From 0d9cab15551be8cdc06969c39eaf851732cda51d Mon Sep 17 00:00:00 2001
From: Mark Andrews <marka@isc.org>
Date: Fri, 25 Oct 2024 14:43:03 +1100
Subject: [PATCH] Process NSID and DNS COOKIE options when returning BADVERS

This will help identify the broken server if we happen to break
EDNS version negotiation.  It will also help protect the client
from spoofed BADVERSION responses.
---
 lib/ns/client.c | 34 +++++++++++++++++++++++-----------
 1 file changed, 23 insertions(+), 11 deletions(-)

diff --git a/lib/ns/client.c b/lib/ns/client.c
index 3eba3c61d9..214e440440 100644
--- a/lib/ns/client.c
+++ b/lib/ns/client.c
@@ -1580,17 +1580,6 @@ process_opt(ns_client_t *client, dns_rdataset_t *opt) {
 	 * XXXRTH need library support for this!
 	 */
 	client->ednsversion = (opt->ttl & 0x00FF0000) >> 16;
-	if (client->ednsversion > DNS_EDNS_VERSION) {
-		ns_stats_increment(client->manager->sctx->nsstats,
-				   ns_statscounter_badednsver);
-		result = ns_client_addopt(client, client->message,
-					  &client->opt);
-		if (result == ISC_R_SUCCESS) {
-			result = DNS_R_BADVERS;
-		}
-		ns_client_error(client, result);
-		return result;
-	}
 
 	/* Check for NSID request */
 	result = dns_rdataset_first(opt);
@@ -1602,6 +1591,17 @@ process_opt(ns_client_t *client, dns_rdataset_t *opt) {
 		while (isc_buffer_remaininglength(&optbuf) >= 4) {
 			optcode = isc_buffer_getuint16(&optbuf);
 			optlen = isc_buffer_getuint16(&optbuf);
+			/*
+			 * When returning BADVERSION, only process
+			 * DNS_OPT_NSID or DNS_OPT_COOKIE options.
+			 */
+			if (client->ednsversion > DNS_EDNS_VERSION &&
+			    optcode != DNS_OPT_NSID &&
+			    optcode != DNS_OPT_COOKIE)
+			{
+				isc_buffer_forward(&optbuf, optlen);
+				continue;
+			}
 			switch (optcode) {
 			case DNS_OPT_NSID:
 				if (!WANTNSID(client)) {
@@ -1683,6 +1683,18 @@ process_opt(ns_client_t *client, dns_rdataset_t *opt) {
 		}
 	}
 
+	if (client->ednsversion > DNS_EDNS_VERSION) {
+		ns_stats_increment(client->manager->sctx->nsstats,
+				   ns_statscounter_badednsver);
+		result = ns_client_addopt(client, client->message,
+					  &client->opt);
+		if (result == ISC_R_SUCCESS) {
+			result = DNS_R_BADVERS;
+		}
+		ns_client_error(client, result);
+		return result;
+	}
+
 	ns_stats_increment(client->manager->sctx->nsstats,
 			   ns_statscounter_edns0in);
 	client->attributes |= NS_CLIENTATTR_WANTOPT;