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tailscale/net/tlsdial/tlsdial.go

296 lines
9.5 KiB
Go

// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
// Package tlsdial generates tls.Config values and does x509 validation of
// certs. It bakes in the LetsEncrypt roots so even if the user's machine
// doesn't have TLS roots, we can at least connect to Tailscale's LetsEncrypt
// services. It's the unified point where we can add shared policy on outgoing
// TLS connections from the three places in the client that connect to Tailscale
// (logs, control, DERP).
package tlsdial
import (
"bytes"
"context"
"crypto/tls"
"crypto/x509"
"errors"
"fmt"
"log"
"net"
"net/http"
"os"
"sync"
"sync/atomic"
"time"
"tailscale.com/envknob"
"tailscale.com/health"
)
var counterFallbackOK int32 // atomic
// If SSLKEYLOGFILE is set, it's a file to which we write our TLS private keys
// in a way that WireShark can read.
//
// See https://developer.mozilla.org/en-US/docs/Mozilla/Projects/NSS/Key_Log_Format
var sslKeyLogFile = os.Getenv("SSLKEYLOGFILE")
var debug = envknob.RegisterBool("TS_DEBUG_TLS_DIAL")
// tlsdialWarningPrinted tracks whether we've printed a warning about a given
// hostname already, to avoid log spam for users with custom DERP servers,
// Headscale, etc.
var tlsdialWarningPrinted sync.Map // map[string]bool
// Config returns a tls.Config for connecting to a server.
// If base is non-nil, it's cloned as the base config before
// being configured and returned.
// If ht is non-nil, it's used to report health errors.
func Config(host string, ht *health.Tracker, base *tls.Config) *tls.Config {
var conf *tls.Config
if base == nil {
conf = new(tls.Config)
} else {
conf = base.Clone()
}
conf.ServerName = host
if n := sslKeyLogFile; n != "" {
f, err := os.OpenFile(n, os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0600)
if err != nil {
log.Fatal(err)
}
log.Printf("WARNING: writing to SSLKEYLOGFILE %v", n)
conf.KeyLogWriter = f
}
if conf.InsecureSkipVerify {
panic("unexpected base.InsecureSkipVerify")
}
if conf.VerifyConnection != nil {
panic("unexpected base.VerifyConnection")
}
// Set InsecureSkipVerify to prevent crypto/tls from doing its
// own cert verification, as do the same work that it'd do
// (with the baked-in fallback root) in the VerifyConnection hook.
conf.InsecureSkipVerify = true
conf.VerifyConnection = func(cs tls.ConnectionState) error {
// Perform some health checks on this certificate before we do
// any verification.
if ht != nil {
if certIsSelfSigned(cs.PeerCertificates[0]) {
// Self-signed certs are never valid.
ht.SetTLSConnectionError(cs.ServerName, fmt.Errorf("certificate is self-signed"))
} else {
// Ensure we clear any error state for this ServerName.
ht.SetTLSConnectionError(cs.ServerName, nil)
}
}
// First try doing x509 verification with the system's
// root CA pool.
opts := x509.VerifyOptions{
DNSName: cs.ServerName,
Intermediates: x509.NewCertPool(),
}
for _, cert := range cs.PeerCertificates[1:] {
opts.Intermediates.AddCert(cert)
}
_, errSys := cs.PeerCertificates[0].Verify(opts)
if debug() {
log.Printf("tlsdial(sys %q): %v", host, errSys)
}
// Always verify with our baked-in Let's Encrypt certificate,
// so we can log an informational message. This is useful for
// detecting SSL MiTM.
opts.Roots = bakedInRoots()
_, bakedErr := cs.PeerCertificates[0].Verify(opts)
if debug() {
log.Printf("tlsdial(bake %q): %v", host, bakedErr)
} else if bakedErr != nil {
if _, loaded := tlsdialWarningPrinted.LoadOrStore(host, true); !loaded {
if errSys == nil {
log.Printf("tlsdial: warning: server cert for %q is not a Let's Encrypt cert", host)
} else {
log.Printf("tlsdial: error: server cert for %q failed to verify and is not a Let's Encrypt cert", host)
}
}
}
if errSys == nil {
return nil
} else if bakedErr == nil {
atomic.AddInt32(&counterFallbackOK, 1)
return nil
}
return errSys
}
return conf
}
func certIsSelfSigned(cert *x509.Certificate) bool {
// A certificate is determined to be self-signed if the certificate's
// subject is the same as its issuer.
return bytes.Equal(cert.RawSubject, cert.RawIssuer)
}
// SetConfigExpectedCert modifies c to expect and verify that the server returns
// a certificate for the provided certDNSName.
