From: Maxim Dounin <mdounin@mdounin.ru>
Date: Tue, 25 May 2021 15:17:36 +0300
Subject: Resolver: fixed off-by-one write in ngx_resolver_copy().
Origin: https://github.com/nginx/nginx/commit/7199ebc203f74fd9e44595474de6bdc41740c5cf
Bug-Debian: https://bugs.debian.org/989095
Bug-Debian-Security: https://security-tracker.debian.org/tracker/CVE-2021-23017

Reported by Luis Merino, Markus Vervier, Eric Sesterhenn, X41 D-Sec GmbH.
---
 src/core/ngx_resolver.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/core/ngx_resolver.c b/src/core/ngx_resolver.c
index 793907010278..63b26193df4f 100644
--- a/src/core/ngx_resolver.c
+++ b/src/core/ngx_resolver.c
@@ -4008,15 +4008,15 @@ done:
             n = *src++;
 
         } else {
+            if (dst != name->data) {
+                *dst++ = '.';
+            }
+
             ngx_strlow(dst, src, n);
             dst += n;
             src += n;
 
             n = *src++;
-
-            if (n != 0) {
-                *dst++ = '.';
-            }
         }
 
         if (n == 0) {
-- 
2.31.1