Practical Guide: Linux System Log Function and Its Management (1)

This document describes the functions of the Linux Log System.

Logs are very important for security. They record all kinds of events that occur in the system every day. You can use them to check the cause of the error or the traces left by the attacker when the system is attacked. The main functions of logs are audit and monitoring. It can also monitor the system status in real time, monitor and track intrusions, and so on.

In Linux, there are three major log subsystems:

Connection time log-the log is executed by multiple programs and written to/var/log/wtmp and/var/run/utmp, login and other programs to update the wtmp and utmp files, enables the system administrator to track who is logged on to the system at any time.

Process statistics-executed by the system kernel. When a process is terminated, write a record for each process to the process Statistics file pacct or acct. Process statistics are used to provide command usage statistics for basic services in the system.

Error Log -- executed by syslogd8. Various system Daemon Processes, user programs, and kernels report noteworthy events to files/var/log/messages through syslog3. In addition, many UNIX programs create logs. Servers that provide network services such as HTTP and FTP also maintain detailed logs.

Common log files are as follows:

Access-log records HTTP/web transmission

Acct/pacct record user commands

Aculog records MODEM activities

Btmp record failure record

Lastlog records the last successful logon events and the last unsuccessful logon events.

Messages records information from syslog and some links to the syslog file)

Sudolog records commands issued by using sudo

Sulog logs the use of the su command

Syslog records information from syslogs, which is usually linked to the messages file)

Utmp records each user currently logged on

Wtmp a permanent record of the entry and exit times of a user upon each login

Xferlog records FTP sessions

Utmp, wtmp, and lastlog log files are the key for most UNIX log subsystems to be reused-keep records of user logon entry and exit. The information about the current logon user is recorded in the file utmp; the logon entry and exit records are recorded in the file wtmp; the last logon file can be viewed using the lastlog command. Data exchange, shutdown, and restart are also recorded in the wtmp file. All records contain timestamps. These files, lastlog is usually not large) are growing rapidly in systems with a large number of users. For example, the wtmp file can grow infinitely unless it is intercepted regularly. Many systems configure wtmp to be used cyclically in units of one day or one week. It is usually modified by the script run by cron. These scripts are renamed and the wtmp file is recycled. Generally, wtmp is named wtmp.1 after the first day, wtmp.1 is changed to wtmp.2 after the second day, and so on until wtmp.7.

Each time a user logs on, the login program checks the user's UID in the lastlog file. If the logon time is found, the user's Last Logon Time, exit time, and host name are written to the standard output, and the login program records the new Logon Time in lastlog. After a new lastlog is written, the utmp file is opened and the user's utmp record is inserted. This record is always used when a user logs on and exits. The utmp file is used by various command files, including who, w, users, and finger.

Next, the login program opens the file wtmp and appends the user's utmp record. When a user logs on and exits, the same utmp record with the updated timestamp is appended to the file. The wtmp file is used by the program last and ac.

Specific commands

Wtmp and utmp files are binary files, and they cannot be cut or combined by tail commands to use cat commands ). You need to use the information contained in the two files by who, w, users, last, and ac.

Who: The who command queries the utmp file and reports to each user currently logged on. The default output of Who includes the user name, terminal type, logon date, and remote host. Example: who press Enter) display

Chyang pts/0 Aug 18 :06

Ynguo pts/2 Aug 18 :32

Ynguo pts/3 Aug 18 :55

Lewis pts/4 Aug 18 :35

Ynguo pts/7 Aug 18

Ylou pts/8 Aug 18

If the wtmp file name is specified, the who command queries all previous records. The command who/var/log/wtmp will report every login since the wtmp file was created or deleted.

The w: w command queries the utmp file and displays information about each user in the current system and the processes it runs. Example: w press Enter): 3: 36pm up 1 day, 6 users, load average: 0.23, 0.29, 0.27.

User ttyfrom login @ IDLE JCPU PCPUWHAT

Chyang pts/0 202.38.68.2423: 06 pm 0.08 s 0.04 s-bash

Ynguo pts/2 202.38.79.47 pm 0.00 s 0.14 s 0.05 w

Lewis pts/3 202.38.64.2331: 55pm 30: 39 0.27 s 0.22 s-bash

Lewis pts/4 202.38.64.2331: 35pm 6.00 s 4.03 s sh/home/users/

Ynguo pts/7 simba. nic. ustc. e pm 0.00 s 0.47 s 0.24 s telnet mail

Yloupts/8 202.38.64.2352: 15 pm 1: 09 m 0.10 s 0.04s-bash