fix(net): resolve external ip on startup (paradigmxyz#19852)

Author: xzav3r

crates/net/discv4/src/config.rs

@@ -101,7 +101,7 @@ impl Discv4Config {
     pub fn resolve_external_ip_interval(&self) -> Option<ResolveNatInterval> {
         let resolver = self.external_ip_resolver?;
         let interval = self.resolve_external_ip_interval?;
-        Some(ResolveNatInterval::interval(resolver, interval))
+        Some(ResolveNatInterval::interval_at(resolver, tokio::time::Instant::now(), interval))
     }
 }
 
@@ -336,4 +336,25 @@ mod tests {
             .enable_lookup(true)
             .build();
     }
+
+    #[tokio::test]
+    async fn test_resolve_external_ip_interval_uses_interval_at() {
+        use reth_net_nat::NatResolver;
+        use std::net::{IpAddr, Ipv4Addr};
+
+        let ip_addr = IpAddr::V4(Ipv4Addr::new(192, 168, 1, 1));
+
+        // Create a config with external IP resolver
+        let mut builder = Discv4Config::builder();
+        builder.external_ip_resolver(Some(NatResolver::ExternalIp(ip_addr)));
+        builder.resolve_external_ip_interval(Some(Duration::from_secs(60 * 5)));
+        let config = builder.build();
+
+        // Get the ResolveNatInterval
+        let mut interval = config.resolve_external_ip_interval().expect("should have interval");
+
+        // Test that first tick returns immediately (interval_at behavior)
+        let ip = interval.tick().await;
+        assert_eq!(ip, Some(ip_addr));
+    }
 }