auth: Add DNS auth (#279)

Fixes #270
Closes #233

Adds support for DNS-based authentication for proving ownership of a
particular domain. E.g. if you can add a TXT record to `anthropic.com`,
you can publish anything at `com.anthropic/*` or `com.anthropic.*`.

Notes:
- Uses Ed25519 signatures with 15-second timestamp windows to prevent
replay attacks
- Supports multiple public keys in DNS TXT records for key rotation
scenarios
- Publisher CLI supports DNS auth via --dns-domain and --dns-private-key
flags
- Includes key generation commands in publisher README for easy setup

## Motivation and Context

This implements DNS-based public/private key authentication to allow
people to actually publish at their own domains. Currently,
authentication requires GitHub accounts which limits publishing to the
io.github.* namespace.

DNS-based auth enables:
- Publishing under your actual domain (e.g., com.anthropic/* for
anthropic.com)
- Independence from GitHub or other external auth providers
- Organizations managing their own keys and access to publishing

## How Has This Been Tested?

- Unit tests cover signature verification, DNS TXT parsing, timestamp
validation, and error cases
- Tested locally by editing my own DNS records
- All existing auth methods covered by passing tests

## Breaking Changes

None

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [x] I have added appropriate error handling
- [x] I have added or updated documentation as needed

## Additional context

Will be adding HTTP challenge auth in a similar way as a separate PR,
e.g. for verifying you own `github.com` by hosting a
`github.com/.well-known/mcp-server-challenge` or similar. Just
mentioning this as we discussed HTTP challenges as closely related to
DNS challenges elsewhere, and want to reassure reviewer that this hasn't
been forgotten with this approach. :) Update: tracking the HTTP auth in
https://github.com/modelcontextprotocol/registry/issues/280

tools/publisher/README.md

@@ -48,7 +48,10 @@ The tool supports two main commands:
 - `--auth-method`: Authentication method to use (default: github-at)
   - `github-at`: Interactive GitHub OAuth device flow authentication
   - `github-oidc`: GitHub Actions OIDC authentication (for CI/CD)
+  - `dns`: DNS-based public/private key authentication
   - `none`: No authentication (for registry contributors testing locally)
+- `--dns-domain`: Domain name for DNS authentication (required for dns auth method)
+- `--dns-private-key`: 64-character hex seed for DNS authentication (required for dns auth method)
 
