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Android KitKat 4.4 Wifi移植之AP模式與網絡共享功能調試記錄

編輯：關於Android編程

Tethering技術在移動平台上已經運用的越來越廣泛了，它可以把移動設備當做一個接入點，其它的設備可以通過Wi-Fi，USB或是Bluetooth等方式連接到此移動設備。在Android中可以將Wifi設為AP模式作為WLAN接入點，從而與其他設備共享Android的互聯網連接。Android成為接入點後，就無法通過WLAN連接使用Android的應用程序訪問互聯網，但可以通過其他方式如以太網或移動網絡訪問互聯網。此時以太網或移動網絡在網絡共享（Tethering）中是作為upstream的角色。

最近在Atmel的SAMA5D3-EK開發板上調試Wifi模塊，需要在Android下實現Tethering，通過Wi-Fi的AP模式，將網絡連接共享給其它設備。

開發板上一個有線網卡(eth0)，一個無線網卡(wlan0)，eth0連接到外網，wlan0作為AP共享給其他設備比如Android手機，使得Android手機可以通過開發板連接到外網。

硬件平台：Atmel SAMA5

軟件平台：Linux 3.10 +Android 4.4

Wifi模組：RTL8723AU(USB接口)

因為使用的內核是廠商基於主線內核開發的，雖然主線內核中加入了Android的基本支持，但並不完全。在做Android移植時，如果發現Android上層的某些功能缺乏內核的支持，可以根據Google維護的AndroidLinux內核將相應的更改應用到廠商Linux內核中，在前文《Android KitKat 4.4平台開發-添加USBADB和MTP功能支持》中就是使用的這種方法。

整個Wifi移植過程，Wifi模組廠商Realtek給出了詳細的過程，但內容只涉及Wifi驅動及Android部分，使用哪個Linux內核以及內核如何配置需要我們自己決定。

按照Realtek提供的移植文檔進行AndroidWifi的移植，在測試Wifi 網絡共享功能時出現如下問題：

在“設置”程序“網絡共享與便攜式熱點”中，打開“便攜式Wi-Fi熱點”，並沒有真正開啟Wifi熱點，而是相應單選框不斷關開，如此反復。

對於在測試Android功能時出現的異常情況，一般在log中會存在相應的錯誤信息，雖然並不是絕對准確，但調試時應該考慮先分析log信息，嘗試從中定位導致異常發生的代碼位置。

經過一番分析推測，如下高亮顯示的log信息很有可能是誘發異常發生的關鍵代碼點。

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(&quota_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(&quota_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

分析這個函數NatController::setTetherCountingRules及log信息，推斷出異常發生的原因是執行命令

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

而且還涉及到路徑/proc/net/xt_quota/，但當前系統下並不存在這個路徑。由此推斷應該是內核缺乏與quota2或xt_quota相關的支持。

找到問題的可能原因，接下來就是驗證了。比較Android Linux內核、廠商Linux內核以及主線Linux內核網絡部分的差異，發現Android Linux內核在主線Linux內核基礎上增加了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 Hj?nnev?g

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 Hj?nnev?g

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相關的commit，制作補丁

$ 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

將這些補丁應用到廠商Linux內核（git am命令）

在內核配置中增加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