Align SDK UDP host selection with CLI

src/client.rs

@@ -155,18 +155,19 @@ impl DoshClientBuilder {
             .cloned()
             .unwrap_or_default();
         let raw_server = host_config.ssh.clone().unwrap_or_else(|| self.host.clone());
-        let udp_host = self
-            .udp_host
-            .clone()
-            .or_else(|| host_config.dosh_host.clone())
-            .or_else(|| self.client.config.dosh_host.clone())
-            .unwrap_or_else(|| destination_host(&raw_server));
+        let ssh_port = host_config.ssh_port.or(self.client.config.ssh_port);
+        let ssh_config = load_sdk_ssh_config(host_config.ssh_config.as_deref(), ssh_port)?;
+        let udp_host = selected_sdk_udp_host(
+            self.udp_host.as_deref(),
+            &host_config,
+            &self.client.config,
+            &raw_server,
+            &ssh_config,
+        )?;
         let udp_port = self
             .udp_port
             .or(host_config.port)
             .unwrap_or(self.client.config.dosh_port);
-        let ssh_port = host_config.ssh_port.or(self.client.config.ssh_port);
-        let ssh_config = load_sdk_ssh_config(host_config.ssh_config.as_deref(), ssh_port)?;
         let requested_user = self
             .user
             .clone()
@@ -740,6 +741,38 @@ fn destination_host(destination: &str) -> String {
         .to_string()
 }
 
+fn selected_sdk_udp_host(
+    explicit: Option<&str>,
+    host: &HostConfig,
+    config: &ClientConfig,
+    raw_server: &str,
+    ssh_config: &SdkSshConfig,
+) -> Result<String> {
+    let configured = explicit
+        .map(str::trim)
+        .filter(|value| !value.is_empty())
+        .map(ToString::to_string)
+        .or_else(|| host.dosh_host.clone())
+        .or_else(|| config.dosh_host.clone());
+    let ssh_host = || {
+        ssh_config
+            .hostname
+            .clone()
+            .unwrap_or_else(|| destination_host(raw_server))
+    };
+    let Some(raw) = configured else {
+        return Ok(ssh_host());
+    };
+    match raw.trim().to_ascii_lowercase().as_str() {
+        "ssh" | "auto" => Ok(ssh_host()),
+        "localhost" => Ok("127.0.0.1".to_string()),
+        "any" => bail!(
+            "dosh_host={raw:?} is a server bind policy, not a client destination; use ssh/auto or a host/IP"
+        ),
+        _ => Ok(raw),
+    }
+}
+
 fn user_from_destination(destination: &str) -> Option<String> {
     destination
         .rsplit_once('@')
@@ -827,6 +860,65 @@ mod tests {
         );
     }
 
+    #[test]
+    fn sdk_udp_host_follows_ssh_config_hostname_by_default() {
+        let ssh_config = SdkSshConfig {
+            hostname: Some("10.0.0.5".to_string()),
+            ..SdkSshConfig::default()
+        };
+
+        assert_eq!(
+            selected_sdk_udp_host(
+                None,
+                &HostConfig::default(),
+                &ClientConfig::default(),
+                "prod",
+                &ssh_config,
+            )
+            .unwrap(),
+            "10.0.0.5"
+        );
+    }
+
+    #[test]
+    fn sdk_udp_host_honors_cli_compatible_special_values() {
+        let ssh_config = SdkSshConfig {
+            hostname: Some("10.0.0.5".to_string()),
+            ..SdkSshConfig::default()
+        };
+        let host = HostConfig {
+            dosh_host: Some("ssh".to_string()),
+            ..HostConfig::default()
+        };
+
+        assert_eq!(
+            selected_sdk_udp_host(None, &host, &ClientConfig::default(), "prod", &ssh_config)
+                .unwrap(),
+            "10.0.0.5"
+        );
+        assert_eq!(
+            selected_sdk_udp_host(
+                Some("localhost"),
+                &host,
+                &ClientConfig::default(),
+                "prod",
+                &ssh_config,
+            )
+            .unwrap(),
+            "127.0.0.1"
+        );
+        assert!(
+            selected_sdk_udp_host(
+                Some("any"),
+                &host,
+                &ClientConfig::default(),
+                "prod",
+                &ssh_config,
+            )
+            .is_err()
+        );
+    }
+
     #[test]
     fn sdk_parses_ssh_config_identity_settings() {
         let parsed = parse_sdk_ssh_config(