feat(netsim/censor): implement blackholing censorship (#34)

This is another common case of censorship where after an offending
packet (or for an offending endpoint) communication cannot actually
proceed and further packets are not allowed.

netsim/censor/ip.go

@@ -0,0 +1,104 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package censor
+
+import (
+	"bytes"
+	"net/netip"
+	"sync"
+	"time"
+
+	"github.com/rbmk-project/x/netsim/packet"
+)
+
+// Blackholer implements connection blackholing with optional pattern matching
+// and connection tracking. Once a connection is blackholed, all packets matching
+// its five-tuple will be dropped for the configured duration.
+type Blackholer struct {
+	// target specifies an optional specific endpoint to filter
+	// if zero, applies to all connections.
+	target netip.AddrPort
+
+	// pattern is an optional byte pattern to match in payload
+	// if nil, only considers the target (if set).
+	pattern []byte
+
+	// duration specifies how long to maintain blackholing state, if set.
+	duration time.Duration
+
+	// mu protects access to blocked.
+	mu sync.Mutex
+
+	// blocked tracks blackholed connections using five-tuple.
+	blocked map[fiveTuple]time.Time
+}
+
+// fiveTuple is the five-tuple identifying a connection.
+type fiveTuple struct {
+	proto   packet.IPProtocol
+	srcAddr netip.Addr
+	srcPort uint16
+	dstAddr netip.Addr
+	dstPort uint16
+}
+
+// NewBlackholer creates a new [*Blackholer] instance.
+//
+// The duration parameter controls how long connections remain blackholed.
+//
+// If target is zero, it applies to all connections.
+//
+// If pattern is nil, it doesn't perform payload matching.
+func NewBlackholer(duration time.Duration, target netip.AddrPort, pattern []byte) *Blackholer {
+	return &Blackholer{
+		target:   target,
+		pattern:  pattern,
+		duration: duration,
+		mu:       sync.Mutex{},
+		blocked:  make(map[fiveTuple]time.Time),
+	}
+}
+
+// Filter implements [packet.Filter].
+func (t *Blackholer) Filter(pkt *packet.Packet) (packet.Target, []*packet.Packet) {
+	// Check if this connection is already blocked
+	tuple := fiveTuple{
+		proto:   pkt.IPProtocol,
+		srcAddr: pkt.SrcAddr,
+		srcPort: pkt.SrcPort,
+		dstAddr: pkt.DstAddr,
+		dstPort: pkt.DstPort,
+	}
+	now := time.Now()
+	t.mu.Lock()
+	deadline, ok := t.blocked[tuple]
+	blocked := ok && now.Before(deadline)
+	if ok && !blocked {
+		delete(t.blocked, tuple)
+	}
+	t.mu.Unlock()
+	if blocked {
+		return packet.DROP, nil
+	}
+
+	// Check if we need to filter specific endpoint
+	if t.target.IsValid() {
+		if pkt.DstAddr != t.target.Addr() || pkt.DstPort != t.target.Port() {
+			return packet.ACCEPT, nil
+		}
+	}
+
+	// If we have a pattern, check payload
+	if t.pattern != nil {
+		if len(pkt.Payload) <= 0 || !bytes.Contains(pkt.Payload, t.pattern) {
+			return packet.ACCEPT, nil
+		}
+	}
+
+	// Block this connection
+	t.mu.Lock()
+	t.blocked[tuple] = now.Add(t.duration)
+	t.mu.Unlock()
+
+	return packet.DROP, nil
+}

netsim/example_censor_tls_test.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"net/http"
 	"net/netip"
+	"time"
 
 	"github.com/rbmk-project/x/netsim"
 	"github.com/rbmk-project/x/netsim/censor"
@@ -53,3 +54,42 @@ func Example_tlsRSTInjection() {
 	// Output:
 	// err: Get "https://dns.google/": connection reset by peer
 }
+
+// This example shows how to use [netsim] to simulate SNI-based TLS blocking
+// using connection blackholing.
+func Example_tlsBlackholing() {
+	// Create a new scenario using the given directory to cache
+	// the certificates used by the simulated PKI
+	scenario := netsim.NewScenario("testdata")
+	defer scenario.Close()
+
+	// Create server stack emulating dns.google.
+	scenario.Attach(scenario.MustNewGoogleDNSStack())
+
+	// Configure blackholing on the scenario router targeting
+	// connections where the SNI matches "dns.google"
+	scenario.Router().AddFilter(censor.NewBlackholer(
+		300*time.Second,      // residual censorship duration
+		netip.AddrPort{},     // match any endpoint
+		[]byte("dns.google"), // match SNI
+	))
+
+	// Create and attach the client stack.
+	clientStack := scenario.MustNewClientStack()
+	scenario.Attach(clientStack)
+
+	// Create the HTTP client with a short timeout
+	clientTxp := scenario.NewHTTPTransport(clientStack)
+	defer clientTxp.CloseIdleConnections()
+	clientHTTP := &http.Client{
+		Transport: clientTxp,
+		Timeout:   200 * time.Millisecond, // short timeout for testing
+	}
+
+	// Attempt the HTTPS request, which should time out
+	_, err := clientHTTP.Get("https://dns.google/")
+	fmt.Printf("err: %v\n", err)
+
+	// Output:
+	// err: Get "https://dns.google/": context deadline exceeded (Client.Timeout exceeded while awaiting headers)
+}