OSSEC是一款开源的多平台的入侵检测系统，可以运行于Windows, Linux, OpenBSD/FreeBSD, 以及 MacOS等操作系统中。包括了日志分析，全面检测，root-kit检测。
1. 测试和验证OSSEC泛化及告警规则
OSSEC默认具有一个ossec-logtest工具用于测试OSSEC的泛化及告警规则。该工具一般默认安装于目录 /var/ossec/bin 中。
使用示例：
/var/ossec/bin/ossec-logtest
2014/06/1113:15:36 ossec-testrule: INFO: Reading local decoder file.
2014/06/11 13:15:36 ossec-testrule: INFO: Started (pid: 26740).
ossec-testrule: Type one log per line.
Jun 10 21:29:33 172.16.25.122/172.16.24.32 sshd[24668]: Accepted publickey for root from 172.16.24.121 port 38720 ssh2
**Phase 1: Completed pre-decoding.
full event: 'Jun 10 21:29:33 172.16.25.122/172.16.24.32 sshd[24668]: Accepted publickey for root from 172.16.24.121 port 38720 ssh2'
hostname: '172.16.25.122/172.16.24.32'
program_name: 'sshd'
log: 'Accepted publickey for root from 172.16.24.121 port 38720 ssh2'
**Phase 2: Completed decoding.
decoder: 'sshd'
dstuser: 'root'
srcip: '172.16.24.121'
**Phase 3: Completed filtering (rules).
Rule id: '10100'
Level: '4'
Description: 'First time user logged in.'
**Alert to be generated.
如上文所示，当输入日志内容：
Jun 1021:29:33 172.16.25.122/172.16.24.32 sshd[24668]: Accepted publickey for rootfrom 172.16.24.121 port 38720 ssh2
该条日志经过三步处理，生成了一条4级告警，规则ID为10100，内容为“First time user logged in.”
使用ossec-logtest-v命令，可获取更详细的日志分析逻辑。
/var/ossec/bin/ossec-logtest -v
2014/06/11 13:44:52 ossec-testrule: INFO: Reading local decoder file.
2014/06/11 13:44:52 ossec-testrule: INFO: Started (pid: 27091).
ossec-testrule: Type one log per line.
Jun 11 21:44:41 172.16.25.122/172.16.24.32 sshd[27743]: Did not receive identification string from 172.16.24.121
**Phase 1: Completed pre-decoding.
full event: 'Jun 11 21:44:41 172.16.25.122/172.16.24.32 sshd[27743]: Did not receive identification string from 172.16.24.121'
hostname: '172.16.25.122/172.16.24.32'
program_name: 'sshd'
log: 'Did not receive identification string from 172.16.24.121'
**Phase 2: Completed decoding.
decoder: 'sshd'
srcip: '172.16.24.121'
**Rule debugging:
Trying rule: 1 - Generic template for all syslog rules.
*Rule 1 matched.
*Trying child rules.
Trying rule: 5500 - Grouping of the pam_unix rules.
Trying rule: 5700 - SSHD messages grouped.
*Rule 5700 matched.
*Trying child rules.
Trying rule: 5709 - Useless SSHD message without an user/ip and context.
Trying rule: 5711 - Useless/Duplicated SSHD message without a user/ip.
Trying rule: 5721 - System disconnected from sshd.
Trying rule: 5722 - ssh connection closed.
Trying rule: 5723 - SSHD key error.
Trying rule: 5724 - SSHD key error.
Trying rule: 5725 - Host ungracefully disconnected.
Trying rule: 5727 - Attempt to start sshd when something already bound to the port.
Trying rule: 5729 - Debug message.
Trying rule: 5732 - Possible port forwarding failure.
Trying rule: 5733 - User entered incorrect password.
Trying rule: 5734 - sshd could not load one or more host keys.
Trying rule: 5735 - Failed write due to one host disappearing.
Trying rule: 5736 - Connection reset or aborted.
Trying rule: 5707 - OpenSSH challenge-response exploit.
Trying rule: 5701 - Possible attack on the ssh server (or version gathering).
