From ec00a16c572e5c80a3c0f6b1e295b831a4638a10 Mon Sep 17 00:00:00 2001
From: Benjamin Valentin <benjamin.valentin@ml-pa.com>
Date: Thu, 7 Aug 2025 17:27:39 +0200
Subject: [PATCH] nanocoap: ensure pkt->hdr equals rbuf in coap_build_reply()

---
 sys/net/application_layer/nanocoap/nanocoap.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/sys/net/application_layer/nanocoap/nanocoap.c b/sys/net/application_layer/nanocoap/nanocoap.c
index a3313ac166..ff64fa81ea 100644
--- a/sys/net/application_layer/nanocoap/nanocoap.c
+++ b/sys/net/application_layer/nanocoap/nanocoap.c
@@ -778,6 +778,9 @@ ssize_t coap_build_reply(coap_pkt_t *pkt, unsigned code,
                    ntohs(pkt->hdr->id));
     len += payload_len;
 
+    /* HACK: many CoAP handlers assume that the pkt buffer is also used for the response */
+    pkt->hdr = (void *)rbuf;
+
     return len;
 }