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// Copyright 2017 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package knownhosts implements a parser for the OpenSSH known_hosts
// host key database, and provides utility functions for writing
// OpenSSH compliant known_hosts files.
package knownhosts
import (
"bufio"
"bytes"
"crypto/hmac"
"crypto/rand"
"crypto/sha1"
"encoding/base64"
"errors"
"fmt"
"io"
"net"
"os"
"strings"
"golang.org/x/crypto/ssh"
)
// See the sshd manpage
// (http://man.openbsd.org/sshd#SSH_KNOWN_HOSTS_FILE_FORMAT) for
// background.
type addr struct{ host, port string }
func (a *addr) String() string {
h := a.host
if strings.Contains(h, ":") {
h = "[" + h + "]"
}
return h + ":" + a.port
}
type matcher interface {
match(addr) bool
}
type hostPattern struct {
negate bool
addr addr
}
func (p *hostPattern) String() string {
n := ""
if p.negate {
n = "!"
}
return n + p.addr.String()
}
type hostPatterns []hostPattern
func (ps hostPatterns) match(a addr) bool {
matched := false
for _, p := range ps {
if !p.match(a) {
continue
}
if p.negate {
return false
}
matched = true
}
return matched
}
// See
// https://android.googlesource.com/platform/external/openssh/+/ab28f5495c85297e7a597c1ba62e996416da7c7e/addrmatch.c
// The matching of * has no regard for separators, unlike filesystem globs
func wildcardMatch(pat []byte, str []byte) bool {
for {
if len(pat) == 0 {
return len(str) == 0
}
if len(str) == 0 {
return false
}
if pat[0] == '*' {
if len(pat) == 1 {
return true
}
for j := range str {
if wildcardMatch(pat[1:], str[j:]) {
return true
}
}
return false
}
if pat[0] == '?' || pat[0] == str[0] {
pat = pat[1:]
str = str[1:]
} else {
return false
}
}
}
func (p *hostPattern) match(a addr) bool {
return wildcardMatch([]byte(p.addr.host), []byte(a.host)) && p.addr.port == a.port
}
type keyDBLine struct {
cert bool
matcher matcher
knownKey KnownKey
}
func serialize(k ssh.PublicKey) string {
return k.Type() + " " + base64.StdEncoding.EncodeToString(k.Marshal())
}
func (l *keyDBLine) match(a addr) bool {
return l.matcher.match(a)
}
type hostKeyDB struct {
// Serialized version of revoked keys
revoked map[string]*KnownKey
lines []keyDBLine
}
func newHostKeyDB() *hostKeyDB {
db := &hostKeyDB{
revoked: make(map[string]*KnownKey),
}
return db
}
func keyEq(a, b ssh.PublicKey) bool {
return bytes.Equal(a.Marshal(), b.Marshal())
}
// IsAuthorityForHost can be used as a callback in ssh.CertChecker
func (db *hostKeyDB) IsHostAuthority(remote ssh.PublicKey, address string) bool {
h, p, err := net.SplitHostPort(address)
if err != nil {
return false
}
a := addr{host: h, port: p}
for _, l := range db.lines {
if l.cert && keyEq(l.knownKey.Key, remote) && l.match(a) {
return true
}
}
return false
}
// IsRevoked can be used as a callback in ssh.CertChecker
func (db *hostKeyDB) IsRevoked(key *ssh.Certificate) bool {
_, ok := db.revoked[string(key.Marshal())]
return ok
}
const markerCert = "@cert-authority"
const markerRevoked = "@revoked"
func nextWord(line []byte) (string, []byte) {
i := bytes.IndexAny(line, "\t ")
if i == -1 {
return string(line), nil
}
return string(line[:i]), bytes.TrimSpace(line[i:])
}
func parseLine(line []byte) (marker, host string, key ssh.PublicKey, err error) {
if w, next := nextWord(line); w == markerCert || w == markerRevoked {
marker = w
line = next
}
host, line = nextWord(line)
if len(line) == 0 {
return "", "", nil, errors.New("knownhosts: missing host pattern")
}
// ignore the keytype as it's in the key blob anyway.
