diff --git a/src/http/modules/ngx_http_gzip_filter_module.c b/src/http/modules/ngx_http_gzip_filter_module.c
index 7210dd6fc..d060190c2 100644
--- a/src/http/modules/ngx_http_gzip_filter_module.c
+++ b/src/http/modules/ngx_http_gzip_filter_module.c
@@ -279,7 +279,7 @@ ngx_http_gzip_header_filter(ngx_http_request_t *r)
         || r->headers_in.accept_encoding == NULL
         || (r->headers_out.content_length_n != -1
             && r->headers_out.content_length_n < conf->min_length)
-        || ngx_strstr(r->headers_in.accept_encoding->value.data, "gzip") == NULL
+        || ngx_strcasestrn(r->headers_in.accept_encoding->value.data, "gzip", 3)
        )
     {
         return ngx_http_next_header_filter(r);
diff --git a/src/http/ngx_http_request.c b/src/http/ngx_http_request.c
index d7843c62a..9e0a41eeb 100644
--- a/src/http/ngx_http_request.c
+++ b/src/http/ngx_http_request.c
@@ -1204,10 +1204,10 @@ static ngx_int_t
 ngx_http_process_connection(ngx_http_request_t *r, ngx_table_elt_t *h,
     ngx_uint_t offset)
 {
-    if (ngx_strstr(h->value.data, "close")) {
+    if (ngx_strcasestrn(h->value.data, "close", 4)) {
         r->headers_in.connection_type = NGX_HTTP_CONNECTION_CLOSE;
 
-    } else if (ngx_strstr(h->value.data, "keep-alive")) {
+    } else if (ngx_strcasestrn(h->value.data, "keep-alive", 9)) {
         r->headers_in.connection_type = NGX_HTTP_CONNECTION_KEEP_ALIVE;
     }
 
@@ -1319,7 +1319,8 @@ ngx_http_process_request_header(ngx_http_request_t *r)
     }
 
     if (r->headers_in.transfer_encoding
-        && ngx_strstr(r->headers_in.transfer_encoding->value.data, "chunked"))
+        && ngx_strcasestrn(r->headers_in.transfer_encoding->value.data,
+                           "chunked", 6))
     {
         ngx_log_error(NGX_LOG_INFO, r->connection->log, 0,
                       "client sent \"Transfer-Encoding: chunked\" header");
@@ -1351,7 +1352,7 @@ ngx_http_process_request_header(ngx_http_request_t *r)
 
         user_agent = r->headers_in.user_agent->value.data;
 
-        ua = (u_char *) ngx_strstr(user_agent, "MSIE");
+        ua = ngx_strstrn(user_agent, "MSIE", 3);
 
         if (ua && ua + 8 < user_agent + r->headers_in.user_agent->value.len) {
 
@@ -1369,7 +1370,7 @@ ngx_http_process_request_header(ngx_http_request_t *r)
 #endif
         }
 
-        if (ngx_strstr(user_agent, "Opera")) {
+        if (ngx_strstrn(user_agent, "Opera", 4)) {
             r->headers_in.opera = 1;
             r->headers_in.msie = 0;
             r->headers_in.msie4 = 0;
@@ -1377,10 +1378,10 @@ ngx_http_process_request_header(ngx_http_request_t *r)
 
         if (!r->headers_in.msie && !r->headers_in.opera) {
 
-            if (ngx_strstr(user_agent, "Gecko/")) {
+            if (ngx_strstrn(user_agent, "Gecko/", 5)) {
                 r->headers_in.gecko = 1;
 
-            } else if (ngx_strstr(user_agent, "Konqueror")) {
+            } else if (ngx_strstrn(user_agent, "Konqueror", 8)) {
                 r->headers_in.konqueror = 1;
             }
         }
diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index 878ca42b3..57f40830b 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -2636,7 +2636,7 @@ ngx_http_upstream_rewrite_refresh(ngx_http_request_t *r, ngx_table_elt_t *h,
 
     if (r->upstream->rewrite_redirect) {
 
-        p = (u_char *) ngx_strstr(ho->value.data, "url=");
+        p = ngx_strcasestrn(ho->value.data, "url=", 3);
 
         if (p) {
             rc = r->upstream->rewrite_redirect(r, ho, p + 4 - ho->value.data);