Use ephemeral SSH keys per rebuild instead of static config keys

Generate a fresh ed25519 key pair at rebuild time, inject the public key
into the Proxmox answer file, use the private key for cluster join over
SSH, then remove the key from both the remote host and the database.
This eliminates the need to manage static SSH keys in config/secrets.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -53,7 +53,6 @@ Edit `provisioning.yaml`:
 - `pxe.subnet` — LAN CIDR for proxy-DHCP
 - `proxmox.existing_node` — IP of any current cluster member
 - `proxmox.join_fingerprint` — from `pvecm status` on an existing node
-- `credentials.ssh_public_key` — public key injected into new hosts
 - `credentials.root_password_hash` — `mkpasswd -m sha-512`
 - `infrastructure.base_url` — URL of the Infrastructure service
 - `infrastructure.server_type_map` — maps local type keys to Infrastructure IDs

deploy/provisioning.example.yaml

@@ -22,8 +22,6 @@ proxmox:
   join_fingerprint: "AA:BB:CC:DD:EE:FF:00:11:22:33:44:55:66:77:88:99:AA:BB:CC:DD:EE:FF:00:11:22:33:44:55:66:77:88:99"
 
 credentials:
-  ssh_private_key_path: "/etc/provisioning/keys/id_ed25519"
-  ssh_public_key: "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAEXAMPLE provisioning@homelab"
   root_password_hash: "$6$rounds=5000$randomsalt$hashedpasswordhere"
 
 infrastructure:

internal/api/boot.go

@@ -82,7 +82,13 @@ func (a *BootAPI) AnswerFile(w http.ResponseWriter, r *http.Request) {
 		a.Runner.Transition(r.Context(), host.ID, statemachine.TriggerAnswerServed)
 	}
 
-	answer := pxe.GenerateAnswerFile(host, st, a.Config)
+	_, pubKey, _ := a.Hosts.GetEphemeralKey(r.Context(), host.ID)
+	if pubKey == "" {
+		http.Error(w, "no ephemeral key for host", http.StatusInternalServerError)
+		return
+	}
+
+	answer := pxe.GenerateAnswerFile(host, st, a.Config, pubKey)
 	w.Header().Set("Content-Type", "application/toml")
 	w.Write([]byte(answer))
 }

internal/api/hosts.go

@@ -23,6 +23,7 @@ type HostAPI struct {
 	Locks        *store.Locks
 	Images       *store.Images
 	Runner       *orchestrator.Runner
+	Orchestrator *orchestrator.HostOrchestrator
 	PXE          *pxe.Supervisor
 	Config       *config.Config
 	ServerTypes  *config.ServerTypeRegistry
@@ -124,6 +125,12 @@ func (a *HostAPI) Rebuild(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	if err := a.Orchestrator.PrepareRebuild(r.Context(), host.ID); err != nil {
+		_ = a.Locks.Release(r.Context(), host.ID)
+		writeJSONErr(w, http.StatusInternalServerError, "failed to generate SSH key: "+err.Error())
+		return
+	}
+
 	if _, err := a.Runner.Transition(r.Context(), host.ID, statemachine.TriggerRebuildRequested); err != nil {
 		_ = a.Locks.Release(r.Context(), host.ID)
 		writeJSONErr(w, http.StatusConflict, err.Error())

internal/api/smoke_test.go

@@ -58,17 +58,6 @@ func newTestServer(t *testing.T) *httptest.Server {
 
 	pxeSupervisor := pxe.NewSupervisor(pxe.SupervisorConfig{Enabled: false})
 
-	hostAPI := &api.HostAPI{
-		Hosts:       hosts,
-		Ops:         ops,
-		Locks:       locks,
-		Images:      images,
-		Runner:      runner,
-		PXE:         pxeSupervisor,
-		Config:      cfg,
-		ServerTypes: serverTypes,
-	}
-
 	hostOrch := &orchestrator.HostOrchestrator{
 		Runner:      runner,
 		Hosts:       hosts,
@@ -79,6 +68,18 @@ func newTestServer(t *testing.T) *httptest.Server {
 		ServerTypes: serverTypes,
 	}
 
