feat: add configurable SSH host key algorithm support

blink-so[bot] · kylecarbs · blink-so[bot] · commit f9f7f5a09436 · 2025-07-15T07:53:18.000Z
Adds support for configurable SSH host key algorithms to improve
compatibility with modern SSH clients like Visual Studio 2022.

Changes:
- Add HostKeyAlgorithm field to agentssh.Config
- Implement Ed25519 key generation in CoderSigner function
- Add CLI flag --ssh-host-key-algorithm with env var support
- Support both 'rsa' (default) and 'ed25519' algorithms
- Maintain backward compatibility with RSA as default

Resolves compatibility issues with SSH clients that no longer
support RSA host keys.

Co-authored-by: kylecarbs &lt;7122116+kylecarbs@users.noreply.github.com&gt;
diff --git a/agent/agent.go b/agent/agent.go
@@ -82,6 +82,7 @@ type Options struct {
 	IgnorePorts                  map[int]string
 	PortCacheDuration            time.Duration
 	SSHMaxTimeout                time.Duration
+	SSHHostKeyAlgorithm          string
 	TailnetListenPort            uint16
 	Subsystems                   []codersdk.AgentSubsystem
 	PrometheusRegistry           *prometheus.Registry
@@ -186,6 +187,7 @@ func New(options Options) Agent {
 		reportMetadataInterval:             options.ReportMetadataInterval,
 		announcementBannersRefreshInterval: options.ServiceBannerRefreshInterval,
 		sshMaxTimeout:                      options.SSHMaxTimeout,
+		sshHostKeyAlgorithm:                options.SSHHostKeyAlgorithm,
 		subsystems:                         options.Subsystems,
 		logSender:                          agentsdk.NewLogSender(options.Logger),
 		blockFileTransfer:                  options.BlockFileTransfer,
@@ -257,6 +259,7 @@ type agent struct {
 	sessionToken                       atomic.Pointer[string]
 	sshServer                          *agentssh.Server
 	sshMaxTimeout                      time.Duration
+	sshHostKeyAlgorithm                string
 	blockFileTransfer                  bool
 
 	lifecycleUpdate            chan struct{}
@@ -292,6 +295,7 @@ func (a *agent) init() {
 	// pass the "hard" context because we explicitly close the SSH server as part of graceful shutdown.
 	sshSrv, err := agentssh.NewServer(a.hardCtx, a.logger.Named("ssh-server"), a.prometheusRegistry, a.filesystem, a.execer, &agentssh.Config{
 		MaxTimeout:          a.sshMaxTimeout,
+		HostKeyAlgorithm:    a.sshHostKeyAlgorithm,
 		MOTDFile:            func() string { return a.manifest.Load().MOTDFile },
 		AnnouncementBanners: func() *[]codersdk.BannerConfig { return a.announcementBanners.Load() },
 		UpdateEnv:           a.updateCommandEnv,
diff --git a/agent/agentssh/agentssh.go b/agent/agentssh/agentssh.go
@@ -3,9 +3,11 @@ package agentssh
 import (
 	"bufio"
 	"context"
+	"crypto/ed25519"
 	"errors"
 	"fmt"
 	"io"
+	"math/rand"
 	"net"
 	"os"
 	"os/exec"
@@ -113,6 +115,9 @@ type Config struct {
 	BlockFileTransfer bool
 	// ReportConnection.
 	ReportConnection reportConnectionFunc
+	// HostKeyAlgorithm specifies the SSH host key algorithm to use.
+	// Valid values: "rsa", "ed25519". Default: "rsa" for backward compatibility.
+	HostKeyAlgorithm string
 	// Experimental: allow connecting to running containers via Docker exec.
 	// Note that this is different from the devcontainers feature, which uses
 	// subagents.
@@ -1312,7 +1317,11 @@ func userHomeDir() (string, error) {
 // UpdateHostSigner updates the host signer with a new key generated from the provided seed.
 // If an existing host key exists with the same algorithm, it is overwritten
 func (s *Server) UpdateHostSigner(seed int64) error {
-	key, err := CoderSigner(seed)
+	algorithm := s.config.HostKeyAlgorithm
+	if algorithm == "" {
+		algorithm = "rsa" // Default to RSA for backward compatibility
+	}
+	key, err := CoderSigner(seed, algorithm)
 	if err != nil {
 		return err
 	}
@@ -1325,14 +1334,29 @@ func (s *Server) UpdateHostSigner(seed int64) error {
 	return nil
 }
 
-// CoderSigner generates a deterministic SSH signer based on the provided seed.
-// It uses RSA with a key size of 2048 bits.
-func CoderSigner(seed int64) (gossh.Signer, error) {
+// CoderSigner generates a deterministic SSH signer based on the provided seed and algorithm.
+// Supported algorithms: "rsa" (2048 bits), "ed25519".
+func CoderSigner(seed int64, algorithm string) (gossh.Signer, error) {
 	// Clients should ignore the host key when connecting.
 	// The agent needs to authenticate with coderd to SSH,
 	// so SSH authentication doesn't improve security.
-	coderHostKey := agentrsa.GenerateDeterministicKey(seed)
-
-	coderSigner, err := gossh.NewSignerFromKey(coderHostKey)
-	return coderSigner, err
+	
+	switch algorithm {
+	case "ed25519":
+		// Generate deterministic Ed25519 key
+		rand := rand.New(rand.NewSource(seed))
+		_, privateKey, err := ed25519.GenerateKey(rand)
+		if err != nil {
+			return nil, err
+		}
+		return gossh.NewSignerFromKey(privateKey)
+	
+	case "rsa", "":
+		// Default to RSA for backward compatibility
+		coderHostKey := agentrsa.GenerateDeterministicKey(seed)
+		return gossh.NewSignerFromKey(coderHostKey)
+	
+	default:
+		return nil, fmt.Errorf("unsupported host key algorithm: %s", algorithm)
+	}
 }
diff --git a/cli/agent.go b/cli/agent.go
@@ -46,6 +46,7 @@ func (r *RootCmd) workspaceAgent() *serpent.Command {
 		pprofAddress        string
 		noReap              bool
 		sshMaxTimeout       time.Duration
+		sshHostKeyAlgorithm string
 		tailnetListenPort   int64
 		prometheusAddress   string
 		debugAddress        string
@@ -356,6 +357,7 @@ func (r *RootCmd) workspaceAgent() *serpent.Command {
 					EnvironmentVariables: environmentVariables,
 					IgnorePorts:          ignorePorts,
 					SSHMaxTimeout:        sshMaxTimeout,
+					SSHHostKeyAlgorithm:  sshHostKeyAlgorithm,
 					Subsystems:           subsystems,
 
 					PrometheusRegistry: prometheusRegistry,
@@ -451,6 +453,13 @@ func (r *RootCmd) workspaceAgent() *serpent.Command {
 			Description: "Specify the max timeout for a SSH connection, it is advisable to set it to a minimum of 60s, but no more than 72h.",
 			Value:       serpent.DurationOf(&sshMaxTimeout),
 		},
+		{
+			Flag:        "ssh-host-key-algorithm",
+			Default:     "rsa",
+			Env:         "CODER_AGENT_SSH_HOST_KEY_ALGORITHM",
+			Description: "Specify the SSH host key algorithm to use. Valid values: rsa, ed25519.",
+			Value:       serpent.StringOf(&sshHostKeyAlgorithm),
+		},
 		{
 			Flag:        "tailnet-listen-port",
 			Default:     "0",
