starssh/agent_forward.go

package starssh

import (
	"errors"
	"fmt"
	"io"
	"strings"
	"time"

	"golang.org/x/crypto/ssh"
	sshagent "golang.org/x/crypto/ssh/agent"
)

var requestSSHAgentForwarding = func(session *ssh.Session) error {
	return sshagent.RequestAgentForwarding(session)
}

var routeSSHAgentForwarding = func(client *ssh.Client, keyring sshagent.Agent) error {
	return sshagent.ForwardToAgent(client, keyring)
}

var newSSHAgentForwarder = func(timeout time.Duration) (sshagent.Agent, io.Closer, error) {
	conn, err := dialSSHAgent(timeout)
	if err != nil {
		return nil, nil, wrapSSHAgentForwardingUnavailable(err)
	}
	if conn == nil {
		return nil, nil, wrapSSHAgentForwardingUnavailable(errors.New("empty agent connection"))
	}
	return sshagent.NewClient(conn), conn, nil
}

var errSSHAgentForwardingDenied = errors.New("ssh-agent forwarding request denied")
var errSSHAgentForwardingUnavailable = errors.New("ssh-agent forwarding unavailable")

func (s *StarSSH) RequestAgentForwarding(session *ssh.Session) error {
	if s == nil {
		return errors.New("ssh client is nil")
	}
	if session == nil {
		return errors.New("ssh session is nil")
	}
	if err := s.ensureAgentForwarding(); err != nil {
		return err
	}
	if err := requestSSHAgentForwarding(session); err != nil {
		if isSSHAgentForwardingDeniedError(err) {
			return fmt.Errorf("%w: %v", errSSHAgentForwardingDenied, err)
		}
		return err
	}
	return nil
}

func (s *StarSSH) maybeRequestAgentForwarding(session *ssh.Session) error {
	if s == nil || !s.LoginInfo.ForwardSSHAgent {
		return nil
	}
	err := s.RequestAgentForwarding(session)
	if isSSHAgentForwardingDeniedError(err) || isSSHAgentForwardingUnavailableError(err) {
		return nil
	}
	return err
}

func (s *StarSSH) ensureAgentForwarding() error {
	if s == nil {
		return errors.New("ssh client is nil")
	}

	s.agentForwardMu.Lock()
	defer s.agentForwardMu.Unlock()

	if s.agentForwarder != nil {
		return nil
	}

	client, err := s.requireSSHClient()
	if err != nil {
		return err
	}

	keyring, closer, err := newSSHAgentForwarder(effectiveDialTimeout(s.LoginInfo))
	if err != nil {
		return wrapSSHAgentForwardingUnavailable(err)
	}
	if s.closing.Load() {
		_ = closer.Close()
		return errSSHClientClosing
	}
	if err := routeSSHAgentForwarding(client, keyring); err != nil {
		_ = closer.Close()
		return err
	}
	if !s.canAttachAgentForwarder(client) {
		_ = closer.Close()
		return errSSHClientClosing
	}
	s.agentForwarder = closer
	return nil
}

func (s *StarSSH) takeAgentForwarder() io.Closer {
	if s == nil {
		return nil
	}

	s.agentForwardMu.Lock()
	defer s.agentForwardMu.Unlock()

	closer := s.agentForwarder
	s.agentForwarder = nil
	return closer
}

func isSSHAgentForwardingDeniedError(err error) bool {
	if err == nil {
		return false
	}
	if errors.Is(err, errSSHAgentForwardingDenied) {
		return true
	}
	message := strings.ToLower(err.Error())
	return strings.Contains(message, "forwarding request denied") ||
		strings.Contains(message, "agent forwarding disabled")
}

func isSSHAgentForwardingUnavailableError(err error) bool {
	if err == nil {
		return false
	}
	if errors.Is(err, errSSHAgentForwardingUnavailable) {
		return true
	}
	message := strings.ToLower(err.Error())
	return strings.Contains(message, "ssh-agent forwarding unavailable") ||
		strings.Contains(message, "ssh-agent unavailable")
}

func wrapSSHAgentForwardingUnavailable(err error) error {
	if err == nil {
		return nil
	}
	if errors.Is(err, errSSHAgentForwardingUnavailable) {
		return err
	}
	if errors.Is(err, errSSHAgentUnavailable) {
		return fmt.Errorf("%w: %w", errSSHAgentForwardingUnavailable, err)
	}
	return fmt.Errorf("%w: %v", errSSHAgentForwardingUnavailable, err)
}