+	hostAPI := &api.HostAPI{
+		Hosts:        hosts,
+		Ops:          ops,
+		Locks:        locks,
+		Images:       images,
+		Runner:       runner,
+		Orchestrator: hostOrch,
+		PXE:          pxeSupervisor,
+		Config:       cfg,
+		ServerTypes:  serverTypes,
+	}
+
 	bootAPI := &api.BootAPI{
 		Hosts:        hosts,
 		Images:       images,
@@ -94,6 +95,7 @@ func newTestServer(t *testing.T) *httptest.Server {
 		Locks:        locks,
 		Images:       images,
 		Runner:       runner,
+		Orchestrator: hostOrch,
 		Hub:          hub,
 		PXE:          pxeSupervisor,
 		Config:       cfg,

internal/api/ui.go

@@ -24,6 +24,7 @@ type UI struct {
 	Locks        *store.Locks
 	Images       *store.Images
 	Runner       *orchestrator.Runner
+	Orchestrator *orchestrator.HostOrchestrator
 	Hub          *events.Hub
 	PXE          *pxe.Supervisor
 	Config       *config.Config
@@ -128,6 +129,13 @@ func (u *UI) TriggerRebuild(w http.ResponseWriter, r *http.Request) {
 		Kind:   model.OpRebuildProxmox,
 	})
 	_ = u.Locks.Acquire(r.Context(), host.ID, opID)
+
+	if err := u.Orchestrator.PrepareRebuild(r.Context(), host.ID); err != nil {
+		_ = u.Locks.Release(r.Context(), host.ID)
+		http.Error(w, "Failed to prepare rebuild: "+err.Error(), http.StatusInternalServerError)
+		return
+	}
+
 	u.Runner.Transition(r.Context(), host.ID, statemachine.TriggerRebuildRequested)
 
 	hosts, _ := u.Hosts.List(r.Context())

internal/config/config.go

@@ -48,8 +48,6 @@ type Proxmox struct {
 }
 
 type Credentials struct {
-	SSHPrivateKeyPath string `yaml:"ssh_private_key_path"`
-	SSHPublicKey      string `yaml:"ssh_public_key"`
 	RootPasswordHash string `yaml:"root_password_hash"`
 }
 

internal/db/migrations/0002_ephemeral_ssh_key.sql

@@ -0,0 +1,2 @@
+ALTER TABLE hosts ADD COLUMN ssh_private_key TEXT;
+ALTER TABLE hosts ADD COLUMN ssh_public_key TEXT;

internal/orchestrator/cluster.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"fmt"
 	"log"
-	"os"
 	"time"
 
 	"golang.org/x/crypto/ssh"
@@ -14,41 +13,39 @@ type ClusterJoiner struct {
 	ExistingNode    string
 	ClusterName     string
 	JoinFingerprint string
-	SSHKeyPath      string
 }
 
-func (c *ClusterJoiner) Join(ctx context.Context, hostIP string) error {
-	client, err := c.connect(hostIP)
+func (c *ClusterJoiner) Join(ctx context.Context, hostIP string, privateKey string, publicKey string) error {
+	client, err := c.connect(hostIP, privateKey)
 	if err != nil {
 		return fmt.Errorf("ssh connect to %s: %w", hostIP, err)
 	}
 	defer client.Close()
 
+	// Join the cluster
 	cmd := fmt.Sprintf("pvecm add %s --force", c.ExistingNode)
 	log.Printf("cluster: running on %s: %s", hostIP, cmd)
-
-	session, err := client.NewSession()
-	if err != nil {
-		return fmt.Errorf("ssh session: %w", err)
-	}
-	defer session.Close()
-
-	output, err := session.CombinedOutput(cmd)
-	if err != nil {
-		return fmt.Errorf("pvecm add failed: %w\noutput: %s", err, string(output))
+	if err := c.runCmd(client, cmd); err != nil {
+		return fmt.Errorf("pvecm add failed: %w", err)
 	}
 	log.Printf("cluster: %s joined successfully", hostIP)
+
+	// Remove the ephemeral key from authorized_keys
+	escaped := escapeForSed(publicKey)
+	removeCmd := fmt.Sprintf(`sed -i '\|%s|d' /root/.ssh/authorized_keys`, escaped)
+	if err := c.runCmd(client, removeCmd); err != nil {
+		log.Printf("cluster: warning: failed to remove ephemeral key from %s: %v", hostIP, err)
+	} else {
+		log.Printf("cluster: ephemeral key removed from %s", hostIP)
+	}
+
 	return nil
 }
 
