diff --git a/src/core/ngx_connection.c b/src/core/ngx_connection.c
index 9d22ac6..87219f2 100644
--- a/src/core/ngx_connection.c
+++ b/src/core/ngx_connection.c
@@ -123,7 +123,7 @@
                                           ntohs(sin->sin_port))
                               - ls[i].addr_text.data;
 
-        ls[i].backlog = -1;
+        ls[i].backlog = NGX_LISTEN_BACKLOG;
 
         olen = sizeof(int);
 
diff --git a/src/http/ngx_http_core_module.c b/src/http/ngx_http_core_module.c
index 0e732cf..f8bd6b9 100644
--- a/src/http/ngx_http_core_module.c
+++ b/src/http/ngx_http_core_module.c
@@ -2219,7 +2219,7 @@
 #endif
         ls->family = AF_INET;
 
-        ls->conf.backlog = -1;
+        ls->conf.backlog = NGX_LISTEN_BACKLOG;
         ls->conf.rcvbuf = -1;
         ls->conf.sndbuf = -1;
     }
@@ -2570,7 +2570,7 @@
     ls->port = u.port;
     ls->file_name = cf->conf_file->file.name;
     ls->line = cf->conf_file->line;
-    ls->conf.backlog = -1;
+    ls->conf.backlog = NGX_LISTEN_BACKLOG;
     ls->conf.rcvbuf = -1;
     ls->conf.sndbuf = -1;
 
diff --git a/src/mail/ngx_mail.c b/src/mail/ngx_mail.c
index ec74e6f..a39bec4 100644
--- a/src/mail/ngx_mail.c
+++ b/src/mail/ngx_mail.c
@@ -300,7 +300,7 @@
                 return NGX_CONF_ERROR;
             }
 
-            ls->backlog = -1;
+            ls->backlog = NGX_LISTEN_BACKLOG;
             ls->rcvbuf = -1;
             ls->sndbuf = -1;
 
diff --git a/src/os/unix/ngx_freebsd_config.h b/src/os/unix/ngx_freebsd_config.h
index 0bf4fd5..818e230 100644
--- a/src/os/unix/ngx_freebsd_config.h
+++ b/src/os/unix/ngx_freebsd_config.h
@@ -77,6 +77,9 @@
 #endif
 
 
+#define NGX_LISTEN_BACKLOG        -1
+
+
 #if (defined SO_ACCEPTFILTER && !defined NGX_HAVE_DEFERRED_ACCEPT)
 #define NGX_HAVE_DEFERRED_ACCEPT  1
 #endif
diff --git a/src/os/unix/ngx_linux_config.h b/src/os/unix/ngx_linux_config.h
index 980e05d..3e0475f 100644
--- a/src/os/unix/ngx_linux_config.h
+++ b/src/os/unix/ngx_linux_config.h
@@ -78,6 +78,9 @@
 #endif
 
 
+#define NGX_LISTEN_BACKLOG        511
+
+
 #if defined TCP_DEFER_ACCEPT && !defined NGX_HAVE_DEFERRED_ACCEPT
 #define NGX_HAVE_DEFERRED_ACCEPT  1
 #endif
diff --git a/src/os/unix/ngx_posix_config.h b/src/os/unix/ngx_posix_config.h
index 8a95099..f796576 100644
--- a/src/os/unix/ngx_posix_config.h
+++ b/src/os/unix/ngx_posix_config.h
@@ -88,6 +88,9 @@
 #endif
 
 
+#define NGX_LISTEN_BACKLOG  511
+
+
 #if (__FreeBSD__) && (__FreeBSD_version < 400017)
 
 #include <sys/param.h>          /* ALIGN() */
diff --git a/src/os/unix/ngx_solaris_config.h b/src/os/unix/ngx_solaris_config.h
index 9183bb3..989a30e 100644
--- a/src/os/unix/ngx_solaris_config.h
+++ b/src/os/unix/ngx_solaris_config.h
@@ -82,6 +82,9 @@
 #endif
 
 
+#define NGX_LISTEN_BACKLOG           511
+
+
 #ifndef NGX_HAVE_INHERITED_NONBLOCK
 #define NGX_HAVE_INHERITED_NONBLOCK  1
 #endif
diff --git a/src/os/win32/ngx_win32_config.h b/src/os/win32/ngx_win32_config.h
index ceb9349..9dc8ca8 100644
--- a/src/os/win32/ngx_win32_config.h
+++ b/src/os/win32/ngx_win32_config.h
@@ -149,6 +149,9 @@
 #define NGX_WIN_NT        200000
 
 
+#define NGX_LISTEN_BACKLOG           511
+
+
 #ifndef NGX_HAVE_INHERITED_NONBLOCK
 #define NGX_HAVE_INHERITED_NONBLOCK  1
 #endif