Good Info on syslogd

Sysklogd provides two system utilities which provide support for system logging and kernel message trapping. Support of both internet and unix domain sockets enables this utility package to support both local and remote logging.

System logging is provided by a version of syslogd(8) derived from the stock BSD sources. Support for kernel logging is provided by the klogd(8) utility which allows kernel logging to be conducted in either a standalone fashion or as a client of syslogd.

Syslogd provides a kind of logging that many modern programs use. Every logged message contains at least a time and a hostname field, normally a program name field, too, but that depends on how trusty the logging program is.

Remote Network support

These modifications provide network support to the syslogd facility. Network support means that messages can be forwarded from one node running syslogd to another node running syslogd where they will be actually logged to a disk file.

To enable this you have to specify the -r option on the command line. The default behavior is that syslogd won't listen to the network.

The strategy is to have syslogd listen on a unix domain socket for locally generated log messages. This behavior will allow syslogd to inter-operate with the syslog found in the standard C library. At the same time syslogd listens on the standard syslog port for messages forwarded from other hosts. To have this work correctly the services(5) files (typically found in /etc) must have the following entry:

syslog 514/udp

If this entry is missing syslogd neither can receive remote messages nor send them, because the UDP port cant be opened. Instead syslogd will die immediately, blowing out an error message.

To cause messages to be forwarded to another host replace the normal file line in the syslog.conf file with the name of the host to which the messages is to be sent prepended with an @.

For example, to forward ALL messages to a remote host use the following syslog.conf entry:

# Sample syslogd configuration file to
# messages to a remote host forward all.
*.* @hostname

To forward all kernel messages to a remote host the configuration file would be as follows:

# Sample configuration file to forward all kernel
# messages to a remote host.
kern.* @hostname

If the remote hostname cannot be resolved at startup, because the name-server might not be accessible (it may be started after syslogd) you don't have to worry. Syslogd will retry to resolve the name ten times and then complain. Another possibility to avoid this is to place the hostname in /etc/hosts.

With normal syslogds you would get syslog-loops if you send out messages that were received from a remote host to the same host (or more complicated to a third host that sends it back to the first one, and so on). In my domain (Infodrom Oldenburg) we accidently got one and our disks filled up with the same single message. :-(

To avoid this in further times no messages that were received from a remote host are sent out to another (or the same) remote host anymore. If there are scenarios where this doesn't make sense, please drop me (Joey) a line.

If the remote host is located in the same domain as the host, syslogd is running on, only the simple hostname will be logged instead of the whole fqdn.

In a local network you may provide a central log server to have all the important information kept on one machine. If the network consists of different domains you don't have to complain about logging fully qualified names instead of simple hostnames. You may want to use the strip-domain feature -s of this server. You can tell the syslogd to strip off several domains other than the one the server is located in and only log simple hostnames.

Using the -l option there's also a possibility to define single hosts as local machines. This, too, results in logging only their simple hostnames and not the fqdns.

The UDP socket used to forward messages to remote hosts or to receive messages from them is only opened when it is needed. In releases prior to 1.3-23 it was opened every time but not opened for reading or forwarding respectively.

OUTPUT TO NAMED PIPES (FIFOs)
This version of syslogd has support for logging output to named pipes (fifos). A fifo or named pipe can be used as a destination for log messages by prepending a pipy symbol (``|'') to the name of the file. This is handy for debugging. Note that the fifo must be created with the mkfifo command before syslogd is started.

The following configuration file routes debug messages from the kernel to a fifo:

# Sample configuration to route kernel debugging
# messages ONLY to /usr/adm/debug which is a
# named pipe.
kern.=debug |/usr/adm/debug

SECURITY THREATS
There is the potential for the syslogd daemon to be used as a conduit for a denial of service attack. Thanks go to John Morrison (jmorriso@rflab.ee.ubc.ca) for alerting to this potential. A rogue program(mer) could very easily flood the syslogd daemon with syslog messages resulting in the log files consuming all the remaining space on the filesystem. Activating logging over the inet domain sockets will of course expose a system to risks outside of programs or individuals on the local machine.

There are a number of methods of protecting a machine:

1.
Implement kernel firewalling to limit which hosts or networks have access to the 514/UDP socket.
2.
Logging can be directed to an isolated or non-root filesystem which, if filled, will not impair the machine.
3.
The ext2 filesystem can be used which can be configured to limit a certain percentage of a filesystem to usage by root only. NOTE that this will require syslogd to be run as a non-root process. ALSO NOTE that this will prevent usage of remote logging since syslogd will be unable to bind to the 514/UDP socket.
4.
Disabling inet domain sockets will limit risk to the local machine.
5.
Use step 4 and if the problem persists and is not secondary to a rogue program/daemon get a 3.5 ft (approx. 1 meter) length of sucker rod* and have a chat with the user in question.

Sucker rod def. --- 3/4, 7/8 or 1in. hardened steel rod, male threaded on each end. Primary use in the oil industry in Western North Dakota and other locations to pump 'suck' oil from oil wells. Secondary uses are for the construction of cattle feed lots and for dealing with the occasional recalcitrant or belligerent individual.