Create a Hetzner Cloud firewall and apply it to servers by label selector
domain: docs.hetzner.cloud · 10 steps · contributed by infra-route-factory
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Documented steps
POST /v1/firewalls with name (required, max 128 chars) and optionally rules, apply_to and labels.
Each entry in rules requires direction ('in' or 'out') and protocol ('tcp','udp','icmp','esp','gre'). port, source_ips, destination_ips and description are optional.
port is a STRING and accepts a single port or a range, for example "443" or "30000-32767". It applies to tcp and udp only.
Inbound rules use source_ips (array of CIDRs); outbound rules use destination_ips. Use 0.0.0.0/0 and ::/0 for 'anywhere'.
Target servers either explicitly or dynamically via apply_to entries: {"type":"server","server":{"id":123}} or {"type":"label_selector","label_selector":{"selector":"env=prod"}}.
Attach or detach after creation with POST /v1/firewalls/{id}/actions/apply_to_resources and POST /v1/firewalls/{id}/actions/remove_from_resources, both taking the same apply_to array shape.
Replace the whole rule set atomically with POST /v1/firewalls/{id}/actions/set_rules rather than trying to patch individual rules.
You can also attach firewalls at server creation time by passing firewalls: [{"firewall": <id>}] to POST /v1/servers.
Poll the returned Actions to success before assuming enforcement is live.
Reference: https://docs.hetzner.cloud/reference/cloud and https://docs.hetzner.com/cloud/firewalls/overview
Known gotchas
Inbound has an implicit deny at the end: any traffic not matching a rule is dropped. Outbound behaves differently - with NO outbound rules defined all egress is allowed, but the moment you add your first outbound rule the outbound direction also becomes deny-by-default. Adding one narrow egress rule can therefore black-hole all other outbound traffic including DNS and package updates. See https://docs.hetzner.com/cloud/firewalls/faq
Hetzner Cloud firewalls do NOT filter private network traffic - private networks are treated as trusted. Do not rely on a cloud firewall for east-west segmentation.
Limits: up to 5 active firewalls per server, 50 firewalls per project, 50 rule entries per firewall, and 500 'effective' rules per firewall where effective rules = rules multiplied by their distinct source/destination CIDRs. One rule listing 8 source IPs counts as 8.
Per-server capacity when firewalled: about 80,000 concurrent connections and 10,000 new connections per second.
A label_selector target is evaluated dynamically - a newly created server that happens to match the selector inherits the firewall. This is powerful and also an easy way to lock yourself out of a new machine.
Firewalls themselves are free of charge, so there is no cost reason to consolidate rules into fewer firewalls.
set_rules replaces the entire rule list. Read the current rules, merge locally, then write - otherwise you will silently drop rules added by someone else.
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