Add experimental support for provisioning users.

api/ssh.go

@@ -14,21 +14,24 @@ import (
 // SSHAuthority is the interface implemented by a SSH CA authority.
 type SSHAuthority interface {
 	SignSSH(key ssh.PublicKey, opts provisioner.SSHOptions, signOpts ...provisioner.SignOption) (*ssh.Certificate, error)
+	SignSSHAddUser(key ssh.PublicKey, cert *ssh.Certificate) (*ssh.Certificate, error)
 }
 
 // SignSSHRequest is the request body of an SSH certificate request.
 type SignSSHRequest struct {
-	PublicKey   []byte       `json:"publicKey"` //base64 encoded
-	OTT         string       `json:"ott"`
-	CertType    string       `json:"certType"`
-	Principals  []string     `json:"principals"`
-	ValidAfter  TimeDuration `json:"validAfter"`
-	ValidBefore TimeDuration `json:"validBefore"`
+	PublicKey        []byte       `json:"publicKey"` //base64 encoded
+	OTT              string       `json:"ott"`
+	CertType         string       `json:"certType,omitempty"`
+	Principals       []string     `json:"principals,omitempty"`
+	ValidAfter       TimeDuration `json:"validAfter,omitempty"`
+	ValidBefore      TimeDuration `json:"validBefore,omitempty"`
+	AddUserPublicKey []byte       `json:"addUserPublicKey,omitempty"`
 }
 
 // SignSSHResponse is the response object that returns the SSH certificate.
 type SignSSHResponse struct {
-	Certificate SSHCertificate `json:"crt"`
+	Certificate        SSHCertificate `json:"crt"`
+	AddUserCertificate SSHCertificate `json:"addUserCrt"`
 }
 
 // SSHCertificate represents the response SSH certificate.
@@ -53,6 +56,10 @@ func (c *SSHCertificate) UnmarshalJSON(data []byte) error {
 	if err := json.Unmarshal(data, &s); err != nil {
 		return errors.Wrap(err, "error decoding certificate")
 	}
+	if s == "" {
+		c.Certificate = nil
+		return nil
+	}
 	certData, err := base64.StdEncoding.DecodeString(s)
 	if err != nil {
 		return errors.Wrap(err, "error decoding certificate")
@@ -105,6 +112,15 @@ func (h *caHandler) SignSSH(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	var addUserPublicKey ssh.PublicKey
+	if body.AddUserPublicKey != nil {
+		addUserPublicKey, err = ssh.ParsePublicKey(body.AddUserPublicKey)
+		if err != nil {
+			WriteError(w, BadRequest(errors.Wrap(err, "error parsing addUserPublicKey")))
+			return
+		}
+	}
+
 	opts := provisioner.SSHOptions{
 		CertType:    body.CertType,
 		Principals:  body.Principals,
@@ -125,9 +141,19 @@ func (h *caHandler) SignSSH(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	var addUserCert *ssh.Certificate
+	if addUserPublicKey != nil && cert.CertType == ssh.UserCert && len(cert.ValidPrincipals) == 1 {
+		addUserCert, err = h.Authority.SignSSHAddUser(addUserPublicKey, cert)
+		if err != nil {
+			WriteError(w, Forbidden(err))
+			return
+		}
+	}
+
 	w.WriteHeader(http.StatusCreated)
 	// logCertificate(w, cert)
 	JSON(w, &SignSSHResponse{
-		Certificate: SSHCertificate{cert},
+		Certificate:        SSHCertificate{cert},
+		AddUserCertificate: SSHCertificate{addUserCert},
 	})
 }

authority/config.go

@@ -101,8 +101,10 @@ func (c *AuthConfig) Validate(audiences provisioner.Audiences) error {
 
 // SSHConfig contains the user and host keys.
 type SSHConfig struct {
-	HostKey string `json:"hostKey"`
-	UserKey string `json:"userKey"`
+	HostKey          string `json:"hostKey"`
+	UserKey          string `json:"userKey"`
+	AddUserPrincipal string `json:"addUserPrincipal"`
+	AddUserCommand   string `json:"addUserCommand"`
 }
 
 // LoadConfiguration parses the given filename in JSON format and returns the

authority/provisioner/sign_ssh_options.go

@@ -14,6 +14,9 @@ const (
 
 	// SSHHostCert is the string used to represent ssh.HostCert.
 	SSHHostCert = "host"
+
+	// sshProvisionerCommand is the provisioner command
+	sshProvisionerCommand = "sudo adduser --quiet --disabled-password --gecos '' %s 2>/dev/null ; nc -q0 localhost 22"
 )
 