Trying rule: 5706 - SSH insecure connection attempt (scan).
*Rule 5706 matched.
**Phase 3: Completed filtering (rules).
Rule id: '5706'
Level: '6'
Description: 'SSH insecure connection attempt (scan).'
**Alert to be generated.
2. 自定义日志泛化规则
2.1 添加日志源
添加日志源的方式很简单，通过修改/var/ossec/etc/ossec.conf 即可实现。
如果日志源是本地文件，可通过添加如下配置实现。
<localfile>
<log_format>syslog</log_format>
<location>/path/to/log/file</location>
</localfile>
如果日志源是远程syslog，可通过添加如下配置实现。
<remote>
<connection>syslog</connection>
<protocol>udp</protocol>
<port>2514</port>
<allowed-ips>172.16.24.0/24</allowed-ips>
</remote>
2.2 创建自定义的日志泛化规则
假如有两条日志如下文：
Jun 11 22:06:30172.17.153.38/172.16.24.32 /usr/bin/auditServerd[25649]: User blackrat loginSUCEESS from 172.17.153.36 to 172.17.153.38 distport 3333 .
Jun 11 22:06:30172.17.153.38/172.16.24.32 /usr/bin/auditServerd[25649]: User blackrat login PWD_ERRORfrom 172.17.153.36 to 172.17.153.38 distport 3333 .
该日志使用ossec-logtest分析之后结果如下：
Jun 11 22:06:30 172.17.153.38/172.16.24.32 /usr/bin/auditServerd[25649]: User blackrat login SUCEESS from 172.17.153.36 to 172.17.153.38 distport 3333 .
**Phase 1: Completed pre-decoding.
full event: 'Jun 11 22:06:30 172.16.25.130/172.16.24.32 /usr/bin/auditServerd[25649]: User blackrat login SUCEESS from 172.17.153.36 to 172.17.153.38 distport 3333 .'
hostname: '172.17.153.38/172.16.24.32'
program_name: '/usr/bin/auditServerd'
log: 'User blackrat login SUCEESS from 172.17.153.36 to 172.17.153.38 distport 3333 .'
**Phase 2: Completed decoding.
No decoder matched.
由此可知OSSEC在分析日志的时候，经过了两个泛化过程：pre-decoding和 decoding。
pre-decoding过程是ossec内置的，只要是标准的syslog日志，都可以解析出如下4个基本信息。
Timestamp:Jun 11 22:06:30
Hostname: 172.17.153.38/172.16.24.32
Programe_name: /usr/bin/auditServerd
Log: User blackrat login SUCEESS from 172.17.153.36 to 172.17.153.38 distport 3333.
在decoding过程，用户可以通过修改/var/ossec/etc/decoder.xml，实现自定义的泛化。例如在该文件中添加如下规则：
<decoder name="auditServerd">
<program_name>/usr/bin/auditServerd</program_name>
</decoder>
再次执行/var/ossec/bin/ossec-logtest
**Phase 1: Completed pre-decoding.
full event: 'Jun 11 22:06:30 172.17.153.38/172.16.24.32 /usr/bin/auditServerd[25649]: User blackrat login SUCEESS from 172.17.153.36 to 172.17.153.38 distport 3333 .'
hostname: '172.17.153.38/172.16.24.32'
program_name: '/usr/bin/auditServerd'
log: 'User blackrat login SUCEESS from 172.17.153.36 to 172.17.153.38 distport 3333 .'
**Phase 2: Completed decoding.
decoder: 'auditServerd'
发现，该条日志成功命中了名为auditServerd的规则，该条规则可以准确的将日志定位为是程序auditServerd所发出的。
除此之外，基于auditServerd这条规则，我们还可以添加更多的子规则，来识别出更多的信息。如:
<decoder name="auditServerd">
<program_name>/usr/bin/auditServerd</program_name>
</decoder>
<decoder name="auditServerd-login">
<parent>auditServerd</parent>
<regex offset="after_parent">^User (\S+) login (\S+) from (\S+) to (\S+) distport (\S+) \.$</regex>
<order>user,status,srcip,dstip,dstport</order>
</decoder>
再次执行/var/ossec/bin/ossec-logtest，可获取更多的信息，如下：
**Phase 1: Completed pre-decoding.
full event: 'Jun 11 22:06:30 172.17.153.38/172.16.24.32/usr/bin/auditServerd[25649]: User blackrat login SUCEESS from 172.17.153.36 to172.17.153.38 distport 3333 .'
