Tethering技术在移动平台上已经运用的越来越广泛了,它可以把移动设备当做一个接入点,其它的设备可以通过Wi-Fi,USB或是Bluetooth等方式连接到此移动设备。在Android中可以将Wifi设为AP模式作为WLAN接入点,从而与其他设备共享Android的互联网连接。Android成为接入点后,就无法通过WLAN连接使用Android的应用程序访问互联网,但可以通过其他方式如以太网或移动网络访问互联网。此时以太网或移动网络在网络共享(Tethering)中是作为upstream的角色。

 

最近在AtmelSAMA5D3-EK开发板上调试Wifi模块,需要在Android下实现Tethering,通过Wi-Fi的AP模式,将网络连接共享给其它设备。

 

开发板上一个有线网卡(eth0),一个无线网卡(wlan0),eth0连接到外网,wlan0作为AP共享给其他设备比如Android手机,使得Android手机可以通过开发板连接到外网。

 

硬件平台:Atmel SAMA5

软件平台:Linux 3.10 +Android 4.4

Wifi模组:RTL8723AU(USB接口)

 

因为使用的内核是厂商基于主线内核开发的,虽然主线内核中加入了Android的基本支持,但并不完全。在做Android移植时,如果发现Android上层的某些功能缺乏内核的支持,可以根据Google维护的AndroidLinux内核将相应的更改应用到厂商Linux内核中,在前文《Android KitKat 4.4平台开发-添加USBADB和MTP功能支持》中就是使用的这种方法。

 

整个Wifi移植过程,Wifi模组厂商Realtek给出了详细的过程,但内容只涉及Wifi驱动及Android部分,使用哪个Linux内核以及内核如何配置需要我们自己决定。

 

按照Realtek提供的移植文档进行AndroidWifi的移植,在测试Wifi 网络共享功能时出现如下问题:

在“设置”程序“网络共享与便携式热点”中,打开“便携式Wi-Fi热点”,并没有真正开启Wifi热点,而是相应单选框不断关开,如此反复。

 

对于在测试Android功能时出现的异常情况,一般在log中会存在相应的错误信息,虽然并不是绝对准确,但调试时应该考虑先分析log信息,尝试从中定位导致异常发生的代码位置。

经过一番分析推测,如下高亮显示的log信息很有可能是诱发异常发生的关键代码点。

V/NatController(972): enableNat(intIface=, extIface=)

V/NatController(972): runCmd(/system/bin/iptables -t nat -A natctrl_nat_POSTROUTING -o eth0 -jMASQUERADE) res=0

V/NatController(972): runCmd(/system/bin/iptables -A natctrl_FORWARD -i eth0 -o wlan0 -m state--state ESTABLISHED,RELATED -g natctrl_tether_counters) res=0

D/dalvikvm(1339): GC_CONCURRENT freed 373K, 54% free 6723K/14460K, paused 44ms+14ms, total219ms

V/NatController(972): runCmd(/system/bin/iptables -A natctrl_FORWARD -i wlan0 -o eth0 -m state--state INVALID -j DROP) res=0

V/NatController(972): runCmd(/system/bin/iptables -A natctrl_FORWARD -i wlan0 -o eth0 -gnatctrl_tether_counters) res=0

V/NatController( 972): runCmd(/system/bin/iptables-A natctrl_tether_counters -i wlan0 -o eth0 -m quota2 --name wlan0_eth0 --grow-j RETURN) res=1

V/NatController(972): runCmd(/system/bin/iptables -D natctrl_FORWARD -i wlan0 -o eth0 -m state--state INVALID -j DROP) res=0

V/NatController(972): runCmd(/system/bin/iptables -D natctrl_FORWARD -i eth0 -o wlan0 -m state--state ESTABLISHED,RELATED -g natctrl_tether_counters) res=0

