I want to provide a flexible, almost universal logging daemon for my netfilter ULOG target. It is not optimized in any way, the goal is to keep as simple as possible. These are my thoughts about how the architecture which is most capable of doing that:

Interpreter lugins It should be possible to add plugins / runtime modules for new protocols, etc. For example the standard logging daemon provides source-ip, dest-ip, source-port, dest-port, etc. Logging for variuos other protocols (GRE, IPsec, ...) may be implemented as modules. Output plugins ... describe how and where to put the information gained by logging plugins. The easiest way is to build a line per packet and fprint it to a file. Some people might want to log into a SQL database or want an output conforming to the intrusion detection systems communication draft from the IETF. DETAILS

The major clue is providing a framework which is as flexible as possible. Nobody knows what strange network protocols are out there :) Flexibility depends on the communication between the output of the logging plugins and input of the output plugins.

Rusty advised me to use some kind of type-key-value triples, but I think this is the total overkill and is too complicated for me to implement it in a reasonable short period of time. (3 hours later) Hmm... Rusty finally convinced me to use linked lists of type-key-value triples - and it wasn't that difficult.

Another issue is, of course, performance. Up to ulogd 0.3, ulogd did several linked list iterations and about 30 malloc() calls _per packet_. This changed with the new 0.9 revision: Not a single dynamic allocation in the core during runtime. Everything is pre-allocated at start of ulogd to provide the highest possible throughput. Hash tables in addition to the linked lists. Linked lists are only traversed if we really want to access each element of the list. INSTALLATION

Linux kernel

First you will need a recent 2.4.x kernel. At the time this document was written, 2.4.0-test11-pre5 was the latest development version. Ulogd should work with all kernels >= 2.4.0-test4. netfilter / iptables

In addition you need the latest iptables package, or even better: the latest CVS snapshot. A description how to obtain this is provided on the netfilter homepage .

ulogd is based on a special netfilter extension, called the netfilter ULOG target module. You have to patch this extension into your kernel, as it has not been integrated into the main kernel yet. To make this as easy as possible, netfilter provides the 'patch-o-matic' subsystem.

To run patch-o-matic, just type make patch-o-matic in the userspace directory of netfilter CVS. ulogd Recompiling the source

Download the ulogd package from and untar it.

Run './configure' and 'make install'.

Copy the configuration file 'ulogd.conf' to /etc Using a precompiled package

I also provide redhat-6.2 and redhat-7.0 RPM's, available at and .

Just download the package and do the usual 'rpm -i <file>'. Configuration netfilter

Just add rules using the ULOG target to your firewalling chain. A very basic example: iptables -A FORWARD -j ULOG --ulog-nlgroup 32 --prefix foo

Of course you can combine the ULOG target with the different netfilter match modules. For a more detailed description, have a look at the netfilter HOWTO's, available on the netfilter homepage. ulogd

All configurable parameters of ulogd are in the configfile '/etc/ulogd.conf'

The following configuration parameters are available: nlgroup The netlink multicast group, which ulgogd should bind to. This is the same as given with the '--ulog-nlgroup' option to iptables. logfile The main logfile, where ulogd reports any errors, warnings and other unexpected conditions. loglevel This specifies, how verbose the logging to logfile is. Currently defined loglevels are: 1=debug information, 3=informational messages, 5=noticable exceptional conditions, 7=error conditions, 8=fatal errors, program abort. plugin This option is followed by a filename of a ulogd plugin, which ulogd shold load upon initialization. This option may appear more than once. Available plugins

ulogd comes with the following plugins: ulogd_BASE.so Basic interpreter plugin for nfmark, timestamp, mac address, ip header, tcp header, udp header, icmp header, ah/esp header. ulogd_PWSNIFF.so Example interpreter plugin to log plaintext passwords as used with FTP and POP3. Don't blame me for writing this plugin! The protocols are inherently insecure, and there are a lot of other tools for sniffing passwords... it's just an example. ulogd_OPRINT.so A very simple output module, dumping all packets in the format ===>PACKET BOUNDARY key=value key=value ... ===>PACKET BOUNDARY ... to a file.

The module defines the following configuration directives: dumpfile The filename where it should log to. The default is /var/log/ulogd.pktlog ulogd_LOGEMU.so An output module which tries to emulate the old syslog-based LOG targed as far as possible. Logging is done to a textfile instead of syslog, though.

The module defines the following configuration directives: syslogfileThe filename where it should log to. The default is /var/log/ulogd.syslogemu ulogd_MYSQL.so An output plugin for logging into a mysql database. This is only compiled if you have the mysql libraries installed, and the configure script was able to detect them. (FIXME: how to do this)

The plugin automagically inserts the data into the configured table; It connects to mysql during the startup phase of ulogd and obtains a list of the columns in the table. Then it tries to resolve the column names against keys of interpreter plugins. This way you can easly select which information you want to log - just by the layout of the table.

If, for example, your table contains a field called 'ip_saddr', ulogd will resolve this against the key 'ip.saddr' and put the ip address as 32bit unsigned integer into the table.

You may want to have a look at the file 'doc/mysql.table' as an example table including fields to log all keys from ulogd_BASE.so. Just delete the fields you are not interested in, and create the table.

The module defines the following configuration directives: mysqltable Name of the table to which ulogd should log mysqldb Name of the mysql database mysqlhost Name of the mysql database host mysqluser Name of the mysql user mysqlpass Password for mysql QUESTIONS / COMMENTS

All comments / questions / ... are appreciated.

Just drop me a note to laforge@gnumonks.org