ReverseHello
ReverseHello is the OPC UA TCP message, type code RHE, that a server sends immediately after dialling out to a listening client. It carries the server's ServerUri and EndpointUrl. The client answers with the ordinary Hello on the same socket and the handshake proceeds exactly as if the client had dialled.
Two fields, both load-bearing
ServerUri is the server's ApplicationUri, which the client uses to decide whether it was expecting this server at all and which configuration entry applies. EndpointUrl is the endpoint the server wants to be reached on, and the client must have that exact string in its configuration. Neither is negotiable after the fact: the client matches on them and either proceeds or drops the socket.
Where it sits among the message types
The OPC UA TCP protocol uses four-character message type codes: HEL and ACK for the handshake, RHE for ReverseHello, ERR for protocol errors, OPN, MSG and CLO for secure channel traffic. RHE is the only one sent server-first. Everything after it - buffer size negotiation in HEL/ACK, then OPN - is the standard sequence, so a packet capture of a reverse connection differs from a normal one by exactly one leading frame.
Rejection semantics
A client that cannot or will not accept the connection replies with BadTcpMessageTypeInvalid, and the server must then stop retrying that client. This makes the code diagnostic: seeing it in a server log means the target port was open and something answered, but that something was not a reverse-connect-capable OPC UA client - frequently a load balancer, a health check, or a client with the feature simply not enabled.
Example
Message sequence, reverse versus normal
Normal connect Reverse connect
--------------- -----------------------------
client dials server dials
-> HEL <- RHE ServerUri, EndpointUrl
<- ACK -> HEL
-> OPN <- ACK
<- OPN -> OPN ...
-> MSG (CreateSession) identical from here on
Client does not support RHE -> BadTcpMessageTypeInvalid, server aborts retries.Defined in: Part 6 - Mappings, OPC UA TCP message types including ReverseHello.
Last verified 7 September 2026 against OPC UA 1.05.07. OPC UA Connect is an educational resource and is not affiliated with the OPC Foundation.