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known_hosts.go
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/
known_hosts.go
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// Copyright 2014 The fleet 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 ssh
import (
"bufio"
"bytes"
"crypto/md5"
"errors"
"fmt"
"net"
"os"
"path"
"strconv"
"strings"
gossh "golang.org/x/crypto/ssh"
"github.com/coreos/fleet/log"
"github.com/coreos/fleet/pkg"
)
const (
DefaultKnownHostsFile = "~/.fleetctl/known_hosts"
sshDefaultPort = 22 // ssh.h
sshHashDelim = "|" // hostfile.h
warningRemoteHostChanged = `@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that a host key has just been changed.
The fingerprint for the %v key sent by the remote host is
%v.
Please contact your system administrator.
Add correct host key in %v to get rid of this message.
Host key verification failed.
`
promptToTrustHost = `The authenticity of host '%v' can't be established.
%v key fingerprint is %v.
Are you sure you want to continue connecting (yes/no)? `
)
// askToTrustHost prompts the user to trust a new key fingerprint while connecting to a host
func askToTrustHost(addr, algo, fingerprint string) bool {
var ans string
fmt.Fprintf(os.Stderr, promptToTrustHost, addr, algo, fingerprint)
fmt.Scanf("%s\n", &ans)
ans = strings.ToLower(ans)
if ans != "yes" && ans != "y" {
return false
}
return true
}
var (
ErrUntrustHost = errors.New("unauthorized host")
ErrUnmatchKey = errors.New("host key mismatch")
)
// HostKeyChecker implements the gossh.HostKeyChecker interface
// It is used for key validation during the cryptographic handshake
type HostKeyChecker struct {
m HostKeyManager
trustHost func(addr, algo, fingerprint string) bool
}
// NewHostKeyChecker returns a new HostKeyChecker
func NewHostKeyChecker(m HostKeyManager) *HostKeyChecker {
return &HostKeyChecker{m, askToTrustHost}
}
// Returns public key algorithms of the remote host that are listed
// inside known_hosts
func (kc *HostKeyChecker) GetHostKeyAlgorithms(addr string) []string {
var results []string
remoteAddr, err := kc.addrToHostPort(addr)
if err != nil {
log.Errorf("Failed to parse address %v: %v", addr, err)
return nil
}
hostKeys, err := kc.m.GetHostKeys()
_, ok := err.(*os.PathError)
if err != nil && !ok {
log.Errorf("Failed to read known_hosts file %v: %v", kc.m.String(), err)
return nil
}
for pattern, keys := range hostKeys {
if !matchHost(remoteAddr, pattern) {
remoteIP, err := net.ResolveTCPAddr("tcp", addr)
if err != nil {
log.Errorf("Failed to resolve TCP address %v: %v", addr, err)
continue
}
ipAddr, err := kc.addrToHostPort(remoteIP.String())
if err != nil {
log.Errorf("Failed to parse address %v: %v", remoteIP.String(), err)
continue
}
if !matchHost(ipAddr, pattern) {
continue
}
}
for _, hostKey := range keys {
results = append(results, hostKey.Type())
}
}
return results
}
// Check is called during the handshake to check the server's public key for
// unexpected changes. The key argument is in SSH wire format. It can be parsed
// using ssh.ParsePublicKey. The address before DNS resolution is passed in the
// addr argument, so the key can also be checked against the hostname.
// It returns any error encountered while checking the public key. A nil return
// value indicates that the key was either successfully verified (against an
// existing known_hosts entry), or accepted by the user as a new key.
func (kc *HostKeyChecker) Check(addr string, remote net.Addr, key gossh.PublicKey) error {
remoteAddr, err := kc.addrToHostPort(remote.String())
if err != nil {
return err
}
algoStr := algoString(key.Type())
keyFingerprintStr := md5String(md5.Sum(key.Marshal()))
hostKeys, err := kc.m.GetHostKeys()
_, ok := err.(*os.PathError)
if err != nil && !ok {
log.Errorf("Failed to read known_hosts file %v: %v", kc.m.String(), err)
}
mismatched := false
for pattern, keys := range hostKeys {
if !matchHost(remoteAddr, pattern) {
continue
}
for _, hostKey := range keys {
// Any matching key is considered a success, irrespective of previous failures
if hostKey.Type() == key.Type() && bytes.Compare(hostKey.Marshal(), key.Marshal()) == 0 {
return nil
}
// TODO(jonboulle): could be super friendly like the OpenSSH client
// and note exactly which key failed (file + line number)
mismatched = true
}
}
if mismatched {
fmt.Fprintf(os.Stderr, warningRemoteHostChanged, algoStr, keyFingerprintStr, kc.m.String())
return ErrUnmatchKey
}
// If we get this far, we haven't matched on any of the hostname patterns,
// so it's considered a new key. Prompt the user to trust it.
if !kc.trustHost(remoteAddr, algoStr, keyFingerprintStr) {
fmt.Fprintln(os.Stderr, "Host key verification failed.")
return ErrUntrustHost
}
if err := kc.m.PutHostKey(remoteAddr, key); err != nil {
fmt.Fprintf(os.Stderr, "Failed to add the host to the list of known hosts (%v).\n", kc.m)
return nil
}
fmt.Fprintf(os.Stderr, "Warning: Permanently added '%v' (%v) to the list of known hosts.\n", remoteAddr, algoStr)
return nil
}
// addrToHostPort takes the given address and parses it into a string suitable
// for use in the 'hostnames' field in a known_hosts file. For more details,
// see the `SSH_KNOWN_HOSTS FILE FORMAT` section of `man 8 sshd`
func (kc *HostKeyChecker) addrToHostPort(a string) (string, error) {
if !strings.Contains(a, ":") {
// No port, so return unadulterated
return a, nil
}
host, p, err := net.SplitHostPort(a)
if err != nil {
log.Debugf("Unable to parse addr %s: %v", a, err)
return "", err
}
port, err := strconv.Atoi(p)
if err != nil {
log.Debugf("Error parsing port %s: %v", p, err)
return "", err
}
// Default port should be omitted from the entry.