E/NatController( 972): Error setting forward rules

V/NatController(972): runCmd(/system/bin/iptables -F natctrl_FORWARD) res=0

V/NatController(972): runCmd(/system/bin/iptables -A natctrl_FORWARD -j DROP) res=0


http://androidxref.com/4.4.2_r1/xref/system/netd/NatController.cpp#294


256int NatController::setTetherCountingRules(bool add, const char *intIface, const char *extIface) {
257
258    /* We only ever add tethering quota rules so thatthey stick. */
259    if (!add) {
260        return 0;
261    }
262    char *quota_name, *proc_path;
263    int quota_fd;
264    asprintf("a_name, "%s_%s", intIface, extIface);
265
266    asprintf(&proc_path, "/proc/net/xt_quota/%s", quota_name);
267    quota_fd = open(proc_path, O_RDONLY);
268    if (quota_fd >= 0) {
269        /* quota for iface pair already exists*/
270        free(proc_path);
271        free(quota_name);
272        return 0;
273    }
274    close(quota_fd);
275    free(proc_path);
276
277    const char *cmd2b[] = {
278            IPTABLES_PATH,
279            "-A",
280            LOCAL_TETHER_COUNTERS_CHAIN,
281            "-i",
282            intIface,
283            "-o",
284            extIface,
285            "-m",
286            "quota2",
287            "--name",
288            quota_name,
289            "--grow",
290            "-j",
291          "RETURN"
292    };
293
294    if (runCmd(ARRAY_SIZE(cmd2b), cmd2b) && add) {
295        free(quota_name);
296        return -1;
297    }
298    free(quota_name);
299
300    asprintf("a_name, "%s_%s", extIface, intIface);
301    asprintf(&proc_path, "/proc/net/xt_quota/%s", quota_name);
302    quota_fd = open(proc_path, O_RDONLY);
303    if (quota_fd >= 0) {
304        /* quota for iface pair already exists*/
305        free(proc_path);
306        free(quota_name);
307        return 0;
308    }
309    close(quota_fd);
310    free(proc_path);
311

 

分析这个函数NatController::setTetherCountingRules及log信息,推断出异常发生的原因是执行命令

/system/bin/iptables -A natctrl_tether_counters -i wlan0 -o eth0 -mquota2 --name wlan0_eth0 --grow -j 失败。

而且还涉及到路径/proc/net/xt_quota/,但当前系统下并不存在这个路径。由此推断应该是内核缺乏与quota2或xt_quota相关的支持。

 

找到问题的可能原因,接下来就是验证了。比较Android Linux内核、厂商Linux内核以及主线Linux内核网络部分的差异,发现Android Linux内核在主线Linux内核基础上增加了quota2的支持。涉及四次提交

$git log --name-only net/netfilter/xt_quota2.cinclude/linux/netfilter/xt_quota2.h net/netfilter/Kconfignet/netfilter/Makefile

commit7d89969ad270d4c535e33830188806233bf35442

Author:Greg Hackmann

Date:Mon Feb 24 09:39:46 2014 -0800

    netfilter: xt_qtaguid: 64-bit warning fixes

    Change-Id:I2adc517c0c51050ed601992fa0ea4de8f1449414

    Signed-off-by: Greg Hackmann

net/netfilter/xt_quota2.c

 

commit73570fe76d3b47e669558f06f1a18e0f02dff606

Author:Arve Hjønnevåg

Date:Mon May 13 20:42:46 2013 -0700

    netfilter: xt_quota2: 3.10 fixes.

    - Stop using obsolete create_proc_entry api.

    - Use proc_set_user instead of directlyaccessing the private structure.

    Signed-off-by: Arve Hjønnevåg

net/netfilter/xt_quota2.c

 

commitea34f99edb73b67ef0a99d304887c64febd4c878

Author:JP Abgrall

Date:Tue Jul 12 12:02:59 2011 -0700

    netfilter: fixup the quota2, and enable.

    The xt_quota2 came from

      http://sourceforge.net/projects/xtables-addons/develop

    It needed tweaking for it to compile withinthe kernel tree.