//
// This is for user-configurable client-side domain fronting support,
// where we send one SNI value but validate a different cert.
func SetConfigExpectedCert(c *tls.Config, certDNSName string) {
if c.ServerName == certDNSName {
return
}
if c.ServerName == "" {
c.ServerName = certDNSName
return
}
if c.VerifyPeerCertificate != nil {
panic("refusing to override tls.Config.VerifyPeerCertificate")
}
// Set InsecureSkipVerify to prevent crypto/tls from doing its
// own cert verification, but do the same work that it'd do
// (but using certDNSName) in the VerifyPeerCertificate hook.
c.InsecureSkipVerify = true
c.VerifyConnection = nil
c.VerifyPeerCertificate = func(rawCerts [][]byte, _ [][]*x509.Certificate) error {
if len(rawCerts) == 0 {
return errors.New("no certs presented")
}
certs := make([]*x509.Certificate, len(rawCerts))
for i, asn1Data := range rawCerts {
cert, err := x509.ParseCertificate(asn1Data)
if err != nil {
return err
}
certs[i] = cert
}
opts := x509.VerifyOptions{
CurrentTime: time.Now(),
DNSName: certDNSName,
Intermediates: x509.NewCertPool(),
}
for _, cert := range certs[1:] {
opts.Intermediates.AddCert(cert)
}
_, errSys := certs[0].Verify(opts)
if debug() {
log.Printf("tlsdial(sys %q/%q): %v", c.ServerName, certDNSName, errSys)
}
if errSys == nil {
return nil
}
opts.Roots = bakedInRoots()
_, err := certs[0].Verify(opts)
if debug() {
log.Printf("tlsdial(bake %q/%q): %v", c.ServerName, certDNSName, err)
}
if err == nil {
return nil
}
return errSys
}
}
// NewTransport returns a new HTTP transport that verifies TLS certs using this
// package, including its baked-in LetsEncrypt fallback roots.
func NewTransport() *http.Transport {
return &http.Transport{
DialTLSContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
host, _, err := net.SplitHostPort(addr)
if err != nil {
return nil, err
}
var d tls.Dialer
d.Config = Config(host, nil, nil)
return d.DialContext(ctx, network, addr)
},
}
}
/*
letsEncryptX1 is the LetsEncrypt X1 root:
Certificate:
Data:
Version: 3 (0x2)
Serial Number:
82:10:cf:b0:d2:40:e3:59:44:63:e0:bb:63:82:8b:00
Signature Algorithm: sha256WithRSAEncryption
Issuer: C = US, O = Internet Security Research Group, CN = ISRG Root X1
Validity
Not Before: Jun 4 11:04:38 2015 GMT
Not After : Jun 4 11:04:38 2035 GMT
Subject: C = US, O = Internet Security Research Group, CN = ISRG Root X1
Subject Public Key Info:
Public Key Algorithm: rsaEncryption
RSA Public-Key: (4096 bit)
We bake it into the binary as a fallback verification root,
in case the system we're running on doesn't have it.
(Tailscale runs on some ancient devices.)
To test that this code is working on Debian/Ubuntu:
$ sudo mv /usr/share/ca-certificates/mozilla/ISRG_Root_X1.crt{,.old}
$ sudo update-ca-certificates
Then restart tailscaled. To also test dnsfallback's use of it, nuke
your /etc/resolv.conf and it should still start & run fine.
