diff --git a/README.md b/README.md
index 51c6013..a3d7cfe 100644
--- a/README.md
+++ b/README.md
@@ -172,3 +172,6 @@ The latest revision of slirp4netns is regularly benchmarked (`make benchmark`) o
 
 ## Acknowledgement
 See [`vendor/README.md`](./vendor/README.md).
+
+## License
+[GPL-2.0-or-later](COPYING)
diff --git a/api.c b/api.c
index 9bacbc1..49350e8 100644
--- a/api.c
+++ b/api.c
@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
 #define _GNU_SOURCE
 #include <stdio.h>
 #include <stdlib.h>
diff --git a/api.h b/api.h
index ace9611..2dd6eb5 100644
--- a/api.h
+++ b/api.h
@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
 #ifndef SLIRP4NETNS_API_H
 # define SLIRP4NETNS_API_H
 int api_bindlisten(const char *api_socket);
diff --git a/main.c b/main.c
index 2fa3c06..9e03eb1 100644
--- a/main.c
+++ b/main.c
@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
 #define _GNU_SOURCE
 #include "config.h"
 #include <stdio.h>
diff --git a/slirp4netns.c b/slirp4netns.c
index f947111..32ccf34 100644
--- a/slirp4netns.c
+++ b/slirp4netns.c
@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
 #define _GNU_SOURCE
 #include <errno.h>
 #include <signal.h>
@@ -80,7 +81,8 @@ static void libslirp_register_poll_fd(int fd, void *opaque)
     /*
      * NOP
      *
-     * This is NOP on QEMU upstream on Linux as well, see:
+     * This is NOP on QEMU@4c76137484878f42a2ce1ae1b888b6a7f66b4053 on Linux as
+     * well, see:
      *  * qemu/net/slirp.c:          net_slirp_register_poll_fd (calls
      * qemu_fd_register)
      *  * qemu/stubs/fd-register.c:  qemu_fd_register (NOP on Linux)
@@ -96,7 +98,8 @@ static void libslirp_unregister_poll_fd(int fd, void *opaque)
     /*
      * NOP
      *
-     * This is NOP on QEMU upstream as well, see:
+     * This is NOP on QEMU@4c76137484878f42a2ce1ae1b888b6a7f66b4053 as well,
+     * see:
      *  * qemu/net/slirp.c:          net_slirp_unregister_poll_fd (NOP)
      */
 }
@@ -107,7 +110,8 @@ static void libslirp_notify(void *opaque)
     /*
      * NOP
      *
-     * This can be NOP on QEMU upstream as well, see:
+     * This can be NOP on QEMU@4c76137484878f42a2ce1ae1b888b6a7f66b4053 as well,
+     * see:
      *  * qemu/net/slirp.c:          net_slirp_notify (calls qemu_notify_event)
      *  * qemu/stubs/notify-event.c: qemu_notify_event (NOP)
      *
@@ -141,6 +145,7 @@ static int libslirp_poll_to_gio(int events)
 /*
  * implements SlirpAddPollCb used in slirp_pollfds_fill.
  * originally from qemu/net/slirp.c:net_slirp_add_poll
+ * (4c76137484878f42a2ce1ae1b888b6a7f66b4053)
  */
 static int libslirp_add_poll(int fd, int events, void *opaque)
 {
@@ -178,6 +183,7 @@ static int libslirp_gio_to_poll(int events)
 /*
  * implements SlirpGetREventsCB used in slirp_pollfds_poll
  * originally from qemu/net/slirp.c:net_slirp_get_revents
+ * (4c76137484878f42a2ce1ae1b888b6a7f66b4053)
  */
 static int libslirp_get_revents(int idx, void *opaque)
 {
diff --git a/slirp4netns.h b/slirp4netns.h
index 1faff77..48f0597 100644
--- a/slirp4netns.h
+++ b/slirp4netns.h
@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
 #ifndef SLIRP4NETNS_H
 # define SLIRP4NETNS_H
 #include <arpa/inet.h>