安装和配置Tripwire,加强你的Linux系统安全

1、为什么要安装 tripwire
在安装完 Linux，做好设定后，建议你马上安装 tripwire 这套软件，它能把文件的特征，如对象大小、拥有者、群组、存取权限等建立成指纹数据库(fingerprints)，并定期执行检查。当发现文件现况与指纹数据库不符合时，tripwire 会提出警告，告知你哪些项目与指纹数据库不符。
2、安装程序
说明：本文档使用的linux发行版本为RedHat Linux 7.3。其他的发行版本设置类似。
1. 安装套件：rpm -Uvh tripwire-版本号码.rpm
2. 切换工作目录到 /etc/tripwire，其中有两个配置文件：
§ twcfg.txt:可用来设定 tripwire 的工作环境，可依照你的习惯来调整，
§ twpol.txt：指定 tripwire 对哪些文件的哪些项目进行监控。
3. 预设的 twcfg.txt其中
ROOT =/usr/sbin
POLFILE =/etc/tripwire/tw.pol
DBFILE =/var/lib/tripwire/$(HOSTNAME).twd
REPORTFILE =/var/lib/tripwire/report/$(HOSTNAME)-$(DATE).twr
SITEKEYFILE =/etc/tripwire/site.key
LOCALKEYFILE =/etc/tripwire/$(HOSTNAME)-local.key
EDITOR =/bin/vi
LATEPROMPTING =false
LOOSEDIRECTORYCHECKING =false
MAILNOVIOLATIONS =true
EMAILREPORTLEVEL =3
REPORTLEVEL =3
MAILMETHOD =SENDMAIL
SYSLOGREPORTING =false
MAILPROGRAM =/usr/sbin/sendmail -oi -t
§ DBFILE 为指纹数据库之文件名。
§ REPORTFILE 为检测报告档之档名。
4. 再来看看 twpol.txt，我们可以设定它来指定 tripwire 对哪些文件的 哪些项目进行监控。tripwire 可监控的项目可在 twpolicy 的 man page 中， 『property masks』一节内找到，如下所示：
- Ignore the following properties
+ Record and check the following properties
a Access timestamp
b Number of blocks allocated
c Inode timestamp (create/modify)
d ID of device on which inode resides
g File owner's group ID
i Inode number
l File is increasing in size (a "growing file")
m Modification timestamp
n Number of links (inode reference count)
p Permissions and file mode bits
r ID of device pointed to by inode
(valid only for device objects)
s File size
t File type
u File owner's user ID
C CRC-32 hash value
H Haval hash value
M MD5 hash value
S SHA hash value
其中『+』与『-』容后说明。
5. 如何要求 tripwire 监控某些文件呢？Red Hat 所附的 twpol.txt 已把重要的配置文件与程序行入监控的范围，你可以找到如附 图内的这一段：(
（
rulename = "Security Control",
severity = $(SIG_HI)
)
{
/etc/group -> $(SEC_CRIT) ;
/etc/security -> $(SEC_CRIT) ;
}
这一段把 /etc/group 以及 /etc/security 这两个对象纳入 『Security Control』这一组，警戒程度为由 SIG_HI 这个变量定义， 值为 100(稍后会介绍)。而 tripwire 会监控 /etc/group 及 /etc/security 的哪些项目呢？则由 SEC_CRIT 这个变量来定义。
6. 从 twpol.txt 的前端往后浏览，可以找到如附图的这一段：
@@section FS
SEC_CRIT = $(IgnoreNone)-SHa ; # Critical files that cannot change
SEC_SUID = $(IgnoreNone)-SHa ; # Binaries with the SUID or SGID flags set
SEC_BIN = $(ReadOnly) ; # Binaries that should not change
SEC_CONFIG = $(Dynamic) ; # Config files that are changed infrequently but accessed often
SEC_LOG = $(Growing) ; # Files that grow, but that should never change ownership
SEC_INVARIANT = +tpug ; # Directories that should never change permission or ownership
SIG_LOW = 33 ; # Non-critical files that are of minimal security impact
SIG_MED = 66 ; # Non-critical files that are of significant security impact
SIG_HI = 100 ; # Critical files that are significant points of vulnerability
你可以发现 SIG_HI 的值就如上一点所提的，为 100。 而 tripwire 会监控 /etc/group 的哪些项目是由 SEC_CRIT 所定义； 在此处你发现 SEC_CRIT 等于『$(IgnoreNone)-SHa』，究竟是哪些项目？
要解开这个谜，必须先找出 IgnoreNone 的变量究竟定义为何。 但找遍 twpol.txt 还是解不开，原来 IgnoreNone 跟下面的 ReadOnly, Dynamic, Growing 等均为 tripwire 预先定义好的变量， 在 twpolicy 的 man page 中『Variables』一节内可以找到如附图的内容：
ReadOnly ReadOnly is good for files that are widely available but are intended to be read-only. Value: +pinugtsdbmCM-rlacSH
Dynamic Dynamic is good for monitoring user direc- tories and files that tend to be dynamic in behavior. value: +pinugtd-srlbamcCMSH
Growing The Growing variable is intended for files that should only get larger. Value: +pinugtdl-srbamcCMSH
Device Device is good for devices or other files that Tripwire should not attempt to open. Value: +pugsdr-intlbamcCMSH
IgnoreAll IgnoreAll tracks a file's presence or absence, but doesn't check any other prop- erties. Value: -pinugtsdrlbamcCMSH
IgnoreNone IgnoreNone turns on all properties and pro-vides a convenient starting point for defining your own property masks. (For example, mymask = $(IgnoreNone) -ar;) Value: +pinugtsdrbamcCMSH-l
从上面得知『IgnoreNone』的值为『+pinugtsdrbamcCMSH-l』， 其中『+』后所列的是要监控的项目，而『-』后所列的则为不监控的项目。那么『$(IgnoreNone)-SHa』呢？就是把 IgnoreNone 内原本列入监控的 SHa 项目改列为不监控。
你可以按照需求来修改这个文件。。
7. 接着在 在 /etc/tripwire 内执行 ./twinstall.sh。 执行过程中会要求你设定两个密码(pass phrase)：
§ site pass phrase ：加密 twpol.txt 及 twcfg.txt 时用。
§ local pass phrase：加密指纹数据库时用。
之后会再要你输入正确的 site pass phrase， 此时会对 twpol.txt 及 twcfg.txt 分别进行加密处理， 由原始文本文件产生 tw.pol 及 tw.cfg。过程如附图所示： [root@localhost tripwire]# ./twinstall.sh

