.\" -*- nroff -*- .\" Copyright 1994-1996 Dr. Greg Wettstein, Enjellic Systems Development. .\" Copyright 1997-2008 Martin Schulze .\" Copyright 2018-2019 Joachim Nilsson .\" .\" May be distributed under the GNU General Public License .\" .Dd Oct 30, 2019 .Dt syslogd 8 .Os "sysklogd (2.0)" .Sh NAME .Nm syslogd .Nd System Log Daemon .Sh SYNOPSIS .Nm .Op Fl ?46Adhnrv .Op Fl b Ar :SVC .Op Fl f Ar FILE .Op Fl l Ar HOST[:HOST] .Op Fl m Ar SECONDS .Op Fl P Ar FILE .Op Fl p Ar SOCK .Op Fl R Ar size[:count] .Op Fl s Ar NAME[:NAME] .Sh DESCRIPTION .Nm support RFC3164 and RFC5424 style log messages for both local and remote logging using Internet and UNIX domain sockets. Differences in style is shown below. The companion daemon, .Xr klogd 8 , is used for trapping kernel messages and events. .Pp .Bl -tag -compact -width "RFC3164:" .It RFC3164: .Cm Aug 24 05:14:15 192.0.2.1 myproc[8710]: Kilroy was here. .It RFC5424: .Cm 2003-08-24T05:14:15.000003-07:00 192.0.2.1 myproc 8710 - - Kilroy was here. .El .Pp Note, for remote logging the messages are prefixed with .Cm or .Cm 1 , respectively. .Pp .Nm is derived from BSD sources, today .Fx is the reference for .Nm and .Nx for the new .Xr syslogp 3 API, which fully supports the new features of RFC5424. Please note; 1) the intention is to follow standard BSD .Nm behavior, 2) despite having a stand-alone .Xr syslog 3 , and .Xr syslogp 3 API in .Lb libsyslog , this version of .Nm interacts transparently with the standard C library .Xr syslog 3 API, as implemented in GLIBC, musl libc, and uClibc. .Pp When .Nm starts up it reads its main configuration file .Pa /etc/syslog.conf , or an alternate file given with the .Fl f Ar FILE option. For details on how to configure syslog priority (facility.severity) filtering, see .Xr syslog.conf 5 . .Sh OPTIONS .Bl -tag -width Ds .It Fl 4 Force .Nm to use IPv4 addresses only. .It Fl 6 Force .Nm to use IPv6 addresses only. .It Fl A Ordinarily, .Nm tries to send the message to only one address even if the host has more than one A or AAAA record. If this option is specified, .Nm tries to send the message to all addresses. .It Fl b Ar :service Bind to a specific port. The port can be specified as a service name or number. The default service is .Ql syslog (514/udp). .It Fl d Turns on debug mode. This implicitly enables .Fl n to prevent .Nm from backgrounding itself. Debug information is written to the current TTY. SIGUSR1 is required to confirm continued debug messages when the daemon has finished started up. See the .Sx DEBUGGING section for more information. .It Fl f Ar FILE Specify an alternative configuration file instead of the default .Pa /etc/syslog.conf . .It Fl h By default syslogd will not forward messages it receives from remote hosts. Specifying this switch on the command line will cause the log daemon to forward any remote messages it receives to forwarding hosts which have been defined. This can cause syslog loops that fill up hard disks quite fast and thus needs to be used with caution. .It Fl l Ar HOST Specify a hostname that should be logged only with its simple hostname and not the fqdn. Multiple hosts may be specified using the colon (':') separator. .It Fl m Ar SECONDS .Nm logs a mark timestamp regularly. The default interval between two .Ql -- MARK -- lines is 20 minutes. This can be changed with this option. Setting this to zero disables log marks entirely. .Pp Depending on other log messages generated these lines may not be written consecutively. The .Ql -- MARK -- message is only written if the log file hasn't been touched in .Ar (SEC * 60) / 2 minutes. .It Fl n Run in foreground, required when run from a modern init/supervisor. See your system .Xr init 1 for details. .It Fl P Ar FILE Specify an alternate file in which to store the process ID. The default is .Pa /var/run/syslog.pid . .It Fl p Ar SOCK Specify an alternate UNIX domain socket instead of the default .Pa /dev/log . When a single .Fl p option is specified, the default pathname is replaced with the specified one. When two or more .Fl p options are specified, the remaining pathnames are treated as additional log sockets. This might be needed when running applications in containers or a .Xr chroot 8 environment. In total 20 UNIX domain sockets are supported. .It Fl R Ar size[:count] Enable built-in support for log rotation of files listed in .Pa /etc/syslog.conf . This feature is particulary