-func (c *ClusterJoiner) connect(hostIP string) (*ssh.Client, error) {
-	keyData, err := os.ReadFile(c.SSHKeyPath)
+func (c *ClusterJoiner) connect(hostIP string, privateKeyPEM string) (*ssh.Client, error) {
+	signer, err := ssh.ParsePrivateKey([]byte(privateKeyPEM))
 	if err != nil {
-		return nil, fmt.Errorf("read ssh key: %w", err)
-	}
-	signer, err := ssh.ParsePrivateKey(keyData)
-	if err != nil {
-		return nil, fmt.Errorf("parse ssh key: %w", err)
+		return nil, fmt.Errorf("parse ephemeral key: %w", err)
 	}
 	config := &ssh.ClientConfig{
 		User:            "root",
@@ -58,3 +55,31 @@ func (c *ClusterJoiner) connect(hostIP string) (*ssh.Client, error) {
 	}
 	return ssh.Dial("tcp", hostIP+":22", config)
 }
+
+func (c *ClusterJoiner) runCmd(client *ssh.Client, cmd string) error {
+	session, err := client.NewSession()
+	if err != nil {
+		return fmt.Errorf("ssh session: %w", err)
+	}
+	defer session.Close()
+	output, err := session.CombinedOutput(cmd)
+	if err != nil {
+		return fmt.Errorf("%w\noutput: %s", err, string(output))
+	}
+	return nil
+}
+
+func escapeForSed(s string) string {
+	// Trim trailing newline and escape sed delimiter
+	result := ""
+	for _, c := range s {
+		if c == '|' {
+			result += `\|`
+		} else if c == '\n' {
+			continue
+		} else {
+			result += string(c)
+		}
+	}
+	return result
+}

internal/orchestrator/host.go

@@ -22,6 +22,16 @@ type HostOrchestrator struct {
 	ServerTypes *config.ServerTypeRegistry
 }
 
+// PrepareRebuild generates an ephemeral SSH key pair and stores it on the host.
+// The public key will be injected into the Proxmox answer file.
+func (o *HostOrchestrator) PrepareRebuild(ctx context.Context, hostID int64) error {
+	kp, err := GenerateEphemeralKey()
+	if err != nil {
+		return err
+	}
+	return o.Hosts.SetEphemeralKey(ctx, hostID, kp.PrivateKey, kp.PublicKey)
+}
+
 func (o *HostOrchestrator) HandlePhoneHome(ctx context.Context, hostID int64, ip string, hardwareID string) {
 	if err := o.Hosts.UpdateIP(ctx, hostID, ip, hardwareID); err != nil {
 		log.Printf("host %d: failed to update IP: %v", hostID, err)
@@ -47,17 +57,27 @@ func (o *HostOrchestrator) postPhoneHome(hostID int64, ip string, hardwareID str
 		return
 	}
 
+	privateKey, publicKey, err := o.Hosts.GetEphemeralKey(ctx, hostID)
+	if err != nil || privateKey == "" {
+		log.Printf("host %d: no ephemeral key available: %v", hostID, err)
+		o.Runner.FailHost(ctx, hostID, "no ephemeral SSH key")
+		return
+	}
+
 	if _, err := o.Runner.Transition(ctx, hostID, statemachine.TriggerClusterJoinStart); err != nil {
 		log.Printf("host %d: cluster join start transition failed: %v", hostID, err)
 		return
 	}
 
-	if err := o.Cluster.Join(ctx, ip); err != nil {
+	if err := o.Cluster.Join(ctx, ip, privateKey, publicKey); err != nil {
 		log.Printf("host %d: cluster join failed: %v", hostID, err)
 		o.Runner.FailHost(ctx, hostID, "cluster join: "+err.Error())
 		return
 	}
 