_, line = nextWord(line)
if len(line) == 0 {
return "", "", nil, errors.New("knownhosts: missing key type pattern")
}
keyBlob, _ := nextWord(line)
keyBytes, err := base64.StdEncoding.DecodeString(keyBlob)
if err != nil {
return "", "", nil, err
}
key, err = ssh.ParsePublicKey(keyBytes)
if err != nil {
return "", "", nil, err
}
return marker, host, key, nil
}
func (db *hostKeyDB) parseLine(line []byte, filename string, linenum int) error {
marker, pattern, key, err := parseLine(line)
if err != nil {
return err
}
if marker == markerRevoked {
db.revoked[string(key.Marshal())] = &KnownKey{
Key: key,
Filename: filename,
Line: linenum,
}
return nil
}
entry := keyDBLine{
cert: marker == markerCert,
knownKey: KnownKey{
Filename: filename,
Line: linenum,
Key: key,
},
}
if pattern[0] == '|' {
entry.matcher, err = newHashedHost(pattern)
} else {
entry.matcher, err = newHostnameMatcher(pattern)
}
if err != nil {
return err
}
db.lines = append(db.lines, entry)
return nil
}
func newHostnameMatcher(pattern string) (matcher, error) {
var hps hostPatterns
for _, p := range strings.Split(pattern, ",") {
if len(p) == 0 {
continue
}
var a addr
var negate bool
if p[0] == '!' {
negate = true
p = p[1:]
}
if len(p) == 0 {
return nil, errors.New("knownhosts: negation without following hostname")
}
var err error
if p[0] == '[' {
a.host, a.port, err = net.SplitHostPort(p)
if err != nil {
return nil, err
}
} else {
a.host, a.port, err = net.SplitHostPort(p)
if err != nil {
a.host = p
a.port = "22"
}
}
hps = append(hps, hostPattern{
negate: negate,
addr: a,
})
}
return hps, nil
}
// KnownKey represents a key declared in a known_hosts file.
type KnownKey struct {
Key ssh.PublicKey
Filename string
Line int
}
func (k *KnownKey) String() string {
return fmt.Sprintf("%s:%d: %s", k.Filename, k.Line, serialize(k.Key))
}
// KeyError is returned if we did not find the key in the host key
// database, or there was a mismatch. Typically, in batch
// applications, this should be interpreted as failure. Interactive
// applications can offer an interactive prompt to the user.
type KeyError struct {
// Want holds the accepted host keys. For each key algorithm,
// there can be one hostkey. If Want is empty, the host is
// unknown. If Want is non-empty, there was a mismatch, which
// can signify a MITM attack.
Want []KnownKey
}
func (u *KeyError) Error() string {
if len(u.Want) == 0 {
return "knownhosts: key is unknown"
}
return "knownhosts: key mismatch"
}
// RevokedError is returned if we found a key that was revoked.
type RevokedError struct {
Revoked KnownKey
}
func (r *RevokedError) Error() string {
return "knownhosts: key is revoked"
}
// check checks a key against the host database. This should not be
// used for verifying certificates.
func (db *hostKeyDB) check(address string, remote net.Addr, remoteKey ssh.PublicKey) error {
if revoked := db.revoked[string(remoteKey.Marshal())]; revoked != nil {
return &RevokedError{Revoked: *revoked}
}
host, port, err := net.SplitHostPort(remote.String())
if err != nil {
return fmt.Errorf("knownhosts: SplitHostPort(%s): %v", remote, err)
}
hostToCheck := addr{host, port}
if address != "" {
// Give preference to the hostname if available.
host, port, err := net.SplitHostPort(address)
if err != nil {
return fmt.Errorf("knownhosts: SplitHostPort(%s): %v", address, err)
}
hostToCheck = addr{host, port}
}
return db.checkAddr(hostToCheck, remoteKey)
}
// checkAddr checks if we can find the given public key for the
// given address. If we only find an entry for the IP address,
// or only the hostname, then this still succeeds.
func (db *hostKeyDB) checkAddr(a addr, remoteKey ssh.PublicKey) error {
// TODO(hanwen): are these the right semantics? What if there
// is just a key for the IP address, but not for the
// hostname?
// Algorithm => key.
knownKeys := map[string]KnownKey{}
for _, l := range db.lines {
if l.match(a) {
typ := l.knownKey.Key.Type()
if _, ok := knownKeys[typ]; !ok {
knownKeys[typ] = l.knownKey
}
}
}
keyErr := &KeyError{}
for _, v := range knownKeys {
keyErr.Want = append(keyErr.Want, v)
}
// Unknown remote host.
if len(knownKeys) == 0 {
return keyErr
}
// If the remote host starts using a different, unknown key type, we
// also interpret that as a mismatch.
if known, ok := knownKeys[remoteKey.Type()]; !ok || !keyEq(known.Key, remoteKey) {
return keyErr
}
return nil
}
// The Read function parses file contents.
