UNIX下的杀毒软件有好多是商业版本的。但和Windows系统下一样，杀毒软件的质量决定于病毒库的量已及更新的速度。在这里，我们使用自由软件 Clam AntiVirus 来建立Linux下的病毒查杀系统。并且为了消除后来的隐患，建议务必在服务器公开以前构建病毒查杀系统。
安装 Clam AntiVirus
由于Clam Antivirus不存在于CentOS中yum的官方库中，所以用yum安装Clam Antivirus需要定义非官方的库。请先确认相应非官方库文件的存在。
[root@sample ~]# ls -l /etc/yum.repos.d/dag.repo ← 确认相应库文件的存在性
-rw-r--r-- 1 root root 143 Oct 1 21:33 /etc/yum.repos.d/dag.repo ← 确认其存在（否则不能通过yum安装Clamd）
如果以上，dag.repo文件不存在，则不能通过yum安装Clam Antivirus，需要定义非官方库。定义非官方库的方法请见 “CentOS的下载、安装及初始环境设置”一节中yum的相关设置。而且，在此前提下也要保证所定义的dag.repo文件的语法的正确性。
然后，通过yum来在线安装 Clam Antivirus 。
[root@sample ~]# yum -y install clamd ← 在线安装 Clam AntiVirus
Setting up Install Process
Setting up repositories
dag 100% |=========================| 1.1 kB 00:00
update 100% |=========================| 951 B 00:00
base 100% |=========================| 1.1 kB 00:00
addons 100% |=========================| 951 B 00:00
extras 100% |=========================| 1.1 kB 00:00
Reading repository metadata in from local files
primary.xml.gz 100% |=========================| 1.6 MB 00:08
dag : ################################################## 4610/4610
Added 4610 new packages, deleted 0 old in 94.91 seconds
primary.xml.gz 100% |=========================| 103 kB 00:05
update : ################################################## 256/256
Added 56 new packages, deleted 0 old in 4.25 seconds
Reducing Dag RPM Repository for Red Hat Enterprise Linux to included packages only
Finished
Parsing package install arguments
Resolving Dependencies
--> Populating transaction set with selected packages. Please wait.
---> Downloading header for clamd to pack into transaction set.
clamd-0.88.4-1.el4.rf.i38 100% |=========================| 5.3 kB 00:00
---> Package clamd.i386 0:0.88.4-1.el4.rf set to be updated
--> Running transaction check
--> Processing Dependency: clamav = 0.88.4-1.el4.rf for package: clamd
--> Processing Dependency: libclamav.so.1 for package: clamd
--> Restarting Dependency Resolution with new changes.
--> Populating transaction set with selected packages. Please wait.
---> Downloading header for clamav to pack into transaction set.
clamav-0.88.4-1.el4.rf.i3 100% |=========================| 8.1 kB 00:00
---> Package clamav.i386 0:0.88.4-1.el4.rf set to be updated
--> Running transaction check
--> Processing Dependency: clamav-db = 0.88.4-1.el4.rf for package: clamav
--> Restarting Dependency Resolution with new changes.
--> Populating transaction set with selected packages. Please wait.
---> Downloading header for clamav-db to pack into transaction set.
