OSSEC日志泛化及告警规则配置

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>   
</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：

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