Scaffold admin app

pkg/proxy/cert.go

@@ -0,0 +1,176 @@
+package proxy
+
+import (
+	"bytes"
+	"crypto"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/sha1"
+	"crypto/tls"
+	"crypto/x509"
+	"crypto/x509/pkix"
+	"errors"
+	"math/big"
+	"net"
+	"time"
+)
+
+// MaxSerialNumber is the upper boundary that is used to create unique serial
+// numbers for the certificate. This can be any unsigned integer up to 20
+// bytes (2^(8*20)-1).
+var MaxSerialNumber = big.NewInt(0).SetBytes(bytes.Repeat([]byte{255}, 20))
+
+// CertConfig is a set of configuration values that are used to build TLS configs
+// capable of MITM
+type CertConfig struct {
+	ca     *x509.Certificate
+	caPriv crypto.PrivateKey
+	priv   *rsa.PrivateKey
+	keyID  []byte
+}
+
+// NewCertConfig creates a MITM config using the CA certificate and
+// private key to generate on-the-fly certificates.
+func NewCertConfig(ca *x509.Certificate, caPrivKey crypto.PrivateKey) (*CertConfig, error) {
+	priv, err := rsa.GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		return nil, err
+	}
+	pub := priv.Public()
+
+	// Subject Key Identifier support for end entity certificate.
+	// https://www.ietf.org/rfc/rfc3280.txt (section 4.2.1.2)
+	pkixPubKey, err := x509.MarshalPKIXPublicKey(pub)
+	if err != nil {
+		return nil, err
+	}
+	h := sha1.New()
+	h.Write(pkixPubKey)
+	keyID := h.Sum(nil)
+
+	return &CertConfig{
+		ca:     ca,
+		caPriv: caPrivKey,
+		priv:   priv,
+		keyID:  keyID,
+	}, nil
+}
+
+// NewCA creates a new CA certificate and associated private key.
+func NewCA(name, organization string, validity time.Duration) (*x509.Certificate, *rsa.PrivateKey, error) {
+	priv, err := rsa.GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		return nil, nil, err
+	}
+	pub := priv.Public()
+
+	// Subject Key Identifier support for end entity certificate.
+	// https://www.ietf.org/rfc/rfc3280.txt (section 4.2.1.2)
+	pkixpub, err := x509.MarshalPKIXPublicKey(pub)
+	if err != nil {
+		return nil, nil, err
+	}
+	h := sha1.New()
+	h.Write(pkixpub)
+	keyID := h.Sum(nil)
+
+	// TODO: keep a map of used serial numbers to avoid potentially reusing a
+	// serial multiple times.
+	serial, err := rand.Int(rand.Reader, MaxSerialNumber)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	tmpl := &x509.Certificate{
+		SerialNumber: serial,
+		Subject: pkix.Name{
+			CommonName:   name,
+			Organization: []string{organization},
+		},
+		SubjectKeyId:          keyID,
+		KeyUsage:              x509.KeyUsageKeyEncipherment | x509.KeyUsageDigitalSignature | x509.KeyUsageCertSign,
+		ExtKeyUsage:           []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
+		BasicConstraintsValid: true,
+		NotBefore:             time.Now().Add(-24 * time.Hour),
+		NotAfter:              time.Now().Add(24 * time.Hour),
+		DNSNames:              []string{name},
+		IsCA:                  true,
+	}
+
+	raw, err := x509.CreateCertificate(rand.Reader, tmpl, tmpl, pub, priv)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	// Parse certificate bytes so that we have a leaf certificate.
+	x509c, err := x509.ParseCertificate(raw)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	return x509c, priv, nil
+}
+
+// TLSConfig returns a *tls.Config that will generate certificates on-the-fly using
+// the SNI extension in the TLS ClientHello.
+func (c *CertConfig) TLSConfig() *tls.Config {
+	return &tls.Config{
+		GetCertificate: func(clientHello *tls.ClientHelloInfo) (*tls.Certificate, error) {
+			if clientHello.ServerName == "" {
+				return nil, errors.New("missing server name (SNI)")
+			}
+			return c.cert(clientHello.ServerName)
+		},
+		NextProtos: []string{"http/1.1"},
+	}
+}
+
+func (c *CertConfig) cert(hostname string) (*tls.Certificate, error) {
+	// Remove the port if it exists.
+	host, _, err := net.SplitHostPort(hostname)
+	if err == nil {
+		hostname = host
+	}
+
+	serial, err := rand.Int(rand.Reader, MaxSerialNumber)
+	if err != nil {
+		return nil, err
+	}
+
+	tmpl := &x509.Certificate{
+		SerialNumber: serial,
+		Subject: pkix.Name{
+			CommonName:   hostname,
+			Organization: []string{"Gurp"},
+		},
+		SubjectKeyId:          c.keyID,
+		KeyUsage:              x509.KeyUsageKeyEncipherment | x509.KeyUsageDigitalSignature,
+		ExtKeyUsage:           []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth},
+		BasicConstraintsValid: true,
+		NotBefore:             time.Now().Add(-24 * time.Hour),
+		NotAfter:              time.Now().Add(24 * time.Hour),
+	}
+
+	if ip := net.ParseIP(hostname); ip != nil {
+		tmpl.IPAddresses = []net.IP{ip}
+	} else {
+		tmpl.DNSNames = []string{hostname}
+	}
+
+	raw, err := x509.CreateCertificate(rand.Reader, tmpl, c.ca, c.priv.Public(), c.caPriv)
+	if err != nil {
+		return nil, err
+	}
+
+	// Parse certificate bytes so that we have a leaf certificate.
+	x509c, err := x509.ParseCertificate(raw)
+	if err != nil {
+		return nil, err
+	}
+
+	return &tls.Certificate{
+		Certificate: [][]byte{raw, c.ca.Raw},
+		PrivateKey:  c.priv,
+		Leaf:        x509c,
+	}, nil
+}