    Fixed kmalloc() and create_proc_entry()invocations within

     a non-interruptible context.

    Removed useless copying of current quotaback to the iptable's

    struct matchinfo:

      - those are per CPU: they will changerandomly based on which

        cpu gets to update the value.

      - they prevent matching a rule: e.g.

          -A chain -m quota2 --name q1 --quota 123

         can't be followed by

          -D chain -m quota2 --name q1 --quota 123

        as the 123 will be compared to the structmatchinfo's quota member.

    Use the NETLINK NETLINK_NFLOG family to loga single message

    when the quota limit is reached.

    It uses the same packet type as ipt_ULOG,but

     - never copies skb data,

     - uses 112 as the event number (ULOG's +1)

    It doesn't log if the module param"event_num" is 0.

    Change-Id:I021d3b743db3b22158cc49acb5c94d905b501492

    Signed-off-by: JP Abgrall

net/netfilter/Kconfig

net/netfilter/Makefile

net/netfilter/xt_quota2.c

 

commit3db08b39ea752748744e9c7733ce9ef54bed9f3b

Author:JP Abgrall

Date:Tue Jun 21 11:14:49 2011 -0700

    netfilter: adding the original quota2 fromxtables-addons

    The original xt_quota in the kernel is plainbroken:

      - counts quota at a per CPU level

        (was written back when ubiquitous SMP wasjust a dream)

      - provides no way to count across IPV4/IPV6.

    This patch is the original unaltered codefrom:

      http://sourceforge.net/projects/xtables-addons

      at commite84391ce665cef046967f796dd91026851d6bbf3

    Change-Id:I19d49858840effee9ecf6cff03c23b45a97efdeb

    Signed-off-by: JP Abgrall

include/linux/netfilter/xt_quota2.h

net/netfilter/xt_quota2.c

 

提取quota2相关的commit,制作补丁

$ git format-patch -n4 net/netfilter/xt_quota2.c include/linux/netfilter/xt_quota2.hnet/netfilter/Kconfig net/netfilter/Makefile 0001-netfilter-adding-the-original-quota2-from-xtables-ad.patch 0002-netfilter-fixup-the-quota2-and-enable.patch 0003-netfilter-xt_quota2-3.10-fixes.patch 0004-netfilter-xt_qtaguid-64-bit-warning-fixes.patch  

将这些补丁应用到厂商Linux内核 git am命令)

 

在内核配置中增加quota2支持

commit4e6bf851ffd340f83062d053a6a20d358def121e

Author:Max Liao

Date:Mon Jun 16 06:08:25 2014 -0400

    ARM: at91/SAMA5: android_ubifs_defconfig:add netfilter quota2 support for sama5d3 and sama5d4

    Signed-off-by: Max Liao

diff--git a/arch/arm/configs/sama5_android_ubifs_defconfigb/arch/arm/configs/sama5_android_ubifs_defconfig

index9881f7d..48c68a1 100644

---a/arch/arm/configs/sama5_android_ubifs_defconfig

+++b/arch/arm/configs/sama5_android_ubifs_defconfig

@@-623,6 +623,7 @@ CONFIG_NETFILTER_XT_MATCH_OSF=y

 CONFIG_NETFILTER_XT_MATCH_OWNER=y

 CONFIG_NETFILTER_XT_MATCH_PKTTYPE=y

 CONFIG_NETFILTER_XT_MATCH_QUOTA=y

+CONFIG_NETFILTER_XT_MATCH_QUOTA2=y

 CONFIG_NETFILTER_XT_MATCH_RATEEST=y

 CONFIG_NETFILTER_XT_MATCH_REALM=y

 CONFIG_NETFILTER_XT_MATCH_RECENT=y

 

编译内核。

 

测试Wifi 络共享功能,之前的异常现象消失,功能测试正常,这说明之前的推导猜测是正确的,异常的原因的确是内核缺乏netfilter quota2支持




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