diff --git a/firewall/dialer.go b/firewall/dialer.go
deleted file mode 100644
index 270b3f9f..00000000
--- a/firewall/dialer.go
+++ /dev/null
@@ -1,46 +0,0 @@
-package firewall
-
-import (
-	"fmt"
-	"net"
-
-	"github.com/safing/portmaster/netenv"
-	"github.com/safing/portmaster/resolver"
-)
-
-func init() {
-	resolver.SetLocalAddrFactory(PermittedAddr)
-	netenv.SetLocalAddrFactory(PermittedAddr)
-}
-
-// PermittedAddr returns an already permitted local address for the given network for reliable connectivity.
-// Returns nil in case of error.
-func PermittedAddr(network string) net.Addr {
-	switch network {
-	case "udp":
-		return PermittedUDPAddr()
-	case "tcp":
-		return PermittedTCPAddr()
-	}
-	return nil
-}
-
-// PermittedUDPAddr returns an already permitted local udp address for reliable connectivity.
-// Returns nil in case of error.
-func PermittedUDPAddr() *net.UDPAddr {
-	addr, err := net.ResolveUDPAddr("udp", fmt.Sprintf(":%d", GetPermittedPort()))
-	if err != nil {
-		return nil
-	}
-	return addr
-}
-
-// PermittedTCPAddr returns an already permitted local tcp address for reliable connectivity.
-// Returns nil in case of error.
-func PermittedTCPAddr() *net.TCPAddr {
-	addr, err := net.ResolveTCPAddr("tcp", fmt.Sprintf(":%d", GetPermittedPort()))
-	if err != nil {
-		return nil
-	}
-	return addr
-}
diff --git a/firewall/interception.go b/firewall/interception.go
index 025a6c55..4edc790c 100644
--- a/firewall/interception.go
+++ b/firewall/interception.go
@@ -3,12 +3,14 @@ package firewall
 import (
 	"context"
 	"errors"
+	"fmt"
 	"net"
 	"os"
 	"sync/atomic"
 	"time"
 
 	"github.com/safing/portmaster/netenv"
+	"golang.org/x/sync/singleflight"
 
 	"github.com/tevino/abool"
 
@@ -63,7 +65,6 @@ func interceptionStart() error {
 
 	interceptionModule.StartWorker("stat logger", statLogger)
 	interceptionModule.StartWorker("packet handler", packetHandler)
-	interceptionModule.StartWorker("ports state cleaner", portsInUseCleaner)
 
 	return interception.Start()
 }
@@ -103,20 +104,60 @@ func handlePacket(ctx context.Context, pkt packet.Packet) {
 	}
 	pkt.SetCtx(traceCtx)
 
-	// associate packet to link and handle
-	conn, ok := network.GetConnection(pkt.GetConnectionID())
-	if ok {
-		tracer.Tracef("filter: assigned to connection %s", conn.ID)
-	} else {
-		conn = network.NewConnectionFromFirstPacket(pkt)
-		tracer.Tracef("filter: created new connection %s", conn.ID)
-		conn.SetFirewallHandler(initialHandler)
+	// Get connection of packet.
+	conn, err := getConnection(pkt)
+	if err != nil {
+		tracer.Errorf("filter: packet %s dropped: %s", pkt, err)
+		_ = pkt.Drop()
+		return
 	}
 
 	// handle packet
 	conn.HandlePacket(pkt)
 }
 
+var getConnectionSingleInflight singleflight.Group
+
+func getConnection(pkt packet.Packet) (*network.Connection, error) {
+	created := false
+
+	// Create or get connection in single inflight lock in order to prevent duplicates.
+	newConn, err, shared := getConnectionSingleInflight.Do(pkt.GetConnectionID(), func() (interface{}, error) {
+		// First, check for an existing connection.
+		conn, ok := network.GetConnection(pkt.GetConnectionID())
+		if ok {
+			return conn, nil
+		}
+
+		// Else create new one from the packet.
+		conn = network.NewConnectionFromFirstPacket(pkt)
+		conn.SetFirewallHandler(initialHandler)
+		created = true
+		return conn, nil
+	})
+	if err != nil {
+		return nil, fmt.Errorf("failed to get connection: %s", err)
+	}
+	if newConn == nil {
+		return nil, errors.New("connection getter returned nil")
+	}
+
+	// Transform and log result.
+	conn := newConn.(*network.Connection)
+	switch {
+	case created && shared:
+		log.Tracer(pkt.Ctx()).Tracef("filter: created new connection %s (shared)", conn.ID)
+	case created:
+		log.Tracer(pkt.Ctx()).Tracef("filter: created new connection %s", conn.ID)
+	case shared:
+		log.Tracer(pkt.Ctx()).Tracef("filter: assigned connection %s (shared)", conn.ID)
+	default:
+		log.Tracer(pkt.Ctx()).Tracef("filter: assigned connection %s", conn.ID)
+	}
+
+	return conn, nil
+}
+
 // fastTrackedPermit quickly permits certain network criticial or internal connections.
 func fastTrackedPermit(pkt packet.Packet) (handled bool) {
 	meta := pkt.Info()
@@ -265,13 +306,12 @@ func fastTrackedPermit(pkt packet.Packet) (handled bool) {
 func initialHandler(conn *network.Connection, pkt packet.Packet) {
 	log.Tracer(pkt.Ctx()).Trace("filter: handing over to connection-based handler")
 
