reverseproxy: Active health checks request body option (#6520)

* Add an option to specify the body used for active health checks

* Replacer on request body

3 files changed, 78 insertions, 12 deletions

diff --git a/caddytest/integration/caddyfile_adapt/reverse_proxy_health_reqbody.caddyfiletest b/caddytest/integration/caddyfile_adapt/reverse_proxy_health_reqbody.caddyfiletest
new file mode 100644
index 000000000..ae5a6791e
--- /dev/null
+++ b/caddytest/integration/caddyfile_adapt/reverse_proxy_health_reqbody.caddyfiletest
@@ -0,0 +1,40 @@
+:8884
+
+reverse_proxy 127.0.0.1:65535 {
+	health_uri /health
+	health_request_body "test body"
+}
+----------
+{
+	"apps": {
+		"http": {
+			"servers": {
+				"srv0": {
+					"listen": [
+						":8884"
+					],
+					"routes": [
+						{
+							"handle": [
+								{
+									"handler": "reverse_proxy",
+									"health_checks": {
+										"active": {
+											"body": "test body",
+											"uri": "/health"
+										}
+									},
+									"upstreams": [
+										{
+											"dial": "127.0.0.1:65535"
+										}
+									]
+								}
+							]
+						}
+					]
+				}
+			}
+		}
+	}
+}
diff --git a/modules/caddyhttp/reverseproxy/caddyfile.go b/modules/caddyhttp/reverseproxy/caddyfile.go
index 4ca5d0e0d..cd0e5d949 100644
--- a/modules/caddyhttp/reverseproxy/caddyfile.go
+++ b/modules/caddyhttp/reverseproxy/caddyfile.go
@@ -69,19 +69,20 @@ func parseCaddyfile(h httpcaddyfile.Helper) (caddyhttp.MiddlewareHandler, error)
 //	    lb_retry_match <request-matcher>
 //
 //	    # active health checking
-//	    health_uri      <uri>
-//	    health_port     <port>
-//	    health_interval <interval>
-//	    health_passes   <num>
-//	    health_fails    <num>
-//	    health_timeout  <duration>
-//	    health_status   <status>
-//	    health_body     <regexp>
+//	    health_uri          <uri>
+//	    health_port         <port>
+//	    health_interval     <interval>
+//	    health_passes       <num>
+//	    health_fails        <num>
+//	    health_timeout      <duration>
+//	    health_status       <status>
+//	    health_body         <regexp>
+//	    health_method       <value>
+//	    health_request_body <value>
 //	    health_follow_redirects
 //	    health_headers {
 //	        <field> [<values...>]
 //	    }
-//	    health_method   <value>
 //
 //	    # passive health checking
 //	    fail_duration     <duration>
@@ -425,6 +426,18 @@ func (h *Handler) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 			}
 			h.HealthChecks.Active.Method = d.Val()
 
+		case "health_request_body":
+			if !d.NextArg() {
+				return d.ArgErr()
+			}
+			if h.HealthChecks == nil {
+				h.HealthChecks = new(HealthChecks)
+			}
+			if h.HealthChecks.Active == nil {
+				h.HealthChecks.Active = new(ActiveHealthChecks)
+			}
+			h.HealthChecks.Active.Body = d.Val()
+
 		case "health_interval":
 			if !d.NextArg() {
 				return d.ArgErr()
diff --git a/modules/caddyhttp/reverseproxy/healthchecks.go b/modules/caddyhttp/reverseproxy/healthchecks.go
index 3b5a6a3af..efa1dbf09 100644
--- a/modules/caddyhttp/reverseproxy/healthchecks.go
+++ b/modules/caddyhttp/reverseproxy/healthchecks.go
@@ -24,6 +24,7 @@ import (
 	"regexp"
 	"runtime/debug"
 	"strconv"
+	"strings"
 	"time"
 
 	"go.uber.org/zap"
@@ -93,6 +94,9 @@ type ActiveHealthChecks struct {
 	// The HTTP method to use for health checks (default "GET").
 	Method string `json:"method,omitempty"`
 
+	// The body to send with the health check request.
+	Body string `json:"body,omitempty"`
+
 	// Whether to follow HTTP redirects in response to active health checks (default off).
 	FollowRedirects bool `json:"follow_redirects,omitempty"`
 
@@ -396,6 +400,16 @@ func (h *Handler) doActiveHealthCheck(dialInfo DialInfo, hostAddr string, networ
 		u.Path = h.HealthChecks.Active.Path
 	}
 
+	// replacer used for both body and headers. Only globals (env vars, system info, etc.) are available
+	repl := caddy.NewReplacer()
+
+	// if body is provided, create a reader for it, otherwise nil
+	var requestBody io.Reader
+	if h.HealthChecks.Active.Body != "" {
+		// set body, using replacer
+		requestBody = strings.NewReader(repl.ReplaceAll(h.HealthChecks.Active.Body, ""))
+	}
+
 	// attach dialing information to this request, as well as context values that
 	// may be expected by handlers of this request
 	ctx := h.ctx.Context
@@ -403,15 +417,14 @@ func (h *Handler) doActiveHealthCheck(dialInfo DialInfo, hostAddr string, networ
 	ctx = context.WithValue(ctx, caddyhttp.VarsCtxKey, map[string]any{
 		dialInfoVarKey: dialInfo,
 	})
-	req, err := http.NewRequestWithContext(ctx, h.HealthChecks.Active.Method, u.String(), nil)
+	req, err := http.NewRequestWithContext(ctx, h.HealthChecks.Active.Method, u.String(), requestBody)
 	if err != nil {
 		return fmt.Errorf("making request: %v", err)
 	}
 	ctx = context.WithValue(ctx, caddyhttp.OriginalRequestCtxKey, *req)
 	req = req.WithContext(ctx)
 
-	// set headers, using a replacer with only globals (env vars, system info, etc.)
-	repl := caddy.NewReplacer()
+	// set headers, using replacer
 	repl.Set("http.reverse_proxy.active.target_upstream", networkAddr)
 	for key, vals := range h.HealthChecks.Active.Headers {
 		key = repl.ReplaceAll(key, "")