Add GSLB external plugin

content/explugins/gslb.md

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+title = "gslb"
+description = "*gslb* (Global Server Load Balancing) functionality in CoreDNS."
+weight = 10
+tags = [  "plugin" , "gslb" ]
+categories = [ "plugin", "external" ]
+date = "2025-02-09T17:00:00+02:00"
+repo = "https://github.com/dmachard/coredns-gslb"
+home = "https://github.com/dmachard/coredns-gslb#readme"
++++
+
+## Description
+
+This plugin provides support for GSLB, enabling advanced load balancing and failover mechanisms based on backend health checks and policies. 
+It is particularly useful for managing geographically distributed services or for ensuring high availability and resilience.
+
+### Features:
+- IPv4 and IPv6 support
+- **Health Checks**:
+  - HTTPS
+  - TCP
+  - ICMP
+- **Selection Modes**:
+  - **Failover**: Routes traffic to the highest-priority available backend.
+  - **Random**: Distributes traffic randomly across backends.
+  - **Round Robin**: Cycles through backends in sequence.
+
+## Syntax
+
+~~~
+gslb DB_YAML_FILE [ZONES...] {
+    max_stagger_start "120s"
+    resolution_idle_timeout "3600s"
+    batch_size_start 100
+}
+~~~
+
+* **DB_YAML_FILE** The GSLB configuration file in YAML format. If the path is relative, the path from the *root*
+  plugin will be prepended to it.
+* **ZONES** Specifies the zones the plugin should be authoritative for. If not provided, the zones from the CoreDNS configuration block are used.
+
+
+## Examples
+
+
+Load the `gslb.example.com` zone from `db.gslb.example.com` and enable GSLB records on it
+
+~~~ corefile
+. {
+    file db.gslb.example.com
+    gslb gslb_config.example.com.yml db.gslb.example.com {
+        max_stagger_start "120s"
+        resolution_idle_timeout "3600s"
+        batch_size_start 100
+    }
+}
+~~~
+
+Where `db.gslb.example.com` would contain 
+
+~~~ text
+$ORIGIN gslb.example.com.
+@       3600    IN      SOA     ns1.example.com. admin.example.com. (
+                                2024010101 ; Serial
+                                7200       ; Refresh
+                                3600       ; Retry
+                                1209600    ; Expire
+                                3600       ; Minimum TTL
+                                )
+        3600    IN      NS      ns1.gslb.example.com.
+        3600    IN      NS      ns2.gslb.example.com.
+~~~
+
+And `gslb_config.example.com.yml` would contain 
+
+~~~ yaml
+records:
+  webapp.gslb.example.com.:
+    mode: "failover"
+    record_ttl: 30
+    scrape_interval: 10s
+    backends:
+    - address: "172.16.0.10"
+      priority: 1
+      healthchecks:
+      - type: http
+        params:
+          port: 443
+          uri: "/"
+          host: "localhost"
+          expected_code: 200
+          enable_tls: true
+    - address: "172.16.0.11"
+      priority: 2
+      healthchecks:
+      - type: http
+        params:
+          port: 443
+          uri: "/"
+          host: "localhost"
+          expected_code: 200
+          enable_tls: true
+~~~