feat: add ip allow-list route-service

ip-allow-listing-route-service/README.md

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+# IP Allow-List Route Service
+
+A simple GoLang application that implements the [route service contract](https://docs.cloudfoundry.org/services/route-services.html)
+of Cloud Foundry and checks the client IP address against an allow-list before passing on the
+request. This requires the `x-cf-true-client-ip` header to be set by the foundation which is the
+case on [SAP BTP, Cloud Foundry runtime](https://www.sap.com/products/technology-platform/btp-cloud-foundry-runtime.html).
+
+## Usage
+
+> [!CAUTION]
+>
+> This is a sample application that is only intended to demonstrate the general idea. It is not
+> production grade.
+
+To deploy this route-service you will first need a separate application you want to put this in
+front of, as well as an allow-list. The allow-list should contain one IP-prefix per line, empty
+lines and lines starting with a `#` are ignored. Example:
+
+```
+# This is a comment
+10.0.0.0/8
+127.0.0.0/8
+# The following lines will be ignored
+
+
+```
+
+Put this file into the app directory next to the manifest file and name it `allowlist.txt`. Now you
+can push the application like this (you may need to adjust the domain depending on the region):
+
+```
+cf push -f ./manifest.yml --var domain=cfapps.example.com --var suffix=foo
+```
+
+This pushes the app and makes it available at `ip-allow-list-rs-foo.cfapps.example.com`.
+Now we can turn it into a route service and bind it to an already existing route, provide the
+domain and host of the route you want restrict access to:
+
+```
+cf create-user-provided-service allow-listing -r https://ip-allow-listing-rs-foo.cfapps.example.com
+cf bind-route-service cfapps.example.com --hostname my-app allow-listing
+```
+
+See the help pages of the individual commands for extended options.
+
+Now every request sent to the route you have bound the route-service to will pass through the
+allow-listing route-service and is only forwarded to the target application if the client IP
+address is allow-listed.

ip-allow-listing-route-service/go.mod

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+module github.com/sap-samples/cf-routing-samples/ip-allow-listing-route-service
+
+go 1.24.0