----------------------------------------------
The Tripwire site and local passphrases are used to
sign a variety of files, such as the configuration,
policy, and database files.

Passphrases should be at least 8 characters in length
and contain both letters and numbers.

See the Tripwire manual for more information.

----------------------------------------------
Creating key files...

(When selecting a passphrase, keep in mind that good passphrases typically
have upper and lower case letters, digits and punctuation marks, and are
at least 8 characters in length.)
Enter the site keyfile passphrase:
Verify the site keyfile passphrase:
Generating key (this may take several minutes)...Key generation complete.
(When selecting a passphrase, keep in mind that good passphrases typically
have upper and lower case letters, digits and punctuation marks, and are
at least 8 characters in length.)

Enter the local keyfile passphrase:
Verify the local keyfile passphrase:
Generating key (this may take several minutes)...Key generation complete.

----------------------------------------------
Signing configuration file...
Please enter your site passphrase:
Wrote configuration file: /etc/tripwire/tw.cfg

A clear-text version of the Tripwire configuration file
/etc/tripwire/twcfg.txt
has been preserved for your inspection. It is recommended
that you delete this file manually after you have examined it.


----------------------------------------------
Signing policy file...
Please enter your site passphrase:
Wrote policy file: /etc/tripwire/tw.pol

A clear-text version of the Tripwire policy file
/etc/tripwire/twpol.txt
has been preserved for your inspection. This implements
a minimal policy, intended only to test essential
Tripwire functionality. You should edit the policy file
to describe your system, and then use twadmin to generate
a new signed copy of the Tripwire policy.

