hamlab/internal/api/kvm.go

package api

// IP-KVMs: a device's own web page, shown inside Ham Lab.
//
// The whole difficulty is that Ham Lab is served over TLS and a KVM usually is not, so a
// browser refuses to put the device's page in an iframe — mixed content, blocked before
// anything loads. The fix is to serve the device from Ham Lab's own origin, which also
// means it inherits Ham Lab's login instead of needing a second one, and makes the iframe
// same-origin.
//
// Proxying an arbitrary web app under a path prefix is the hard part, and nothing here
// knows anything about any particular device. A device UI asks for its assets by absolute
// path — "/assets/app.js", "/api/whatever" — which a prefix cannot rewrite away. So the
// prefix is only the entry point: any request Ham Lab does not serve itself is routed to
// whichever KVM the asking page belongs to, worked out from the Referer, and from a cookie
// when there is no Referer (a WebSocket upgrade, say). Ham Lab's own routes always win, so
// a device can never shadow the console.

import (
	"crypto/rand"
	"encoding/hex"
	"encoding/json"
	"errors"
	"io"
	"log"
	"net/http"
	"net/http/httputil"
	"net/url"
	"strings"
	"sync"
	"time"

	"hamlab/internal/config"
)

const kvmPrefix = "/kvm/"
const kvmCookie = "hamlab_kvm"

type kvmProxies struct {
	mu sync.Mutex
	m  map[string]*httputil.ReverseProxy
}

func (s *Server) kvm(id string) (config.KVM, bool) {
	var found config.KVM
	ok := false
	s.cfg.View(func(c *config.Config) {
		for _, k := range c.KVMs {
			if k.ID == id {
				found, ok = k, true
			}
		}
	})
	return found, ok
}

// kvmProxy is the reverse proxy for one device, made once and kept.
func (s *Server) kvmProxy(k config.KVM) (*httputil.ReverseProxy, error) {
	s.kvms.mu.Lock()
	defer s.kvms.mu.Unlock()
	if s.kvms.m == nil {
		s.kvms.m = map[string]*httputil.ReverseProxy{}
	}
	if p, ok := s.kvms.m[k.ID]; ok {
		return p, nil
	}
	target, err := url.Parse(k.URL)
	if err != nil || target.Host == "" {
		return nil, errors.New("this KVM's address is not a URL")
	}
	prefix := kvmPrefix + k.ID
	p := &httputil.ReverseProxy{
		Director: func(r *http.Request) {
			r.URL.Scheme, r.URL.Host = target.Scheme, target.Host
			// Entry through the prefix keeps the path after it; anything else arrived by its
			// own absolute path and is passed through untouched.
			if rest, ok := strings.CutPrefix(r.URL.Path, prefix); ok {
				if rest == "" {
					rest = "/"
				}
				r.URL.Path = rest
			}
			r.URL.RawPath = ""
			r.Host = target.Host
			// The device gets its own host and none of Ham Lab's session.
			r.Header.Del("Origin")
			stripCookie(r, cookieName, kvmCookie)
		},
		FlushInterval: -1, // a video stream must not be buffered
		ModifyResponse: func(resp *http.Response) error {
			// The device's own framing and transport policy would stop it being embedded, or
			// send a browser off to plain HTTP. Ham Lab is the origin now, and says what it
			// serves; its own headers are added on the way out.
			for _, h := range []string{"X-Frame-Options", "Content-Security-Policy", "Content-Security-Policy-Report-Only", "Strict-Transport-Security", "Referrer-Policy"} {
				resp.Header.Del(h)
			}
			// A redirect to the device's own root has to come back through the prefix.
			if loc := resp.Header.Get("Location"); loc != "" {
				if u, err := url.Parse(loc); err == nil && (u.Host == "" || u.Host == target.Host) && strings.HasPrefix(u.Path, "/") && !strings.HasPrefix(u.Path, prefix) {
					u.Scheme, u.Host = "", ""
					u.Path = prefix + u.Path
					resp.Header.Set("Location", u.String())
				}
			}
			return nil
		},
		ErrorHandler: func(w http.ResponseWriter, r *http.Request, err error) {
			w.Header().Set("Content-Type", "application/json")
			w.WriteHeader(http.StatusBadGateway)
			_ = json.NewEncoder(w).Encode(map[string]string{"error": k.Name + " unreachable: " + err.Error()})
		},
	}
	s.kvms.m[k.ID] = p
	return p, nil
}

// stripCookie removes named cookies from a request, so Ham Lab's session is never handed to
// a device and the device never sees which KVM the browser is looking at.
func stripCookie(r *http.Request, names ...string) {
	cs := r.Cookies()
	kept := cs[:0]
	for _, c := range cs {
		drop := false
		for _, n := range names {
			if c.Name == n {
				drop = true
			}
		}
		if !drop {
			kept = append(kept, c)
		}
	}
	r.Header.Del("Cookie")
	for _, c := range kept {
		r.AddCookie(c)
	}
}

func (s *Server) forgetKVMProxy(id string) {
	s.kvms.mu.Lock()
	delete(s.kvms.m, id)
	s.kvms.mu.Unlock()
}

// serveKVM is the entry point: /kvm/<id>/… . It also drops a cookie naming the device, so
// that requests the page makes later by absolute path can be traced back to it.
func (s *Server) serveKVM(w http.ResponseWriter, r *http.Request) {
	id := r.PathValue("id")
	k, ok := s.kvm(id)
	if !ok {
		jsonErr(w, 404, "no such KVM")
		return
	}
	p, err := s.kvmProxy(k)
	if err != nil {
		jsonErr(w, 400, err.Error())
		return
	}
	http.SetCookie(w, &http.Cookie{Name: kvmCookie, Value: id, Path: "/", HttpOnly: true, Secure: secure(r), SameSite: http.SameSiteLaxMode})
	p.ServeHTTP(w, r)
}

// kvmFor works out which device a request belongs to when it arrived by absolute path
// rather than through the prefix: the Referer names the page that asked, and a cookie
// covers what has no Referer at all. Nothing device-specific: it never has to know which
// paths a given KVM uses.
func (s *Server) kvmFor(r *http.Request) (config.KVM, bool) {
	if ref := r.Header.Get("Referer"); ref != "" {
		if u, err := url.Parse(ref); err == nil {
			if rest, ok := strings.CutPrefix(u.Path, kvmPrefix); ok {
				if id, _, _ := strings.Cut(rest, "/"); id != "" {
					if k, ok := s.kvm(id); ok {
						return k, true
					}
				}
			}
		}
		// asked for by a page that is not a KVM: not ours to answer
		return config.KVM{}, false
	}
	if c, err := r.Cookie(kvmCookie); err == nil && c.Value != "" {
		return s.kvm(c.Value)
	}
	return config.KVM{}, false
}

// kvmFallback answers a request Ham Lab does not serve itself, when it came from inside a
// KVM's page. It returns false when the request has nothing to do with a KVM, and the
// caller carries on with its own 404.
func (s *Server) kvmFallback(w http.ResponseWriter, r *http.Request) bool {
	k, ok := s.kvmFor(r)
	if !ok {
		return false
	}
	p, err := s.kvmProxy(k)
	if err != nil {
		return false
	}
	p.ServeHTTP(w, r)
	return true
}

// ---- the list ----

func (s *Server) listKVMs(w http.ResponseWriter, _ *http.Request) {
	out := []config.KVM{}
	s.cfg.View(func(c *config.Config) { out = append(out, c.KVMs...) })
	jsonOK(w, 200, out)
}

func (s *Server) addKVM(w http.ResponseWriter, r *http.Request) {
	var body struct{ Name, URL, Machine string }
	if err := json.NewDecoder(io.LimitReader(r.Body, 4096)).Decode(&body); err != nil {
		jsonErr(w, 400, "bad request")
		return
	}
	raw := strings.TrimSpace(body.URL)
	if raw != "" && !strings.Contains(raw, "://") {
		raw = "http://" + raw // what you would type: an address, not a scheme
	}
	u, err := url.Parse(raw)
	if err != nil || (u.Scheme != "http" && u.Scheme != "https") || u.Host == "" {
		jsonErr(w, 400, "address: http(s)://host, or just the host")
		return
	}
	name := strings.TrimSpace(body.Name)
	if name == "" {
		name = u.Host
	}
	if len(name) > maxNameLen {
		jsonErr(w, 400, "that name is too long")
		return
	}
	b := make([]byte, 6)
	if _, err := rand.Read(b); err != nil {
		jsonErr(w, 500, err.Error())
		return
	}
	k := config.KVM{ID: hex.EncodeToString(b), Name: name, URL: u.Scheme + "://" + u.Host + strings.TrimSuffix(u.Path, "/"), Machine: strings.TrimSpace(body.Machine), Added: time.Now()}
	if err := s.cfg.Update(func(c *config.Config) { c.KVMs = append(c.KVMs, k) }); err != nil {
		jsonErr(w, 500, err.Error())
		return
	}
	log.Printf("kvm %q added (%s) by %s", k.Name, k.URL, clientIP(r))
	jsonOK(w, 201, k)
}

func (s *Server) removeKVM(w http.ResponseWriter, r *http.Request) {
	id := r.PathValue("id")
	found := false
	if err := s.cfg.Update(func(c *config.Config) {
		kept := c.KVMs[:0]
		for _, k := range c.KVMs {
			if k.ID == id {
				found = true
				continue
			}
			kept = append(kept, k)
		}
		c.KVMs = kept
	}); err != nil {
		jsonErr(w, 500, err.Error())
		return
	}
	if !found {
		jsonErr(w, 404, "no such KVM")
		return
	}
	s.forgetKVMProxy(id)
	log.Printf("kvm %s removed by %s", id, clientIP(r))
	w.WriteHeader(204)
}