useful for small and embedded systems that do not want the overhead of .Xr cron 8 and .Xr logrotate 8 . .Pp The option controls the max size and number of backup files kept by the built-in log-rotation. When present on the command line it activates log rotation of all files with the given maximum size. It is also possible to control log rotate per log file, see .Xr syslog.conf 5 for details. .Pp The size argument takes optional modifiers; k, M, G. E.g., 100M is 100MB, 42k is 42 kB, etc. .Pp The optional number of files kept include both gzipped files and the first rotated (not zipped) file. The default for this, when omitted, is 5. .It Fl r This option enables support for receiving syslog messages over the network using an internet domain socket with the .Ql syslog (514/udp) service, see .Xr services 5 . The default is to not listen for incoming network connections. Use the .Fl b option to override the default listening port. .It Fl s NAME Specify domain name(s) to be stripped off before logging. Multiple domains may be specified using the colon (':') separator. Note, no sub-domains may be specified but only entire domains. For example if .Fl s Ar north.de is specified and the host logging resolves to .Ql satu.infodrom.north.de nothing is stripped, instead two domains must be specified: .Fl s Ar north.de:infodrom.north.de . .It Fl v Print .Nm version and exit. .Sh CONFIGURATION FILE SYNTAX DIFFERENCES .Nm uses a slightly different syntax for its configuration file than the original BSD sources. First, rules may now also have a third field .Cm ;OPTION . Several options are supported, comma separated, that control formatting and log rotation, for more on this see .Xr syslog.conf 5 . Second, originally all messages of a specific priority and above were forwarded to the log file. .Pp For example the following line send all output from daemons using the daemon facilities (debug is the lowest priority, so every higher will also match) to go into .Pa /var/log/daemons : .Bd -literal -offset indent # Sample syslog.conf daemon.debug /var/log/daemons .Ed .Pp With .Nm this behavior remains the same. The difference is the addition of four additional specifiers, the asterisk ('*') wildcard, the equation sign ('='), the exclamation mark ('!'), and the minus sign ('-'). .Pp The '*' specifies that all messages for the specified facility are to be directed to the destination. Note that this behavior is degenerate with specifying a priority level of debug. Users have indicated that the asterisk notation is more intuitive. .Pp The '=' wildcard is used to restrict logging to the specified priority class. This allows, for example, routing only debug messages to a particular logging source. .Pp For example, the following line in .Pa /etc/syslog.conf directs debug messages from all sources to the .Pa /var/log/debug file in RFC5424 format, with log rotation every 512 kiB, saving only 20 files in total (including the non-rotated file): .Bd -literal -offset indent # Sample syslog.conf *.=debug -/var/log/debug ;RFC5424,rotate=512k:20 .Ed .Pp .\" The '!' as the first character of a priority inverts the above .\" mentioned interpretation. The '!' is used to exclude logging of the specified priorities. This affects all (!) possibilities of specifying priorities. .Pp For example the following lines in .Pa syslog.conf log all messages of facility .Ql mail except those with priority .Ql info to the .Pa /var/log/mail file. All messages from .Ql news.info (including) to .Ql news.crit (excluding) are logged to the .Pa /var/log/news file. .Bd -literal -offset indent # Sample syslog.conf mail.*;mail.!=info /var/log/mail news.info;news.!crit /var/log/news .Ed .Pp You may use it intuitively as an exception specifier. The above mentioned interpretation is simply inverted. For example, to skip every message with facility .Ar mail : .Bd -literal -offset indent mail.none .Ed or .Bd -literal -offset indent mail.!