You have new mail in /var/spool/mail/root
在 twinstall.sh 执行完毕后，建议把 twpol.txt 及 twcfg.txt 这两个文本文件删除或移至别处。
8. 执行 tripwire -m i 来建立指纹数据库，它会要求你输入 local pass phase。
[root@localhost tripwire]# tripwire -m i
Please enter your local passphrase:
Parsing policy file: /etc/tripwire/tw.pol
Generating the database...
*** Processing Unix File System ***
### Warning: File system error.
### Filename: /proc/scsi
### No such file or directory
### Continuing...
### Warning: File system error.
### Filename: /usr/sbin/fixrmtab
### No such file or directory
### Continuing...
Wrote database file: /var/lib/tripwire/localhost.localdomain.twd
The database was successfully generated.
You have new mail in /var/spool/mail/root
9. 或许你怀疑 tripwire 真的能侦测出文件最细微的改变吗？ 以下来做个实验，我们把 /etc/group 中第一行第二个字段的『x』改成『X』：
[root@localhost tripwire]#head –1 /etc/group
root:x:0:root
[root@localhost tripwire]#vi /etc/group
[root@localhost tripwire]#head –1 /etc/group
root:X:0:root
10. 接下来执行『tripwire -m c --interactive』进行检查，最后出现报告清单 (内定使用 vi)，有：
§ 『Rule Summary』：列出所有组别的检查结果。
===============================================================================
Rule Summary:
===============================================================================

-------------------------------------------------------------------------------
Section: Unix File System
-------------------------------------------------------------------------------

Rule Name Severity Level Added Removed Modified
--------- -------------- ----- ------- --------
Invariant Directories 66 0 0 0
Temporary directories 33 0 0 0
* Tripwire Data Files 100 1 0 0
Critical devices 100 0 0 0
User binaries 66 0 0 0
Tripwire Binaries 100 0 0 0
Critical configuration files 100 0 0 0
Libraries 66 0 0 0
Operating System Utilities 100 0 0 0
Critical system boot files 100 0 0 0
File System and Disk Administraton Programs
100 0 0 0
Kernel Administration Programs 100 0 0 0
Networking Programs 100 0 0 0
System Administration Programs 100 0 0 0
Hardware and Device Control Programs
100 0 0 0
System Information Programs 100 0 0 0
Application Information Programs
100 0 0 0
Shell Related Programs 100 0 0 0
Critical Utility Sym-Links 100 0 0 0
Shell Binaries 100 0 0 0
System boot changes 100 0 0 0
OS executables and libraries 100 0 0 0
* Security Control 100 0 0 1
Login Scripts 100 0 0 0
Root config files 100 0 0 0
Total objects scanned: 15675
Total violations found: 2
有两组有异动，一为 tripwire 的资料文件(新增)，另一为 『System Control』这一组(/etc/group 属于这一组!)。
『Object Summary』：列出有异动的对象清单。
===============================================================================
Object Summary:
===============================================================================
-------------------------------------------------------------------------------
# Section: Unix File System
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Rule Name: Tripwire Data Files (/var/lib/tripwire)
Severity Level: 100
-------------------------------------------------------------------------------
Remove the "x" from the adjacent box to prevent updating the database
with the new values for this object.
Added:
[x] "/var/lib/tripwire/localhost.localdomain.twd"

-------------------------------------------------------------------------------
Rule Name: Security Control (/etc/group)
Severity Level: 100
-------------------------------------------------------------------------------
Remove the "x" from the adjacent box to prevent updating the database
with the new values for this object.

Modified:
[x] "/etc/group"

===============================================================================
Object Detail:
===============================================================================
-------------------------------------------------------------------------------
Section: Unix File System
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Rule Name: Tripwire Data Files (/var/lib/tripwire)
/etc/group 被发现有异动了。如果要把指纹数据库内 /etc/group 的资料 更新为目前的状态，请保留 /etc/group 前方 [ ] 内的 x， 否则把它改为空格。
『Object Detail』：异动对象的详细信息，如哪些项目有异动等。===============================================================================
Object Summary:
===============================================================================
-------------------------------------------------------------------------------
# Section: Unix File System
-------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Rule Name: Tripwire Data Files (/var/lib/tripwire)
Severity Level: 100
-------------------------------------------------------------------------------
Remove the "x" from the adjacent box to prevent updating the database
with the new values for this object.

Added:
[x] "/var/lib/tripwire/localhost.localdomain.twd"
-------------------------------------------------------------------------------
Rule Name: Security Control (/etc/group)
Severity Level: 100
-------------------------------------------------------------------------------
Remove the "x" from the adjacent box to prevent updating the database
with the new values for this object.

Modified:[x] "/etc/group"
虽然仅仅是把 x 改成 X，但有四个项目受到影响。其中最重要的是 MD5 的值。一个文件的 MD5 值只要跟原先的值不一样，就可以断定文件的内容 已被修改过。
§ tripwire 每日均会自动执行检查，并寄 e-mail 给 root 告知结果。 你也应定期执行『tripwire -m c --interactive』来更新指纹数据库。