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Author(s) Olivier Bonaventure
Deadline No deadline
Submission limit No limitation
Category tags tls

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SMTP over TLS


Question 1: The SMTP server

What is the name of the SMTP server ?

Question 2: Version of TLS

What is the version of the TLS protocol used for this session ?

Question 3: Latency

How many messages are sent by the client before transmitting its first encrypted record ?

Question 4: A simple SMTP over TLS trace

The SMTP protocol was originally used directly above TCP. All emails were sent in clear text between SMTP servers. Today, most SMTP servers run SMTP over TLS over TCP. The attached trace provides a sample connection.


0000  0004fffe00000000  00000000000086dd  
0010  600b987300280640  200106a83081a001  
0020  0130010402400230  2603102602070014  
0030  0000000000000002  aa14024b84ad8857  
0040  00000000a0007080  9ad50000          

0000  ..ÿþ...........Ý
0010  `..s.(.@ ..¨0...
0020  .0...@.0&..&....
0030  ........ª..K...W
0040  ......p..Õ..    
  • Frame 1: 96 bytes on wire (768 bits), 96 bytes captured (768 bits)
    • Linux cooked capture
      • Packet type: Sent by us (4)
        • Link-layer address type: 65534
          • Link-layer address length: 0
            • Unused: 0000000000000000
              • Protocol: IPv6 (0x86dd)
              • Internet Protocol Version 6, Src: 2001:6a8:3081:a001:130:104:240:230, Dst: 2603:1026:207:14::2
                • 0110 .... = Version: 6
                  • .... 0000 0000 .... .... .... .... .... = Traffic Class: 0x00 (DSCP: CS0, ECN: Not-ECT)
                    • .... 0000 00.. .... .... .... .... .... = Differentiated Services Codepoint: Default (0)
                      • .... .... ..00 .... .... .... .... .... = Explicit Congestion Notification: Not ECN-Capable Transport (0)
                      • .... .... .... 1011 1001 1000 0111 0011 = Flow Label: 0xb9873
                        • Payload Length: 40
                          • Next Header: TCP (6)
                            • Hop Limit: 64
                              • Source: 2001:6a8:3081:a001:130:104:240:230
                                • Destination: 2603:1026:207:14::2
                                • Transmission Control Protocol, Src Port: 43540, Dst Port: 587, Seq: 0, Len: 0
                                  • Source Port: 43540
                                    • Destination Port: 587
                                      • TCP Segment Len: 0
                                        • Sequence number: 0 (relative sequence number)
                                          • Acknowledgment number: 0
                                            • 1010 .... = Header Length: 40 bytes (10)
                                              • Window size value: 28800
                                                • Calculated window size: 28800
                                                  • Checksum: 0x9ad5 [unverified]
                                                    • Urgent pointer: 0