Question

I have an open TCP connection and read from it with a for loop like so

for {
  // tx.Text is of type textproto.Conn
  // the underlying connection is stored in tx.Conn
  l, err := tx.Text.Reader.ReadLine()

  // do stuff with the text line ...
}

Now I want to upgrade the connection to TLS like this (TlsConf contains a certificate loaded with tls.LoadX509KeyPair)

tx.Conn = tls.Server(tx.Conn, tx.Server.Conf.TlsConf)
tx.Text = textproto.NewConn(tx.Conn)

When I do this I get a segmentation fault on the client when the server attempts a handshake. I’m implementing a SMTP server and am testing it with swaks using the -tls flag. The terminal output of swaks is the following

-> STARTTLS
<-  220 Start TLS
Segmentation fault: 11

Since swaks is a tested tool, and worked with the nodeJS SMTP implementation I had before, I don’t suspect the error is on the client side.

What did I do wrong or what is missing?

PS: When a TLS connection is started from an existing insecure connection, what does exactly happen? Does the client establish a new connection on a different port or is the connection reused?

Answer 1

Ditched swaks, built a small tool to test TLS using Go’s own smtp.SendMail:

package main

import (
  "fmt"
  "net/smtp"
)

func main() {
  err := smtp.SendMail(
    "127.0.0.1:2525",
    nil,
    "[e-mail]",
    []string{"[e-mail]"},
    []byte("Hello! Just testing."),
  )
  if err != nil {
    panic(err)
  }
}