Android VpnService: Reading from Tun device hangs

Question

My app is using VpnService for traffic interception.

What it does:
1.Reads from Tun device in a loop:

while (started && tunDevice.valid()) {
    final byte[] bytes = tunDevice.read();
    IpPacket packet = PacketFactory.createPacket(bytes);
    if (packet == null) {
        Thread.yield();
    } else {
        proxyService.handlePacket(packet);
    }
}

TunDevice.read:

@Override
public byte[] read() throws IOException {
    if (!valid()) {
        LOG.warn("TUN: file descriptor is not valid any more");
        return null;
    }

    int length = tunInputStream.read(readBuffer);
    LOG.debug("TUN: Received packet length={}", length);
    if (length < 0) {
        throw new IOException("Tun device is closed");
    }
    if (length == 0) {
        return null;
    }
    return Arrays.copyOfRange(readBuffer, 0, length);
}

2.Proxifies data to the protected socket.

The problem is that after some time it stops reading from TUN device. Read method just hangs and waits for some time (like 3-5 minutes).

Using netstat I see that all new connections are in SYN_SENT state and I can understand why - they cannot receive ACK from my code because I cannot receive these SYN packets.

The question is: what could it be? When TUN device could behave like this?

Answer 1

In our case the problem was in our TCP implementation. We have written more data than TCP could receive (advertised window).