 // SSHCertificateModifier is the interface used to change properties in an SSH
@@ -188,6 +191,18 @@ func (m *sshDefaultExtensionModifier) Modify(cert *ssh.Certificate) error {
 	}
 }
 
+type sshProvisionerExtensionModifier string
+
+func (m sshProvisionerExtensionModifier) Modify(cert *ssh.Certificate) error {
+	if cert.CertType == ssh.UserCert {
+		if cert.CriticalOptions == nil {
+			cert.CriticalOptions = make(map[string]string)
+		}
+		cert.CriticalOptions["force-command"] = fmt.Sprintf(sshProvisionerCommand, m)
+	}
+	return nil
+}
+
 // sshCertificateValidityModifier is a SSHCertificateModifier checks the
 // validity bounds, setting them if they are not provided. It will fail if a
 // CertType has not been set or is not valid.

authority/ssh.go

@@ -2,9 +2,7 @@ package authority
 
 import (
 	"crypto/rand"
-	"crypto/sha256"
 	"encoding/binary"
-	"encoding/hex"
 	"net/http"
 	"strings"
 
@@ -14,10 +12,17 @@ import (
 	"golang.org/x/crypto/ssh"
 )
 
-func generateSSHPublicKeyID(key ssh.PublicKey) string {
-	sum := sha256.Sum256(key.Marshal())
-	return strings.ToLower(hex.EncodeToString(sum[:]))
-}
+const (
+	// SSHAddUserPrincipal is the principal that will run the add user command.
+	// Defaults to "provisioner" but it can be changed in the configuration.
+	SSHAddUserPrincipal = "provisioner"
+
+	// SSHAddUserCommand is the default command to run to add a new user.
+	// Defaults to "sudo useradd -m <principal>; nc -q0 localhost 22" but it can be changed in the
+	// configuration. The string "<principal>" will be replace by the new
+	// principal to add.
+	SSHAddUserCommand = "sudo useradd -m <principal>; nc -q0 localhost 22"
+)
 
 // SignSSH creates a signed SSH certificate with the given public key and options.
 func (a *Authority) SignSSH(key ssh.PublicKey, opts provisioner.SSHOptions, signOpts ...provisioner.SignOption) (*ssh.Certificate, error) {
@@ -38,7 +43,7 @@ func (a *Authority) SignSSH(key ssh.PublicKey, opts provisioner.SSHOptions, sign
 		// validate the given SSHOptions
 		case provisioner.SSHCertificateOptionsValidator:
 			if err := o.Valid(opts); err != nil {
-				return nil, &apiError{err: err, code: http.StatusUnauthorized}
+				return nil, &apiError{err: err, code: http.StatusForbidden}
 			}
 		default:
 			return nil, &apiError{
@@ -50,12 +55,15 @@ func (a *Authority) SignSSH(key ssh.PublicKey, opts provisioner.SSHOptions, sign
 
 	nonce, err := randutil.ASCII(32)
 	if err != nil {
-		return nil, err
+		return nil, &apiError{err: err, code: http.StatusInternalServerError}
 	}
 
 	var serial uint64
 	if err := binary.Read(rand.Reader, binary.BigEndian, &serial); err != nil {
-		return nil, errors.Wrap(err, "error reading random number")
+		return nil, &apiError{
+			err:  errors.Wrap(err, "signSSH: error reading random number"),
+			code: http.StatusInternalServerError,
+		}
 	}
 
 	// Build base certificate with the key and some random values
@@ -67,13 +75,13 @@ func (a *Authority) SignSSH(key ssh.PublicKey, opts provisioner.SSHOptions, sign
 
 	// Use opts to modify the certificate
 	if err := opts.Modify(cert); err != nil {
-		return nil, err
+		return nil, &apiError{err: err, code: http.StatusForbidden}
 	}
 
 	// Use provisioner modifiers
 	for _, m := range mods {
 		if err := m.Modify(cert); err != nil {
-			return nil, &apiError{err: err, code: http.StatusInternalServerError}
+			return nil, &apiError{err: err, code: http.StatusForbidden}
 		}
 	}
 
