Layered DDoS Response Plan for MU Online Servers

A distributed denial-of-service (DDoS) attack is, for most MU Online server administrators, a matter of when, not if. Competition between projects, grudges from banned players, and even attacks for entertainment make the GameServer and ConnectServer ports frequent targets. The difference between a server that survives these attacks with its credibility intact and one that loses its player base lies in having a layered response plan prepared before the incident, not improvised during it. This tutorial presents a plan structured around five defense layers, the role each team member plays during the attack, and the steps for communicating with the community — all designed for the reality of mid-sized private servers.

Why MU servers are frequent targets

Private MU Online has an unusual competitive dynamic: servers compete directly for the same player base, and population rankings move donation revenue. This creates a real incentive for sabotage. On top of that, players banned for cheating or toxic behavior often have enough technical knowledge to hire cheap, publicly available "booters" (stress/DDoS services). Launch events, season resets, and weekends with paid events are the highest-risk moments, because that's when the damage from an attack is maximized.

The five defense layers

The core idea of a "layered" plan is that no single layer solves everything on its own — each one covers a different attack type and reduces the surface that reaches the next layer.

Layer 1 — Network edge protection (upstream)

This is the defense against volumetric attacks (hundreds of Mbps to several Gbps), which your server alone will never be able to absorb, regardless of local firewall rules. The solution is to contract hosting with built-in anti-DDoS at the network layer — providers like OVH (with its "Game" anti-DDoS), Voxility, or dedicated game-scrubbing services. The typical setup involves announcing your IP via BGP through the protection provider, which filters malicious traffic before routing clean traffic to your real server.

A common mistake is assuming a generic cloud VPS (without game-specific anti-DDoS) solves the problem just because it has "basic DDoS protection" — that protection usually covers HTTP/HTTPS only, not game traffic on custom ports like 44405 (ConnectServer) or 55901+ (GameServer).

Layer 2 — Dedicated game proxy

Between the internet and your real GameServer, place a proxy that absorbs and filters connections before forwarding to the game server. This hides your server's real IP (making targeted attacks harder) and lets you terminate suspicious connections without overloading the emulator process. A common setup uses iptables with a reverse-proxy IP on a cheap VPS dedicated to that function, redirecting with DNAT:

# Simplified DNAT example to redirect proxy traffic to the real server
iptables -t nat -A PREROUTING -p tcp --dport 44405 -j DNAT --to-destination 10.0.0.5:44405
iptables -t nat -A POSTROUTING -j MASQUERADE

If the proxy falls under attack, you swap the public IP and update DNS/launcher quickly — the real server is never directly exposed.

Layer 3 — Local firewall and connection limiting

Even with upstream protection in place, configure local limits to contain smaller-scale attacks (connection floods, mass login attempts). An example iptables rule limiting new connections per IP on the ConnectServer port:

iptables -A INPUT -p tcp --dport 44405 -m connlimit --connlimit-above 10 -j DROP
iptables -A INPUT -p tcp --syn -m limit --limit 1/s --limit-burst 5 -j ACCEPT
iptables -A INPUT -p tcp --syn -j DROP

Complement this with fail2ban watching ConnectServer logs to ban IPs attempting repeated authentication in a short interval — a common pattern for both account bots and flood testing.

Layer 4 — Application (emulator) tuning

Most emulators (IGCN, MuEMU, X-Team) have parameters for packets-per-second limits and simultaneous connections per IP on the ConnectServer. Enable them even if the default seems sufficient — during a real attack, the volume of fake connections can saturate the process before the firewall reacts. Also configure a limit on login attempts per IP per minute, to prevent an authentication flood attack from taking down the account database.

Team roles during the incident

A real attack lasts anywhere from minutes to days — having defined roles avoids panic-driven decisions.

Step-by-step activation playbook

1. Detect: monitoring flags an unusual spike in bandwidth/connections (ideally with an automatic alert via Zabbix, Grafana, or a simple threshold script).
2. Confirm: distinguish a real DDoS from a legitimate player spike (e.g. launch with a big queue) by checking source IPs and traffic patterns.
3. Isolate: if the game proxy (layer 2) is active, swap the exposed public IP and update the launcher/DNS.
4. Escalate: open a ticket with the anti-DDoS provider stating the attacked IP and the attack signature, if identifiable.
5. Communicate: post the first message on the status channel within 10 minutes of the perceived start.
6. Mitigate locally: apply more aggressive temporary firewall rules (layer 3) while upstream mitigation stabilizes.
7. Monitor recovery: confirm the malicious traffic has dropped before fully reopening access.
8. Record: document duration, attack vector, and actions taken for the post-mortem.

Metrics to monitor continuously

Communicating with the community during the attack

Never let silence be the default message. Players tolerate technical instability far better than the feeling of being abandoned. Keep a status channel outside the potentially attacked infrastructure — a Discord webhook posting to a fixed channel, or a status page on an external service (e.g. status.io or a static page hosted with another provider). Post regular updates even with no concrete news: "attack under mitigation, provider engaged, no exact ETA" already reduces anxiety and prevents "the server shut down" rumors.

Post-incident compensation plan

For servers with VIP systems or paid packages, define the compensation policy in advance — this prevents improvised decisions under pressure from unhappy players. A common scale:

Post-mortem and recurrence prevention

After every significant incident, produce a short summary (even if internal) with: start/end time, identified vector, layers that worked, layers that failed, and improvement actions. Servers suffering recurring attacks from the same actor should consider additional measures like temporary geographic whitelisting or a partnership with a more robust anti-DDoS provider — the upgrade cost is usually lower than that of another prolonged incident.

Common errors and fixes

DDoS response preparation checklist

• Game-specific network-layer anti-DDoS contract active.
• Dedicated game proxy configured and tested.
• Local firewall rules (connlimit, rate limit) applied on critical ports.
• Connection/login limits configured on ConnectServer.
• Team roles defined and documented.
• Status channel external to the main infrastructure created.
• Post-incident compensation policy defined before the first attack.
• Post-mortem process documented for every significant incident.

With the layered plan in place, the next step is to review your infrastructure's overall resilience — including backups and failover — so that an attack isn't the only risk scenario covered. Also check out the MU Online server creation tutorial to review the infrastructure foundation this response plan should be built on.

Frequently asked questions