Hey , So in last days/weeks 🙂 I work quite a lot with ELK stack. Especially in getting data from my systems into Elastic. There would not be any problem if not the fact that default parsing did not quite do work. But what would be IT life without challenges ?

So in this post I will explain in short how I have overcome this problem. And I’m sure you would be able to use this or event make it better.

We will look into following:

* Incoming raw data

* Creating filter

* Enjoying results

Incoming raw data:

So you got your vyos box doing the hard work on the edge of your network. And now you would like to have control when someone is knocking to your door or to find root cause when troubleshooting firewall rules.

Example of incoming data from my box looks similar to the following :

<4>Dec  6 01:36:00 myfwname kernel: [465183.670329] 
[internet_local-default-D]IN=eth2 OUT= 
MAC=00:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:00 
SRC=5.6.7.8 DST=1.2.3.4 LEN=64 TOS=0x00 
PREC=0x00 TTL=56 ID=10434 DF PROTO=TCP 
SPT=51790 DPT=80 WINDOW=65535 RES=0x00 SYN URGP=0

and if we just apply basic syslog filtering it will not give out all required fields. The problem we challenge here is we need to get out internet rule. Then following that we can see use case for key value filter.

Creating filter:

So time has come to use some magical skills of creating configuration for Logstash filter. I would like to put stress that using different approaches can have impact on performance. Both negative and positive. As yet I’m getting familiar with Logstash this might not be the best solution but I will definitely explore this.

You will notice that my filters use conditional statements so I do not process data unnecessary. In my case vyoss traffic is tagged as syslog and contains specific string in the message.

So without further bubbling … We begin with parsing out data from the message that will get for sure extracted.

  if [type] == "syslog" and [message] =~ "myfw" {
    grok {
      break_on_match => false
      match => [
      "message",  "<%{POSINT:syslog_pri}>%{SYSLOGTIMESTAMP:syslog_timestamp} %{SYSLOGHOST:syslog_hostname} %{DATA:syslog_program}(?:\[%{POSINT:syslog_pid}\])?: \[(?<syslog_pid>.*?)\] %{GREEDYDATA:syslog_message}",
      "syslog_message", "\[(?<firewall_rule>.*?)\]"
      ]
    }

Points of interest here :

• Grok filter DOES NOT break on match
• We do match on message and further on extracted syslog_message to get our firewall rule from [ ]

Next we will do changes on fly to our fields using mutate

    # mutate our values
    mutate {
      add_field => [ "event_type", "firewall" ]
      rename => { "firewall_rule" => "[firewall][rule]" }
      gsub => [ "message", "= ", "=xxx" ]            # Here we remove scenario where this value is empty
    }

Points of interest :

• I add a field event_type  called firewall so in future I would be able to quickly query for those events.
• I rename my previous field ( firewall_rule ) to nested field
• And lastly I use gsub to mitigate problem of missing values in key pair

Once this is done I extract remaining values using kv filter which is configured as follow :

    # Apply key value pair
    kv {
      include_keys => ["SRC","DST","PROTO","IN","MAC","SPT","DPT"]
      field_split => " \[\]"
      add_field => {
        "[firewall][source_address]" => "%{SRC}"
        "[firewall][destination_address]" => "%{DST}"
        "[firewall][protocol]" => "%{PROTO}"
        "[firewall][source_port]" => "%{SPT}"
        "[firewall][destination_port]" => "%{DPT}"
        "[firewall][interface_in]" => "%{IN}"
        "[firewall][mac_address]" => "%{MAC}"
      }
    }

Points of interest :

• I use include_keys so only fields in array will be extracted ( positive impact on performance )
• I tried field_split to help out with one of previous challenges but that did not make a lot of difference
• And lastly I specify my new nested fields for extracted values

So thats it! The complete file looks following :

filter {
  if [type] == "syslog" and [message] =~ "vmfw" {
    grok {
      break_on_match => false
      match => [
      "message",  "<%{POSINT:syslog_pri}>%{SYSLOGTIMESTAMP:syslog_timestamp} %{SYSLOGHOST:syslog_hostname} %{DATA:syslog_program}(?:\[%{POSINT:syslog_pid}\])?: \[(?<syslog_pid>.*?)\] %{GREEDYDATA:syslog_message}",
      "syslog_message", "\[(?<firewall_rule>.*?)\]"
      ]
    }
    
    # mutate our values
    mutate {
      add_field => [ "event_type", "firewall" ]
      rename => { "firewall_rule" => "[firewall][rule]" }
      gsub => [ "message", "OUT= MAC=", "MAC=" ]            # Here we remove scenario where this value is empty
    }

    # Apply key value pair
    kv {
      include_keys => ["SRC","DST","PROTO","IN","MAC","SPT","DPT"]
      field_split => " \[\]"
      add_field => {
        "[firewall][source_address]" => "%{SRC}"
        "[firewall][destination_address]" => "%{DST}"
        "[firewall][protocol]" => "%{PROTO}"
        "[firewall][source_port]" => "%{SPT}"
        "[firewall][destination_port]" => "%{DPT}"
        "[firewall][interface_in]" => "%{IN}"
        "[firewall][mac_address]" => "%{MAC}"
      }
    }
  }
}

Enjoying results:

We now would need to test this if it really works as we expect it to work. For this to check we of course will use Docker

First I will create the afore mentioned config file and name it logstash.conf . Once thats done we bring up container up with the following command :

docker run -d -p 25666:25666 -v "$PWD":/config-dir logstash logstash -f /config-dir/logstash.conf

This creates container for me which I can then test locally. Now for this to work you need input source ( i.e. tcp / udp and stdout i.e. codec ruby )

Then I will split my screen using tmux and will execute request while looking at results from docker logs

And thats it! You have beautifully working parsing for your vyos box ! If you have any comments / or improvements – feel free to share!