-	// check for internal firewall bypass
-	ps := getPortStatusAndMarkUsed(pkt.Info().LocalPort())
-	if ps.isMe {
-		// approve
+	// Check for pre-authenticated port.
+	if localPortIsPreAuthenticated(conn.Entity.Protocol, conn.LocalPort) {
+		// Approve connection.
 		conn.Accept("connection by Portmaster", noReasonOptionKey)
 		conn.Internal = true
-		// finish
+		// Finalize connection.
 		conn.StopFirewallHandler()
 		issueVerdict(conn, pkt, 0, true)
 		return
diff --git a/firewall/ports.go b/firewall/ports.go
deleted file mode 100644
index d8a61660..00000000
--- a/firewall/ports.go
+++ /dev/null
@@ -1,95 +0,0 @@
-package firewall
-
-import (
-	"context"
-	"sync"
-	"time"
-
-	"github.com/safing/portbase/log"
-	"github.com/safing/portbase/rng"
-)
-
-type portStatus struct {
-	lastSeen time.Time
-	isMe     bool
-}
-
-var (
-	portsInUse     = make(map[uint16]*portStatus)
-	portsInUseLock sync.Mutex
-
-	cleanerTickDuration = 10 * time.Second
-	cleanTimeout        = 10 * time.Minute
-)
-
-func getPortStatusAndMarkUsed(port uint16) *portStatus {
-	portsInUseLock.Lock()
-	defer portsInUseLock.Unlock()
-
-	ps, ok := portsInUse[port]
-	if ok {
-		ps.lastSeen = time.Now()
-		return ps
-	}
-
-	new := &portStatus{
-		lastSeen: time.Now(),
-		isMe:     false,
-	}
-	portsInUse[port] = new
-	return new
-}
-
-// GetPermittedPort returns a local port number that is already permitted for communication.
-// This bypasses the process attribution step to guarantee connectivity.
-// Communication on the returned port is attributed to the Portmaster.
-func GetPermittedPort() uint16 {
-	portsInUseLock.Lock()
-	defer portsInUseLock.Unlock()
-
-	for i := 0; i < 1000; i++ {
-		// generate port between 10000 and 65535
-		rN, err := rng.Number(55535)
-		if err != nil {
-			log.Warningf("filter: failed to generate random port: %s", err)
-			return 0
-		}
-		port := uint16(rN + 10000)
-
-		// check if free, return if it is
-		_, ok := portsInUse[port]
-		if !ok {
-			portsInUse[port] = &portStatus{
-				lastSeen: time.Now(),
-				isMe:     true,
-			}
-			return port
-		}
-	}
-
-	return 0
-}
-
-func portsInUseCleaner(ctx context.Context) error {
-	for {
-		select {
-		case <-ctx.Done():
-			return nil
-		case <-time.After(cleanerTickDuration):
-			cleanPortsInUse()
-		}
-	}
-}
-
-func cleanPortsInUse() {
-	portsInUseLock.Lock()
-	defer portsInUseLock.Unlock()
-
-	threshold := time.Now().Add(-cleanTimeout)
-
-	for port, status := range portsInUse {
-		if status.lastSeen.Before(threshold) {
-			delete(portsInUse, port)
-		}
-	}
-}
diff --git a/firewall/preauth.go b/firewall/preauth.go
new file mode 100644
index 00000000..a2ab7322
--- /dev/null
+++ b/firewall/preauth.go
@@ -0,0 +1,113 @@
+package firewall
+
+import (
+	"strconv"
+	"sync"
+
+	"github.com/safing/portmaster/network"
+	"github.com/safing/portmaster/network/packet"
+
+	"fmt"
+	"net"
+
+	"github.com/safing/portmaster/netenv"
+	"github.com/safing/portmaster/resolver"
+)
+
+var (
+	preAuthenticatedPorts     = make(map[string]struct{})
+	preAuthenticatedPortsLock sync.Mutex
+)
+
+func init() {
+	resolver.SetLocalAddrFactory(PermittedAddr)
+	netenv.SetLocalAddrFactory(PermittedAddr)
+}
+
+// PermittedAddr returns an already permitted local address for the given network for reliable connectivity.
+// Returns nil in case of error.
+func PermittedAddr(network string) net.Addr {
+	switch network {
+	case "udp":
+		return PermittedUDPAddr()