hostname: '172.17.153.38/172.16.24.32'
program_name: '/usr/bin/auditServerd'
log: 'User blackrat login SUCEESS from 172.17.153.36 to 172.17.153.38distport 3333 .'
**Phase 2: Completed decoding.
decoder: 'auditServerd'
dstuser: 'blackrat'
status:'SUCEESS'
srcip: '172.17.153.36'
dstip: '172.17.153.
用户通过配置上述正则表达式，获取特定字段，用于后续的关联分析。OSSEC一共内置了14个用户可解析的字段：
- location - where the log came from (only on FTS)
- srcuser - extracts the source username
- dstuser - extracts the destination (target) username
- user - an alias to dstuser (only one of the two can be used)
- srcip - source ip
- dstip - dst ip
- srcport - source port
- dstport - destination port
- protocol - protocol
- id - event id
- url - url of the event
- action - event action (deny, drop, accept, etc)
- status - event status (success, failure, etc)
- extra_data - Any extra data
3. 自定义日志告警规则
3.1 规则文件路径配置
OSSEC的规则配置文件默认路径为/var/ossec/rules/，要加载规则文件，需要在/var/ossec/etc/ossec.conf 中配置，默认的配置如下：
<ossec_config> <!-- rules global entry -->
<rules>
<include>rules_config.xml</include>
<include>pam_rules.xml</include>
<include>sshd_rules.xml</include>
<include>telnetd_rules.xml</include>
<include>syslog_rules.xml</include>
<include>arpwatch_rules.xml</include>
......
<include>clam_av_rules.xml</include>
<include>bro-ids_rules.xml</include>
<include>dropbear_rules.xml</include>
<include>local_rules.xml</include>
</rules>
</ossec_config> <!-- rules global entry -->
其实通过下列配置，可以实现加载/var/ossec/rules 下的所有规则文件：
<ossec_config>
<rules>
<rule_dir pattern=".xml$">rules</rule_dir>
</rules>
</ossec_config>
于泛化规则，也可以通过配置decoder_dir域来实现，如：
<ossec_config>
<rules>
<decoder_dir pattern=".xml$">rules/plugins/decoders</decoder_dir>
</rules>
</ossec_config>
上述配置可将/var/ossec/rules/plugins/plugins/decoders目录下所有的xml文件都添加为OSSEC日志泛化规则。
对于更详细的配置及语法，可参考下列文档：
http://ossec-docs.readthedocs.org/en/latest/syntax/head_ossec_config.rules.html#element-rule_dir
3.2 OSSEC告警规则配置
例如，我们需要增加对程序auditServerd的告警规则，我们需要针对auditServerd程序新建一个规则文件，对于OSSEC中已经存在的规则文件如sshd, openbsd, vsftpd等，我们只需要在对应的文件中进行新增或修改。
首先我们新建文件
/var/ossec/rules/auditServerd_rules.xml
添加如下内容：
<group name="auditServer,">
<rule id="80000" level="0" noalert="1">
<decoded_as>auditServerd</decoded_as>
<description>Grouping for the auditServerd rules.</description>
</rule>
<rule id="80001" level="10">
<if_sid>80000</if_sid>
<user>blackrat</user>
<srcip>172.17.153.36</srcip>
<description>User blackrat is not allowed login from 172.17.153.36!</description>
</rule>
</group>
上述规则中，规则id 80000 用于对日志进行分组计数，假如日志中出现了泛化为auditServerd的日志，则对该日志分组为auditServer，且状态机计数加1.
