Using ulogd and JSON output

Ulogd and JSON output

In February 2014, I’ve commited a new output plugin to ulogd, the userspace logging daemon for Netfilter. This is a JSON output plugin which output logs into a file in JSON format. The interest of the JSON format is that it is easily parsed by software just as logstash. And once data are understood by logstash, you can get some nice and useful dashboard in Kibana:

Screenshot from 2014-02-02 13:22:34

This post explains how to configure ulogd and iptables to do packet logging and differentiate accepted and blocked packets. If you want to see how cool is the result, just check my post: Investigation on an attack tool used in China.


At the time of this writing, the JSON output plugin for ulogd is only available in the git tree. Ulogd 2.0.4 will contain the feature.

If you need to get the source, you can do:

git clone git://

Then the build is standard:

sudo make install

Please note that at the end of the configure, you must see:

Ulogd configuration:
  Input plugins:
    NFLOG plugin:			yes
    NFACCT plugin:			yes
  Output plugins:
    PCAP plugin:			yes
    JSON plugin:			yes

If the JSON plugin is not build, you need to install libjansson devel files on your system and rerun configure.


Ulogd configuration

All the edits are made in the ulogd.conf file. With default configure option the file is in /usr/local/etc/.

First, you need to activate the JSON plugin:


Then we define two stacks for logging. It will be used to differentiate accepted packets from dropped packets:


The first stack will be used to log accepted packet, so we the numeric_label to 1 in set in [log2].
In [log3], we use a numeric_label of 0.

group=1 # Group has to be different from the one use in log1

group=2 # Group has to be different from the one use in log1/log2
numeric_label=0 # you can label the log info based on the packet verdict

The last thing to edit is the configuration of the JSON instance:

device="My awesome FW"

Here we say we want log and write on disk configuration (via sync) and we named our device My awesome FW.
Last value boolean_label is the most tricky. It this configuration variable is set to 1, the numeric_label will
be used to decide if a packet has been accepted or blocked. It this variable is set non null, then the packet is seen as allowed.
If not, then it is seen as blocked.

Sample Iptables rules

In this example, packets to port 22 are logged and accepted and thus are logged in nflog-group 1. Packet in the default drop rule are sent to group 2 because they are dropped.

iptables -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A INPUT ! -i lo -p tcp -m tcp --dport 22 --tcp-flags FIN,SYN,RST,ACK SYN -m state --state NEW -j NFLOG --nflog-prefix  "SSH Attempt" --nflog-group 1
iptables -A INPUT -i lo -j ACCEPT
iptables -A INPUT -p tcp -m tcp --dport 22 -m state --state NEW -j ACCEPT
iptables -A INPUT -j NFLOG --nflog-prefix  "Input IPv4 Default DROP" --nflog-group 2

There is no difference in IPv6, we just use nflog-group 1 and 2 with the same purpose:

ip6tables -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
ip6tables -A INPUT ! -i lo -p tcp -m tcp --dport 22 --tcp-flags FIN,SYN,RST,ACK SYN -m state --state NEW -j NFLOG --nflog-prefix  "SSH Attempt" --nflog-group 1
ip6tables -A INPUT ! -i lo -p ipv6-icmp -m icmp6 --icmpv6-type 128 -m state --state NEW -j NFLOG --nflog-prefix  "Input ICMPv6" --nflog-group 1
ip6tables -A INPUT -p ipv6-icmp -j ACCEPT
ip6tables -A INPUT -p tcp -m tcp --dport 22 -m state --state NEW -j ACCEPT
ip6tables -A INPUT -i lo -j ACCEPT
ip6tables -A INPUT -j NFLOG --nflog-prefix  "Input IPv6 Default DROP" --nflog-group 2

Logstash configuration

Logstash configuration is simple. You must simply declare the ulogd.json file as input and optionaly you can activate geoip on the src_ip key:

input {
   file { 
      path => [ "/var/log/ulogd.json"]
      codec =>   json 

filter {
  if [src_ip]  {
    geoip {
      source => "src_ip"
      target => "geoip"
      add_field => [ "[geoip][coordinates]", "%{[geoip][longitude]}" ]
      add_field => [ "[geoip][coordinates]", "%{[geoip][latitude]}"  ]
    mutate {
      convert => [ "[geoip][coordinates]", "float" ]

output { 
  stdout { codec => rubydebug }
  elasticsearch { embedded => true }


To start ulogd in daemon mode, simply run:

ulogd -d

You can download logstash from their website and start it with the following command line:

java -jar logstash-1.3.3-flatjar.jar agent -f etc/logstash.conf --log log/logstash-indexer.out -- web

Once done, just point your browser to localhost:9292 and enjoy nice and interesting graphs.

Screenshot from 2014-02-02 13:57:19

16 thoughts on “Using ulogd and JSON output”

  1. Neat, but more “bleeding-edge” examples would be even better: how do I log into 2 different groups for ulogd using nftables?

    I tried “udp dport sip iifname “eth0″ log group 2 counter” and I see counter updated with nftables list but ulogd seems to ignore this completely 🙁

  2. The logstash web interface complains that “Could not contact Elasticsearch at…” while java command complains to console that “superclass mismatch for class Error”. Is there some additional configuration required besides what’s mentioned in the article?

  3. Hi,

    It seems this is a bug at init, restart logstash, check if the message is there. If not, restart again…

  4. I tried to restart several times – it looks the same 🙁
    Is there some debug options combo to help with troubleshooting?

  5. Wow, that magic helped.
    Do you know if there .service file to automate this with systemd?

  6. One more thing – using json is truly awesome, but I’d like to get .pcap output t the same time as well. Simply adding [pcap1] section seems to have no effect at all. Do I have to modify log stacks?

  7. Hi Lorks,

    Yes you have to simply add a new stack. Same entry but different filter and output (here pcap1).

  8. Hello volga629,

    No, it is not implemented, you need to use logstash in transport mode to send data to distant server.

  9. Hi,
    thanks you for this doc. It’s really interesting.

    Your kibana dashboard looks awesome. Can you give me your configuration ?

    I am new to ELK and I want to monitor what ip is doing a lot of traffic. A dashboard like yours will be useful.

    Can you tell if it is possible to set a threshold and have an email alert when this threshold is crossed ?


  10. Great article.
    If ulogd could send UDP json log to remote would be better

  11. Great article. I’m using the logd2 that comes with Ubuntu 18.04. When I start the service it says:

    Apr 16 08:27:35 vpn-2 ulogd[18416]: can’t find requested plugin JSON

    How can it be that the JSON module is not included? Am I missing something? Do I really have to remove it, get the source and compile it myself?

    Many Thanks

  12. @Houman
    install ulogd2-json
    for debian :
    apt-get install ulogd2-json


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