+	case "tcp":
+		return PermittedTCPAddr()
+	}
+	return nil
+}
+
+// PermittedUDPAddr returns an already permitted local udp address for reliable connectivity.
+// Returns nil in case of error.
+func PermittedUDPAddr() *net.UDPAddr {
+	preAuthdPort := GetPermittedPort(packet.UDP)
+	if preAuthdPort == 0 {
+		return nil
+	}
+
+	addr, err := net.ResolveUDPAddr("udp", fmt.Sprintf(":%d", preAuthdPort))
+	if err != nil {
+		return nil
+	}
+
+	return addr
+}
+
+// PermittedTCPAddr returns an already permitted local tcp address for reliable connectivity.
+// Returns nil in case of error.
+func PermittedTCPAddr() *net.TCPAddr {
+	preAuthdPort := GetPermittedPort(packet.TCP)
+	if preAuthdPort == 0 {
+		return nil
+	}
+
+	addr, err := net.ResolveTCPAddr("tcp", fmt.Sprintf(":%d", preAuthdPort))
+	if err != nil {
+		return nil
+	}
+
+	return addr
+}
+
+// GetPermittedPort returns a local port number that is already permitted for communication.
+// This bypasses the process attribution step to guarantee connectivity.
+// Communication on the returned port is attributed to the Portmaster.
+// Every pre-authenticated port is only valid once.
+// If no unused local port number can be found, it will return 0, which is
+// expected to trigger automatic port selection by the underlying OS.
+func GetPermittedPort(protocol packet.IPProtocol) uint16 {
+	port, ok := network.GetUnusedLocalPort(uint8(protocol))
+	if !ok {
+		return 0
+	}
+
+	preAuthenticatedPortsLock.Lock()
+	defer preAuthenticatedPortsLock.Unlock()
+
+	// Save generated port.
+	key := generateLocalPreAuthKey(uint8(protocol), port)
+	preAuthenticatedPorts[key] = struct{}{}
+
+	return port
+}
+
+// localPortIsPreAuthenticated checks if the given protocol and port are
+// pre-authenticated and should be attributed to the Portmaster itself.
+func localPortIsPreAuthenticated(protocol uint8, port uint16) bool {
+	preAuthenticatedPortsLock.Lock()
+	defer preAuthenticatedPortsLock.Unlock()
+
+	// Check if the given protocol and port are pre-authenticated.
+	key := generateLocalPreAuthKey(protocol, port)
+	_, ok := preAuthenticatedPorts[key]
+	if ok {
+		// Immediately remove pre authenticated port.
+		delete(preAuthenticatedPorts, key)
+	}
+
+	return ok
+}
+
+// generateLocalPreAuthKey creates a map key for the pre-authenticated ports.
+func generateLocalPreAuthKey(protocol uint8, port uint16) string {
+	return strconv.Itoa(int(protocol)) + ":" + strconv.Itoa(int(port))
+}
diff --git a/network/ports.go b/network/ports.go
new file mode 100644
index 00000000..0a4fa43d
--- /dev/null
+++ b/network/ports.go
@@ -0,0 +1,49 @@
+package network
+
+import (
+	"github.com/safing/portbase/log"
+	"github.com/safing/portbase/rng"
+)
+
+// GetUnusedLocalPort returns a local port of the specified protocol that is
+// currently unused and is unlikely to be used within the next seconds.
+func GetUnusedLocalPort(protocol uint8) (port uint16, ok bool) {
+	allConns := conns.clone()
+
+	tries := 1000
+	hundredth := tries / 100
+
+	// Try up to 1000 times to find an unused port.
+nextPort:
+	for i := 0; i < tries; i++ {
+		// Generate random port between 10000 and 65535
+		rN, err := rng.Number(55535)
+		if err != nil {
+			log.Warningf("network: failed to generate random port: %s", err)
+			return 0, false
+		}
+		port := uint16(rN + 10000)
+
+		// Shrink range when we chew through the tries.
+		portRangeStart := port - uint16(100-(i/hundredth))
