diff --git a/README.md b/README.md
index 277356d..e378d6d 100644
--- a/README.md
+++ b/README.md
@@ -7,3 +7,142 @@ RUST_LOG=info cargo watch --ignore-nothing -w harmony -w private_repos/ -x 'run
 ```
 
 This will run the nationtech bin (likely `private_repos/nationtech/src/main.rs`) on any change in the harmony or private_repos folders.
+
+## Supporting a new field in OPNSense `config.xml`
+
+Two steps:
+- Supporting the field in `opnsense-config-xml`
+- Enabling Harmony to control the field
+
+We'll use the `filename` field in the `dhcpcd` section of the file as an example.
+
+### Supporting the field
+
+As type checking if enforced, every field from `config.xml` must be known by the code. Each subsection of `config.xml` has its `.rs` file. For the `dhcpcd` section, we'll modify `opnsense-config-xml/src/data/dhcpd.rs`.
+
+When a new field appears in the xml file, an error like this will be thrown and Harmony will panic :
+```
+     Running `/home/stremblay/nt/dir/harmony/target/debug/example-nanodc`
+Found unauthorized element filename
+thread 'main' panicked at opnsense-config-xml/src/data/opnsense.rs:54:14:
+OPNSense received invalid string, should be full XML: ()
+
+```
+
+Define the missing field (`filename`) in the `DhcpInterface` struct of `opnsense-config-xml/src/data/dhcpd.rs`:
+```
+pub struct DhcpInterface {
+    ...
+    pub filename: Option<String>,
+```
+
+Harmony should now be fixed, build and run.
+
+### Controlling the field
+
+Define the `xml field setter` in `opnsense-config-xml/src/data/dhcpd.rs`.
+```
+impl<'a> DhcpConfig<'a> {
+    ...
+    pub fn set_filename(&mut self, filename: &str) {
+        self.enable_netboot();
+        self.get_lan_dhcpd().filename = Some(filename.to_string());
+    },
+    ...
+```
+
+Define the `value setter` in the `DhcpServer trait`  in `domain/topology/network.rs`
+```
+#[async_trait]
+pub trait DhcpServer: Send + Sync {
+    ...
+    async fn set_filename(&self, filename: &str) -> Result<(), ExecutorError>;
+    ...
+```
+
+Implement the `value setter` in each `DhcpServer` implementation.
+`infra/opnsense/dhcp.rs`:
+```
+#[async_trait]
+impl DhcpServer for OPNSenseFirewall {
+    ...
+    async fn set_filename(&self, filename: &str) -> Result<(), ExecutorError> {
+        {
+            let mut writable_opnsense = self.opnsense_config.write().await;
+            writable_opnsense.dhcp().set_filename(filename);
+            debug!("OPNsense dhcp server set filename {filename}");
+        }
+
+        Ok(())
+    },
+    ...
+```
+
+`domain/topology/ha_cluster.rs`
+```
+#[async_trait]
+impl DhcpServer for DummyInfra {
+    ...
+    async fn set_filename(&self, _filename: &str) -> Result<(), ExecutorError> {
+        unimplemented!("{}", UNIMPLEMENTED_DUMMY_INFRA)
+    },
+    ...
+```
+
+Add the new field to the DhcpScore in `modules/dhcp.rs`
+```
+pub struct DhcpScore {
+    ...
+    pub filename: Option<String>,
+```
+
+Define it in its implementation in `modules/okd/dhcp.rs`
+```
+impl OKDDhcpScore {
+        ...
+        Self {
+            dhcp_score: DhcpScore {
+                ...
+                filename: Some("undionly.kpxe".to_string()),
+```
+
+Define it in its implementation in `modules/okd/bootstrap_dhcp.rs`
+```
+impl OKDDhcpScore {
+        ...
+        Self {
+            dhcp_score: DhcpScore::new(
+                ...
+                Some("undionly.kpxe".to_string()),
+```
+
+Update the interpret (function called by the `execute` fn of the interpret) so it now updates the `filename` field value in `modules/dhcp.rs`
+```
+impl DhcpInterpret {
+        ...
+        let filename_outcome = match &self.score.filename {
+            Some(filename) => {
+                let dhcp_server = Arc::new(topology.dhcp_server.clone());
+                dhcp_server.set_filename(&filename).await?;
+                Outcome::new(
+                    InterpretStatus::SUCCESS,
+                    format!("Dhcp Interpret Set filename to {filename}"),
+                )
+            }
+            None => Outcome::noop(),
+        };
+
+        if next_server_outcome.status == InterpretStatus::NOOP
+            && boot_filename_outcome.status == InterpretStatus::NOOP
+            && filename_outcome.status == InterpretStatus::NOOP
+
+            ...
+
+            Ok(Outcome::new(
+            InterpretStatus::SUCCESS,
+            format!(
+                "Dhcp Interpret Set next boot to [{:?}], boot_filename to [{:?}], filename to [{:?}]",
+                self.score.boot_filename, self.score.boot_filename, self.score.filename
+            ),
+            ...
+```