规则80001描述了假如user为blackrat，srcip为172.17.153.36 则命中，并发出“User blackrat is not allowed login from 172.17.153.36!”的告警。
将该文件路径加入到文件/var/ossec/etc/ossec.conf中
…
<include>dropbear_rules.xml</include>
<include>local_rules.xml</include>
<include>auditServerd_rules.xml</include>
</rules>
</ossec_config>
执行/var/ossec/bin/ossec-logtest，结果如下：
**Phase 1: Completed pre-decoding.
full event: 'Jun 11 22:06:30 172.17.153.38/172.16.24.32 /usr/bin/auditServerd[25649]: User blackrat login SUCEESS from 172.17.153.36 to 172.17.153.38 distport 3333 .'
hostname: '172.17.153.38/172.16.24.32'
program_name: '/usr/bin/auditServerd'
log: 'User blackrat login SUCEESS from 172.17.153.36 to 172.17.153.38 distport 3333 .'
**Phase 2: Completed decoding.
decoder: 'auditServerd'
dstuser: 'blackrat'
status: 'SUCEESS'
srcip: '172.17.153.36'
dstip: '172.17.153.38'
dstport: '3333'
**Phase 3: Completed filtering (rules).
Rule id: '80001'
Level: '10'
Description: 'User blackrat is not allowed login from 172.17.153.36!'
**Alert to be generated.
3.3 关联分析告警规则
OSSEC可以实现基于因果关系、事件频次的关联分析告警，具体实现方式如下。
假如我们想要实现当来自同一IP的用户登陆auditServerd，在1分钟内达到5次登录失败时，进行告警，我们可以配置规则如下：
<group name="auditServer,">
<rule id="80000" level="0" noalert="1">
<decoded_as>auditServerd</decoded_as>
<description>Grouping for the auditServerd rules.</description>
</rule>
<rule id="80001" level="10">
<if_sid>80000</if_sid>
<match>SUCEESS</match>
<user>blackrat</user>
<srcip>172.17.153.36</srcip>
<description>User blackrat is not allowed login from 172.17.153.36!</description>
</rule>
<rule id="80002" level="1">
<if_sid>80000</if_sid>
<match>PWD_ERROR</match>
<group>authServer_login_failures,</group>
<description>login auditServerd password error.</description>
</rule>
<rule id="80003" level="15" frequency="5" timeframe="60" ignore="30">
<if_matched_group>authServer_login_failures</if_matched_group>
<description>auditServerd brute force trying to get access to </description>
<description>the audit system.</description>
<same_source_ip />
<group>authentication_failures,</group>
</rule>
</group>
执行/var/ossec/bin/ossec-logtest，连续五次输入日志：
Jun 11 22:06:30 172.17.153.38/172.16.24.32 /usr/bin/auditServerd[25649]: User blackrat login PWD_ERROR from 172.17.153.36 to 172.17.153.38 distport 3333 .
结果如下：
**Phase 1: Completed pre-decoding.
full event: 'Jun 11 22:06:30 172.17.153.38/172.16.24.32 /usr/bin/auditServerd[25649]: User blackrat login PWD_ERROR from 172.17.153.36 to 172.17.153.38 distport 3333 .'
hostname: '172.17.153.38/172.16.24.32'
program_name: '/usr/bin/auditServerd'
log: 'User blackrat login PWD_ERROR from 172.17.153.36 to 172.17.153.38 distport 3333 .'
**Phase 2: Completed decoding.
decoder: 'auditServerd'
dstuser: 'blackrat'
status: 'PWD_ERROR'
srcip: '172.17.153.36'
dstip: '172.17.153.38'
dstport: '3333'
**Phase 3: Completed filtering (rules).
Rule id: '80003'
Level: '15'
Description: 'auditServerd brute force trying to get access to the audit system.'
**Alert to be generated.
对于OSSEC日志告警规则更详细的语法，参见：
http://ossec-docs.readthedocs.org/en/latest/syntax/head_rules.html
对于OSSEC中正则表达式的语法，参加：
http://ossec-docs.readthedocs.org/en/latest/syntax/regex.html
文章出处：
http://www.freebuf.com/articles/network/36484.html