ip-allow-listing-route-service/main.go

@@ -0,0 +1,179 @@
+package main
+
+import (
+	"bufio"
+	"errors"
+	"fmt"
+	"log/slog"
+	"net/http"
+	"net/http/httputil"
+	"net/netip"
+	"net/url"
+	"os"
+	"strings"
+)
+
+var (
+	ErrBadRequest                   = fmt.Errorf("bad request")
+	ErrForbidden                    = fmt.Errorf("forbidden")
+	ErrMissingOrInvalidForwardedURL = fmt.Errorf("%w: invalid or missing x-cf-forwarded-url header", ErrBadRequest)
+	ErrMissingTrueClientIP          = fmt.Errorf("%w: missing x-cf-true-client-ip header", ErrBadRequest)
+	ErrInvalidTrueClientIP          = fmt.Errorf("%w: invalid x-cf-true-client-ip header", ErrBadRequest)
+)
+
+func main() {
+	slog.SetDefault(slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelDebug})))
+
+	err := Main()
+	if err != nil {
+		slog.Error("route-service failed", "error", err)
+		os.Exit(1)
+	}
+}
+
+func Main() error {
+	allowListFile := os.Getenv("ALLOW_LIST_FILE")
+	if allowListFile == "" {
+		return fmt.Errorf("no file to read prefixes from provided")
+	}
+
+	allowedPrefixes, err := loadPrefixes(allowListFile)
+	if err != nil {
+		return err
+	}
+
+	s := http.Server{
+		Addr: ":" + os.Getenv("PORT"),
+		Handler: &httputil.ReverseProxy{
+			Director:     proxyDirector,
+			ErrorHandler: proxyErrorHandler,
+			Transport: &transport{
+				allowedPrefixes: allowedPrefixes,
+				roundTripper:    &http.Transport{},
+			},
+		},
+	}
+
+	slog.Info("starting server", "allow-list", allowedPrefixes, "addr", s.Addr)
+
+	return s.ListenAndServe()
+}
+
+func loadPrefixes(path string) (prefixes []netip.Prefix, err error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+
+	s := bufio.NewScanner(f)
+	for s.Scan() {
+		l := strings.TrimSpace(s.Text())
+		if len(l) == 0 || l[0] == '#' {
+			continue
+		}
+
+		p, err := netip.ParsePrefix(l)
+		if err != nil {
+			return nil, err
+		}
+
+		prefixes = append(prefixes, p)
+	}
+
+	if s.Err() != nil {
+		return nil, s.Err()
+	}
+
+	return prefixes, nil
+}
+
+func proxyDirector(req *http.Request) {
+	log := slog.With("component", "proxy-director", "vcap-id", req.Header.Get("x-vcap-request-id"))
+
+	log.Info("handling request", "client-ip", req.RemoteAddr)
+
+	forwardedURL := req.Header.Get("x-cf-forwarded-url")
+	if forwardedURL == "" {
+		log.Error("missing x-cf-forwarded-url header")
+		req.URL = nil
+		req.Host = ""
+		return // the transport will deal with that
+	}
+
+	u, err := url.Parse(forwardedURL)
+	if err != nil {
+		log.Error("invalid x-cf-forwarded-url header", "error", err)
+		req.URL = nil
+		req.Host = ""
+		return // the transport will deal with that
+	}
+
+	log.Debug("obtained forwarded URL", "forwarded-url", u.String())
+
+	req.URL = u
+	req.Host = u.Host
+}
+
+func proxyErrorHandler(w http.ResponseWriter, r *http.Request, handleErr error) {
+	log := slog.With("component", "proxy-error-handler", "vcap-id", r.Header.Get("x-vcap-request-id"))
+
+	if handleErr == nil {
+		panic("received nil error")
+	}
+
+	var status int
+	switch {
+	case errors.Is(handleErr, ErrBadRequest):
+		status = http.StatusBadRequest
+	case errors.Is(handleErr, ErrForbidden):
+		status = http.StatusForbidden
+	default:
+		status = http.StatusBadGateway
+	}
+
+	log.Warn("handling error", "error", handleErr, "status", status)
+
+	w.WriteHeader(status)
+	_, err := fmt.Fprintf(w, "error: %s", handleErr.Error())
+	if err != nil {
+		log.Warn("failed to send handleError to client", "handleError", handleErr, "error", err, "status", status)
+	}
+}
+
+type transport struct {
+	allowedPrefixes []netip.Prefix
+	roundTripper    http.RoundTripper
+}
+
+func (t *transport) RoundTrip(req *http.Request) (*http.Response, error) {
+	// This indicates that an issue was encountered while trying to read the forwarded URL.
+	if req.URL == nil {
+		return nil, ErrMissingOrInvalidForwardedURL
+	}
+
+	addrString := req.Header.Get("x-cf-true-client-ip")
+	if addrString == "" {
+		return nil, ErrMissingTrueClientIP
+	}
+
+	addr, err := netip.ParseAddr(addrString)
+	if err != nil {
+		return nil, fmt.Errorf("%w: %w", ErrInvalidTrueClientIP, err)
+	}
+
+	// This is a bit inefficient but for a good enough for a sample. A proper implementation should
+	// use some form of trie for efficient lookups.
+	allowed := false
+	for _, p := range t.allowedPrefixes {
+		if p.Contains(addr) {
+			allowed = true
+			break
+		}
+	}
+
+	if !allowed {
+		return nil, fmt.Errorf("%w: address '%s' is not in allow-list", ErrForbidden, addr.String())
+	}
+
+	return t.roundTripper.RoundTrip(req)
+}

ip-allow-listing-route-service/manifest.yml

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+applications:
+- name: ip-allow-listing-route-service
+  buildpacks:
+  - go_buildpack
+  memory: 128M
+  instances: 1
+  env:
+    ALLOW_LIST_FILE: allowlist.txt
+  routes:
+  - route: ip-allow-list-rs-((suffix)).((domain))