This article is contributed. See the original author and article here.

 


Hello everybody, Simone here to tell you about a situation that happened many times to my customers: understanding how the syslog ingestion works.


To make subject clear make sure you have clear in mind the below references:



Most of the time nobody knows what needs to be collected and how hence, with this article, I just want to make some clarification on what is behind the scenes.


Starting from RFC, it is mentioned that we have a list of “Facility” like in the screenshot below:


sifriger_0-1608569858351.png


 


And for each of them we could have a specific “Severity” (see the corresponding picture below):


sifriger_1-1608569858358.png


 


Back to the situation, the natural question that comes up is: how can we clearly understand who is using who if we have no information about facilities and severities about related products we are using?


To find the information we need, we must capture some TCP/UDP packets from the syslog server and rebuild the packets in wireshark and then analyze the results.


Let’s start with first step: packets capture. Below you have the macro steps to be followed:



  • From the syslog server (in this case a Linux server) we will use the tcpdump command,
    if not available follow this link on how to setup
    https://opensource.com/article/18/10/introduction-tcpdump

  • the command could be for example
    tcpdump -i any -c50 -nn src xxx.xxx.xxx.xxx (replace with source IPADDRESS under analysis)

  • the results after the rebuilt with wireshark, should be something similar the following image:sifriger_2-1608569858371.png


The header of every row contains exactly the information that we are looking for; how to deal with this piece of info? Easy; use the formula contained in the following part directly taken from RFC:


“The Priority value is calculated by first multiplying the Facility number by 8 and then adding the numerical value of the Severity.  For example, a kernel message (Facility=0) with a Severity of Emergency (Severity=0) would have a Priority value of 0.  Also, a “local use 4” message (Facility=20) with a Severity of Notice (Severity=5) would have a Priority value of 165.  In the PRI of a syslog message, these values would be placed between the angle brackets as <0> and <165> respectively.
The only time a value of “0” follows the “<” is for the Priority value of “0”.  Otherwise, leading “0”s MUST NOT be used.”


 


In the example above, we have the value of <46>. According to the above-mentioned RFC, the formula used to translate that number into something human readable is the following:


8 x facility + severity


 


We now must look for the formula result in the following matrix:



































































































































































































































































































 

Emergency



Alert



Critical



Error



Warning



Notice



Informational



Debug


                 

Kernel



0



1



2



3



4



5



6



7



user-level



8



9



10



11



12



13



14



15



mail



16



17



18



19



20



21



22



23



system



24



25



26



27



28



29



30



31



security/auth



32



33



34



35



36



37



38



39



message



40



41



42



43



44



45



46



47



printer



48



49



50



51



52



53



54



55



network news



56



57



58



59



60



61



62



63



UUCP



64



65



66



67



68



69



70



71



clock



72



73



74



75



76



77



78



79



security/auth



80



81



82



83



84



85



86



87



FTP deamon



88



89



90



91



92



93



94



95



NTP



96



97



98



99



100



101



102



103



Log Audit



104



105



106



107



108



109



110



111



Log Alert



112



113



114



115



116



117



118



119



Clock



120



121



122



123



124



125



126



127



local0



128



129



130



131



132



133



134



135



local1



136



137



138



139



140



141



142



143



local2



144



145



146



147



148



149



150



151



local3



152



153



154



155



156



157



158



159



local4



160



161



162



163



164



165



166



167



local5



168



169



170



171



172



173



174



175



local6



176



177



178



179



180



181



182



183



local7



184



185



186



187



188



189



190



191



 


So now, let’s make one step back to customer’ question and “guess” what the “Facility” and the “Severity” are in the provided example.


Since header was 46, the result was:



  • Facility = message

  • Severity = Informational


Once we understood what to deal with, it’s time to configure Log Analytics / Sentinel enabling the Syslog data sources in Azure Monitor.


All we have to do is to:



  • add the facilities (by entering its name and leveraging the intellisense) to the workspace.
    sifriger_3-1608569858377.png

     



  • select what severity(ies) to import.
    sifriger_4-1608569858407.png


 



  •  and click Save.
    sifriger_5-1608569858411.png


 


Using some real-life example, if we want to collect the logs for FTP, the corresponding facility to be entered is “ftp” and the following logs will be imported:


























Syslog file



Log Path



ftp.info; ftp.notice



/log/ftplog/ftplog.info



ftp.warning



/log/ftplog/ftplog.warning



ftp.debug



/log/ftplog/ftplog.debug



ftp.err; ftp.crit; ftp.emerg



/log/ftplog/ftplog.err



 


Differently, talking about Users, the facility is “user” and the imported logs will be:


























Syslog file



Log Path



user.info;user.notice



/log/user/user.info



user.warning



/log/user/user.warning



user.debug



/log/user/user.debug



user.err;user.crit;user.emerg



/log/user/user.err



 


Another one: for Apache, the facility is “local0” and the logs will be:


























Syslog file



Log Path



local0.info;local0.notice



/log/httpd/httpd.



local0.warning



/log/httpd/httpd.warning



local0.debug



/log/httpd/httpd.debug



local0.err; local0.crit;local0.emerg



/log/httpd/httpd.err



 


We have everything in place, but are we really sure that info is produced?
What if you would like to effectively test that data is flowing in the corresponding facility?
We can leverage the following sample commands for CEF & Syslog using the logger built-in utility:



logger -p auth.notice “Some message for the auth.log file”


logger -p local0.info “Some message for the local0.log file”


logger “CEF:0|Microsoft|MOCK|1.9.0.0|SuspiciousActivity|Demo suspicious activity|5|start=2020-12-12T18:52:58.0000000Z app=mock suser=simo msg=Demo suspicious activity externalId=2024 cs1Label=tag cs1=my test”



Note pay attention to time when you query for this result!!! ;)


That’s it from my side, thank you for reading my article till the end.


Special thanks go to Bruno Gabrielli for review


Simone


 


 

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