pkg/proxy/modify.go

@@ -0,0 +1,24 @@
+package proxy
+
+import "net/http"
+
+var (
+	nopReqModifier = func(req *http.Request) {}
+	nopResModifier = func(res *http.Response) error { return nil }
+)
+
+// RequestModifyFunc defines a type for a function that can modify a HTTP
+// request before it's proxied.
+type RequestModifyFunc func(req *http.Request)
+
+// RequestModifyMiddleware defines a type for chaining request modifier
+// middleware.
+type RequestModifyMiddleware func(next RequestModifyFunc) RequestModifyFunc
+
+// ResponseModifyFunc defines a type for a function that can modify a HTTP
+// response before it's written back to the client.
+type ResponseModifyFunc func(res *http.Response) error
+
+// ResponseModifyMiddleware defines a type for chaining response modifier
+// middleware.
+type ResponseModifyMiddleware func(ResponseModifyFunc) ResponseModifyFunc

pkg/proxy/net.go

@@ -0,0 +1,47 @@
+package proxy
+
+import (
+	"errors"
+	"net"
+)
+
+var ErrAlreadyAccepted = errors.New("listener already accepted")
+
+// OnceListener implements net.Listener.
+//
+// Accepts a connection once and returns an error on subsequent
+// attempts.
+type OnceAcceptListener struct {
+	c net.Conn
+}
+
+func (l *OnceAcceptListener) Accept() (net.Conn, error) {
+	if l.c == nil {
+		return nil, ErrAlreadyAccepted
+	}
+
+	c := l.c
+	l.c = nil
+
+	return c, nil
+}
+
+func (l *OnceAcceptListener) Close() error {
+	return nil
+}
+
+func (l *OnceAcceptListener) Addr() net.Addr {
+	return l.c.LocalAddr()
+}
+
+// ConnNotify embeds net.Conn and adds a channel field for notifying
+// that the connection was closed.
+type ConnNotify struct {
+	net.Conn
+	closed chan struct{}
+}
+
+func (c *ConnNotify) Close() {
+	c.Conn.Close()
+	c.closed <- struct{}{}
+}

