Android VpnService 保护 fd 是如何工作的？

Question

根据https://developer.android.com/reference/android/net/VpnService.html#protect(int)

保护套接字免受 VPN 连接后，通过此套接字发送的数据将直接进入底层网络，因此其流量不会通过 VPN 转发。如果某些连接需要保留在 VPN 之外，此方法很有用。例如，如果 VPN 的目的地被 VPN 路由覆盖，VPN 隧道应该保护自己。否则，它的传出数据包将被发送回 VPN 接口并导致无限循环。如果应用程序未准备好或被撤销，此方法将失败。

我知道 Android 是建立在 Linux 之上的，这个函数背后使用了哪些 Linux 机制或实用程序，使通过这个套接字发送的数据绕过 VPN，而所有其他都通过 VPN？

Answer 1

简而言之，Android VPNService 通过策略路由保护 fd 工作，所有通过protected fd 的数据包都会被标记一个特殊的fwmark，所有带有这个fwmark 的数据包都会绕过VPN。

键码sn-ps如下：

// android/frameworks/base/core/java/android/net/VpnService.java

 /**
     * Protect a socket from VPN connections. After protecting, data sent
     * through this socket will go directly to the underlying network,
     * so its traffic will not be forwarded through the VPN.
     * This method is useful if some connections need to be kept
     * outside of VPN. For example, a VPN tunnel should protect itself if its
     * destination is covered by VPN routes. Otherwise its outgoing packets
     * will be sent back to the VPN interface and cause an infinite loop. This
     * method will fail if the application is not prepared or is revoked.
     *
     * <p class="note">The socket is NOT closed by this method.
     *
     * @return {@code true} on success.
     */
    public boolean protect(int socket) {
        return NetworkUtils.protectFromVpn(socket);
    }


// android/frameworks/base/core/java/android/net/VpnService.java

    /**
     * Protect {@code fd} from VPN connections.  After protecting, data sent through
     * this socket will go directly to the underlying network, so its traffic will not be
     * forwarded through the VPN.
     */
    public static boolean protectFromVpn(FileDescriptor fd) {
        return protectFromVpn(fd.getInt$());
    }

// android/system/netd/server/FwmarkServer.cpp

    fwmark.permission = permission;

    if (setsockopt(*socketFd, SOL_SOCKET, SO_MARK, &fwmark.intValue,
                   sizeof(fwmark.intValue)) == -1) {
        return -errno;
    }


// android/system/netd/include/Fwmark.h 

union Fwmark {
    uint32_t intValue;
    struct {
        unsigned netId          : 16;
        bool explicitlySelected :  1;
        bool protectedFromVpn   :  1;
        Permission permission   :  2;
    };
    Fwmark() : intValue(0) {}
};

static const unsigned FWMARK_NET_ID_MASK = 0xffff;

还有一个路由策略示例，在打开 VPN 服务的应用后：

root@CP8692:/ # ip rule
0:  from all lookup local 
10000:  from all fwmark 0xc0000/0xd0000 lookup legacy_system 
11000:  from all iif tun0 lookup local_network 
12000:  from all fwmark 0xc0072/0xcffff lookup tun0 
12000:  from all fwmark 0x0/0x20000 uidrange 0-99999 lookup tun0 
13000:  from all fwmark 0x10063/0x1ffff lookup local_network 
13000:  from all fwmark 0x10071/0x1ffff lookup wlan0 
13000:  from all fwmark 0x10072/0x1ffff uidrange 0-0 lookup tun0 
13000:  from all fwmark 0x10072/0x1ffff uidrange 0-99999 lookup tun0 
14000:  from all oif wlan0 lookup wlan0 
14000:  from all oif tun0 uidrange 0-99999 lookup tun0 
15000:  from all fwmark 0x0/0x10000 lookup legacy_system 
16000:  from all fwmark 0x0/0x10000 lookup legacy_network 
17000:  from all fwmark 0x0/0x10000 lookup local_network 
19000:  from all fwmark 0x71/0x1ffff lookup wlan0 
21000:  from all fwmark 0x72/0x1ffff lookup wlan0 
22000:  from all fwmark 0x0/0xffff lookup wlan0 
23000:  from all fwmark 0x0/0xffff uidrange 0-0 lookup main 
32000:  from all unreachable