diff --git a/dnsapi.md b/dnsapi.md
index f5805aa..8fdf794 100644
--- a/dnsapi.md
+++ b/dnsapi.md
@@ -274,6 +274,8 @@ See [How to use OVH domain API](https://github.com/acmesh-official/acme.sh/wiki/
 <a name="dns_nsupdate" />
 
 ## 7. Use nsupdate to automatically issue cert
+ncupdate is [RFC 2136 Dynamic DNS Update](https://datatracker.ietf.org/doc/html/rfc2136.html) client.
+[Man page](https://bind9.readthedocs.io/en/v9.18.19/manpages.html#nsupdate-dynamic-dns-update-utility)
 
 First, generate a key for updating the zone
 ```sh