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.
1. Port Accessibility Test
:: 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
pathping -n -q 10 -p 250 serverip
Packet loss >5% at any hop indicates ISP routing issue.
3. DNS Resolution Verification
nslookup serverdomain nslookup serverdomain 8.8.8.8
Mismatched results indicate DNS poisoning.
1. Windows Firewall Rules
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
:: 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
netsh winsock reset catalog netsh int ip reset reset.log ipconfig /flushdns
1. Endpoint Visibility
-- server.cfg set sv_endpointprivacy false set sv_listingIPOverride "serverip:30120" set sv_forceIndirectListing true
2. Connection Queue Management
-- Increase slots and timeout set sv_maxClients 128 set sv_connectTimeout 300 set sv_authMaxRetries 10
3. Rate Limiting Adjustments
-- Prevent false positive DoS detection set sv_requestParanoia 0 set sv_rateLimit 0
1. Nucleus Server Connectivity
:: 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
-- server.cfg set sv_licenseKey "cfxk_your_32_char_key" restart sessionmanager
3. Cache Corruption
:: Client-side nucleus cache rmdir /s /q "%appdata%CitizenFXkvs" del "%localappdata%FiveMFiveM.appcaches.xml"
1. CGNAT Bypass
:: 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
:: 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:
-- server.cfg endpoint_add_tcp "0.0.0.0:30121" endpoint_add_udp "0.0.0.0:30121"
Client connects via: connect serverip:30121
1. Packet Capture Analysis
:: Wireshark filter tcp.port == 30120 or udp.port >= 30110 and udp.port <= 30125
Look for RST packets or missing ACKs.
2. Connection State Monitoring
-- F8 console during connection netstats 1 netgraph 1
3. TLS Certificate Issues
:: Verify certificate chain openssl s_client -connect servers-frontend.fivem.net:443 -servername servers-frontend.fivem.net
VPN Conflicts
- Disable split tunneling
- Exclude FiveM from VPN:
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:
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.
Related resources
Official references and next steps
Use these official references to verify the technical steps before changing a live FiveM server.
Related FiveMX guides: