doc: document IPv4-only relays

Author: missytake

doc/source/getting_started.rst

@@ -14,8 +14,6 @@ Minimal requirements and prerequisites
 
 You will need the following:
 
--  Control over a domain through a DNS provider of your choice.
-
 -  A Debian 12 **deployment server** with reachable SMTP/SUBMISSIONS/IMAPS/HTTPS ports.
    IPv6 is encouraged if available. Chatmail relay servers only require
    1GB RAM, one CPU, and perhaps 10GB storage for a few thousand active
@@ -28,6 +26,9 @@ You will need the following:
    (An ed25519 private key is required due to an `upstream bug in
    paramiko <https://github.com/paramiko/paramiko/issues/2191>`_)
 
+-  Control over a domain through a DNS provider of your choice
+   (there is experimental support for `DNS-less relays <iponly.html>`_).
+
 
 Setup with ``scripts/cmdeploy``
 -------------------------------------

doc/source/index.rst

@@ -18,3 +18,4 @@ Contributions and feedback welcome through the https://github.com/chatmail/relay
     overview
     related
     faq
+    iponly

doc/source/iponly.rst

@@ -0,0 +1,24 @@
+
+Hosting without DNS records
+===========================
+
+In case you don't have a domain,
+for example in a local network,
+you can run a chatmail relay with only an IPv4 address as well.
+
+To deploy a relay without a domain,
+run ``cmdeploy init`` with only the IPv4 address
+during the `installation steps <getting_started.html>`_,
+for example ``cmdeploy init 13.12.23.42``.
+
+Drawbacks
+---------
+
+- your transport encryption will only use self-signed TLS certificates,
+  which are vulnerable against MITM attacks.
+  the chatmail core's end-to-end encryption should suffice in most scenarios though.
+
+- your messages will not be DKIM-signed;
+  experimentally, most chatmail relays accept non-DKIM-signed messages from IPv4-only relays,
+  but some relays might not accept messages from yours.
+