Generally speaking, the overall goal is to provide centralised logging services for the purposes of operations, compliance and audit. All systems should send log messages to the central server, where the logs will be kept in original form for the required period of time and the logs will be available for event, incident and problem management purposes.

Centralised Logging Architecture

Architectural Considerations

There are several considerations for creating a centralised logging solution.

  • What devices and operating systems will be sending logs.
  • What applications will be sending logs.
  • What protocol will be used to send log messages.
  • What timezone are each of the devices sending logs in.
  • What criticality of logs is required.

Devices Sending Logs

You should complete a table as below to catalogue all the devices which will be sending logs.

NamePurposeLogging System
Windows Event ManagerEvent and AuditWindows Event Logging
CentOS Linux 5.xEvent and Auditsyslog
CentOS Linux 6.xEvent and Auditsyslog
Cisco IOS SwitchesEvent and AuditCisco IOS syslog
Cisco IOS RoutersEvent and AuditCisco IOS syslog

Applications Sending Logs

The following applications logs need to be send centrally.

Application NamePurposeFileDevice
Monkey Auth SystemAuditC:\Program Files\MAS\logs\monkeyauth.logWindows 2008 Servers
Elephant FinancialAudit/data/elefin/log/app.logCentOS Linux 6.8

Logging Protocol

syslog has proven to be a very robust protocol for large scale log management. 

TCP should where reliable logging is required, UDP works very well, 99.99% of the time.

Logging Severity Levels

The requirement is to send level 6 and above.


Full details for syslog severity levels

Handy cross reference: opEvents priority levels vs. NMIS and Syslog levels

Time Handling

There are two primary concerns about time, one clock drift the second is multiple timezones.

There are several options for handling timezone, the following are considered best practices for IT management in this regard:

  • All devices should use NTP to ensure accurate time is set
  • All devices should have their timezone set to their localtime or to a common timezone, e.g. UTC
  • All devices should include the timezone when timestamping, ideally as an offset, e.g. +00

However it is often difficult to get all devices especially devices already installed to do all these things, so a great option is to make the logging server the authoritive time, NTP is setup a timezone is selected, and all logs received by it will be stamped with the time when the log is received.

Centralised Logging and Archiving Solution

The following diagram shows how a system can be implemented using syslog and Opmantek opEvents.

Centralised Logging Design

Syslog Method and Transport

The following table summarises how the logs will get from the source into the opEvents server.  Three basic techniques exist, native syslog transport supported by the operating system, a logging agent which monitors for data and sends the resulting data/events/log as a syslog.

There are several good choices for Windows, but NXLOG has proven to meet all the requirements, almost all other systems include embedded syslog systems


Method and Transport

Windows 2003 Servers

nxlog monitoring Windows Event log, transport over syslog

Windows 2008 Servers

nxlog monitoring Windows Event log, transport over syslog

Windows 2012 Servers

nxlog monitoring Windows Event log, transport over syslog

CentOS Linux 5.xrsyslog 3.x
CentOS Linux 6.xrsyslog 7.6

Cisco IOS Switches

Native IOS syslog

Cisco IOS Routers

Native IOS syslog

Monkey Auth System

nxlog running on Windows.

Elephant Financialsrsyslog running on Linux

syslog Facility

The best reference is:

We are primarily concerned with the facilities localX facilities.  Logs will also grow at different rates and having them in separate files will allow for more granular control.  The following table summarises which log files will end up in which files.

Device Type

syslog facility

Log file



Log server to log server (future)



Application logging e.g. MonkeyAuth



Windows servers (nxlog default)



Cisco ASA default (VMware ESXi default)





Linux syslog



Cisco Routers and Switches



Alternate file naming can be supported if required, e.g. cisco.log instead of local7.log.

Centralised Logging Implementation

Central rsyslog Server Configuration

Translating all the above into the configuration the following are the most important parts.

It should be noted that trying to use the /etc/rsyslog.d scheme did not work.

Enable rsyslog to receive UDP and TCP syslogs

By default (to prevent DOS) rsyslog is configured to not receive syslogs from remote servers.

# Provides UDP syslog reception
$UDPServerRun 514

# Provides TCP syslog reception
$InputTCPServerRun 514

Configure each Facility to be saved into Files

Based on the table above the following would be the configuration

local0.*        /usr/local/nmis8/logs/local0.log
local1.*        /usr/local/nmis8/logs/local1.log
local2.*        /usr/local/nmis8/logs/local2.log
local3.*        /usr/local/nmis8/logs/local3.log
local4.*        /usr/local/nmis8/logs/local4.log
local5.*        /usr/local/nmis8/logs/local5.log
local6.*        /usr/local/nmis8/logs/local6.log
local7.*        /usr/local/nmis8/logs/local7.log