+	// Key has been removed from the remote host; clear it from the DB
+	_ = o.Hosts.ClearEphemeralKey(ctx, hostID)
+
 	if err := o.registerInfra(ctx, host, ip, hardwareID); err != nil {
 		log.Printf("host %d: infra registration failed: %v", hostID, err)
 		o.Runner.FailHost(ctx, hostID, "infra registration: "+err.Error())

internal/orchestrator/sshkey.go

@@ -0,0 +1,39 @@
+package orchestrator
+
+import (
+	"crypto/ed25519"
+	"crypto/rand"
+	"encoding/pem"
+	"fmt"
+
+	"golang.org/x/crypto/ssh"
+)
+
+type KeyPair struct {
+	PrivateKey string
+	PublicKey  string
+}
+
+func GenerateEphemeralKey() (*KeyPair, error) {
+	pub, priv, err := ed25519.GenerateKey(rand.Reader)
+	if err != nil {
+		return nil, fmt.Errorf("generate ed25519 key: %w", err)
+	}
+
+	sshPub, err := ssh.NewPublicKey(pub)
+	if err != nil {
+		return nil, fmt.Errorf("ssh public key: %w", err)
+	}
+	pubStr := string(ssh.MarshalAuthorizedKey(sshPub))
+
+	privBytes, err := ssh.MarshalPrivateKey(priv, "")
+	if err != nil {
+		return nil, fmt.Errorf("marshal private key: %w", err)
+	}
+	privStr := string(pem.EncodeToMemory(privBytes))
+
+	return &KeyPair{
+		PrivateKey: privStr,
+		PublicKey:  pubStr,
+	}, nil
+}

internal/pxe/answer.go

@@ -8,7 +8,7 @@ import (
 	"provisioning/internal/model"
 )
 
-func GenerateAnswerFile(host *model.Host, serverType model.ServerType, cfg *config.Config) string {
+func GenerateAnswerFile(host *model.Host, serverType model.ServerType, cfg *config.Config, sshPublicKey string) string {
 	var b strings.Builder
 
 	b.WriteString("[global]\n")
@@ -18,7 +18,7 @@ func GenerateAnswerFile(host *model.Host, serverType model.ServerType, cfg *conf
 	b.WriteString(`mailto = "admin@thewrightserver.net"` + "\n")
 	b.WriteString(`timezone = "America/Indiana/Indianapolis"` + "\n")
 	b.WriteString(fmt.Sprintf("root-password-hashed = \"%s\"\n", cfg.Credentials.RootPasswordHash))
-	b.WriteString(fmt.Sprintf("root-ssh-keys = [\"%s\"]\n", cfg.Credentials.SSHPublicKey))
+	b.WriteString(fmt.Sprintf("root-ssh-keys = [\"%s\"]\n", strings.TrimSpace(sshPublicKey)))
 	b.WriteString("\n")
 
 	b.WriteString("[network]\n")

internal/store/hosts.go

@@ -109,6 +109,25 @@ func (s *Hosts) UpdateInfraID(ctx context.Context, id int64, infraHostID int64)
 	return err
 }
 
+func (s *Hosts) SetEphemeralKey(ctx context.Context, id int64, privateKey, publicKey string) error {
+	_, err := s.DB.ExecContext(ctx, `UPDATE hosts SET ssh_private_key = ?, ssh_public_key = ? WHERE id = ?`, privateKey, publicKey, id)
+	return err
+}
+
+func (s *Hosts) GetEphemeralKey(ctx context.Context, id int64) (privateKey, publicKey string, err error) {
+	var priv, pub sql.NullString
+	err = s.DB.QueryRowContext(ctx, `SELECT ssh_private_key, ssh_public_key FROM hosts WHERE id = ?`, id).Scan(&priv, &pub)
+	if err != nil {
+		return "", "", err
+	}
+	return priv.String, pub.String, nil
+}
+
+func (s *Hosts) ClearEphemeralKey(ctx context.Context, id int64) error {
+	_, err := s.DB.ExecContext(ctx, `UPDATE hosts SET ssh_private_key = NULL, ssh_public_key = NULL WHERE id = ?`, id)
+	return err
+}
+
 func (s *Hosts) Delete(ctx context.Context, id int64) error {
 	res, err := s.DB.ExecContext(ctx, `DELETE FROM hosts WHERE id = ?`, id)
 	if err != nil {