Manually hook libc connect() to capture even more traffic

native-hook.js

@@ -0,0 +1,105 @@
+/**
+ * In some cases, proxy configuration by itself won't work. This notably includes Flutter apps (which ignore
+ * system/JVM configuration entirely) and plausibly other apps intentionally ignoring proxies. To handle that
+ * we hook libc's connect() directly to redirect traffic on all recognized ports.
+ *
+ * This handles all calls to connect an outgoing socket, and for all TCP connections opened on recognized ports,
+ * it will manually replace the connect parameters, so that the socket connects to the proxy instead of the
+ * 'real' destination.
+ *
+ * This doesn't help with certificate trust (you still need some kind of certificate setup) but it does ensure
+ * the proxy receives all connections (and so will see if connections don't trust its CA). It's still useful
+ * to do proxy config alongside this, primarily to capture traffic that might be sent on any non-standard ports.
+ */
+
+const PROXY_PORT = 8000;
+const PROXY_HOST = '127.0.0.1';
+
+// Ports which we recognize, and forcibly redirect to capture, if not captured already.
+const RECOGNIZED_PORTS = [
+    80,
+    443,
+    4443,
+    8000,
+    8080,
+    8443,
+    8888,
+    9000
+];
+
+const PROXY_HOST_IPv4_BYTES = PROXY_HOST.split('.').map(part => parseInt(part, 10));
+const IPv6_MAPPING_PREFIX_BYTES = [0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0xff, 0xff];
+const PROXY_HOST_IPv6_BYTES = IPv6_MAPPING_PREFIX_BYTES.concat(PROXY_HOST_IPv4_BYTES);
+
+const connectFn = (
+    Module.findExportByName('libc.so', 'connect') ?? // Android
+    Module.findExportByName('libc.so.6', 'connect') // Linux
+);
+
+if (!connectFn) { // Should always be set, but just in case
+    console.warn('Could not find libc connect() function to hook raw traffic');
+} else {
+    Interceptor.attach(connectFn, {
+        onEnter(args) {
+            const fd = this.sockFd = args[0].toInt32();
+            const sockType = Socket.type(fd);
+
+            const addrPtr = ptr(args[1]);
+            const addrLen = args[2].toInt32(); // TODO: Probably not right?
+            const addrData = addrPtr.readByteArray(addrLen);
+
+            if (sockType === 'tcp' || sockType === 'tcp6') {
+                const portAddrBytes = new DataView(addrData.slice(2, 4));
+                const port = portAddrBytes.getUint16(0, false); // Big endian!
+
+                const shouldBeIntercepted = RECOGNIZED_PORTS.includes(port);
+                if (!shouldBeIntercepted) return; // Unrecognized port - probably not HTTP(S)
+
+                const isIPv6 = sockType === 'tcp6';
+
+                const hostBytes = isIPv6
+                    // 16 bytes offset by 8 (2 for family, 2 for port, 4 for flowinfo):
+                    ? new Uint8Array(addrData.slice(8, 8 + 16))
+                    // 4 bytes, offset by 4 (2 for family, 2 for port)
+                    : new Uint8Array(addrData.slice(4, 4 + 4));
+
+                const isIntercepted = port === PROXY_PORT && areArraysEqual(hostBytes,
+                    isIPv6
+                        ? PROXY_HOST_IPv6_BYTES
+                        : PROXY_HOST_IPv4_BYTES
+                );
+
+                if (isIntercepted) return;
+
+                console.log(`Manually intercepting connection to ${
+                    isIPv6
+                        ? `[${[...hostBytes].map(x => x.toString(16)).join(':')}]`
+                        : [...hostBytes].map(x => x.toString()).join('.')
+                }:${port}`);
+
+                // Overwrite the port with the proxy port:
+                portAddrBytes.setUint16(0, PROXY_PORT, false); // Big endian
+                addrPtr.add(2).writeByteArray(portAddrBytes.buffer);
+
+                // Overwrite the address with the proxy address:
+                if (isIPv6) {
+                    // Skip 8 bytes: 2 family, 2 port, 4 flowinfo
+                    addrPtr.add(8).writeByteArray(PROXY_HOST_IPv6_BYTES);
+                } else {
+                    // Skip 4 bytes: 2 family, 2 port
+                    addrPtr.add(4).writeByteArray(PROXY_HOST_IPv4_BYTES);
+                }
+            }
+
+            // N.b. we ignore all non-TCP connections: both UDP and Unix streams
+        }
+    });
+
+    console.log(`Hooked connect() at ${connectFn}`);
+}
+
+const areArraysEqual = (arrayA, arrayB) => {
+    if (arrayA.length !== arrayB.length) return false;
+    return arrayA.every((x, i) => arrayB[i] === x);
+};
+