@@ -108,7 +116,7 @@ func (a *Authority) SignSSH(key ssh.PublicKey, opts provisioner.SSHOptions, sign
 		}
 	default:
 		return nil, &apiError{
-			err:  errors.Errorf("unexpected ssh certificate type: %d", cert.CertType),
+			err:  errors.Errorf("signSSH: unexpected ssh certificate type: %d", cert.CertType),
 			code: http.StatusInternalServerError,
 		}
 	}
@@ -121,16 +129,111 @@ func (a *Authority) SignSSH(key ssh.PublicKey, opts provisioner.SSHOptions, sign
 	// Sign the certificate
 	sig, err := signer.Sign(rand.Reader, data)
 	if err != nil {
-		return nil, err
+		return nil, &apiError{
+			err:  errors.Wrap(err, "signSSH: error signing certificate"),
+			code: http.StatusInternalServerError,
+		}
 	}
 	cert.Signature = sig
 
 	// User provisioners validators
 	for _, v := range validators {
 		if err := v.Valid(cert); err != nil {
-			return nil, &apiError{err: err, code: http.StatusUnauthorized}
+			return nil, &apiError{err: err, code: http.StatusForbidden}
 		}
 	}
 
 	return cert, nil
 }
+
+// SignSSHAddUser signs a certificate that provisions a new user in a server.
+func (a *Authority) SignSSHAddUser(key ssh.PublicKey, subject *ssh.Certificate) (*ssh.Certificate, error) {
+	if a.sshCAUserCertSignKey == nil {
+		return nil, &apiError{
+			err:  errors.New("signSSHProxy: user certificate signing is not enabled"),
+			code: http.StatusNotImplemented,
+		}
+	}
+	if subject.CertType != ssh.UserCert {
+		return nil, &apiError{
+			err:  errors.New("signSSHProxy: certificate is not a user certificate"),
+			code: http.StatusForbidden,
+		}
+	}
+	if len(subject.ValidPrincipals) != 1 {
+		return nil, &apiError{
+			err:  errors.New("signSSHProxy: certificate does not have only one principal"),
+			code: http.StatusForbidden,
+		}
+	}
+
+	nonce, err := randutil.ASCII(32)
+	if err != nil {
+		return nil, &apiError{err: err, code: http.StatusInternalServerError}
+	}
+
+	var serial uint64
+	if err := binary.Read(rand.Reader, binary.BigEndian, &serial); err != nil {
+		return nil, &apiError{
+			err:  errors.Wrap(err, "signSSHProxy: error reading random number"),
+			code: http.StatusInternalServerError,
+		}
+	}
+
+	signer, err := ssh.NewSignerFromSigner(a.sshCAUserCertSignKey)
+	if err != nil {
+		return nil, &apiError{
+			err:  errors.Wrap(err, "signSSHProxy: error creating signer"),
+			code: http.StatusInternalServerError,
+		}
+	}
+
+	principal := subject.ValidPrincipals[0]
+	addUserPrincipal := a.getAddUserPrincipal()
+
+	cert := &ssh.Certificate{
+		Nonce:           []byte(nonce),
+		Key:             key,
+		Serial:          serial,
+		CertType:        ssh.UserCert,
+		KeyId:           principal + "-" + addUserPrincipal,
+		ValidPrincipals: []string{addUserPrincipal},
+		ValidAfter:      subject.ValidAfter,
+		ValidBefore:     subject.ValidBefore,
+		Permissions: ssh.Permissions{
+			CriticalOptions: map[string]string{
+				"force-command": a.getAddUserCommand(principal),
+			},
+		},
+		SignatureKey: signer.PublicKey(),
+	}
+
+	// Get bytes for signing trailing the signature length.
+	data := cert.Marshal()
+	data = data[:len(data)-4]
+
+	// Sign the certificate
+	sig, err := signer.Sign(rand.Reader, data)
+	if err != nil {
+		return nil, err
+	}
+	cert.Signature = sig
+	return cert, nil
+}
+
+func (a *Authority) getAddUserPrincipal() (cmd string) {
+	if a.config.SSH.AddUserPrincipal == "" {
+		return SSHAddUserPrincipal
+	}
+	return a.config.SSH.AddUserPrincipal
+}
+
+func (a *Authority) getAddUserCommand(principal string) string {
+	var cmd string
+	if a.config.SSH.AddUserCommand == "" {
+		cmd = SSHAddUserCommand
+	} else {
+		cmd = a.config.SSH.AddUserCommand
+	}
+	return strings.Replace(cmd, "<principal>", principal, -1)
+}