为了定位问题，需要在Linux上使用tcpdump并且保存到文件，遇到了如下问题：
tcpdump port 9001 -w xx
tcpdump: xx: Permission denied
因为已经是root用户，所以判断不是文件系统权限的问题。
在网上查了一下，找到解决方法：将tcpdump的模式从enforce改为complain模式。
修改过程如下：
先查看处在那个模式：
grep tcpdump /sys/kernel/security/apparmor/profiles
/usr/sbin/tcpdump (enforce)
果然不是complain模式。
修改为complain模式：
aa-complain /usr/sbin/tcpdump
The program 'aa-complain' is currently not installed. You can install it by typing:
apt-get install apparmor-utils
出现错误，按照提示安装apparmor-utils，过程略，然后再次执行：
aa-complain /usr/sbin/tcpdump
Setting /usr/sbin/tcpdump to complain mode.
查看：
grep tcpdump /sys/kernel/security/apparmor/profiles
/usr/sbin/tcpdump (complain)
完成
参考：
tcpdump permission denied：https://ubuntuforums.org/showthread.php?t=1501339
页面内容比较多，关键信息如下：
This is probably caused by AppArmor. You need to switch from 'enforcement' mode to 'complain' mode on 'tcpdump'. Run the following command as root:
Code:
aa-complain /usr/sbin/tcpdump
You can check by running the following command as root:
Code:
grep tcpdump /sys/kernel/security/apparmor/profiles
You should see (enforce) or (complain). You want it to say (complain).
AppArmor介绍：http://www.tuicool.com/articles/aeiIzq
Linux版本：Linux 10-6-13-14 3.2.0-85-generic #122-Ubuntu SMP Tue May 26 16:14:57 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux