[Bug]: TCP Port Forwarding Fails: Connection succeeds but data stream times out

[Lukas2019]

I have done the following

• I have searched the existing issues
• If possible, I've reproduced the issue using the 'main' branch of this project

Steps to reproduce

The following steps use a public mail server container to reliably reproduce the issue.

1. On macOS 26, run the marlonb/mailcrab:latest container and map the SMTP port:

   container run --rm -p 1025:1025 marlonb/mailcrab:latest

2. From the host terminal, confirm that the port is open and the TCP handshake succeeds using nc. This step will report instant success.

   nc -vz -4 localhost 1025

   Successful Output: Connection to localhost port 1025 [tcp/blackjack] succeeded!

3. Next, attempt to communicate with the service using a protocol-aware client like curl. This step will hang for 2 minutes and fail with a timeout.

   # This command attempts to send a basic email via SMTP
   curl -v smtp://127.0.0.1:1025 --mail-from "test@example.com" --mail-rcpt "test@example.com" --upload-file /dev/null

Current behavior

The curl command successfully establishes the TCP connection but then times out waiting for the initial server response, which never arrives. The connection is held open for 2 minutes before failing.

The exact verbose output is:

  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0*   Trying 127.0.0.1:1025...
* Connected to 127.0.0.1 (127.0.0.1) port 1025
  0     0    0     0    0     0      0      0 --:--:--  0:01:59 --:--:--     0* server response timeout
  0     0    0     0    0     0      0      0 --:--:--  0:02:00 --:--:--     0
* Closing connection
curl: (28) server response timeout

This demonstrates that the initial connection is accepted (* Connected to ...), but the data proxy is failing, causing the timeout.

Expected behavior

The curl command should successfully connect and communicate with the SMTP server inside the container, receiving a 220 mailcrab welcome banner and subsequent protocol messages, and then exit cleanly.

Environment

- OS: 26.0.1 
- Container: 0.5.0

Relevant log output

N/A

Code of Conduct

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