SMTP client TLS handshake not sent

[gawelkot]

Hello,
I have problem with running SMTP client when using TLS. Everything is fine when use SMTP server without TLS and just omit authorization part of transmission.

To simplify things I created empty project with Mongoose Wizard on NUCLEO-H753ZI. The only think I added is 5s timer which connects to smtp server exactly as in smtp client tutorial. I am using built-in TLS.

From the mongoose log and wireshark it's visible that the client handshake is not being sent and the connection closes due to timeout. Am I missing something or is there any way to debug this a bit more to find the reason? I tried with 3 different smtp servers and situation is the same for each. I will appreciate any help, thank you.

Mongoose log:

2712 3 mongoose.c:3971:mg_connect 3 -1 tcp://smtp.office365.com:587
2720 3 mongoose.c:288:dns_cb 3 ooc-g2.tm-4.office.com is 40.99.218.146
2720 3 mongoose.c:5162:mg_connect_res 3 192.168.1.100:43447 -> 40.99.218.146:587
2739 2 mongoose_impl.c:630:fn <-- 220 WA1P291CA0013.outlook.office365.com Microsoft ESMTP MAIL Service ready at Thu, 23 Jan 2025 10:25:36 +0000 [08DD3B7858DD3449]
2739 2 mongoose_impl.c:676:fn --> EHLO myname
2745 2 mongoose_impl.c:630:fn <-- 250-WA1P291CA0013.outlook.office365.com Hello [83.144.90.202]
250-SIZE 157286400
250-PIPELINING
250-DSN
250-ENHANCEDSTATUSCODES
250-STARTTLS
250-8BITMIME
250-BINARYMIME
250-CHUNKING
250 SMTPUTF8
2745 2 mongoose_impl.c:676:fn --> STARTTLS
274f 2 mongoose_impl.c:630:fn <-- 220 2.0.0 SMTP server ready
2751 2 mongoose_impl.c:676:fn -->

Wireshark log:

[scaprile]

Did you try the tutorial example as is before trying to add it into your Wizard code ?

[gawelkot]

I did try tutorial from 7.16 tag in repo almost as is. I had to do 2 modifications to make it running.

1. I had to wait for resolving IP by DHCP because raw code from tutorial fails every time as DNS is not available yet.
2. I replaced ca.pem file from filesystem with C string.

But the result is exactly the same. I made it through wizard to make sure everything else is working except of smtp client.

[scaprile]

The tutorial has been written as a simple example on a platform with an OS, it is not production code for embedded, hence your need to make mods.

I just tried the example as is, with Google. It works with MbedTLS, as it always did.
It fails with our bultin TLS right where the tutorial explains. A verbose log shows

133bd869 2 main_gmail_cesanta.c:23:fn   <-- 220 2.0.0 Ready to start TLS
133bd869 4 tls_builtin.c:1303:mg_tls_in no certificate provided
133bd869 2 main_gmail_cesanta.c:65:fn   --> 

Given that the certificate set is the same, it is probably the server not supporting our TLS1.3. To pinpoint the exact reason, more in depth debugging should be done.
I'll mark this internally as a probable issue to verify, we'll eventually get there on a priority basis. If you are a paying customer, please contact Support as per your contract and we'll raise its priority.
Thank you

[gawelkot]

Thank you, switching to MbedTLS helped and it fails on cert validation as in tutorial. Looks like built-in TLS is not handling it correctly, at least without any required modifications.

[scaprile]

Here's the fix #3023