FiveM Connection Failed: Diagnostic and Resolution Guide

FiveM connection failures occur at TCP handshake (30120), UDP stream initialization (30110-30125), or authentication timeout with nucleus servers. Error presents as Failed to connect to server: Connection timed out after 15-second threshold.

Diagnostic Framework

Network path validation
Server availability confirmation
Client-side interference elimination
Authentication chain verification

If you are server-owner, make sure to host your server on a good host.

Network Diagnostics

1. Port Accessibility Test

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:: TCP connectivity
powershell -command "Test-NetConnection -ComputerName serverip -Port 30120"

:: UDP requires nmap
nmap -sU -p 30110-30125 serverip

Expected: TcpTestSucceeded: True

2. Traceroute Analysis

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pathping -n -q 10 -p 250 serverip

Packet loss >5% at any hop indicates ISP routing issue.

3. DNS Resolution Verification

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nslookup serverdomain
nslookup serverdomain 8.8.8.8

Mismatched results indicate DNS poisoning.

Client-Side Solutions

1. Windows Firewall Rules

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New-NetFirewallRule -DisplayName "FiveM TCP" -Direction Outbound -Protocol TCP -LocalPort 30120 -Action Allow
New-NetFirewallRule -DisplayName "FiveM UDP" -Direction Outbound -Protocol UDP -LocalPort 30110-30125 -Action Allow
New-NetFirewallRule -DisplayName "FiveM Inbound" -Direction Inbound -Protocol UDP -LocalPort 30110-30125 -Action Allow

2. IPv6 Interference Fix

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:: Disable IPv6 for FiveM
netsh int ipv6 set prefixpolicy ::ffff:0:0/96 46 4
netsh int ipv6 set prefixpolicy ::1/128 45 4

3. Winsock Reset

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netsh winsock reset catalog
netsh int ip reset reset.log
ipconfig /flushdns

Server Configuration Fixes

1. Endpoint Visibility

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-- server.cfg
set sv_endpointprivacy false
set sv_listingIPOverride "serverip:30120"
set sv_forceIndirectListing true

2. Connection Queue Management

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-- Increase slots and timeout
set sv_maxClients 128
set sv_connectTimeout 300
set sv_authMaxRetries 10

3. Rate Limiting Adjustments

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-- Prevent false positive DoS detection
set sv_requestParanoia 0
set sv_rateLimit 0

Authentication Chain Repair

1. Nucleus Server Connectivity

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:: Test Cfx.re services
curl -I https://servers-frontend.fivem.net/api/servers/single/serverid
curl -I https://nucleus.cfx.re/

HTTP 200 required for both.

2. License Key Validation

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-- server.cfg
set sv_licenseKey "cfxk_your_32_char_key"
restart sessionmanager

3. Cache Corruption

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:: Client-side nucleus cache
rmdir /s /q "%appdata%\CitizenFX\kvs"
del "%localappdata%\FiveM\FiveM.app\caches.xml"

ISP-Specific Workarounds

1. CGNAT Bypass

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:: Force IPv4 stack
netsh int ipv6 6to4 set state disabled
netsh int ipv6 isatap set state disabled
netsh int ipv6 set teredo disabled

2. MTU Discovery

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:: Find optimal MTU
ping -f -l 1500 serverip
:: Decrease by 28 until successful
netsh int ipv4 set subinterface "Ethernet" mtu=1472 store=persistent

3. Alternative Port Configuration

Server owner must configure:

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-- server.cfg
endpoint_add_tcp "0.0.0.0:30121"
endpoint_add_udp "0.0.0.0:30121"

Client connects via: connect serverip:30121

Advanced Debugging

1. Packet Capture Analysis

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:: Wireshark filter
tcp.port == 30120 or udp.port >= 30110 and udp.port <= 30125

Look for RST packets or missing ACKs.

2. Connection State Monitoring

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-- F8 console during connection
netstats 1
netgraph 1

3. TLS Certificate Issues

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:: Verify certificate chain
openssl s_client -connect servers-frontend.fivem.net:443 -servername servers-frontend.fivem.net

Platform-Specific Issues

VPN Conflicts

Disable split tunneling
Exclude FiveM from VPN:

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route add serverip mask 255.255.255.255 192.168.1.1

University/Corporate Networks

Request firewall exceptions for:
- TCP 30120 outbound
- UDP 30110-30125 bidirectional
- TCP 443 to nucleus.cfx.re
  Mobile Hotspot Limitations
  - Carrier-grade NAT blocks P2P
  - Use TCP-only endpoint:

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connect serverip:30120+tcp

Uncertainties

Cloudflare Workers intermittently block FiveM API calls (false positive bot detection)
Windows 11 Network Location Awareness service may require manual restart
Some Xfinity modems silently drop UDP packets >1400 bytes

Standards Reference

RFC 4787 (NAT Behavioral Requirements)
CitizenFX Protocol Specification v2.4.1
QUIC-like UDP streaming (custom implementation)

Conclusion: Connection failures stem from blocked ports, MTU mismatches, or authentication timeouts requiring systematic network path validation.

Diagnostic Framework

Network Diagnostics

1. Port Accessibility Test

2. Traceroute Analysis

3. DNS Resolution Verification

Client-Side Solutions

1. Windows Firewall Rules

2. IPv6 Interference Fix

3. Winsock Reset

Server Configuration Fixes

1. Endpoint Visibility

2. Connection Queue Management

3. Rate Limiting Adjustments

Authentication Chain Repair

1. Nucleus Server Connectivity

2. License Key Validation

3. Cache Corruption

ISP-Specific Workarounds

1. CGNAT Bypass

2. MTU Discovery

3. Alternative Port Configuration

Advanced Debugging

1. Packet Capture Analysis

2. Connection State Monitoring

3. TLS Certificate Issues

Platform-Specific Issues

VPN Conflicts

University/Corporate Networks

Mobile Hotspot Limitations

Uncertainties

Standards Reference

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