SYSLOG.CONF(5)

File Formats Manual

SYSLOG.CONF(5)

NAME

syslog.conf — syslogd(8) configuration file

DESCRIPTION

The syslog.conf file is the configuration file for the syslogd(8) program. It consists of extended options (lines with one key="value" assignment) and blocks of lines separated by program and hostname specifications, with each line containing two fields: the selector field which specifies the types of messages and priorities to which the line applies, and an action field which specifies the action to be taken if a message syslogd(8) receives matches the selection criteria. The selector field is separated from the action field by one or more tab characters.

The Selectors function are encoded as a facility, a period (‘.'), an optional set of comparison flags ([!] [<=>]), and a level, with no intervening white-space. Both the facility and the level are case insensitive.

The facility describes the part of the system generating the message, and is one of the following keywords: auth, authpriv, cron, ftp, daemon, kern, lpr, mail, mark, news, syslog, user, uucp and local0 through local7. These keywords (with the exception of mark) correspond to the similar “LOG_” values specified to the openlog(3) and syslog(3) library routines.

The comparison flags may be used to specify exactly what levels are logged. If unspecified, the default comparison is ‘>=' (greater than or equal to), or, if the -U option is passed to syslogd(8), ‘=' (equal to). Comparison flags beginning with ‘!' will have their logical sense inverted. Thus, ‘!=info' means all levels except info and ‘!notice' has the same meaning as ‘<notice'.

The level describes the severity of the message, and is a keyword from the following ordered list (higher to lower): emerg, alert, crit, err, warning, notice, info and debug. These keywords correspond to the similar (LOG_) values specified to the syslog(3) library routine.

Each block of lines is separated from the previous block by a program or hostname specification. A block will only log messages corresponding to the most recent program and hostname specifications given. Consider the case of a block that selects ‘pppd' as the program, directly followed by a block that selects messages from the hostname ‘dialhost'. The second block will log only messages from the pppd(8) program from the host ‘dialhost'.

A program specification of the form ‘#!+prog1,prog2' or ‘!+prog1,prog2' will cause subsequent blocks to be applied to messages logged by the specified programs. A program specification of the form ‘#!-prog1,prog2' or ‘!-prog1,prog2' will cause subsequent blocks to be applied to messages logged by programs other than the ones specified. A program specification of the form ‘#!prog1,prog2' or ‘!prog1,prog2' is equivalent to ‘!+prog1,prog2'. Program selectors may also match kernel-generated messages. For example, a program specification of ‘!+subsys' will match kernel-generated messages of the form ‘subsys: here is a message'. The special specification ‘!*' will cause subsequent blocks to apply to all programs.

A hostname specification of the form ‘#+host1,host2' or ‘+host1,host2' will cause subsequent blocks to be applied to messages received from the specified hosts. A hostname specification of the form ‘#-host1,host2' or ‘-host1,host2' will cause subsequent blocks to be applied to messages from hosts other than the ones specified. If the hostname is given as ‘@', the local hostname will be used. The special specification ‘+*' will cause subsequent blocks to apply to all hosts.

See syslog(3) for a further descriptions of both the facility and level keywords and their significance. It is preferred that selections be made based on facility rather than program, since the latter can vary in a networked environment. However, there are cases where a facility may be too broadly defined.

If a received message matches the specified facility, and the specified level comparison is true, and the first word in the message after the date matches the program, the action specified in the action field will be taken.

Multiple selectors may be specified for a single action by separating them with semicolon (‘;') characters. It is important to note, however, that each selector can modify the ones preceding it.

Multiple facilities may be specified for a single level by separating them with comma (‘,') characters.

An asterisk (‘*') can be used to specify all facilities or all levels.

The special facility “mark” receives a message at priority “info” every 20 minutes (see syslogd(8)). This is not enabled by a facility field containing an asterisk.

The special level “none” disables a particular facility.

