Sunday, January 27, 2019

rsyslog fun - Basic Splunk Log Collection and Forwarding - Part I

By Tony Lee

We found it a bit surprising that there are so few articles on how to use an rsyslog server to forward logs to Splunk. This provided the motivation to write this article and hopefully save others some Googling. Choosing between rsyslog, syslog-ng, or other software is entirely up to the reader and may depend on their environment and approved/available software. We realize that this is not the only option for log architecture or collection, but it may help those faced with this task—especially if rsyslog is the standard in their environment.


Before we jump in, we wanted to remind you of three potential gotchas that may thwart your success and give you a troubleshooting migraine.
  1. Network firewalls – You may not own this, but make sure the network path is clear
  2. iptables – Complex rule sets can throw you for a loop
  3. SE Linux – Believe it or not, SE Linux when locked down can prevent the writing of the log files

If something is not working the way you expect it to work, it is most likely due to one of the three items mentioned above. It could be worth temporarily disabling them until you get everything working. Just don’t forget to go back and lock it down.

Note:  We will also be using Splunk Universal Forwarders (UF) in this article.  Universal Forwarders have very little pre-processing or filtering capabilities when compared to Heavy Forwarders.  If significant filtering is necessary, consider using a Splunk Heavy Forwarder in the same fashion as we are using the UFs below.


Whether your Splunk instance is on-prem or it is in the cloud, you will most likely need syslog collectors and forwarders at some point. The architecture diagram below shows one potential configuration. The number of each component is configurable and dependent upon the volume of traffic.

Figure 1:  Architecture diagram illustrating traffic flow from data sources to the Index Cluster

Rsyslog configuration

Rsyslog is a flexible service, but in this case rsyslog’s primary role will be to:

  • Open the sockets to accept data from the sources
  • Properly route traffic to local temporary files that Splunk will forward on to the indexers

If you are fortunate enough to be able to route network traffic to different ports, you may be able to reduce the if-then logic shown below for routing the events to separate files. In this case, we were not able to open separate ports from the load balancer, thus we needed to do the routing on our end. In the next article we will cover more advanced routing to include regex and traffic coming in on different ports.

Note:  Modern rsyslog is designed to run extra config files that exist in the /etc/rsyslog.d/ directory. If that directory exists, place the following 15-splunk-rsyslog.conf file in that directory. Otherwise, the /etc/rsyslog.conf file is interpreted from top to bottom, so make a copy of your current config file (cp /etc/rsyslog.conf /etc/rsyslog.bak) and selectively add the following at the top of the new active rsyslog.conf file. This addition to the rsyslog configuration will do the following (assuming the day is 2018-06-01:
  • Open TCP and UDP 514
  • Write all data from to:  /rsyslog/cisco/
  • Write all data from to:  /rsyslog/cisco/
  • Write all data from 10.1.1.* to /rsyslog/pan/10.1.1.*/2018-06-01.log (where * is the last octet of the source IP
  • Write all remaining data to /rsyslog/unclaimed/<host>/2018-06-01.log (where <host> is the source IP or hostname of the sender)
Note:  If the rsyslog server sees the hosts by their hostname instead of IP address, feel free to use $fromhost == '<hostname>' in the configuration file below.


$ModLoad imtcp
$ModLoad imudp
$UDPServerRun 514
$InputTCPServerRun 514

# do this in FRONT of the local/regular rules

$template ciscoFile,"/rsyslog/cisco/%fromhost%/%$YEAR%-%$MONTH%-%$DAY%.log"
$template PANFile,"/rsyslog/pan/%fromhost%/%$YEAR%-%$MONTH%-%$DAY%.log"
$template unclaimedFile,"/rsyslog/unclaimed/%fromhost%/%$YEAR%-%$MONTH%-%$DAY%.log"

if ($fromhost-ip == '' or $fromhost-ip == '') then ?ciscoFile
& stop

if $fromhost-ip startswith '10.1.1' then ?PANFile
& stop

else ?unclaimedFile
& stop

# local/regular rules, like
*.* /var/log/syslog.log

Note:  Rsyslog should create directories that don't already exist, but just in case it doesn't, you need to create the directories and make them writable.  For example:

mkdir -p /rsyslog/cisco/
mkdir -p /rsyslog/pan/
mkdir -p /rsyslog/unclaimed/

Pro tip:  After making changes to the rsyslog config file, you can verify that there are no syntax errors BEFORE you restart the rsyslog daemon.  For a simple rsyslog config validation.  Try using the following command: 

rsyslogd -N 1

If there are no errors, then you should be good to restart the rsyslog service so your changes take effect:

service rsyslog restart

Log cleanup

The rsyslog servers in our setup are not intended to store the data permanently. They are intended to act as a caching server for temporary storage before shipping the logs off to the Splunk Indexers for proper long-term storage. Since disk space is not unlimited on these caching servers we will need to implement log rotation and deletion so we do not fill up the hard disk. Our rsyslog config file already takes care of the log rotation with the template parameter specifying the name of the file as “%$YEAR%-%$MONTH%-%$DAY%.log", however, we still need to clean up the files, so they don’t sit there indefinitely. One possible solution is to use a daily cron job to clean up files in the /rsyslog/ directory that are more than x days old (where x is defined by the organization). Once you have some files in the /rsyslog/ directory, try the following command to see what would potentially be deleted. The command below lists files in the rsyslog directory that are older than two days.

find /rsyslog/ -type f -mtime +1 -exec ls -l "{}" \;

If you are happy with a two-day cache period, add it to a daily cron job (as shown below).  Otherwise feel free to play with the +1 until you are comfortable with what it will delete and use that for your cron job.


find /rsyslog/ -type f -mtime +1 -delete

Splunk Universal Forwarder (UF) Configuration

Splunk Forwarders are very flexible in terms of data ingest. For example, they can create listening ports, monitor directories, run scripts, etc. In this case, since rsyslog is writing the information to a directory, we will use a Splunk UF to monitor those directories and send them to the appropriate indexes and label them with the appropriate sourcetypes.  See our example configuration below.

Note:  Make sure the indexes mentioned below exist prior to trying to send data there. These will need to be created within Splunk.  Also ensure that the UF is configured to forward data to indexers (out of the scope of this write up).


whitelist = \.log$
sourcetype = cisco:ios
index = cisco

whitelist = \.log$
sourcetype = pan:traffic
index = pan_logs

whitelist = \.log$
sourcetype = syslog
index = lastchanceindex

Pro tip:  Remember to restart the Splunk UF after modifying files.  

/opt/splunkforwarder/bin/splunk restart


A simple Splunk search of index=cisco, index=pan_logs, or index=lastchanceindex should be able to confirm that you are now receiving data in Splunk. Keep monitoring the lastchanceindex to move hosts to where they need to go as they come on-line. Moving the hosts is accomplished by editing the rsyslog.conf file and possibly adding another monitor stanza within the Splunk UF config. This process can be challenging to create, but once it is going, it just needs a little care from time to time to make sure that all is well.  We hope you found this article helpful.  Happy Splunking!

Basic Troubleshooting

So you did everything above and you are still not seeing data...  Walk through some of these steps:

  1. Ensure all network firewalls permit the traffic
  2. Ensure iptables allows traffic to the rsyslog server
  3. Run tcpdump on the source to ensure it is sending data
  4. Run tcpdump on the rsyslog server to ensure it is receiving data
  5. Verify permissions when writing the files to disk
  6. Attempt to telnet (or even web browse) to the rsyslog server port to see if anything is written to the directories



  1. Very comprehensive and easy to understand! Thank you

  2. Everything That is Required, To-The-Point. !! Great Work !!