*/
const letsEncryptX1 = `
-----BEGIN CERTIFICATE-----
MIIFazCCA1OgAwIBAgIRAIIQz7DSQONZRGPgu2OCiwAwDQYJKoZIhvcNAQELBQAw
TzELMAkGA1UEBhMCVVMxKTAnBgNVBAoTIEludGVybmV0IFNlY3VyaXR5IFJlc2Vh
cmNoIEdyb3VwMRUwEwYDVQQDEwxJU1JHIFJvb3QgWDEwHhcNMTUwNjA0MTEwNDM4
WhcNMzUwNjA0MTEwNDM4WjBPMQswCQYDVQQGEwJVUzEpMCcGA1UEChMgSW50ZXJu
ZXQgU2VjdXJpdHkgUmVzZWFyY2ggR3JvdXAxFTATBgNVBAMTDElTUkcgUm9vdCBY
MTCCAiIwDQYJKoZIhvcNAQEBBQADggIPADCCAgoCggIBAK3oJHP0FDfzm54rVygc
h77ct984kIxuPOZXoHj3dcKi/vVqbvYATyjb3miGbESTtrFj/RQSa78f0uoxmyF+
0TM8ukj13Xnfs7j/EvEhmkvBioZxaUpmZmyPfjxwv60pIgbz5MDmgK7iS4+3mX6U
A5/TR5d8mUgjU+g4rk8Kb4Mu0UlXjIB0ttov0DiNewNwIRt18jA8+o+u3dpjq+sW
T8KOEUt+zwvo/7V3LvSye0rgTBIlDHCNAymg4VMk7BPZ7hm/ELNKjD+Jo2FR3qyH
B5T0Y3HsLuJvW5iB4YlcNHlsdu87kGJ55tukmi8mxdAQ4Q7e2RCOFvu396j3x+UC
B5iPNgiV5+I3lg02dZ77DnKxHZu8A/lJBdiB3QW0KtZB6awBdpUKD9jf1b0SHzUv
KBds0pjBqAlkd25HN7rOrFleaJ1/ctaJxQZBKT5ZPt0m9STJEadao0xAH0ahmbWn
OlFuhjuefXKnEgV4We0+UXgVCwOPjdAvBbI+e0ocS3MFEvzG6uBQE3xDk3SzynTn
jh8BCNAw1FtxNrQHusEwMFxIt4I7mKZ9YIqioymCzLq9gwQbooMDQaHWBfEbwrbw
qHyGO0aoSCqI3Haadr8faqU9GY/rOPNk3sgrDQoo//fb4hVC1CLQJ13hef4Y53CI
rU7m2Ys6xt0nUW7/vGT1M0NPAgMBAAGjQjBAMA4GA1UdDwEB/wQEAwIBBjAPBgNV
HRMBAf8EBTADAQH/MB0GA1UdDgQWBBR5tFnme7bl5AFzgAiIyBpY9umbbjANBgkq
hkiG9w0BAQsFAAOCAgEAVR9YqbyyqFDQDLHYGmkgJykIrGF1XIpu+ILlaS/V9lZL
ubhzEFnTIZd+50xx+7LSYK05qAvqFyFWhfFQDlnrzuBZ6brJFe+GnY+EgPbk6ZGQ
3BebYhtF8GaV0nxvwuo77x/Py9auJ/GpsMiu/X1+mvoiBOv/2X/qkSsisRcOj/KK
NFtY2PwByVS5uCbMiogziUwthDyC3+6WVwW6LLv3xLfHTjuCvjHIInNzktHCgKQ5
ORAzI4JMPJ+GslWYHb4phowim57iaztXOoJwTdwJx4nLCgdNbOhdjsnvzqvHu7Ur
TkXWStAmzOVyyghqpZXjFaH3pO3JLF+l+/+sKAIuvtd7u+Nxe5AW0wdeRlN8NwdC
jNPElpzVmbUq4JUagEiuTDkHzsxHpFKVK7q4+63SM1N95R1NbdWhscdCb+ZAJzVc
oyi3B43njTOQ5yOf+1CceWxG1bQVs5ZufpsMljq4Ui0/1lvh+wjChP4kqKOJ2qxq
4RgqsahDYVvTH9w7jXbyLeiNdd8XM2w9U/t7y0Ff/9yi0GE44Za4rF2LN9d11TPA
mRGunUHBcnWEvgJBQl9nJEiU0Zsnvgc/ubhPgXRR4Xq37Z0j4r7g1SgEEzwxA57d
emyPxgcYxn/eR44/KJ4EBs+lVDR3veyJm+kXQ99b21/+jh5Xos1AnX5iItreGCc=
-----END CERTIFICATE-----
`
var bakedInRootsOnce struct {
sync.Once
p *x509.CertPool
}
func bakedInRoots() *x509.CertPool {
bakedInRootsOnce.Do(func() {
p := x509.NewCertPool()
if !p.AppendCertsFromPEM([]byte(letsEncryptX1)) {
panic("bogus PEM")
}
bakedInRootsOnce.p = p
})
return bakedInRootsOnce.p
}