The action field of each line specifies the action to be taken when the selector field selects a message. There are five forms:

A pathname (beginning with a leading slash). Selected messages are appended to the file.
To ensure that kernel messages are written to disk promptly, syslogd(8) calls fsync(2) after writing messages from the kernel. Other messages are not synced explcitly. You may disable syncing of files specified to receive kernel messages by prefixing the pathname with a minus sign ‘-'. Note that use of this option may cause the loss of log information in the event of a system crash immediately following the write attempt. However, using this option may prove to be useful if your system's kernel is logging many messages.
Normally the priority and version is not written to file. In order to use syslog-sign you may prefix a pathname with the plus sign ‘+'. If both switches are used the order has to be ‘+-'.
A hostname (preceded by an at (‘@') sign). Selected messages are forwarded to the syslogd(8) program on the named host with UDP.
A hostname preceded by an at (‘@') sign and enclosed in brackets (‘[]') Selected messages are forwarded with TLS to the syslogd(8) program on the named host. After the closing bracket a colon (‘:') and a port or service name may be appended. Additional options are configured in parantheses in the form of key="value". Recognized keywords are subject, fingerprint, cert, and verify.
A comma separated list of users. Selected messages are written to those users if they are logged in.
An asterisk. Selected messages are written to all logged-in users.
A vertical bar (‘|') followed by a command to which to pipe the selected messages. The command string is passed to /bin/sh for evaluation, so the usual shell metacharacters or input/output redirection can occur. (Note that redirecting stdio(3) buffered output from the invoked command can cause additional delays, or even lost output data in case a logging subprocess exits with a signal.) The command itself runs with stdout and stderr redirected to /dev/null. Upon receipt of a SIGHUP, syslogd(8) will close the pipe to the process. If the process does not exit voluntarily, it will be sent a SIGTERM signal after a grace period of up to 60 seconds.
The command will only be started once data arrives that should be piped to it. If the command exits, it will be restarted as necessary.
If it is desired that the subprocess should receive exactly one line of input, this can be achieved by exiting after reading and processing the single line. A wrapper script can be used to achieve this effect, if necessary. Note that this method can be very resource-intensive if many log messages are being piped through the filter.
Unless the command is a full pipeline, it may be useful to start the command with exec so that the invoking shell process does not wait for the command to complete. Note that the command is started with the UID of the syslogd(8) process, normally the superuser.
Just like with files a plus sign ‘+' will leave the priority and version information intact.

Blank lines and lines whose first non-blank character is a hash (‘#') character are ignored.

TLS OPTIONS

Additional options are used for TLS configuration:

tls_server: Enables TLS server mode.
tls_bindport: Service name or port number to bind to. Default is ‘syslog'. As long as no official port is assigned this option is required for TLS servers.
tls_bindhost: Hostname or IP to bind to.
tls_gen_cert: Automatically generate a private key and certificate.
tls_key: File with private key. Default is ‘/etc/openssl/default.key'
tls_cert: File with certificate to use. Default is ‘/etc/openssl/default.crt'
tls_ca: File with CA certificate to use.
tls_cadir: Directory containing CA certificates.
tls_verify: If set to ‘off' then certificate authentication is skipped.
tls_allow_fingerprints: List of fingerprints of trusted client certificates.
tls_allow_clientcerts: List of filenames with trusted client certificates.

TLS AUTHENTICATION

One function of TLS is mutual authentication of client and server. Unless authentication is disabled by setting ‘tls_verify=off' the following rules are used:

As client:

A client can be configured not to check a server's certificate by setting the parameter verify to ‘off'. If the server's certificate is signed by a trusted CA then it is checked if its hostname or IP is given in its certificate (as a CommonName, as a DNS SubjectAltName, or as an IP SubjectAltName). If any match is found then the server is authenticated. If a subject parameter is given then it is can satisfy this test as well. This allows DNS-independent configurations using the server's IP address in the destination and adding its hostname as subject to authenticate the TLS connection without having to add the IP to the X.509 certificate.

If no CA is used or no trust path between CA and server certificate exists, then hash value of the server's certificate is compared with the hash given in fingerprint and the hash of the certificate in cert. If the hashes are equal then the server is authenticated.

As server:

If using a CA and the client's certificate is signed by it then the client is authenticated. Otherwise the hash of the client's certificate is compared with the hashes given in tls_allow_fingerprints and the hashes of the certificates given in tls_allow_clientcerts. On any match the client is authenticated.

