support kamal-proxy --health-check-host

lib/kamal/configuration/docs/proxy.yml

@@ -114,10 +114,16 @@ proxy:
   # the deploy timeout, with a 5-second timeout for each request.
   #
   # Once the app is up, the proxy will stop hitting the healthcheck endpoint.
+  #
+  # The `host` option allows setting a custom Host header for health check requests.
+  # This is useful when your application validates the Host header (e.g., Django's
+  # ALLOWED_HOSTS, Rails host authorization). Without this, health checks using
+  # internal IPs may be rejected even when the application is healthy.
   healthcheck:
     interval: 3
     path: /health
     timeout: 3
+    host: example.com
 
   # Buffering
   #

lib/kamal/configuration/proxy.rb

@@ -78,6 +78,7 @@ def deploy_options
       "health-check-interval": seconds_duration(proxy_config.dig("healthcheck", "interval")),
       "health-check-timeout": seconds_duration(proxy_config.dig("healthcheck", "timeout")),
       "health-check-path": proxy_config.dig("healthcheck", "path"),
+      "health-check-host": proxy_config.dig("healthcheck", "host"),
       "target-timeout": seconds_duration(proxy_config["response_timeout"]),
       "buffer-requests": proxy_config.fetch("buffering", { "requests": true }).fetch("requests", true),
       "buffer-responses": proxy_config.fetch("buffering", { "responses": true }).fetch("responses", true),

test/configuration/proxy_test.rb

@@ -105,6 +105,35 @@ class ConfigurationProxyTest < ActiveSupport::TestCase
     end
   end
 
+  test "health check host in deploy options" do
+    @deploy[:proxy] = {
+      "host" => "example.com",
+      "healthcheck" => {
+        "path" => "/up",
+        "host" => "example.com"
+      }
+    }
+
+    proxy = config.proxy
+    options = proxy.deploy_options
+    assert_equal "example.com", options[:"health-check-host"]
+    assert_equal "/up", options[:"health-check-path"]
+  end
+
+  test "health check without custom host" do
+    @deploy[:proxy] = {
+      "host" => "example.com",
+      "healthcheck" => {
+        "path" => "/up"
+      }
+    }
+
+    proxy = config.proxy
+    options = proxy.deploy_options
+    assert_nil options[:"health-check-host"]
+    assert_equal "/up", options[:"health-check-path"]
+  end
+
   private
     def config
       Kamal::Configuration.new(@deploy)