 ## Creating a server.json file
 
@@ -182,6 +185,31 @@ jobs:
             --auth-method github-oidc
 ```
 
+### DNS Authentication (`dns`)
+
+For domain-based authentication using public/private key cryptography:
+
+1. **Generate Ed25519 keypair**: 
+   ```bash
+   openssl genpkey -algorithm Ed25519 -out /tmp/key.pem && \
+   echo "\n\nDNS record to add to your domain:" && \
+   echo "  Type: TXT" && \
+   echo "  Value: v=MCPv1; k=ed25519; p=$(openssl pkey -in /tmp/key.pem -pubout -outform DER | tail -c 32 | base64)" && \
+   echo "" && \
+   echo "Private key for --dns-private-key flag:" && \
+   echo "  $(openssl pkey -in /tmp/key.pem -noout -text | grep -A3 "priv:" | tail -n +2 | tr -d ' :\n')\n" && \
+   rm /tmp/key.pem
+   ```
+2. **Add DNS TXT record**: Add a TXT record to your domain with format: `v=MCPv1; k=ed25519; p=<base64-public-key>`
+3. **Use CLI arguments**: Provide domain and private key via command line flags
+
+```bash
+./bin/mcp-publisher publish --registry-url <REGISTRY_URL> --mcp-file <PATH_TO_MCP_FILE> \
+  --auth-method dns --dns-domain example.com --dns-private-key abc123...
+```
+
+This grants publishing permissions for both `example.com/*` and `*.example.com/*` namespaces.
+
 ### No Authentication (`none`)
 
 Mainly for registry contributors, for testing locally:

tools/publisher/auth/dns.go

@@ -0,0 +1,133 @@
+package auth
+
+import (
+	"bytes"
+	"context"
+	"crypto/ed25519"
+	"encoding/hex"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"time"
+)
+
+type DNSProvider struct {
+	registryURL string
+	domain      string
+	hexSeed     string
+}
+
+//nolint:ireturn
+func NewDNSProvider(registryURL, domain, hexSeed string) Provider {
+	return &DNSProvider{
+		registryURL: registryURL,
+		domain:      domain,
+		hexSeed:     hexSeed,
+	}
+}
+
+// GetToken retrieves the registry JWT token using DNS authentication
+func (d *DNSProvider) GetToken(ctx context.Context) (string, error) {
+	if d.domain == "" {
+		return "", fmt.Errorf("DNS domain is required")
+	}
+
+	if d.hexSeed == "" {
+		return "", fmt.Errorf("DNS private key (hex seed) is required")
+	}
+
+	// Decode hex seed to private key
+	seedBytes, err := hex.DecodeString(d.hexSeed)
+	if err != nil {
+		return "", fmt.Errorf("invalid hex seed format: %w", err)
+	}
+
+	if len(seedBytes) != ed25519.SeedSize {
+		return "", fmt.Errorf("invalid seed length: expected %d bytes, got %d", ed25519.SeedSize, len(seedBytes))
+	}
+
+	privateKey := ed25519.NewKeyFromSeed(seedBytes)
+
+	// Generate current timestamp
+	timestamp := time.Now().UTC().Format(time.RFC3339)
+
+	// Sign the timestamp
+	signature := ed25519.Sign(privateKey, []byte(timestamp))
+	signedTimestamp := hex.EncodeToString(signature)
+
+	// Exchange signature for registry token
+	registryToken, err := d.exchangeDNSTokenForRegistry(ctx, d.domain, timestamp, signedTimestamp)
+	if err != nil {
+		return "", fmt.Errorf("failed to exchange DNS signature: %w", err)
+	}
+
+	return registryToken, nil
+}
+
+// NeedsLogin always returns false for DNS since no interactive login is needed
+func (d *DNSProvider) NeedsLogin() bool {
+	return false
+}
+
+// Login is not needed for DNS since authentication is cryptographic
+func (d *DNSProvider) Login(_ context.Context) error {
+	return nil
+}
+
+// Name returns the name of this auth provider
+func (d *DNSProvider) Name() string {
+	return "dns"
+}
+
+// exchangeDNSTokenForRegistry exchanges DNS signature for a registry JWT token
+func (d *DNSProvider) exchangeDNSTokenForRegistry(ctx context.Context, domain, timestamp, signedTimestamp string) (string, error) {
+	if d.registryURL == "" {
+		return "", fmt.Errorf("registry URL is required for token exchange")
+	}
+
+	// Prepare the request body
+	payload := map[string]string{
+		"domain":           domain,
+		"timestamp":        timestamp,
+		"signed_timestamp": signedTimestamp,
+	}
+
+	jsonData, err := json.Marshal(payload)
+	if err != nil {
+		return "", fmt.Errorf("failed to marshal request: %w", err)
+	}
+
+	// Make the token exchange request
+	exchangeURL := d.registryURL + "/v0/auth/dns"
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, exchangeURL, bytes.NewBuffer(jsonData))
+	if err != nil {
+		return "", fmt.Errorf("failed to create request: %w", err)
+	}
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Accept", "application/json")
+
+	client := &http.Client{}
+	resp, err := client.Do(req)
+	if err != nil {
+		return "", fmt.Errorf("failed to send request: %w", err)
+	}
+	defer resp.Body.Close()
+
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return "", fmt.Errorf("failed to read response: %w", err)
+	}
+
+	if resp.StatusCode != http.StatusOK {
+		return "", fmt.Errorf("token exchange failed with status %d: %s", resp.StatusCode, body)
+	}
+
+	var tokenResp RegistryTokenResponse
+	err = json.Unmarshal(body, &tokenResp)
+	if err != nil {
+		return "", fmt.Errorf("failed to unmarshal response: %w", err)
+	}
+
+	return tokenResp.RegistryToken, nil
+}

tools/publisher/main.go

@@ -97,12 +97,16 @@ func publishCommand() error {
 	var mcpFilePath string
 	var forceLogin bool
 	var authMethod string
+	var dnsDomain string
+	var dnsPrivateKey string
 
 	// Command-line flags for configuration
 	publishFlags.StringVar(&registryURL, "registry-url", "", "URL of the registry (required)")
 	publishFlags.StringVar(&mcpFilePath, "mcp-file", "", "path to the MCP file (required)")
 	publishFlags.BoolVar(&forceLogin, "login", false, "force a new login even if a token exists")
 	publishFlags.StringVar(&authMethod, "auth-method", "github-at", "authentication method (default: github-at)")
+	publishFlags.StringVar(&dnsDomain, "dns-domain", "", "domain name for DNS authentication (required for dns auth method)")
+	publishFlags.StringVar(&dnsPrivateKey, "dns-private-key", "", "64-character hex seed for DNS authentication (required for dns auth method)")
 
 	// Set custom usage function
 	publishFlags.Usage = func() {
@@ -111,10 +115,12 @@ func publishCommand() error {
 		fmt.Fprint(os.Stdout, "Publish a server.json file to the registry\n")
 		fmt.Fprint(os.Stdout, "\n")
 		fmt.Fprint(os.Stdout, "Flags:\n")
-		fmt.Fprint(os.Stdout, "  --registry-url string    URL of the registry (required)\n")
-		fmt.Fprint(os.Stdout, "  --mcp-file string        path to the MCP file (required)\n")
-		fmt.Fprint(os.Stdout, "  --login                  force a new login even if a token exists\n")
-		fmt.Fprint(os.Stdout, "  --auth-method string     authentication method (default: github-at)\n")
+		fmt.Fprint(os.Stdout, "  --registry-url string       URL of the registry (required)\n")
+		fmt.Fprint(os.Stdout, "  --mcp-file string           path to the MCP file (required)\n")
+		fmt.Fprint(os.Stdout, "  --login                     force a new login even if a token exists\n")
+		fmt.Fprint(os.Stdout, "  --auth-method string        authentication method (default: github-at)\n")
+		fmt.Fprint(os.Stdout, "  --dns-domain string         domain name for DNS authentication\n")
+		fmt.Fprint(os.Stdout, "  --dns-private-key string    64-character hex seed for DNS authentication\n")
 	}
 
 	if err := publishFlags.Parse(os.Args[2:]); err != nil {
@@ -140,6 +146,9 @@ func publishCommand() error {
 	case "github-oidc":
 		log.Println("Using GitHub Actions OIDC for authentication")
 		authProvider = auth.NewGitHubOIDCProvider(registryURL)
+	case "dns":
+		log.Println("Using DNS-based authentication")
+		authProvider = auth.NewDNSProvider(registryURL, dnsDomain, dnsPrivateKey)
 	case "none":
 		log.Println("Using anonymous authentication")
 		authProvider = auth.NewNoneProvider(registryURL)