BUFFERING

syslogd(8) is able to buffer temporary not writeable messages in memory. To limit the memory consumed for this buffering the following optons may be given:

file_queue_length
pipe_queue_length
tls_queue_length: The maximum number of messages buffered for one destination of type tls, file, or pipe respectively. Defaults are ‘1024', ‘1024', and ‘-1' (no limit).
file_queue_size
pipe_queue_size
tls_queue_size: The maximum memory usage in bytes of messages buffered for one destination. Defaults are ‘1M', ‘1M', and ‘16M'.

SIGNING

syslogd(8) is able to digitally sign all processed messages. The used protocol is defined by RFC nnnn (syslog-sign): at the start of a session the signing sender sends so called certificate blocks containing its public key; after that it periodically sends a signed message containing hashes of previous messages.

To detect later manipulation one has to keep a copy of the key used for signing (otherwise an attacker could alter the logs and sign them with his his own key). If TLS is used with a DSA key then the same key will be used for signing. This is the recommended setup because it makes it easy to have copies of the certificate (with the public key) in backups. Otherwise new keys are generated on every restart and for certain verification it is necessary to have copies of all used keys. So logging only to a local file is not secure; at least the used keys should be logged to another host.

sign_sg

Enables signing. Set this option to enable syslog-sign and select how to assign messages to signature groups (subsets of messages that are signed together). To enable later signature verification and detection of lost messages the assignment should be chosen such that all messages of one signature group are written to the same file. Four possible values for this option are:

0: Use one global signature group for all messages.
1: Use one signature group per priority.
2: Use signature groups for ranges of priorities.
3: Use one signature group per destination. This is a custom strategy not defined by the standard. With this setting one signature group is set up for every file and network action.

sign_delim_sg2

This option is only evaluated with ‘sign_sg=2' and allows to configure the priority ranges for signature groups. The parameters are numerical values used as the maximum priority for one group. The default is to use one signature groups per facility, which is equal to setting ‘sign_delim_sg2=7 15 23 31 39 ...'.

FILES

EXAMPLES

A configuration file might appear as follows:

# Log all kernel messages, authentication messages of 
# level notice or higher and anything of level err or 
# higher to the console. 
# Don't log private authentication messages! 
*.err;kern.*;auth.notice;authpriv.none	/dev/console 
 
# Log anything (except mail) of level info or higher. 
# Don't log private authentication messages! 
*.info;mail.none;authpriv.none		/var/log/messages 
 
# Log daemon messages at debug level only 
daemon.=debug				/var/log/daemon.debug 
 
# The authpriv file has restricted access. 
# Write logs with priority for later verification with syslog-sign. 
authpriv.*				+/var/log/secure 
 
# Log all the mail messages in one place. 
mail.*					/var/log/maillog 
 
# Everybody gets emergency messages, plus log them on another 
# machine. 
*.emerg					* 
*.emerg					@arpa.berkeley.edu 
 
# Log all messages of level info or higher to another 
# machine using TLS with an alternative portname and a 
# fingerprint for athentication 
*.info			@[logserver]:1234(fingerprint="SHA1:01:02:...") 
 
# Root and Eric get alert and higher messages. 
*.alert					root,eric 
 
# Save mail and news errors of level err and higher in a 
# special file. 
mail,news.err				/var/log/spoolerr 
 
# Pipe all authentication messages to a filter. 
auth.*					|exec /usr/local/sbin/authfilter 
 
# Log kernel messages to a separate file without syncing each message. 
kern.*					-/var/log/kernlog 
 
# Save ftpd transactions along with mail and news. 
!ftpd 
*.*					/var/log/spoolerr 
 
# Send all error messages from a RAID array through a filter. 
!raid0 
kern.err				|exec /usr/local/sbin/raidfilter 
 
# Save pppd messages from dialhost to a separate file. 
!pppd 
+dialhost 
*.*					/var/log/dialhost-pppd 
 
# Save non-local log messages from all programs to a separate file. 
!* 
-@ 
*.*					/var/log/foreign 
 
# Generate digital signatures for all messages 
# to each file or network destination. 
sign_sg=3

HISTORY

The syslog.conf file appeared in 4.3BSD, along with syslogd(8).

BUGS

The effects of multiple selectors are sometimes not intuitive. For example “mail.crit;*.err” will select “mail” facility messages at the level of “err” or higher, not at the level of “crit” or higher.

January 1, 2010

NetBSD 6.1