errorf("loading TLS storage module: %s", err)
}
cmStorage, err := val.(caddy.StorageConverter).CertMagicStorage()
if err != nil {
return fmt.Errorf("creating TLS storage configuration: %v", err)
}
s.storage = cmStorage
s.Storage = nil // allow GC to deallocate
}

// otherwise, use default storage
if s.storage == nil {
s.storage = ctx.Storage()
}

return nil
}

// Initialize sets the configuration for s and returns the starting keys.
func (s *Provider) Initialize(config *caddytls.SessionTicketService) ([][32]byte, error) {
// keep a reference to the config; we'll need it when rotating keys
s.stekConfig = config

dstek, err := s.getSTEK()
if err != nil {
return nil, err
}

// create timer for the remaining time on the interval;
// this timer is cleaned up only when rotate() returns
s.timer = time.NewTimer(time.Until(dstek.NextRotation))

return dstek.Keys, nil
}

// Next returns a channel which transmits the latest session ticket keys.
func (s *Provider) Next(doneChan <-chan struct{}) <-chan [][32]byte {
keysChan := make(chan [][32]byte)
go s.rotate(doneChan, keysChan)
return keysChan
}

func (s *Provider) loadSTEK() (distributedSTEK, error) {
var sg distributedSTEK
gobBytes, err := s.storage.Load(stekFileName)
if err != nil {
return sg, err // don't wrap, in case error is certmagic.ErrNotExist
}
dec := gob.NewDecoder(bytes.NewReader(gobBytes))
err = dec.Decode(&sg)
if err != nil {
return sg, fmt.Errorf("STEK gob corrupted: %v", err)
}
return sg, nil
}

func (s *Provider) storeSTEK(dstek distributedSTEK) error {
var buf bytes.Buffer
err := gob.NewEncoder(&buf).Encode(dstek)
if err != nil {
return fmt.Errorf("encoding STEK gob: %v", err)
}
err = s.storage.Store(stekFileName, buf.Bytes())
if err != nil {
return fmt.Errorf("storing STEK gob: %v", err)
}
return nil
}

// getSTEK locks and loads the current STEK from storage. If none
// currently exists, a new STEK is created and persisted. If the
// current STEK is outdated (NextRotation time is in the past),
// then it is rotated and persisted. The resulting STEK is returned.
func (s *Provider) getSTEK() (distributedSTEK, error) {
s.storage.Lock(stekLockName)
defer s.storage.Unlock(stekLockName)

// load the current STEKs from storage
dstek, err := s.loadSTEK()
if _, isNotExist := err.(certmagic.ErrNotExist); isNotExist {
// if there is none, then make some right away
dstek, err = s.rotateKeys(dstek)
if err != nil {
return dstek, fmt.Errorf("creating new STEK: %v", err)
}
} else if err != nil {
// some other error, that's a problem
return dstek, fmt.Errorf("loading STEK: %v", err)
} else if time.Now().After(dstek.NextRotation) {
// if current STEKs are outdated, rotate them
dstek, err = s.rotateKeys(dstek)
if err != nil {
return dstek, fmt.Errorf("rotating keys: %v", err)
}
}

return dstek, nil
}

// rotateKeys rotates the keys of oldSTEK and returns the new distributedSTEK
// with updated keys and timestamps. It stores the returned STEK in storage,
// so this function must only be called in a storage-provided lock.
func (s *Provider) rotateKeys(oldSTEK distributedSTEK) (distributedSTEK, error) {
var newSTEK distributedSTEK
var err error

newSTEK.Keys, err = s.stekConfig.RotateSTEKs(oldSTEK.Keys)
if err != nil {
return newSTEK, err
}

now := time.Now()
newSTEK.LastRotation = now
newSTEK.NextRotation = now.Add(time.Duration(s.stekConfig.RotationInterval))

err = s.storeSTEK(newSTEK)
if err != nil {
return newSTEK, err
}

return newSTEK, nil
}

// rotate rotates keys on a regular basis, sending each updated set of
// keys down keysChan, until doneChan is closed.
func (s *Provider) rotate(doneChan <-chan struct{}, keysChan chan<- [][32]byte) {
for {
select {
case <-s.timer.C:
dstek, err := s.getSTEK()
if err != nil {
// TODO: improve this handling
log.Printf("[ERROR] Loading STEK: %v", err)
continue
}

// send the updated keys to the service
keysChan <- dstek.Keys

// timer channel is already drained, so reset directly (see godoc)
s.timer.Reset(time.Until(dstek.NextRotation))

case <-doneChan:
// again, see godocs for why timer is stopped this way
if !s.timer.Stop() {
<-s.timer.C
}
return
}
}
}

type distributedSTEK struct {
Keys [][32]byte
LastRotation, NextRotation time.Time
}

const (
stekLockName = "stek_check"
stekFileName = "stek/stek.bin"
)

// Interface guard
var _ caddytls.STEKProvider = (*Provider)(nil)

0 comments on commit dedcfd4

Please sign in to comment.