cloudflared-mirror/sshserver/sshserver_unix.go

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//+build !windows
package sshserver
import (
"bufio"
"errors"
"fmt"
"io"
"os"
"os/exec"
"os/user"
"strconv"
"syscall"
"time"
"unsafe"
"github.com/creack/pty"
"github.com/gliderlabs/ssh"
"github.com/sirupsen/logrus"
)
type SSHServer struct {
ssh.Server
logger *logrus.Logger
shutdownC chan struct{}
caCert ssh.PublicKey
getUserFunc func(string) (*User, error)
}
func New(logger *logrus.Logger, address string, shutdownC chan struct{}, shortLivedCertAuth bool, idleTimeout, maxTimeout time.Duration) (*SSHServer, error) {
currentUser, err := user.Current()
if err != nil {
return nil, err
}
if currentUser.Uid != "0" {
return nil, errors.New("cloudflared SSH server needs to run as root")
}
sshServer := SSHServer{
Server: ssh.Server{Addr: address, MaxTimeout: maxTimeout, IdleTimeout: idleTimeout},
logger: logger,
shutdownC: shutdownC,
getUserFunc: lookupUser,
}
if err := sshServer.configureHostKeys(); err != nil {
return nil, err
}
if shortLivedCertAuth {
caCert, err := getCACert()
if err != nil {
return nil, err
}
sshServer.caCert = caCert
sshServer.PublicKeyHandler = sshServer.shortLivedCertHandler
} else {
sshServer.PublicKeyHandler = sshServer.authorizedKeyHandler
}
return &sshServer, nil
}
func (s *SSHServer) Start() error {
s.logger.Infof("Starting SSH server at %s", s.Addr)
go func() {
<-s.shutdownC
if err := s.Close(); err != nil {
s.logger.WithError(err).Error("Cannot close SSH server")
}
}()
s.Handle(s.connectionHandler)
return s.ListenAndServe()
}
func (s *SSHServer) connectionHandler(session ssh.Session) {
// Get uid and gid of user attempting to login
sshUser, ok := session.Context().Value("sshUser").(*User)
if !ok || sshUser == nil {
s.logger.Error("Error retrieving credentials from session")
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s.CloseSession(session)
return
}
// Spawn shell under user
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cmd := exec.Command(sshUser.Shell)
ptyReq, winCh, isPty := session.Pty()
if !isPty {
if _, err := io.WriteString(session, "No PTY requested.\n"); err != nil {
s.logger.WithError(err).Error("No PTY requested: Failed to write to SSH session")
}
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s.CloseSession(session)
return
}
uidInt, err := stringToUint32(sshUser.Uid)
if err != nil {
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s.logger.WithError(err).Error("Invalid user")
s.CloseSession(session)
return
}
gidInt, err := stringToUint32(sshUser.Gid)
if err != nil {
s.logger.WithError(err).Error("Invalid user group")
s.CloseSession(session)
return
}
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cmd.SysProcAttr = &syscall.SysProcAttr{Credential: &syscall.Credential{Uid: uidInt, Gid: gidInt}}
cmd.Env = append(cmd.Env, fmt.Sprintf("TERM=%s", ptyReq.Term))
cmd.Env = append(cmd.Env, fmt.Sprintf("USER=%s", sshUser.Username))
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cmd.Env = append(cmd.Env, fmt.Sprintf("HOME=%s", sshUser.HomeDir))
cmd.Dir = sshUser.HomeDir
psuedoTTY, err := pty.Start(cmd)
if err != nil {
s.logger.WithError(err).Error("Failed to start pty session")
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s.CloseSession(session)
close(s.shutdownC)
return
}
// Handle terminal window size changes
go func() {
for win := range winCh {
if errNo := setWinsize(psuedoTTY, win.Width, win.Height); errNo != 0 {
s.logger.WithError(err).Error("Failed to set pty window size: ", err.Error())
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s.CloseSession(session)
close(s.shutdownC)
return
}
}
}()
// Write incoming commands to PTY
go func() {
if _, err := io.Copy(psuedoTTY, session); err != nil {
s.logger.WithError(err).Error("Failed to write incoming command to pty")
}
}()
pr, pw := io.Pipe()
scanner := bufio.NewScanner(pr)
go func() {
for scanner.Scan() {
s.logger.Info(scanner.Text())
}
}()
// Write outgoing command output to both the command recorder, and remote user
mw := io.MultiWriter(pw, session)
if _, err := io.Copy(mw, psuedoTTY); err != nil {
s.logger.WithError(err).Error("Failed to write command output to user")
}
if err := pw.Close(); err != nil {
s.logger.WithError(err).Error("Failed to close pipe writer")
}
if err := pr.Close(); err != nil {
s.logger.WithError(err).Error("Failed to close pipe reader")
}
// Wait for all resources associated with cmd to be released
// Returns error if shell exited with a non-zero status or received a signal
if err := cmd.Wait(); err != nil {
s.logger.WithError(err).Debug("Shell did not close correctly")
}
}
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func (s *SSHServer) CloseSession(session ssh.Session) {
if err := session.Exit(1); err != nil {
s.logger.WithError(err).Error("Failed to close SSH session")
}
}
// Sets PTY window size for terminal
func setWinsize(f *os.File, w, h int) syscall.Errno {
_, _, errNo := syscall.Syscall(syscall.SYS_IOCTL, f.Fd(), uintptr(syscall.TIOCSWINSZ),
uintptr(unsafe.Pointer(&struct{ h, w, x, y uint16 }{uint16(h), uint16(w), 0, 0})))
return errNo
}
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func stringToUint32(str string) (uint32, error) {
uid, err := strconv.ParseUint(str, 10, 32)
return uint32(uid), err
}