* .Ed or .Bd -literal -offset indent mail.!debug .Ed .Pp The '-' may only be used to prefix a filename if you want to omit sync'ing the file after every write to it. .Sh REMOTE LOGGING The following modifications provide network support to the .Nm facility. Network support means that messages can be forwarded from one node running .Nm to another node running .Nm where they will be actually logged to a disk file. .Pp This feature is enabled using the .Fl r option on the command line. The default behavior to not listen to network connections. .Pp 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 .Xr services 5 files (typically found in .Pa /etc/services ) must have the following entry: .Bd -literal -offset indent syslog 514/udp .Ed .Pp If this entry is missing .Nm cannot receive remote messages, or send them, because the UDP port cannot be determined. Instead .Nm will die immediately with an error message. .Pp To forward messages to to a remote host, replace the file line in the .Pa syslog.conf file with the name of the hostname to which the messages is to be sent prepended with an at ('@') sign. For remote logging the hostname can also be appended with the flag .Ql ;RFC5424 to enable RFC5424 style formatting which includes RFC3339 timestamp and hostname information, which is not included in the default BSD .Nm . .Pp For example, to forward .Sy ALL messages to a remote host use the following .Pa syslog.conf entry: .Bd -literal -offset indent # Sample syslogd configuration file to forward all message # messages to a remote host using RFC5424 style formatting *.* @hostname;RFC5424 .Ed .Pp To forward all .Ql kernel messages to a remote host the configuration file would be as follows: .Bd -literal -offset indent # Sample configuration file to forward all kernel # messages to a remote host. kern.* @hostname .Ed .Pp If the remote hostname cannot be resolved at startup, because the name-server might not be accessible (it may be started after .Nm ), .Nm will retry resolving the name ten times before logging the error. Another possibility to avoid this is to place the hostname in .Pa /etc/hosts . .Pp To avoid syslog-loops (messages that were received from a remote host are sent back to the same host, or more complicated to a third host that sends it back to the first one, and so on), .Nm by default does not forward remote messages to another remote server. If this for some reason is required, use the .Fl h option on the command line. However, this option needs to be handled with caution since a syslog loop can fill up hard disks quite fast. .Pp If the remote host is located in the same domain as the host, .Nm is running on, only the simple hostname will be logged instead of the whole FQDN. .Pp 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 may want to use the strip-domain feature .Fl s . See above. .Pp Using the .Fl l option it is possibile to define single hosts as local machines. This also results in logging only their simple hostnames and not the FQDNs. .Pp 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. .Sh 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 very handy for debugging. Note, the FIFO must be created with the .Xr mkfifo 1 command before .Nm is started. .Pp The following configuration file routes debug messages from the kernel to a FIFO: .Bd -literal -offset indent # Sample configuration to route kernel debugging # messages ONLY to /var/log/debug which is a # named pipe. kern.=debug |/var/log/debug .Ed .Sh CONCERNS There is probably one important consideration when installing this version of syslogd. This version of syslogd is dependent on proper formatting of messages by the syslog function. The functioning of the syslog function in the shared libraries changed somewhere in the region of libc.so.4.[2-4].n. The specific change was to null-terminate the message before transmitting it to the .Pa /dev/log socket. Proper functioning of this version of .Nm is dependent on null-termination of the message. .Pp This problem will typically manifest itself if old statically linked binaries are being used on the system. Binaries using old versions of the syslog function will cause empty lines to be logged followed by the message with the first character in the message removed. Relinking these binaries to newer versions of the shared libraries will correct this problem. .Sh SECURITY There is the potential for .Nm to be used as a conduit for a denial of service attack. Thanks go to .An John Morrison Aq Mt jmorriso@rflab.ee.ubc.ca for alerting the project of this. A rogue program(mer) could very easily flood .Nm with syslog messages resulting in the log files consuming all the remaining space on the filesystem. Activating logging over network domain sockets will of course expose a system to risks outside of programs or individuals on the local machine. .Pp There are a number