Linux系统日志rsyslogd
Linux系统日志
Linux上使用rsyslogd守护进程接收用户进程输出的日志和接收内核日志。
用户进程是通过syslogd函数生成系统日志。该函数将日志输出到一个UNIX本地域socket类型(AF_UNIX）的文件/dev/log中，rsyslogd则监听该文件以获取用户进程的输出。
内核日志是如何进行管理的，在这里我们不进行关系。
rsyslogd守护进程在接收到用户进程或内核输入的日志后，会把它们输出至某些特定的日志文件。默认情况下，调试信息会保存至/var/log/debug文件，普通信息保存至/var/log/messages文件，内核消息则保存至/var/log/kern.log文件。
不过，日志信息具体如何分发，可以在rsyslogd的配置文件中设置。rsyslogd 的主配置文件是/etc/rsyslog.conf，其中主要可以设置的项包括:内核日志输入路径，是否接收UDP日志及其监听端口（默认是514，见/etc/services文件)，是否接收TCP日志及其监听端口，日志文件的权限，包含哪些子配置文件(比如 /etc/rsyslog.d/*.conf)。rsyslogd的子配置文件则指定各类日志的目标存储文件。
rsyslogd系统日志功能比较复杂，有facility、priority、action等概念。还有Input模块、Filetr模块、Output模块等模块内容，目前还未弄清楚相关的知识。
查看了一下/etc/rsyslog.conf的配置文件
# /etc/rsyslog.conf configuration file for rsyslog
#
# For more information install rsyslog-doc and see
# /usr/share/doc/rsyslog-doc/html/configuration/index.html
#
# Default logging rules can be found in /etc/rsyslog.d/50-default.conf#################
#### MODULES ####
#################module(load="imuxsock") # provides support for local system logging
# module(load="immark") # provides --MARK-- message capability# provides UDP syslog reception
# module(load="imudp")
# input(type="imudp" port="514")# provides TCP syslog reception
# module(load="imtcp")
# input(type="imtcp" port="514")# provides kernel logging support and enable non-kernel klog messages
module(load="imklog" permitnonkernelfacility="on")###########################
#### GLOBAL DIRECTIVES ####
############################
# Use traditional timestamp format.
# To enable high precision timestamps, comment out the following line.
#
$ActionFileDefaultTemplate RSYSLOG_TraditionalFileFormat# Filter duplicated messages