clamav-db-0.88.4-1.el4.rf 100% |=========================| 3.2 kB 00:00
---> Package clamav-db.i386 0:0.88.4-1.el4.rf set to be updated
--> Running transaction check
Dependencies Resolved
=============================================================================
Package Arch Version Repository Size
=============================================================================
Installing:
clamd i386 0.88.4-1.el4.rf dag 64 k
Installing for dependencies:
clamav i386 0.88.4-1.el4.rf dag 724 k
clamav-db i386 0.88.4-1.el4.rf dag 5.6 M
Transaction Summary
=============================================================================
Install 3 Package(s)
Update 0 Package(s)
Remove 0 Package(s)
Total download size: 6.4 M
Downloading Packages:
(1/3): clamd-0.88.4-1.el4 100% |=========================| 64 kB 00:01
(2/3): clamav-0.88.4-1.el 100% |=========================| 724 kB 00:04
(3/3): clamav-db-0.88.4-1 100% |=========================| 5.6 MB 00:25
Running Transaction Test
Finished Transaction Test
Transaction Test Succeeded
Running Transaction
Installing: clamav-db ######################### [1/3]
Installing: clamav ######################### [2/3]
Installing: clamd ######################### [3/3]
Installed: clamd.i386 0:0.88.4-1.el4.rf
Dependency Installed: clamav.i386 0:0.88.4-1.el4.rf clamav-db.i386 0:0.88.4-1.el4.rf
Complete! ← 安装完毕！
配置 Clam AntiVirus
接下来配置 Clam Antivirus 。
[root@sample ~]# vi /etc/clamd.conf ← 修改clamd的配置文件
ArchiveBlockMax ← 找到这一行，在行首加上“#”（不把大容量的压缩文件看作被感染病毒的文件）
↓
#ArchiveBlockMax ← 变为此状态
User clamav ← 找到这一行，在行首加上“#”（不允许一般用户控制）
↓
#User clamav ← 变为此状态
运行 Clam AntiVirus
让 Clam Antivirus 开始运行，并设置其为自启动。
[root@sample ~]# chkconfig clamd on ← 将其设置为自系统启动后启动
[root@sample ~]# chkconfig --list clamd
clamd 0:off 1:off 2:on 3:on 4:on 5:on 6:off ← 确认2--5为on的状态就OK
[root@sample ~]# /etc/rc.d/init.d/clamd start ← 启动clamd服务（运行Clam AntiVirus）
Starting Clam AntiVirus Daemon: [ OK ] ← 启动成功
更新 Clam AntiVirus 的病毒库
安装后建议立即更新病毒库，以保证最新病毒的查杀。
[root@sample ~]# freshclam ← 更新clam的病毒库
ClamAV update process started at Fri Aug 25 18:39:26 2006
Downloading main.cvd
main.cvd updated (version: 40, sigs: 64138, f-level: 8, builder: tkojm)
Downloading daily.cvd
daily.cvd updated (version: 1728, sigs: 2565, f-level: 8, builder: ccordes)
Database updated (66703 signatures) from db.cn.clamav.net (IP: 58.221.253.171)
Clamd successfully notified about the update.
病毒扫描
然后进行病毒扫描的测试，在这里，我们首先下载测试用的病毒文件。
[root@sample ~]# clamdscan ← 进行病毒扫描
/root: OK
----------- SCAN SUMMARY -----------
Infected files: 0 ← 没有发现病毒
Time: 5.074 sec (0 m 5 s)
[root@sample ~]# wget http://www.eicar.org/download/eicar.com ← 下载带毒文件
[root@sample ~]# wget http://www.eicar.org/download/eicar_com.zip ← 下载带毒文件
然后，再次进行病毒到描。附加“remove”选项后，会在查出病毒后自动删除染毒文件。
[root@sample ~]# clamdscan --remove ← 再次进行病毒扫描，并附加删除选项
/root/eicar.com: Eicar-Test-Signature FOUND ← 发现被病毒感染的文件
/root/eicar.com: Removed. ← 删除被病毒感染的文件
/root/eicar_com.zip: Eicar-Test-Signature FOUND ← 发现被病毒感染的文件
/root/eicar_com.zip: Removed. ← 删除被病毒感染的文件 ----------- SCAN SUMMARY -----------
Infected files: 2
Time: 2.201 sec (0 m 2 s)
让病毒扫描定期运行
[root@sample ~]# vi scan.sh ← 建立自动扫描脚本，如下：
#!/bin/bash
PATH=/usr/bin:/bin
CLAMSCANTMP=`mktemp`
clamdscan --recursive --remove / > $CLAMSCANTMP
[ ! -z "$(grep FOUND$ $CLAMSCANTMP)" ] && \
grep FOUND $CLAMSCANTMP | mail -s "Virus Found in `hostname`" root
rm -f $CLAMSCANTMP
[root@sample ~]# chmod 700 scan.sh ← 赋予脚本可被执行的权限
[root@sample ~]# crontab -e ← 编辑计划任务，添加如下行
00 03 * * * /root/scan.sh ← 添加这一行，让其在每天3点钟执行扫描