Get client IP address while client connects to smtp server?

[iredmail]

Dear @emersion

First of all, thanks for the great work. 👍

We're trying to implement an internal SMTP server, and we'd like to reject client as earlier as possible if client IP address is not whitelisted. Currently it's ok to get client IP in Mail() method of Session interface, but we'd like to reject it even earlier. For example, when the client connects to the smtp server.

Could you share some idea of how we can achieve this? Thank you. :)

[emersion]

Maybe we could add a Serve.ServeConn(c net.Conn) function like x/net/http2 has? An alternative would be to add hooks to the Server configuration like net/http has.

Note, you should already be able to do this by implementing a custom net.Listener that filters incoming connections, however it's somewhat boilerplate-y.

[iredmail]

1. Is it better / easier to add a new method for the Session struct? e.g. Connect(addr net.Addr) error, it's called when client connected.
2. Another question: we want to reply with a short message, how to send reply to client like conn.WriteResponse()? i mistakenly used return &smtp.SMTPError{...} even it returns 200 smtp code, client considers it's a failure. :(

[Savageman]

Sorry to intrude, But I'm interested in the same kind of feature for the Helo command, if it can help.

The session is not yet created at connection time (neither Login() or AnonymousLogin() was called). Maybe this hook to the Backend interface instead?

[emersion]

Can you open a new issue an explain your use-case?

[foxcpp]

There is no way to terminate the connection on NewSession though. That's probably can be done via #147. Needs discussion.

[emersion]

Wouldn't an error returned by NewSession terminate the connection?

[foxcpp]

It will return an error response to the client without terminating the connection. Client can issue EHLO/HELO/QUIT in this case.