Packetbl is a user space RBL client that works with netfilter
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This Packetbl software is a new NON-OFFICIAL version of a not maintained project with this same name, this project was developed by Russell Miller. Later, it was incluided in CentOS repositories and now, it has been improved by Alvaro cano of Zevenet team. The goal of this program is to connect netfilter with user space to check if a origin IP is malicious using a DNS server as a realtime database. Packetbl uses netfilter-queue feature to receive a packet from netfilter to user space.

Packetbl will reject or will drop a connection if a DNS server resolves the queried client IP. So the DNS server will be used as a service of dynamic black lists. Those black lists are supported by specialist security providers, you can find some of them here:

Some RBL domains are:



RBL or DNSBL is a tech that does DNS queries about a source IP to determinate if the IP is blacklisted. If the source IP is resolved by a remote DNS serve, the malicious packet can be dropped, rejected or logged to mitigate a cyber-attack. RBL technology is typically used to mitigate email threats as spam or phishing. But it is useful for another services as FTP, SIP, SSH, web...

New features

  • Multi-thread, Packetbl will create a thread for each checking packet.
  • Direct DNS queries, Packetbl can use a determinate DNS server to resolve a specific domain.
  • Fail-over, This option is available since Linux 3.6 and allows to accept packet instead of dropping them when the netfilter queue is full.
  • Queuesize, this parameter allows to set Netfilter queue size.
  • Select a config file, it is possible to choose a packetbl config file with the "-f" command line option.

Installation and runtime

Packetbl has been tested in stable debian distributions: Wheezy, Jessie and Stretch


1. Dot.conf (required)
	Dot.conf is used to handle the configuration data and is
	required for PacketBL to operate. 
	Dot.conf uses "Apache-style" configuration files so logical
	hierarchal configuration files can be used.

2. libldns (required)
	This library is used to send DNS queries to a DNS server.

3. iptables (required)
	Iptables' library "ipq" (IP Queuing library) is required, it
	provides the necessary hooks to allow PacketBL to accept
	packets from the QUEUE target and process them.

4. FireDNS (optional)
	FireDNS is a library that queries all configured nameservers in
	parallel and once it gets an answer from one of them reports
	this.  It can make name resolution MUCH faster, especially when
	a configured nameserver is unreachable or down.


  • Compiling dependencies:
libldns-dev libdotconf-dev libnetfilter-queue-dev libnfnetlink-dev
  • Executing dependencies


libdotconf1.0 libldns1 libnetfilter-queue1 libnfnetlink0


libdotconf0 libldns1 libnetfilter-queue1 libnfnetlink0 libdotconf-dev


libdotconf0 libldns2 libnetfilter-queue1 libnfnetlink0 libdotconf-dev

Installation on Debian

PacketBL uses a GNU autoconf style configure' script for configuration. To invoke this script run the configure' script within the top-level source directory, for example: ./configure There are a few options that can be passed to the configure' script that will affect the way PacketBL is built (in addition to the standard autoconf configure' script options):

a. --with-cache
	This option will enable the experimental caching
	mechanism.  This may introduce unexpected problems.
	If you encounter any problems you should post a bug
	report to the PacketBL mailing list (for details, see
b. --with-firedns
	This option will cause PacketBL to use FireDNS's name
	resolution routines when testing IPs against DNS RBLs.
	Read above for more information on FireDNS.
c. --with-stats
	This option will enable the experimental statistic
	gathering code, which will require an extra thread to
	handle incoming connections to a UNIX domain socket.
d. --with-stats-socket=/path/to/socket
	This option allows one to specify the path to the UNIX
	domain socket that is used for communications between
	the PacketBL daemon and the "packetbl_getstat" process.
	Default is /tmp/.packetbl.sock.

To install Packetbl, execute the following commands:

git clone
cd /packetbl/src
./configure [OPTIONS]
make deb
sudo dpkg -i ../packetbl.deb

Config file

1. The configuration file (packetbl.conf) is in "Apache-style" format.
   An example configuration file might look something like this:
		# blacklistbl/whitelistbl domain
		blacklistbl    my.own.dnsbl    # this domain is a example
		whitelistbl    # this domain is a example
		# whitelist/blacklist cidr
		blacklist  # this IP is a example
	FallthroughAccept    yes
	AllowNonPort25        yes
	AllowNonSyn        no
	DryRun            no
	CacheSize        8192
	CacheTTL        3600
	LogFacility        daemon
	loglevel            5
	AlternativeDomain    my.own.domain
	AlternativeResolveFile    /usr/local/etc/packetbl/optional_resolvers
	Quiet            no
	Queueno        0
	Queuesize    2048
	Threadmax    700

