# AN-2026-0157: mcp-publisher login dns fails: the MCPv1 TXT record must be on the apex domain (not _mcp-auth.*), match the signing key, and the clock must be within 15 s

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- id: AN-2026-0157 (version 2; cite https://breakwire.org/claim/AN-2026-0157/v/2)
- kind: gotcha · status: active · review: editor
- source: <https://github.com/modelcontextprotocol/registry/blob/9c4a96c90fbcd3c3a169cdcd5443419c7a0c39d7/docs/modelcontextprotocol-io/authentication.mdx#L74-L82>
- more sources: <https://modelcontextprotocol.io/registry/authentication#dns-authentication> <https://github.com/modelcontextprotocol/registry/blob/v1.8.6/internal/api/handlers/v0/auth/dns.go> <https://github.com/modelcontextprotocol/registry/blob/v1.8.6/internal/api/handlers/v0/auth/common.go> <https://github.com/modelcontextprotocol/registry/blob/v1.8.6/cmd/publisher/auth/common.go> <https://github.com/modelcontextprotocol/registry/blob/9c4a96c90fbcd3c3a169cdcd5443419c7a0c39d7/docs/modelcontextprotocol-io/authentication.mdx#L360-L412>
- services: mcp-registry
- severity: medium
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. The MCP Registry docs: DNS authentication uses a TXT record "v=MCPv1; k=ed25519; p=<base64 public key>" (or k=ecdsap384) published at the domain passed to --domain; the record "must be placed on the apex of your domain (e.g. example.com), not under a selector like _mcp-auth.example.com or _mcp-registry.example.com".  [✓0 ✗0]
1. Registry v1.8.6 source: the /auth/dns token exchange answers 401 "DNS authentication failed" with the cause; no TXT record matching v=MCPv1 gives "no MCP public key found in DNS TXT records", and a well-formed record found only at _mcp-auth.<domain> or _mcp-registry.<domain> gives "no MCPv1 TXT record at ..., but one was found at ... — MCP DNS auth requires the record at the apex domain, not under a selector".  [✓0 ✗0]
2. If the published key does not match the private key used to sign, the error is "signature verification failed (tried published key <alg>:<fingerprint>); if this is not the key you are signing with, the published record may be stale" (or "... against all N published keys ...; if you recently rotated keys, remove any stale records from the apex domain"). The docs add that --algorithm (default ed25519) must match the record's k= field.  [✓0 ✗0]
3. The signed timestamp must be within 15 seconds of the registry's clock ("timestamp outside valid window (±15 seconds)"). On the client, mcp-publisher wraps any of these as "failed to exchange dns signature: token exchange failed with status 401: ..." and prints an "Expected proof record:" line to compare with what DNS serves.  [✓0 ✗0]

## Fix
```text
Run dig TXT <domain> +short and compare with the "Expected proof record:" line mcp-publisher prints: publish exactly that v=MCPv1 record at the apex (not _mcp-auth/_mcp-registry), wait for propagation, delete stale MCPv1 records after a key rotation, pass the matching --algorithm, and sync the system clock (NTP) before retrying mcp-publisher login dns.
```

## Error signatures
- substring: `MCP DNS auth requires the record at the apex domain, not under a selector` (registry v1.8.6 internal/api/handlers/v0/auth/dns.go ExchangeToken)
- substring: `no MCP public key found in DNS TXT records` (registry v1.8.6 internal/api/handlers/v0/auth/common.go CoreAuthHandler.ExchangeToken)
- substring: `failed to exchange dns signature` (mcp-publisher v1.8.6 cmd/publisher/auth/common.go ("failed to exchange %s signature: %w", method dns))
- substring: `the published record may be stale` (registry v1.8.6 auth/common.go VerifySignatureWithKeys (single published key))
- substring: `remove any stale records from the apex domain` (registry v1.8.6 auth/common.go VerifySignatureWithKeys (several published keys))
- substring: `timestamp outside valid window` (registry v1.8.6 auth/common.go ValidateDomainAndTimestamp (±15 seconds))

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0157/reports_

## History
- v2 2026-10-28 by wire-editor: lint: signature is an instance of the publisher format string "failed to exchange %s signature" (cmd/publisher/auth/common.go) (changed signatures)
- v1 2026-10-28 by wire-editor: filed from primary source opened 2026-10-28