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

保护套接字免受 VPN 连接后，通过该套接字发送的数据将直接到达底层网络，因此其流量不会通过 VPN 转发。如果某些连接需要保持在 VPN 之外，则此方法很有用。例如，如果 VPN 隧道的目的地被 VPN 路由覆盖，则 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(interface)并导致无限循环。如果申请未准备好或被撤销，此方法将失败。

我知道 Android 是基于 Linux 构建的，此功能背后使用了哪些 Linux 机制或实用程序，使通过此套接字发送的数据仅绕过 VPN，但所有其他数据都通过 VPN？

最佳答案

简而言之，Android VPNService protected fd working by policy routing，所有通过protected fd的数据包都会被标记一个特殊的fwmark，所有带有这个的数据包fwmark 将绕过 VPN。

关键代码 fragment 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 服务的应用程序之后:

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

关于android - Android VpnService是如何保护fd工作的？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/38606036/