pkg/proxy/proxy.go

@@ -0,0 +1,151 @@
+package proxy
+
+import (
+	"context"
+	"crypto"
+	"crypto/tls"
+	"crypto/x509"
+	"fmt"
+	"log"
+	"net"
+	"net/http"
+	"net/http/httputil"
+)
+
+// Proxy implements http.Handler and offers MITM behaviour for modifying
+// HTTP requests and responses.
+type Proxy struct {
+	certConfig *CertConfig
+	handler    http.Handler
+
+	// TODO: Add mutex for modifier funcs.
+	reqModifiers []RequestModifyMiddleware
+	resModifiers []ResponseModifyMiddleware
+}
+
+// NewProxy returns a new Proxy.
+func NewProxy(ca *x509.Certificate, key crypto.PrivateKey) (*Proxy, error) {
+	certConfig, err := NewCertConfig(ca, key)
+	if err != nil {
+		return nil, err
+	}
+
+	p := &Proxy{
+		certConfig:   certConfig,
+		reqModifiers: make([]RequestModifyMiddleware, 0),
+		resModifiers: make([]ResponseModifyMiddleware, 0),
+	}
+
+	p.handler = &httputil.ReverseProxy{
+		Director:       p.modifyRequest,
+		ModifyResponse: p.modifyResponse,
+		ErrorHandler:   errorHandler,
+	}
+
+	return p, nil
+}
+
+func (p *Proxy) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	if r.Method == http.MethodConnect {
+		p.handleConnect(w, r)
+		return
+	}
+
+	p.handler.ServeHTTP(w, r)
+}
+
+func (p *Proxy) UseRequestModifier(fn ...RequestModifyMiddleware) {
+	p.reqModifiers = append(p.reqModifiers, fn...)
+}
+
+func (p *Proxy) UseResponseModifier(fn ...ResponseModifyMiddleware) {
+	p.resModifiers = append(p.resModifiers, fn...)
+}
+
+func (p *Proxy) modifyRequest(r *http.Request) {
+	// Fix r.URL for HTTPS requests after CONNECT.
+	if r.URL.Scheme == "" {
+		r.URL.Host = r.Host
+		r.URL.Scheme = "https"
+	}
+
+	fn := nopReqModifier
+
+	for i := len(p.reqModifiers) - 1; i >= 0; i-- {
+		fn = p.reqModifiers[i](fn)
+	}
+
+	fn(r)
+}
+
+func (p *Proxy) modifyResponse(res *http.Response) error {
+	fn := nopResModifier
+
+	for i := len(p.resModifiers) - 1; i >= 0; i-- {
+		fn = p.resModifiers[i](fn)
+	}
+
+	return fn(res)
+}
+
+// handleConnect hijacks the incoming HTTP request and sets up an HTTP tunnel.
+// During the TLS handshake with the client, we use the proxy's CA config to
+// create a certificate on-the-fly.
+func (p *Proxy) handleConnect(w http.ResponseWriter, r *http.Request) {
+	hj, ok := w.(http.Hijacker)
+	if !ok {
+		log.Printf("[ERROR] handleConnect: ResponseWriter is not a http.Hijacker (type: %T)", w)
+		writeError(w, r, http.StatusServiceUnavailable)
+		return
+	}
+
+	w.WriteHeader(http.StatusOK)
+
+	clientConn, _, err := hj.Hijack()
+	if err != nil {
+		log.Printf("[ERROR] Hijacking client connection failed: %v", err)
+		writeError(w, r, http.StatusServiceUnavailable)
+		return
+	}
+	defer clientConn.Close()
+
+	// Secure connection to client.
+	clientConn, err = p.clientTLSConn(clientConn)
+	if err != nil {
+		log.Printf("[ERROR] Securing client connection failed: %v", err)
+		return
+	}
+	clientConnNotify := ConnNotify{clientConn, make(chan struct{})}
+
+	l := &OnceAcceptListener{clientConnNotify.Conn}
+
+	err = http.Serve(l, p.handler)
+	if err != nil && err != ErrAlreadyAccepted {
+		log.Printf("[ERROR] Serving HTTP request failed: %v", err)
+	}
+	<-clientConnNotify.closed
+}
+
+func (p *Proxy) clientTLSConn(conn net.Conn) (*tls.Conn, error) {
+	tlsConfig := p.certConfig.TLSConfig()
+
+	tlsConn := tls.Server(conn, tlsConfig)
+	if err := tlsConn.Handshake(); err != nil {
+		tlsConn.Close()
+		return nil, fmt.Errorf("handshake error: %v", err)
+	}
+
+	return tlsConn, nil
+}
+
+func errorHandler(w http.ResponseWriter, r *http.Request, err error) {
+	if err == context.Canceled {
+		return
+	}
+	log.Printf("[ERROR]: Proxy error: %v", err)
+	w.WriteHeader(http.StatusBadGateway)
+}
+
+func writeError(w http.ResponseWriter, r *http.Request, code int) {
+	http.Error(w, http.StatusText(code), code)
+}