[gnutls-devel] GnuTLS | DTLS Handshake Failure: Error in the push/pull function during communication over UDP on port 12345 (#1707)

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Jennifer-first created an issue: https://gitlab.com/gnutls/gnutls/-/issues/1707



## Description of problem:
I am attempting to establish a DTLS connection over UDP between a client and server using GnuTLS. The client successfully sends a message to the server, and the server receives and prints "Hello, server!". However, when attempting to send additional data after this initial message, the DTLS handshake fails with the following errors:
Client Side:
Fatal error: Error in the push function
Could not connect to 127.0.0.1:12345: Connection refused
Server Side:
DTLS Handshake failed: Error in the pull function.

I have verified that the server listens on port 12345.
I checked for firewall or network issues that could block UDP communication.
I attempted to modify the MTU settings in the GnuTLS session.
I also tried running the server without using the certificates to isolate the problem.

## Version of gnutls used:
gnutls 3.7.3

## Distributor of gnutls (e.g., Ubuntu, Fedora, RHEL)
Ubuntu

## How reproducible:

Steps to Reproduce:

 * server:gcc dtls_server.c -o dtls_server $(pkg-config --cflags --libs gnutls)
 * server:./dtls_server
 * client:gnutls-cli --udp --port 12345 127.0.0.1 --insecure

## Actual results:
-port 12345 127.0.0.1 --insecure
Processed 0 CA certificate(s).
Resolving '127.0.0.1:12345'...
Connecting to '127.0.0.1:12345'...
*** Fatal error: Error in the push function.
Could not connect to 127.0.0.1:12345: Connection refused
Listening on UDP port 12345...
Received initial packet from client 127.0.0.1:53900
DTLS Handshake failed: Error in the pull function.

## Expected results:
DTLS Handshake succeeded!

[26973.zip](/uploads/9e3af574dd8190fcd80e845b1c43b47b/26973.zip)

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