$RepeatedMsgReduction on#
# Set the default permissions for all log files.
#
$FileOwner syslog
$FileGroup adm
$FileCreateMode 0640
$DirCreateMode 0755
$Umask 0022
$PrivDropToUser syslog
$PrivDropToGroup syslog#
# Where to place spool and state files
#
$WorkDirectory /var/spool/rsyslog#
# Include all config files in /etc/rsyslog.d/
#
$IncludeConfig /etc/rsyslog.d/*.conf
我们可以看到模块module(load="imuxsock")，这是一个输入模块
然后看到默认规则在/etc/rsyslog.d/50-default.conf，查看一下
# Default rules for rsyslog.
#
# For more information see rsyslog.conf(5) and /etc/rsyslog.conf#
# First some standard log files. Log by facility.
#
auth,authpriv.* /var/log/auth.log
*.*;auth,authpriv.none -/var/log/syslog
#cron.* /var/log/cron.log
#daemon.* -/var/log/daemon.log
kern.* -/var/log/kern.log
#lpr.* -/var/log/lpr.log
mail.* -/var/log/mail.log
#user.* -/var/log/user.log#
# Logging for the mail system. Split it up so that
# it is easy to write scripts to parse these files.
#
#mail.info -/var/log/mail.info
#mail.warn -/var/log/mail.warn
mail.err /var/log/mail.err#
# Some "catch-all" log files.
#
#
#*.=debug;\
# auth,authpriv.none;\
# news.none;mail.none -/var/log/debug
#*.=info;*.=notice;*.=warn;\
# auth,authpriv.none;\
# cron,daemon.none;\
# mail,news.none -/var/log/messages#
# Emergencies are sent to everybody logged in.
#
*.emerg :omusrmsg:*#
# I like to have messages displayed on the console, but only on a virtual
# console I usually leave idle.
#
#daemon,mail.*;\
# news.=crit;news.=err;news.=notice;\
# *.=debug;*.=info;\
# *.=notice;*.=warn /dev/tty8
其中auth这种就是facility（设施），mail.err这种就是priority（等级），而这种设置日志记录的位置就是action
*.*;auth,authpriv.none -/var/log/syslog
从上面这一个action可以看出普通的日志（"."），设置日志记录位置是/var/log/syslog，前面-表示异步写入
我们使用logger命令测试一下
logger -i -t "my_test" "test_log"
使用vim查看/var/log/syslog可以看到我们的logger的结果
rsyslogd的内容比较多，后续再补。
syslog函数
应用程序使用syslog函数和rsyslogd守护进程进行通讯。
#include <syslog.h>void openlog(const char *ident, int option, int facility);
void syslog(int priority, const char *format, ...);
void closelog(void);
openlog用于改变syslog默认的输出方式，进行日志结构化
ident参数指定的字符串将被添加到日志消息的日期和时间之后，它通常被设置为程序的名字。
option参数对后续syslog调用的行为进行配置，它可取下列的值
#define LOG_PID 0x01 /* log the pid with each message */
#define LOG_CONS 0x02 /* log on the console if errors in sending */
#define LOG_ODELAY 0x04 /* delay open until first syslog() (default) */
#define LOG_NDELAY 0x08 /* don't delay open */
#define LOG_NOWAIT 0x10 /* don't wait for console forks: DEPRECATED */
#define LOG_PERROR 0x20 /* log to stderr as well */
facility参数可以修改syslog函数中的默认设施值
/* facility codes */
#define LOG_KERN (0<<3) /* kernel messages */
#define LOG_USER (1<<3) /* random user-level messages */
#define LOG_MAIL (2<<3) /* mail system */
#define LOG_DAEMON (3<<3) /* system daemons */
#define LOG_AUTH (4<<3) /* security/authorization messages */
#define LOG_SYSLOG (5<<3) /* messages generated internally by syslogd */
#define LOG_LPR (6<<3) /* line printer subsystem */
#define LOG_NEWS (7<<3) /* network news subsystem */
#define LOG_UUCP (8<<3) /* UUCP subsystem */
#define LOG_CRON (9<<3) /* clock daemon */
#define LOG_AUTHPRIV (10<<3) /* security/authorization messages (private) */
#define LOG_FTP (11<<3) /* ftp daemon *//* other codes through 15 reserved for system use */
#define LOG_LOCAL0 (16<<3) /* reserved for local use */
#define LOG_LOCAL1 (17<<3) /* reserved for local use */
#define LOG_LOCAL2 (18<<3) /* reserved for local use */
#define LOG_LOCAL3 (19<<3) /* reserved for local use */
#define LOG_LOCAL4 (20<<3) /* reserved for local use */
#define LOG_LOCAL5 (21<<3) /* reserved for local use */
#define LOG_LOCAL6 (22<<3) /* reserved for local use */
#define LOG_LOCAL7 (23<<3) /* reserved for local use */
syslog用于输出日志。
priority参数是设施值和日志级别的按位与，默认是LOG_USER。日志级别有下面几个
#define LOG_EMERG 0 /* system is unusable */
#define LOG_ALERT 1 /* action must be taken immediately */
#define LOG_CRIT 2 /* critical conditions */
#define LOG_ERR 3 /* error conditions */
#define LOG_WARNING 4 /* warning conditions */
#define LOG_NOTICE 5 /* normal but significant condition */
#define LOG_INFO 6 /* informational */
#define LOG_DEBUG 7 /* debug-level messages */
第二个参数message和第三个参数...来结构化输出。
closelog用于关闭日志
小例子：
#include <syslog.h>
int main(int argc, char **argv)
{openlog(argv[0], LOG_CONS | LOG_PID, LOG_USER);syslog(LOG_DEBUG, "This is a syslog test message generated by program '%s'\n", argv[0]);closelog();return 0;
}
结果