2. Explanation of configuration elements:
	a. <host>
		This element begins the HOST section of the
		configuration.  You must define your Blacklists/Whitelist DNS
		RBLs and Whitelist/Balcklist addresses in the HOST section.
	b. BlackListBL
		The "BlackListBL" element defines a DNS RBL which is
		checked to determine whether or not packets are
		dropped.  This particular example configures
		"" as an RBL to use.
	c. WhiteList
		The "WhiteList" element defines a range in (CIDR
		format) of IP address to always accept and never check
		the configured "BlackListBL" elements.  You should
		usually leave at least "" there for safety.
	d. FallthroughAccept yes
		The "FallthroughAccept" element tells PacketBL how to
		handle packets that are neither listed in a configured
		DNS RBL ("BlackListBL" element) nor match a configured
		whitelist ("WhiteList" element).  Usually you should
		leave this as "yes" (the default).
	e. AllowNonPort25 no
		The "AllowNonPort25" element controls whether or not
		PacketBL will examine packets that are passed it that
		do not have a "Destination Port" of 25 (SMTP).  This
		is probably not something you want, leaving it "no"
		is safe.  Enabling this and mis-configuring your
		iptables configuration could cause a LOT of load on the
		configured DNS RBLs and may cause you to lose access to
	f. AllowNonSyn no
		The "AllowNonSyn" element controls whether or not
		PacketBL will examine packets that are passed it that
		do not have the SYN flag set (i.e, incoming TCP
		connections). This is probably not something you want,
		leaving it "no" is safe.  Enabling this and
		mis-configuring your iptables configuration could cause
		a LOT of load on the configured DNS RBLs and may cause
		you to lose access to them!
	g. DryRun no
		The "DryRun" element controls whether or not PacketBL
		actually rejects (DROPs) the packets that match a
		configured DNS RBL.  Setting this to "yes" will cause
		all packets to be ACCEPTed.  The default is "no"
		which causes normal operation.
	h. CacheSize 8192
		The "CacheSize" element determines the size of the
		cache (in entries, not bytes or bits) if cache has
		been enabled at compile time.  A setting of "0" causes
		caching to be disabled.  The largest reasonable value
		is currently 21675, anything above that will be wasted.
		it is necessary execute configure with option "--with-cache".
	i. CacheTTL 3600
		The "CacheTTL" element determines the length of time
		(in seconds) that cached entries are considered valid.
		Once an entry is looked up through a configured DNS RBL
		it will not need to be looked up again until after its
		"Time To Live" has been exceeded.
		it is necessary execute configure with option "--with-cache".
	j. LogFacility daemon
		The "LogFacility" element controls which syslog facility
		PacketBL sends its information to.  The default is
		probably fine for most people.
	k. Quiet no
		The "Quiet" element controls whether PacketBL writes
		a message to syslog() every time it accepts or rejects
		a packet.  The safe choice (and default) is "no"
		meaning that PacketBL writes a message to syslog about
		every packet.
	l. loglevel 5
		Log lvl, syslog log levels from 0 to 7: 0 Emergency, 1 Alert, 2 
		Critical, 3 Error, 4 Warning, 5 Notice, 6 Informational, or 7 Debug
	m. AlternativaDomain my.own.domain
		This parameter is related with next one. Queries to this domain 
		will be done using the resovlers of the file "AlternativeresolveFiles"
	n. AlternativeresolveFiles /usr/local/etc/packetbl/optional_resolvers
		This file is like /etc/resolv.conf. It will be used to get a lookup for 
		the domain "AlternativaDomain". In this file must be appear the 
		IP of the DNS server that resolves the domain.
	o. Queueno        0
		Specify a netfilter queue to packetbl
	p. Queuesize    2048
		Number of packets that can be stored in the netfilter queue.
	q. Threadmax    700
		Maximum number of threads. One thread is required to manage a 
		packet. If this parameter is commented then there is not limit in the threads number.


1. PacketBL supports a minimal number of command line arguments, since
   most configuration should be done in the configuration file (see
   previous section).  The following is a complete list of supported
   command line arguments:
	a. "-q"
		The "-q" option causes PacketBL to be quiet, it is
		identical to setting "Quiet" to "yes" in the
		configuration file.
	b. "-V"
		The "-V" option causes PacketBL to print out its
		version number and other relevant information to
		standard output and exit successfully.
	c. "-h"
		Show the command line options.
	d. "-f <file>"	
		Run packetbl using the file as configuration file.
	e. "-p <file>"
		Set a file where the PID is saved.
	f. "-d <level>"
		Run packetbl in debug mode. Level is a number between 0 and 3, 
		been 3 the level with most information details.
   Command line arguments always override their configuration file
   counter-parts where appropriate.  Unknown command line arguments
   cause PacketBL to terminate in error immediately at startup.


Once packetbl has been installed, run packetbl with a config file:

/usr/local/bin/packetbl -f /usr/local/etc/packetbl/packetbl_configfile.conf

It is necessary to fordward the origin packet to packetbl in order to query a DNS server. It must be done with an IPtables rule, an example could be:

iptables -A INPUT -t filter -p tcp --dport 25 -j NFQUEUE --queue-num 0

With this rule, all SMPT input traffic will be fordwarded to netfilter queue 0 where packetbl receives the packets and query to the DNS servers. If the origin IP is resolved by some of the DNS servers, Packetbl will apply an action to the packet (for example: drop, log or reject).

The "queue-num" parameter of IPtables must be the same than the "queueno" directive of Packetbl config file.


For the test, a Nginx server has been stressed using the tool "wrk". The following graph shows the number of HTTP resquest that Nginx has responsed with and without Packetbl. This test has been done without configuring the Packetbl cache to simule that each HTTP request is from a new client and to stress to Packetbl.

In this benchmark packetbl was compiled without firedns. Packetbl was running in a Debian Stretch.


  • The vertical axis shows the number of success HTTP requests per second.
  • The horizontal axis shows the number of concurrent clients doing HTTP requests.
  • The domains number is the number of RBL domains where Packetbl is looking for the source IP (Packetbl does one DNS query for each domain).


  • Using Packetbl to check the origin IP in 4 domains, it has an average performance of 90%.
  • Using Packetbl to check the origin IP in 10 domains, it has an average performance of 77%.
  • Using Packetbl to check the origin IP in 15 domains, it has an average performance of 67%.

How to Contribute

All reported bugs, new feature and patches are welcome.


Please use the GitHub project Issues to report any issue or bug with the software. Try to describe the problem and a way to reproduce it. It'll be useful to attach the service and network configurations as well as system and services logs.