of methods of protecting a machine: .Bl -enum .It Implement kernel firewalling to limit which hosts or networks have access to the 514/UDP socket. .It Logging can be directed to an isolated or non-root filesystem which, if filled, will not impair the machine. .It The ext2 filesystem can be used which can be configured to limit a certain percentage of a filesystem to usage by root only. .Sy NOTE: this requires .Nm to be run as a non-root process. Also, this prevents usage of remote logging since .Nm will be unable to bind to the 514/UDP socket. .It Disabling inet domain sockets will limit risk to the local machine. .El .Sh DEBUGGING When debug mode ( .Fl d ) is enabled .Nm is very verbose, writing most of what it does on stdout. Whenever the configuration file is reread and re-parsed you'll see a tabular, corresponding to the internal data structure. This tabular consists of four fields: .Pp .Bl -tag -width arguments .It number This field contains a serial number starting by zero. This number represents the position in the internal data structure (i.e. the array). If one number is left out then there might be an error in the corresponding line in .Pa /etc/syslog.conf . .It pattern This field is tricky and represents the internal structure exactly. Every column stands for a facility, refer to .Xr syslogp 3 . As you can see, there are still some facilities left free for former use, only the left most are used. Every field in a column represents the priorities, refer to .Xr syslogp 3 . .It action This field describes the particular action that takes place whenever a message is received that matches the pattern. Refer to the .Xr syslog.conf 5 manpage for all possible actions. .It arguments This field shows additional arguments to the actions in the last field. For file-logging this is the filename for the logfile; for user-logging this is a list of users; for remote logging this is the hostname of the machine to log to; for console-logging this is the used console; for tty-logging this is the specified tty; wall has no additional arguments. .El .Sh SIGNALS .Nm supports the following signals: .Pp .Bl -tag -width "TERM, QUIT" .It HUP This lets .Nm perform a re-initialization. All open files are closed, the configuration file (see above) is reread and the .Xr syslog 3 facility is started again. .It TERM This tells .Nm to exit gracefully. Flushing any log files to disk. .It INT, QUIT In debug mode these are ignored. In normal operation they act as SIGTERM. .It USR1 In debug mode this switches debugging on/off. In normal operation it is ignored. .El .Pp For convenience the PID is, by default, stored in .Pa /var/run/syslogd.pid . Example usage: .Bd -literal -offset indent kill -SIGNAL `cat /var/run/syslogd.pid` .Ed .Sh FILES .Bl -tag -width TERM .It Pa /etc/syslog.conf Configuration file for .Nm . See .Xr syslog.conf 5 for more information. .It Pa /dev/log The UNIX domain socket to from where local syslog messages are read. .It Pa /var/run/syslogd.pid The file containing the process id of .Nm . .El .Sh BUGS As mentioned in the .Sx DESCRIPTION , .Nm transparently supports the standard C library .Xr syslog 3 API. If a binary linked to the standard C libraries does not operate correctly, this should be reported as a bug to this project. See below for contact details. .Pp .Nm doesn't change the file mode of opened log files at any stage. If a file is created it is world readable. If you want to avoid this, you have to create it and change permissions on your own. This could be done in combination with rotating logfiles using the .Xr savelog 8 program that is shipped in the .Nm smail 3.x distribution. Remember that it might be a security hole if everybody is able to read .Ql auth.* messages as these might contain passwords. .Sh SEE ALSO .Xr syslog.conf 5 , .Xr klogd 8 , .Xr logger 1 , .Xr syslog 2 , .Xr syslogp 3 , .Xr services 5 , .Xr savelog 8 . .Sh AUTHORS The system log daemon .Nm is originally taken from BSD sources and later updated with new funcitonality from .Fx and .Nx . .An -nosplit .An Greg Wettstein Aq Mt greg@wind.enjellic.com performed the initial port to Linux. .An Martin Schulze Aq Mt joey@infodrom.org fixed some bugs, added several new features and took over maintenance. .An Joachim Nilsson Aq Mt troglobit@gmail.com later picked up the aging .Nm sysklogd and gave it a home at GitHub with new features imported from .Fx and .Nx .