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How to Configure Firewall and Anti-DDoS Settings

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Michael M.
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Objective

To protect its global infrastructure and its customers’ servers, OVHcloud offers a firewall that can be configured and integrated into the Anti-DDoS (VAC) solution: the Network Firewall. This is an option that will enable you to limit how much your service is exposed to attacks from the public network.

This guide will take you through the steps for its configuration.

VAC: For more information on VAC, please check out this page about our protection system against DDoS attacks: https://us.ovhcloud.com/products/security/anti-ddos

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Requirements

Instructions

Enable the Network Firewall

The Network Firewall protects the IPs that are associated with a machine. You must therefore configure each IP separately; it is not possible to configure the server as a whole.

You can enable and configure it manually from the Control Panel in the IP section, by clicking on the gear icon to the right of the relevant IPv4.

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  • You will then be asked for confirmation:

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  • You can then Enable the firewall and Configure the Firewall by clicking once more on the gear icon next to the IPv4:

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You can set up to 20 rules per IP.

The firewall is enabled automatically upon each DDoS attack, and cannot be disabled before the attack ends. This is why it is important to keep the firewall rules up to date. As a default setting you do not have any configured rules, so all connections can be set up. If you do have any, remember to check your firewall rules regularly, even if you disable it.

  • The UDP fragmentation is blocked (DROP) as a default setting. When you enable the Firewall Network, if you use a VPN, remember to correctly configure your maximum transmission unit (MTU). For example, on OpenVPN, you can tick MTU test.
  • The Network Firewall is not taken into account within the OVH network, so the rules set up do not affect the connections in this internal network.

Configuring the Network Firewall

To add a rule, right-click on Add a rule:

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For each rule you must choose:

  • a priority (from 0 to 19, 0 being the first rule to be applied, followed by the others);
  • an action (Authorize or Refuse);
  • the protocol;
  • an IP (optional);
  • the source port (TCP only)
  • the destination port (TCP only)
  • the TCP options (TCP only)

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  • Priority 0: we advise that you authorize the TCP protocol on all the IPs with an established option. The established option enables you to verify that the packet is part of a session that has previously been opened (already started). If you do not authorize it, the server will not receive the TCP protocol feedback from the SYN/ACK requests.
  • Priority 19: refuses all of the IPv4 protocol if any rules before 19th (the last possible) are not filled in.

Configuration example

To make sure that only the SSH (22), HTTP (80), HTTPS (443), and UDP (on port 10000) ports are left open when authorizing the ICMP, you need to follow the rules below:

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The rules are sorted chronologically from 0 (the first rule read) to 19 (the last). The chain stops being scanned as soon as a rule is applied to the packet.

For example, a packet for TCP port 80 will be captured by rule 2 and the rules that come after will not be tested. A packet for TCP port 25 will only be captured at the last rule (19) which will block it, because OVHcloud does not authorize communication on port 25 in the previous rules.

If anti-DDoS mitigation is enabled, your Network Firewall rules will be applied, even if you have disabled them. If you wish to disable it, remember to delete your rules.

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