Hello,
 I'm trying to leverage stunnel with smpp to communicate over by smppS protocol.
I saw an older message from Pierre Delaage (2019)  outlining the process (https://www.stunnel.org/pipermail/stunnel-users/2010-September/002752.html).

Unfortunately, that is not working for me. What may I be overlooking?

Data:
Fedora 39, stunnel 5.72 on x86_64-redhat-linux-gnu platform

stunnel config from /etc/stunnel:

cat ./stunnel.conf
output = /var/log/stunnel.log
foreground = yes

[smpps]
client = yes
accept = 3775
connect = <vendor dnsname>:3550
cert = /tmp/nginx-selfsigned.crt
key=/tmp/nginx-selfsigned.key
sslVersion=all

[root@dell5000 stunnel]# stunnel  ./stunnel.conf
2024.10.16 13:52:44 LOG5[ui]: stunnel 5.72 on x86_64-redhat-linux-gnu platform
2024.10.16 13:52:44 LOG5[ui]: Compiled with OpenSSL 3.1.1 30 May 2023
2024.10.16 13:52:44 LOG5[ui]: Running  with OpenSSL 3.1.4 24 Oct 2023
2024.10.16 13:52:44 LOG5[ui]: Threading:PTHREAD Sockets:POLL,IPv6,SYSTEMD TLS:ENGINE,FIPS,OCSP,PSK,SNI
2024.10.16 13:52:44 LOG5[ui]: Reading configuration from file /etc/stunnel/stunnel.conf
2024.10.16 13:52:44 LOG5[ui]: UTF-8 byte order mark not detected
2024.10.16 13:52:44 LOG5[ui]: FIPS mode disabled
2024.10.16 13:52:44 LOG4[ui]: Service [smpps] needs authentication to prevent MITM attacks
2024.10.16 13:52:44 LOG5[ui]: Configuration successful
2024.10.16 13:53:02 LOG5[0]: Service [smpps] accepted connection from 192.168.1.114:11270
2024.10.16 13:53:02 LOG5[0]: s_connect: connected <vendor IP>:3550
2024.10.16 13:53:02 LOG5[0]: Service [smpps] connected remote server from 192.168.1.114:55508
2024.10.16 13:53:07 LOG5[0]: Connection closed: 293 byte(s) sent to TLS, 0 byte(s) sent to socket

the smpp client is pointed to localhost:3775 and I running in the container. As such the "localhost" is "host.containers.internal".

 Here is the smpp  log:

2024-10-16 17:53:02 INFO     1 Establishing SSL connection to host.containers.internal:3775
2024-10-16 17:53:02 INFO     1 Connecting to IPv4Address(type='TCP', host='host.containers.internal', port=3775) ...
2024-10-16 17:53:02 WARNING  1 SMPP connection established from 192.168.1.114 to port 40554
2024-10-16 17:53:02 INFO     1 Connection made to host.containers.internal:3775
2024-10-16 17:53:02 WARNING  1 Requesting bind as transceiver
2024-10-16 17:53:07 WARNING  1 SMPP 192.168.1.114 disconnected from port 40554: [Failure instance: Traceback (failure with no frames): <class 'twisted.internet.error.ConnectionLost'>: Connection to the other side was lost in a non-clean fashion: Connection lost.
]
2024-10-16 17:53:07 ERROR    1 Connection lost. Reason: [Failure instance: Traceback (failure with no frames): <class 'twisted.internet.error.ConnectionDone'>: Connection was closed cleanly.
]
2024-10-16 17:53:07 INFO     1 Reconnecting after 10 seconds ...
2024-10-16 17:53:17 INFO     1 Connecting to IPv4Address(type='TCP', host='host.containers.internal', port=3775) ...
2024-10-16 17:53:17 WARNING  1 SMPP connection established from 192.168.1.114 to port 54806
2024-10-16 17:53:17 INFO     1 Connection made to host.containers.internal:3775
2024-10-16 17:53:17 WARNING  1 Requesting bind as transceiver
2024-10-16 17:53:22 WARNING  1 SMPP 192.168.1.114 disconnected from port 54806: [Failure instance: Traceback (failure with no frames): <class 'twisted.internet.error.ConnectionLost'>: Connection to the other side was lost in a non-clean fashion: Connection lost.
]
2024-10-16 17:53:22 ERROR    1 Connection lost. Reason: [Failure instance: Traceback (failure with no frames): <class 'twisted.internet.error.ConnectionDone'>: Connection was closed cleanly.
]
2024-10-16 17:53:22 INFO     1 Reconnecting after 10 seconds ...
2024-10-16 17:53:32 ERROR    1 Request timed out after 30 secs: PDU [command: CommandId.bind_transceiver, sequence_number: 1, command_status: CommandStatus.ESME_ROK
system_id: b'XXXXX'
password: b'XXXXX'
system_type: b''
interface_version: 52
addr_ton: <AddrTon.UNKNOWN: 1>
addr_npi: <AddrNpi.UNKNOWN: 1>
address_range: None
]
2024-10-16 17:53:32 ERROR    1 Bind failed [[Failure instance: Traceback (failure with no frames): <class 'smpp.pdu.error.SMPPRequestTimoutError'>: Request timed out after 30 secs: PDU [command: CommandId.bind_transceiver, sequence_number: 1, command_status: CommandStatus.ESME_ROK
system_id: b'XXXX'
password: b'XXXXX'
system_type: b''
interface_version: 52
addr_ton: <AddrTon.UNKNOWN: 1>
addr_npi: <AddrNpi.UNKNOWN: 1>
address_range: None
]
]]. Disconnecting...