That's what is enforced by the kernel: the 'port' is used to create a
new listening socket on that port, not to create a new subflow from/to
that port. It then requires the 'signal' flag.

Acked-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 man/man8/ip-mptcp.8 | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/man/man8/ip-mptcp.8 b/man/man8/ip-mptcp.8
index 2b693564..11df43ce 100644
--- a/man/man8/ip-mptcp.8
+++ b/man/man8/ip-mptcp.8
@@ -133,7 +133,10 @@ is 0.
 When a port number is specified, incoming MPTCP subflows for already
 established MPTCP sockets will be accepted on the specified port, regardless
 the original listener port accepting the first MPTCP subflow and/or
-this peer being actually on the client side.
+this peer being actually on the client side. This option has to be used in
+combination with the
+.BR signal
+flag.
 
 .TP
 .IR IFNAME

-- 
2.45.2