func (db *hostKeyDB) Read(r io.Reader, filename string) error {
scanner := bufio.NewScanner(r)
lineNum := 0
for scanner.Scan() {
lineNum++
line := scanner.Bytes()
line = bytes.TrimSpace(line)
if len(line) == 0 || line[0] == '#' {
continue
}
if err := db.parseLine(line, filename, lineNum); err != nil {
return fmt.Errorf("knownhosts: %s:%d: %v", filename, lineNum, err)
}
}
return scanner.Err()
}
// New creates a host key callback from the given OpenSSH host key
// files. The returned callback is for use in
// ssh.ClientConfig.HostKeyCallback. By preference, the key check
// operates on the hostname if available, i.e. if a server changes its
// IP address, the host key check will still succeed, even though a
// record of the new IP address is not available.
func New(files ...string) (ssh.HostKeyCallback, error) {
db := newHostKeyDB()
for _, fn := range files {
f, err := os.Open(fn)
if err != nil {
return nil, err
}
defer f.Close()
if err := db.Read(f, fn); err != nil {
return nil, err
}
}
var certChecker ssh.CertChecker
certChecker.IsHostAuthority = db.IsHostAuthority
certChecker.IsRevoked = db.IsRevoked
certChecker.HostKeyFallback = db.check
return certChecker.CheckHostKey, nil
}
// Normalize normalizes an address into the form used in known_hosts
func Normalize(address string) string {
host, port, err := net.SplitHostPort(address)
if err != nil {
host = address
port = "22"
}
entry := host
if port != "22" {
entry = "[" + entry + "]:" + port
} else if strings.Contains(host, ":") && !strings.HasPrefix(host, "[") {
entry = "[" + entry + "]"
}
return entry
}
// Line returns a line to add append to the known_hosts files.
func Line(addresses []string, key ssh.PublicKey) string {
var trimmed []string
for _, a := range addresses {
trimmed = append(trimmed, Normalize(a))
}
return strings.Join(trimmed, ",") + " " + serialize(key)
}
// HashHostname hashes the given hostname. The hostname is not
// normalized before hashing.
func HashHostname(hostname string) string {
// TODO(hanwen): check if we can safely normalize this always.
salt := make([]byte, sha1.Size)
_, err := rand.Read(salt)
if err != nil {
panic(fmt.Sprintf("crypto/rand failure %v", err))
}
hash := hashHost(hostname, salt)
return encodeHash(sha1HashType, salt, hash)
}
func decodeHash(encoded string) (hashType string, salt, hash []byte, err error) {
if len(encoded) == 0 || encoded[0] != '|' {
err = errors.New("knownhosts: hashed host must start with '|'")
return
}
components := strings.Split(encoded, "|")
if len(components) != 4 {
err = fmt.Errorf("knownhosts: got %d components, want 3", len(components))
return
}
hashType = components[1]
if salt, err = base64.StdEncoding.DecodeString(components[2]); err != nil {
return
}
if hash, err = base64.StdEncoding.DecodeString(components[3]); err != nil {
return
}
return
}
func encodeHash(typ string, salt []byte, hash []byte) string {
return strings.Join([]string{"",
typ,
base64.StdEncoding.EncodeToString(salt),
base64.StdEncoding.EncodeToString(hash),
}, "|")
}
// See https://android.googlesource.com/platform/external/openssh/+/ab28f5495c85297e7a597c1ba62e996416da7c7e/hostfile.c#120
func hashHost(hostname string, salt []byte) []byte {
mac := hmac.New(sha1.New, salt)
mac.Write([]byte(hostname))
return mac.Sum(nil)
}
type hashedHost struct {
salt []byte
hash []byte
}
const sha1HashType = "1"
func newHashedHost(encoded string) (*hashedHost, error) {
typ, salt, hash, err := decodeHash(encoded)
if err != nil {
return nil, err
}
// The type field seems for future algorithm agility, but it's
// actually hardcoded in openssh currently, see
// https://android.googlesource.com/platform/external/openssh/+/ab28f5495c85297e7a597c1ba62e996416da7c7e/hostfile.c#120
if typ != sha1HashType {
return nil, fmt.Errorf("knownhosts: got hash type %s, must be '1'", typ)
}
return &hashedHost{salt: salt, hash: hash}, nil
}
func (h *hashedHost) match(a addr) bool {
return bytes.Equal(hashHost(Normalize(a.String()), h.salt), h.hash)
}
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