Configure syslog reception for remote Linux hosts

Based on the table above the following would be the configuration

$template LinuxLogs,"%timegenerated%.%timegenerated:::date-subseconds% %HOSTNAME% %syslogtag%%msg%\n"
if      $fromhost-ip != '' \
        and $syslogseverity <= '6' \
        and $syslogfacility <= '15' \
then    /usr/local/nmis8/logs/linux.log;LinuxLogs

Optionally handling things with the WORD LINUX in the tag

if      $fromhost-ip != '' \
        and $syslogtag contains 'LINUX' \
        and $syslogseverity <= '6' \
        and $syslogfacility <= '15' \
then    /usr/local/nmis8/logs/linux.log;LinuxLogs

Handling Different Times and Time Zones

The following configuration shows how to create an rsyslog template and apply that to the logs being received, this example also adds high precision time, which is supported by opEvents.

$template ServerTime,"%timegenerated%.%timegenerated:::date-subseconds% %HOSTNAME% %syslogtag%%msg%\n"
local5.*        /usr/local/nmis8/logs/local5.log;ServerTime

All syslog received to the facility local5 will be timestamped with the receiving syslog servers high precision time.

Sample Configuration for rsyslog 7.6

The following is a config which sends logs from /var/log/messages using facility local6.

# enable the imfile module for file monitoring
$ModLoad imfile
$WorkDirectory /var/spool/rsyslog

# Monitor the file
$InputFileName /var/log/messages
$InputFileTag :
$InputFileStateFile messages_log
$InputFileSeverity error
$InputFileFacility local6
$InputFilePollInterval 1
$InputFilePersistStateInterval 1
# forward these logs to another server
local6.*                                @

Remote Linux Server

The following rsyslog config will send all syslogs which are sourced localy with a severity 0-6 to the remote server.

$template LinuxMnemonic,"%timereported% %HOSTNAME% %syslogfacility-text%-%syslogseverity%-%syslogtag%%msg%\n"
if $fromhost-ip == '' and $syslogseverity <= '6' then @;LinuxMnemonic

Here we are using a Linux Mnemonic like the Cisco Syslog so that we do not lose the original facility and severity when the message is fowarded.

Optionally send everything with the WORD LINUX in the tag

$template LinuxMnemonic,"%timereported% %HOSTNAME% LINUX-%syslogfacility-text%-%syslogseverity%-%syslogtag%%msg%\n"

Example Topology

In the example above all syslog messages received with a facility of local7 will be forwarded to the Primary server at  When this message is forwarded from the poller to the Primary, the poller will insert its own timestamp into the message.

### syslog message from the node to the poller server

02:23:37.250516 IP > SYSLOG local7.notice, length: 100
..../...l./<189>90: *Feb  1 11:23:35.623: %SYS-5-CONFIG_I: Configured from console by hero on vty0 (

### syslog message from the poller server to the primary server

11:23:37.273514 IP > SYSLOG local7.notice, length: 126
....>.....j<189>Feb  1 02:23:37 90: *Feb  1 11:23:35.623: %SYS-5-CONFIG_I: Configured from console by hero on vty0 (

If the servers/nodes are in differnt time zones or the clocks are not correct opEvents may not by default accept the syslog message.  Setting the opEvents debug option to '1' will show the following message in /usr/local/omk/log/opEvents.log if this is the case.

[Wed Feb  1 09:08:49 2017] [debug] worker[4697] event 1485875324 R1 Feb  1 00:08:44 66: *Feb  1 09:08:42.711: %OSPF-5-ADJCHG: Process 1, Nbr on FastEthernet1/0 from FULL to DOWN, Neighbor Down: Interface down or detached is older than opevents_max_event_age, skipping!

As of opEvents 2.2.1 we can provision opEvents to allow these wildly out of date syslog messages and replace the origin timestamp with its own.

### /usr/local/omk/config/opCommon.nmis

    'opevents_max_action_queue_age' => 3600,
    'opevents_max_event_age' => 7200,
    'opevents_max_time_delta' => 1800,
    'opevents_monthly_report_recipients' => [],
    'opevents_monthly_report_title' => 'Monthly Summary Report',

Setting the 'opevents_max_time_delta' to a value of 1800 seconds will instruct opEvents to do the following:

  • If the syslog message has a timestamp that is more than 1800 seconds off from the current server time:
    • Accept the syslog message
    • Remove and replace the timestamp with its own time stamp.


Appendix A: Upgrading rsyslog of RHEL and CentOS

The version of rsyslog which is included in older CentOS and RedHat systems is problematic and should be upgraded, the steps to do so are included below.

Create/Edit rsyslog.repo

cat /etc/yum.repos.d/rsyslog.repo 


name=Adiscon CentOS-$releasever - local packages for $basearch

Test Yum

yum search rsyslog

Install rsyslog

yum install rsyslog

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