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From: NeilBrown <neilb@suse.de>
To: Steve Dickson <steved@redhat.com>
Cc: linux-nfs@vger.kernel.org,
	Petr Vorel <pvorel@suse.cz>
Subject: [PATCH 1/4] manpage: describe use of extra port for broadcast rpc
Date: Mon, 26 Feb 2024 10:53:53 +1100
Message-ID: <20240225235628.12473-2-neilb@suse.de>
In-Reply-To: <20240225235628.12473-1-neilb@suse.de>
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From: NeilBrown <neilb@suse.com>

Some people notice the extra privileged UDP port that
rpcbind creates, and wonder what it is for.  So add
a section to the man page to explain it.

Signed-off-by: NeilBrown <neilb@suse.com>
---
 man/rpcbind.8 | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/man/rpcbind.8 b/man/rpcbind.8
index fbf0ace24b27..6ba318f5ff77 100644
--- a/man/rpcbind.8
+++ b/man/rpcbind.8
@@ -66,6 +66,25 @@ reports the condition and terminates.
 The
 .Nm
 utility can only be started by the super-user.
+.Sh "BROADCAST RPC"
+.Nm
+supports a little-used part of the ONC-RPC specification known as
+Broadcast RPC.
+A client can send a UDP broadcast message to
+.Nm
+on every host on a local subnetwork, and each
+.Nm
+will forward the request to the local service if available.
+Should the service reply,
+.Nm
+will forward that reply back to the originator.
+To support this,
+.Nm
+creates an extra UDP socket bound to an arbitrary privileged port
+number, and uses it to forward requests to local services and to
+receive replies from them.
+When configuring a firewall, the "port 111" sockets may need to
+be accessible through the firewall, but the extra UDP socket does not.
 .Sh OPTIONS
 .Bl -tag -width indent
 .It Fl a
-- 
2.43.0