// (see `put_host_port` in openssh/misc.c)
if port == 0 || port == sshDefaultPort {
// IPv6 addresses must be enclosed in square brackets
if strings.Contains(host, ":") {
host = fmt.Sprintf("[%s]", host)
}
return host, nil
}
return fmt.Sprintf("[%s]:%d", host, port), nil
}
// HostKeyManager defines an interface for managing "known hosts" keys
type HostKeyManager interface {
String() string
// GetHostKeys returns a map from host patterns to a list of PublicKeys
GetHostKeys() (map[string][]gossh.PublicKey, error)
// put new host key under management
PutHostKey(addr string, hostKey gossh.PublicKey) error
}
// HostKeyFile is an implementation of HostKeyManager that saves and loads
// "known hosts" keys from a file
type HostKeyFile struct {
path string
}
// NewHostKeyFile returns a new HostKeyFile using the given file path
func NewHostKeyFile(path string) *HostKeyFile {
return &HostKeyFile{pkg.ParseFilepath(path)}
}
func (f *HostKeyFile) String() string {
return f.path
}
func (f *HostKeyFile) GetHostKeys() (map[string][]gossh.PublicKey, error) {
in, err := os.Open(f.path)
if err != nil {
return nil, err
}
defer in.Close()
hostKeys := make(map[string][]gossh.PublicKey)
n := 0
s := bufio.NewScanner(in)
for s.Scan() {
n++
line := s.Bytes()
hosts, key, err := parseKnownHostsLine(line)
if err != nil {
log.Warningf("%v:%d - %v\n", f.path, n, err)
continue
}
if hosts == "" {
// Comment/empty line
continue
}
// It is permissible to have several lines for the same host name(s)
hostKeys[hosts] = append(hostKeys[hosts], key)
}
return hostKeys, nil
}
// parseKnownHostsLine parses a line from a known hosts file. It returns a
// string containing the hosts section of the line, a gossh.PublicKey parsed
// from the line, and any error encountered during the parsing.
func parseKnownHostsLine(line []byte) (string, gossh.PublicKey, error) {
// Skip any leading whitespace.
line = bytes.TrimLeft(line, "\t ")
// Skip comments and empty lines.
if bytes.HasPrefix(line, []byte("#")) || len(line) == 0 {
return "", nil, nil
}
// Skip markers.
if bytes.HasPrefix(line, []byte("@")) {
return "", nil, errors.New("marker functionality not implemented")
}
// Find the end of the host name(s) portion.
end := bytes.IndexAny(line, "\t ")
if end <= 0 {
return "", nil, errors.New("bad format (insufficient fields)")
}
hosts := string(line[:end])
keyBytes := line[end+1:]
// Check for hashed host names.
if strings.HasPrefix(hosts, sshHashDelim) {
return "", nil, errors.New("hashed hosts not implemented")
}
// Finally, actually try to extract the key.
key, _, _, _, err := gossh.ParseAuthorizedKey(keyBytes)
if err != nil {
return "", nil, fmt.Errorf("error parsing key: %v", err)
}
return hosts, key, nil
}
func (f *HostKeyFile) PutHostKey(addr string, hostKey gossh.PublicKey) error {
// Make necessary directories if needed
err := os.MkdirAll(path.Dir(f.path), 0700)
if err != nil {
return err
}
out, err := os.OpenFile(f.path, os.O_WRONLY|os.O_CREATE|os.O_APPEND, 0644)
if err != nil {
return err
}
defer out.Close()
_, err = out.Write(renderHostLine(addr, hostKey))
if err != nil {
return err
}
return nil
}
func renderHostLine(addr string, key gossh.PublicKey) []byte {
keyByte := gossh.MarshalAuthorizedKey(key)
// allocate line space in advance
length := len(addr) + 1 + len(keyByte)
line := make([]byte, 0, length)
w := bytes.NewBuffer(line)
w.Write([]byte(addr))
w.WriteByte(' ')
w.Write(keyByte)
return w.Bytes()
}
// algoString returns a short-name representation of an algorithm type
func algoString(algo string) string {
switch algo {
case gossh.KeyAlgoRSA:
return "RSA"
case gossh.KeyAlgoDSA:
return "DSA"
case gossh.KeyAlgoECDSA256, gossh.KeyAlgoECDSA384, gossh.KeyAlgoECDSA521:
return "ECDSA"
}
return algo
}
// md5String returns a formatted string representing the given md5Sum in hex
func md5String(md5Sum [16]byte) string {
md5Str := fmt.Sprintf("% x", md5Sum)
md5Str = strings.Replace(md5Str, " ", ":", -1)
return md5Str
}