一、爲何要安裝 tripwire
在安裝完 Linux，作好設定後，建議你立刻安裝 tripwire 這套軟件，它能把文件的特徵，如對象大小、擁有者、羣組、存取權限等創建成指紋數據庫(fingerprints)，並按期執行檢查。當發現文件現況與指紋數據庫不符合時，tripwire 會提出警告，告知你哪些項目與指紋數據庫不符。
二、安裝程序
說明：本文檔使用的linux發行版本爲RedHat Linux 7.3。其餘的發行版本設置相似。
1. 安裝套件：rpm -Uvh tripwire-版本號碼.rpm
2. 切換工做目錄到 /etc/tripwire，其中有兩個配置文件：
§ twcfg.txt:可用來設定 tripwire 的工做環境，可依照你的習慣來調整，
§ twpol.txt：指定 tripwire 對哪些文件的哪些項目進行監控。
3. 預設的 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 爲指紋數據庫之文件名。
§ REPORTFILE 爲檢測報告檔之檔名。
4. 再來看看 twpol.txt，咱們能夠設定它來指定 tripwire 對哪些文件的 哪些項目進行監控。tripwire 可監控的項目可在 twpolicy 的 man page 中， 『property masks』一節內找到，以下所示：
- 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『+』與『-』容後說明。
5. 如何要求 tripwire 監控某些文件呢？Red Hat 所附的 twpol.txt 已把重要的配置文件與程序行入監控的範圍，你能夠找到如附 圖內的這一段：(
（
rulename = "Security Control",
severity = $(SIG_HI)
)
{
/etc/group -> $(SEC_CRIT) ;
/etc/security -> $(SEC_CRIT) ;
}
這一段把 /etc/group 以及 /etc/security 這兩個對象歸入 『Security Control』這一組，警惕程度爲由 SIG_HI 這個變量定義， 值爲 100(稍後會介紹)。而 tripwire 會監控 /etc/group 及 /etc/security 的哪些項目呢？則由 SEC_CRIT 這個變量來定義。
6. 從 twpol.txt 的前端日後瀏覽，能夠找到如附圖的這一段：
@@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 = $(Growing) ; # Files that grow, but that should never change ownership
SEC_INVARIANT = +tpug ; # Directories that should never change permission or ownership
SIG_LOW = 33 ; # Non-critical files that are of minimal security impact
SIG_MED = 66 ; # Non-critical files that are of significant security impact
SIG_HI = 100 ; # Critical files that are significant points of vulnerability
你能夠發現 SIG_HI 的值就如上一點所提的，爲 100。 而 tripwire 會監控 /etc/group 的哪些項目是由 SEC_CRIT 所定義； 在此處你發現 SEC_CRIT 等於『$(IgnoreNone)-SHa』，到底是哪些項目？
要解開這個謎，必須先找出 IgnoreNone 的變量究竟定義爲什麼。 但找遍 twpol.txt 仍是解不開，原來 IgnoreNone 跟下面的 ReadOnly, Dynamic, Growing 等均爲 tripwire 預先定義好的變量， 在 twpolicy 的 man page 中『Variables』一節內能夠找到如附圖的內容：
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 = $(IgnoreNone) -ar;) Value: +pinugtsdrbamcCMSH-l
從上面得知『IgnoreNone』的值爲『+pinugtsdrbamcCMSH-l』， 其中『+』後所列的是要監控的項目，而『-』後所列的則爲不監控的項目。那麼『$(IgnoreNone)-SHa』呢？就是把 IgnoreNone 內本來列入監控的 SHa 項目改列爲不監控。
你能夠按照需求來修改這個文件。。
7. 接着在 在 /etc/tripwire 內執行 ./twinstall.sh。 執行過程當中會要求你設定兩個密碼(pass phrase)：
§ site pass phrase ：加密 twpol.txt 及 twcfg.txt 時用。
§ local pass phrase：加密指紋數據庫時用。
以後會再要你輸入正確的 site pass phrase， 此時會對 twpol.txt 及 twcfg.txt 分別進行加密處理， 由原始文本文件產生 tw.pol 及 tw.cfg。過程如附圖所示：[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 執行完畢後，建議把 twpol.txt 及 twcfg.txt 這兩個文本文件刪除或移至別處。
8. 執行 tripwire -m i 來創建指紋數據庫，它會要求你輸入 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. 或許你懷疑 tripwire 真的能偵測出文件最細微的改變嗎？ 如下來作個實驗，咱們把 /etc/group 中第一行第二個字段的『x』改爲『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. 接下來執行『tripwire -m c --interactive』進行檢查，最後出現報告清單 (內定使用 vi)，有：
§ 『Rule Summary』：列出全部組別的檢查結果。
===============================================================================
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
有兩組有異動，一爲 tripwire 的資料文件(新增)，另外一爲 『System Control』這一組(/etc/group 屬於這一組!)。
『Object Summary』：列出有異動的對象清單。
===============================================================================
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 被發現有異動了。若是要把指紋數據庫內 /etc/group 的資料 更新爲目前的狀態，請保留 /etc/group 前方 [ ] 內的 x， 不然把它改成空格。
『Object Detail』：異動對象的詳細信息，如哪些項目有異動等。===============================================================================
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"
雖然僅僅是把 x 改爲 X，但有四個項目受到影響。其中最重要的是 MD5 的值。一個文件的 MD5 值只要跟原先的值不同，就能夠判定文件的內容 已被修改過。
§ tripwire 每日均會自動執行檢查，並寄 e-mail 給 root 告知結果。 你也應按期執行『tripwire -m c --interactive』來更新指紋數據庫。