It was missing, while it is a very important option.

Indeed, without it, the kernel might not pick the right interface to
send packets for additional subflows. Mention that in the man page.

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

diff --git a/man/man8/ip-mptcp.8 b/man/man8/ip-mptcp.8
index 6c708957..89fcb64f 100644
--- a/man/man8/ip-mptcp.8
+++ b/man/man8/ip-mptcp.8
@@ -135,6 +135,14 @@ 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.
 
+.TP
+.IR IFNAME
+is the network interface name attached to the endpoint. It is important to
+specify this device name linked to the address to make sure the system knows how
+to route packets from the specified IP address to the correct network interface.
+Without this, it might be required to add IP rules and routes to have the
+expected behavior.
+
 .TP
 .IR ID
 is a unique numeric identifier for the given endpoint

-- 
2.45.2