When setting up OpenVPN 2.3.10 on Windows Server 2012 behind a NAT router, clients encounter TLS handshake timeouts despite proper port forwarding (UDP 1194). The server logs show these telltale messages:

Mon Mar 21 11:12:48 2016 XX.XX.XX.XX:57804 TLS Error: TLS key negotiation failed to occur within 60 seconds
Mon Mar 21 11:12:48 2016 XX.XX.XX.XX:57804 TLS Error: TLS handshake failed

Before diving deeper, let's verify basic connectivity:

# On client machine (Linux example):
nc -vu [SERVER_PUBLIC_IP] 1194

# Windows equivalent:
Test-NetConnection -ComputerName [SERVER_PUBLIC_IP] -Port 1194 -Udp

If these succeed but OpenVPN still fails, we're likely facing a protocol/configuration issue rather than a network one.

Your server.ovpn needs these essential directives:

port 1194
proto udp
dev tun
tls-server
tls-auth ta.key 0
cipher AES-256-CBC
auth SHA512
tls-timeout 120  # Extended from default 60 seconds
keepalive 10 60
persist-key
persist-tun

The client configuration must mirror these settings:

client
dev tun
proto udp
remote [SERVER_PUBLIC_IP] 1194
resolv-retry infinite
nobind
tls-client
tls-auth ta.key 1
cipher AES-256-CBC
auth SHA512

When basic checks don't resolve the issue:

# On server:
openvpn --verb 4 --config server.ovpn

# On client:
openvpn --verb 4 --config client.ovpn

Key things to verify in verbose logs:

• Certificate validity periods
• Clock synchronization (NTP drift)
• MTU mismatches (try tun-mtu 1500)
• Enterprise firewall deep packet inspection

For Windows Server 2012 deployments:

# In server.ovpn:
server 10.8.0.0 255.255.255.0
route 192.168.1.0 255.255.255.0
push "route 192.168.1.0 255.255.255.0"
push "dhcp-option DNS 8.8.8.8"

Don't forget Windows Firewall rules for OpenVPN:

netsh advfirewall firewall add rule name="OpenVPN" dir=in action=allow protocol=UDP localport=1194

As last resort, try TCP fallback with these changes:

# server.ovpn:
proto tcp-server

# client.ovpn:
proto tcp-client

This often works where UDP gets blocked by intermediate devices.

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When your OpenVPN server (v2.3.10) on Windows Server 2012 shows these logs:

Mon Mar 21 11:12:48 2016 XX.XX.XX.XX:57804 TLS Error: TLS key negotiation failed to occur within 60 seconds
Mon Mar 21 11:12:48 2016 XX.XX.XX.XX:57804 TLS Error: TLS handshake failed

It indicates the initial TCP connection succeeds (port forwarding works), but the TLS handshake times out. This suggests either cryptographic configuration mismatches or network-level interference.

First verify these elements in both server and client configs:

# Server configuration minimum requirements (server.ovpn)
proto udp
port 1194
dev tun
tls-server
tls-auth ta.key 0
cipher AES-256-CBC
auth SHA256
dh dh2048.pem
ca ca.crt
cert server.crt
key server.key

The client must mirror these cryptographic settings:

# Client configuration (client.ovpn)
proto udp
remote your.server.ip 1194
tls-client
tls-auth ta.key 1
cipher AES-256-CBC
auth SHA256
ca ca.crt
cert client.crt
key client.key

1. Test UDP connectivity (OpenVPN's default):

nc -uvz your.server.ip 1194

2. Verify MTU settings - Add this to both configs:

mssfix 1200
fragment 1300

3. Check for NAT traversal issues:

# Add to server config
push "redirect-gateway def1 bypass-dhcp"
push "dhcp-option DNS 8.8.8.8"

Enable verbose logging by adding to both configs:

verb 4
mute 10

For Windows-specific issues, ensure:

# In server.ovpn
server-bridge
route-method exe
route-delay 2

If UDP continues failing, test with TCP (temporarily):

# In both configs
proto tcp
# Then test connection:
telnet your.server.ip 1194

Remember to revert to UDP for production after testing.

Generate fresh certificates if needed:

# On Linux build machine
./easyrsa build-client-full client nopass
./easyrsa gen-dh
openvpn --genkey --secret ta.key

Ensure all certs are transferred with proper permissions (Windows often needs admin rights for key files).