Add HMAC-SHA-384 TSIG support, document HMAC-SHA-512 truncation limitation

BIND 9.18+ truncates HMAC-SHA-512 MACs by default, and hickory-dns
does not support verifying truncated HMACs. Document the workaround
(use hmac-sha256) and add hmac-sha384 as an additional option.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -11,6 +11,12 @@
 
 ### Fixes
 
+- Add HMAC-SHA-384 TSIG algorithm support
+
+### Known limitations
+
+- TSIG with HMAC-SHA-512 may fail with BIND 9.18+ which truncates MACs by default. Use HMAC-SHA-256 as a workaround (hickory-dns does not support truncated HMAC verification).
+
 ### Changes
 
 - **Breaking**: Configuration format overhauled

README.md

@@ -125,7 +125,7 @@ server = "ns1.example.com"
 zone = "example.com"
 tsig_key_path = "/etc/certforge/tsig.key"
 tsig_key_name = "certforge."
-tsig_algorithm = "hmac-sha256"   # or hmac-sha512
+tsig_algorithm = "hmac-sha256"   # hmac-sha256, hmac-sha384, or hmac-sha512
 ```
 
 ### Challenge solvers
@@ -231,6 +231,12 @@ systemctl enable --now certforge.timer
 
 For encrypted credentials, add `LoadCredentialEncrypted=` directives to service units that need access to TLS keys. Run `certforge init` to see the required directives.
 
+## Known limitations
+
+- **TSIG HMAC-SHA-512**: BIND 9.18+ truncates HMAC-SHA-512 MACs by default, and the underlying DNS library (hickory-dns) does not support verifying truncated HMACs. Use `hmac-sha256` instead.
+- RSA key types not yet supported (ECDSA only).
+- `dane-check` shows expected TLSA records but does not query DNS to verify.
+
 ## License
 
 MIT

src/config.rs

@@ -62,6 +62,7 @@ pub struct DnsClientConfig {
 #[serde(rename_all = "kebab-case")]
 pub enum TsigAlgorithm {
     HmacSha256,
+    HmacSha384,
     HmacSha512,
 }
 

src/dns/tsig.rs

@@ -26,6 +26,7 @@ pub async fn load_tsig_signer(
 
     let algorithm = match dns_config.tsig_algorithm {
         config::TsigAlgorithm::HmacSha256 => TsigAlgorithm::HmacSha256,
+        config::TsigAlgorithm::HmacSha384 => TsigAlgorithm::HmacSha384,
         config::TsigAlgorithm::HmacSha512 => TsigAlgorithm::HmacSha512,
     };