不知道啥原因。google了半天。终于找到解决方法。
OK。

ADSL Server出现如下大量提示:/var/log/messages
Mar 19 16:36:43 ADSLserver kernel: ll header: ff:ff:ff:ff:ff:ff:00:48:54:5b:0d:3f:08:00
Mar 19 16:36:43 ADSLserver kernel: martian source 192.168.6.255 from 192.168.6.2, on dev eth0
Mar 19 16:36:43 ADSLserver kernel: ll header: ff:ff:ff:ff:ff:ff:00:48:54:5b:0d:3f:08:00
Mar 19 16:36:43 ADSLserver kernel: martian source 192.168.6.255 from 192.168.6.2, on dev eth0
Mar 19 16:36:43 ADSLserver kernel: ll header: ff:ff:ff:ff:ff:ff:00:48:54:5b:0d:3f:08:00

解决:
echo 0 >/proc/sys/net/ipv4/conf/eth0/rp_filter

原因:
http://linux.chinaitlab.com/server/534526_6.html
22.4 战神攻击
如果tcpdump显示Linux服务器看起来没有对来自Windows/Mac客户端的数据包做出响应，那么可能没有禁用反向路径过滤（Reverse Path Filtering）。如果你在/var/log/messages里看到这样：
martian destination 127.0.0.1 from 192.168.0.222, dev ipsec0
martian source 192.168.0.200 from 192.168.0.222, on dev eth0
ll header: 00:00:00:00:00:00:00:40:33:2c:70:c8:08:00
那么你需要清空rp_filter。如果在/var/log/messages向回翻，你可以看到FreeS/WAN有这样的提示：
ipsec_setup: (/proc/sys/net/ipv4/conf/eth0/rp_filter = `1', should be 0)
如是eth0设备，使用echo 0 >; /proc/ss/net/ipv4/conf/eth0/rp_filter可以快速修复这个问题。作为选择，你可以在/etc/sysctl.conf中添加net.ipv4.conf.default.rp_filter=0。