fix(remote): validate cloud DNS hostnames (#3845)

## Summary

- validate the generated LanceDB Cloud hostname during connection setup
- return a clear invalid-input error for empty, overlong, or oversized
DNS names before network resolution
- add Rust and Python regression coverage for malformed `db://`
authorities

## Root cause

The `db://` authority and region were interpolated into the Cloud API
hostname without DNS length validation. Empty or overlong labels
therefore reached the resolver and surfaced as an opaque IDNA
`UnicodeError` instead of a useful connection error.

## Validation

- `cargo test --quiet --features remote -p lancedb
test_rejects_invalid_cloud_dns_hostname --lib`
- `cargo check --quiet --features remote --tests --examples`
- `uv run --no-sync --extra tests pytest
python/tests/test_remote_db.py::test_async_remote_db
python/tests/test_remote_db.py::test_connect_rejects_invalid_cloud_dns_hostname
-q`
- `cargo fmt --all -- --check`
- `ruff check .`
- `ruff format --check python/python/tests/test_remote_db.py`

Fixes #799

<!-- lance-gatekeeper-fix:v1 agent=4d1597b3d244b58f0603ed40a8a59cf9
generation=1 -->

Co-authored-by: Gatefixer <313497061+lancedb-gatefixer[bot]@users.noreply.github.com>

python/python/tests/test_remote_db.py

@@ -35,6 +35,12 @@ def make_mock_http_handler(handler):
     return MockLanceDBHandler
 
 
+@pytest.mark.parametrize("db_name", ["a" * 64, "invalid..database"])
+def test_connect_rejects_invalid_cloud_dns_hostname(db_name):
+    with pytest.raises(ValueError, match="DNS labels must contain 1 to 63 bytes"):
+        lancedb.connect(f"db://{db_name}", api_key="fake")
+
+
 @contextlib.contextmanager
 def mock_lancedb_connection(handler):
     with http.server.HTTPServer(

rust/lancedb/src/remote/client.rs

@@ -373,6 +373,37 @@ pub fn parse_db_url(db_url: &str) -> Result<ParsedDbUrl> {
     Ok(ParsedDbUrl { db_name, db_prefix })
 }
 
+fn validate_dns_hostname(hostname: &str) -> Result<()> {
+    let ascii_hostname = match url::Host::parse(hostname) {
+        Ok(url::Host::Domain(hostname)) => hostname,
+        Ok(_) => {
+            return Err(Error::InvalidInput {
+                message: "LanceDB Cloud database URI or region produced a non-DNS hostname"
+                    .to_string(),
+            });
+        }
+        Err(err) => {
+            return Err(Error::InvalidInput {
+                message: format!(
+                    "LanceDB Cloud database URI or region produced an invalid hostname: {err}"
+                ),
+            });
+        }
+    };
+
+    if ascii_hostname.len() > 253
+        || ascii_hostname
+            .split('.')
+            .any(|label| label.is_empty() || label.len() > 63)
+    {
+        return Err(Error::InvalidInput {
+            message: "LanceDB Cloud database URI or region produced an invalid hostname: DNS labels must contain 1 to 63 bytes and the full hostname must not exceed 253 bytes".to_string(),
+        });
+    }
+
+    Ok(())
+}
+
 impl RestfulLanceDbClient<Sender> {
     fn get_timeout(passed: Option<Duration>, env_var: &str) -> Result<Option<Duration>> {
         if let Some(passed) = passed {
@@ -480,7 +511,11 @@ impl RestfulLanceDbClient<Sender> {
 
         let host = match host_override {
             Some(host_override) => host_override,
-            None => format!("https://{}.{}.api.lancedb.com", parsed_url.db_name, region),
+            None => {
+                let hostname = format!("{}.{}.api.lancedb.com", parsed_url.db_name, region);
+                validate_dns_hostname(&hostname)?;
+                format!("https://{hostname}")
+            }
         };
         debug!("Created client for host: {}", host);
         let retry_config = client_config.retry_config.clone().try_into()?;
@@ -1157,6 +1192,29 @@ mod tests {
         assert_eq!(headers.get("x-api-key").unwrap(), "api-key");
     }
 
+    #[test]
+    fn test_rejects_invalid_cloud_dns_hostname() {
+        let invalid_database_names = ["a".repeat(64), "invalid..database".to_string()];
+
+        for db_name in invalid_database_names {
+            let parsed_url = parse_db_url(&format!("db://{db_name}")).unwrap();
+            let error = RestfulLanceDbClient::<Sender>::try_new(
+                &parsed_url,
+                "us-east-1",
+                None,
+                HeaderMap::new(),
+                ClientConfig::default(),
+                None,
+            )
+            .unwrap_err();
+
+            assert!(
+                matches!(error, Error::InvalidInput { ref message } if message.contains("DNS labels must contain 1 to 63 bytes")),
+                "unexpected error: {error}"
+            );
+        }
+    }
+
     // Test implementation of HeaderProvider
     #[derive(Debug, Clone)]
     struct TestHeaderProvider {