+
+		// Check if the generated port is unused.
+	nextConnection:
+		for _, conn := range allConns {
+			// Skip connection if the protocol does not match the protocol of interest.
+			if conn.Entity.Protocol != protocol {
+				continue nextConnection
+			}
+			// Skip port if the local port is in dangerous proximity.
+			// Consecutive port numbers are very common.
+			if conn.LocalPort <= port && conn.LocalPort >= portRangeStart {
+				continue nextPort
+			}
+		}
+
+		// The checks have passed. We have found a good unused port.
+		return port, true
+	}
+
+	return 0, false
+}
diff --git a/ui/serve.go b/ui/serve.go
index 1e054c81..b7d0d401 100644
--- a/ui/serve.go
+++ b/ui/serve.go
@@ -3,7 +3,6 @@ package ui
 import (
 	"fmt"
 	"io"
-	"mime"
 	"net/http"
 	"net/url"
 	"path/filepath"
@@ -143,7 +142,7 @@ func ServeFileFromBundle(w http.ResponseWriter, r *http.Request, bundleName stri
 	// set content type
 	_, ok := w.Header()["Content-Type"]
 	if !ok {
-		contentType := mime.TypeByExtension(filepath.Ext(path))
+		contentType := mimeTypeByExtension(filepath.Ext(path))
 		if contentType != "" {
 			w.Header().Set("Content-Type", contentType)
 		}
@@ -179,3 +178,72 @@ func redirectToDefault(w http.ResponseWriter, r *http.Request) {
 func redirAddSlash(w http.ResponseWriter, r *http.Request) {
 	http.Redirect(w, r, r.RequestURI+"/", http.StatusPermanentRedirect)
 }
+
+// We now do our mimetypes ourselves, because, as far as we analyzed, a Windows
+// update screwed us over here and broke all the mime typing.
+// (April 2021)
+
+var (
+	defaultMimeType = "application/octet-stream"
+
+	mimeTypes = map[string]string{
+		".7z":    "application/x-7z-compressed",
+		".atom":  "application/atom+xml",
+		".css":   "text/css; charset=utf-8",
+		".csv":   "text/csv; charset=utf-8",
+		".deb":   "application/x-debian-package",
+		".epub":  "application/epub+zip",
+		".es":    "application/ecmascript",
+		".flv":   "video/x-flv",
+		".gif":   "image/gif",
+		".gz":    "application/gzip",
+		".htm":   "text/html; charset=utf-8",
+		".html":  "text/html; charset=utf-8",
+		".jpeg":  "image/jpeg",
+		".jpg":   "image/jpeg",
+		".js":    "text/javascript; charset=utf-8",
+		".json":  "application/json",
+		".m3u":   "audio/mpegurl",
+		".m4a":   "audio/mpeg",
+		".md":    "text/markdown; charset=utf-8",
+		".mjs":   "text/javascript; charset=utf-8",
+		".mov":   "video/quicktime",
+		".mp3":   "audio/mpeg",
+		".mp4":   "video/mp4",
+		".mpeg":  "video/mpeg",
+		".mpg":   "video/mpeg",
+		".ogg":   "audio/ogg",
+		".ogv":   "video/ogg",
+		".otf":   "font/otf",
+		".pdf":   "application/pdf",
+		".png":   "image/png",
+		".qt":    "video/quicktime",
+		".rar":   "application/rar",
+		".rtf":   "application/rtf",
+		".svg":   "image/svg+xml",
+		".tar":   "application/x-tar",
+		".tiff":  "image/tiff",
+		".ts":    "video/MP2T",
+		".ttc":   "font/collection",
+		".ttf":   "font/ttf",
+		".txt":   "text/plain; charset=utf-8",
+		".wasm":  "application/wasm",
+		".wav":   "audio/x-wav",
+		".webm":  "video/webm",
+		".webp":  "image/webp",
+		".woff":  "font/woff",
+		".woff2": "font/woff2",
+		".xml":   "text/xml; charset=utf-8",
+		".xz":    "application/x-xz",
+		".zip":   "application/zip",
+	}
+)
+
+func mimeTypeByExtension(ext string) string {
+	mimeType, ok := mimeTypes[ext]
+	if ok {
+		return mimeType
+	}
+
+	return defaultMimeType
+}