diff --git a/content/blog/cat.md b/content/blog/cat.md
index 6ec584b..798754f 100644
--- a/content/blog/cat.md
+++ b/content/blog/cat.md
@@ -106,6 +106,8 @@ Linux 2.6.33.  Unfortunately, sendfile is not perfect: because it
 only supports file descriptors which can be memory mapped, we 
 must use a different strategy when using copying from `stdin`.
 
+   [sf-origin]: https://yarchive.net/comp/linux/sendfile.html
+
 ```c
 /* This program is released into the public domain. */
 #include <stdbool.h>