What is system logs in linux

How to find and interpret system log files on Linux

Posted: May 18, 2022 |

Log files and journals are important to a system administrator’s work. They reveal a great deal of information about a system and are instrumental during troubleshooting and auditing.

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Log files contain events and messages generated by the kernel, applications, and users that log into the system.

Use rsyslog

Syslog and rsyslog have long been used to provide logging on Linux servers. Systemd became the default service manager with Red Hat Enterprise Linux (RHEL) 7, and it introduced its own logging system called systemd-journald. systemd-journald continues to be the logging mechanism on RHEL 8 and 9 while keeping rsyslog for backward compatibility.

The rsyslog service keeps various log files in the /var/log directory. You can open these files using native commands such as tail , head , more , less , cat , and so forth, depending on what you are looking for.

For example, to display boot and other kernel messages, view /var/log/messages :

Use grep and other filtering tools to gather more specific events from a file. You can also use tail to view files as they are updated:

In the command above, the -f option updates the output when new log file entries are added.

Check the /var/log/secure file to view users and their activities:

Use systemd-journald

The systemd-journald service does not keep separate files, as rsyslog does. The idea is to avoid checking different files for issues. Systemd-journald saves the events and messages in a binary format that cannot be read with a text editor. You can query the journal with the journalctl command.

To show all event messages, use:

This is similar to the /var/log/messages in the rsyslog service.

To view the last 10 event messages, use:

You can view the last n entries by using journalctl -n . For example, to view the last 20 entries, type:

To output new journal entries as they are written to the journal, use:

Run the following command to display the kernel message log from the last boot:

The journalctl command has several choices that can make querying the journal easier. You can query the log based on applications, time frame, systemd units, priority, and many other options. Run the journalctl –help command to list the available options.

To view journal entries based on their critical priority, use:

To query all messages related to a particular user, find the user’s ID (UID) and use that to perform the query. For example, to check all logs related to the sadmin user, run:

To view journal entries for today, use:

To view journal entries related to the sshd daemon, run:

The same applies to other services running under systemd that can be stopped and started with systemctl .

To check for messages related to the httpd service for the past hour, you can run:

More Linux resources

Manage log forwarding

RHEL 8 and 9 servers use both rsyslog and systemd-journald, and they complement each other to perform logging. Systemd-journald does not have a mechanism to forward logs to external systems and monitoring applications. A configuration modifies this in the /etc/systemd/journald.conf . The ForwardToSyslog parameter defines whether entries in the journal should be forwarded to syslog. When enabled, syslog then captures the entries as they come through systemd-journald and forwards them accordingly.

Wrap up

Current RHEL distributions rely on systemd and the related journald logging tool. However, rsyslog still plays a major role in logging for many administrators—particularly when it comes to log forwarding and centralization. Sysadmins must know how to use both log mechanisms effectively. These commands will help you learn and use system logging for troubleshooting and audits. Work with them both and you will have a much better understanding of what is happening on your Linux systems.

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Лог файлы Linux по порядку

Невозможно представить себе пользователя и администратора сервера, или даже рабочей станции на основе Linux, который никогда не читал лог файлы. Операционная система и работающие приложения постоянно создают различные типы сообщений, которые регистрируются в различных файлах журналов. Умение определить нужный файл журнала и что искать в нем поможет существенно сэкономить время и быстрее устранить ошибку.

Журналирование является основным источником информации о работе системы и ее ошибках. В этом кратком руководстве рассмотрим основные аспекты журналирования операционной системы, структуру каталогов, программы для чтения и обзора логов.

Основные лог файлы

Все файлы журналов, можно отнести к одной из следующих категорий:

Большинство же лог файлов содержится в директории /var/log .

  • /var/log/syslog или /var/log/messages содержит глобальный системный журнал, в котором пишутся сообщения с момента запуска системы, от ядра Linux, различных служб, обнаруженных устройствах, сетевых интерфейсов и много другого.
  • /var/log/auth.log или /var/log/secure — информация об авторизации пользователей, включая удачные и неудачные попытки входа в систему, а также задействованные механизмы аутентификации.
  • /var/log/dmesg — драйвера устройств. Одноименной командой можно просмотреть вывод содержимого файла. Размер журнала ограничен, когда файл достигнет своего предела, старые сообщения будут перезаписаны более новыми. Задав ключ —level= можно отфильтровать вывод по критерию значимости.
  • /var/log/alternatives.log — Вывод программы update-alternatives , в котором находятся символические ссылки на команды или библиотеки по умолчанию.
  • /var/log/anaconda.log — Записи, зарегистрированные во время установки системы.
  • /var/log/audit — Записи, созданные службой аудита auditd .
  • /var/log/boot.log — Информация, которая пишется при загрузке операционной системы.
  • /var/log/cron — Отчет службы crond об исполняемых командах и сообщения от самих команд.
  • /var/log/cups — Все, что связано с печатью и принтерами.
  • /var/log/faillog — Неудачные попытки входа в систему. Очень полезно при проверке угроз в системе безопасности, хакерских атаках, попыток взлома методом перебора. Прочитать содержимое можно с помощью команды faillog .
  • var/log/kern.log — Журнал содержит сообщения от ядра и предупреждения, которые могут быть полезны при устранении ошибок пользовательских модулей встроенных в ядро.
  • /var/log/maillog/ или /var/log/mail.log — Журнал почтового сервера, используемого на ОС.
  • /var/log/pm-powersave.log — Сообщения службы экономии заряда батареи.
  • /var/log/samba/ — Логи файлового сервера Samba , который используется для доступа к общим папкам Windows и предоставления доступа пользователям Windows к общим папкам Linux.
  • /var/log/spooler — Для представителей старой школы, содержит сообщения USENET. Чаще всего бывает пустым и заброшенным.
  • /var/log/Xorg.0.log — Логи X сервера. Чаще всего бесполезны, но если в них есть строки начинающиеся с EE, то следует обратить на них внимание.
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Для каждого дистрибутива будет отдельный журнал менеджера пакетов.

  • /var/log/yum.log — Для программ установленных с помощью Yum в RedHat Linux.
  • /var/log/emerge.log — Для ebuild -ов установленных из Portage с помощью emerge в Gentoo Linux.
  • /var/log/dpkg.log — Для программ установленных с помощью dpkg в Debian Linux и всем семействе родственных дистрибутивах.

И немного бинарных журналов учета пользовательских сессий.

  • /var/log/lastlog — Последняя сессия пользователей. Прочитать можно командой last .
  • /var/log/tallylog — Аудит неудачных попыток входа в систему. Вывод на экран с помощью утилиты pam_tally2 .
  • /var/log/btmp — Еже один журнал записи неудачных попыток входа в систему. Просто так, на всякий случай, если вы еще не догадались где следует искать следы активности взломщиков.
  • /var/log/utmp — Список входов пользователей в систему на данный момент.
  • /var/log/wtmp — Еще один журнал записи входа пользователей в систему. Вывод на экран командой utmpdump .

И другие журналы

Так как операционная система, даже такая замечательная как Linux, сама по себе никакой ощутимой пользы не несет в себе, то скорее всего на сервере или рабочей станции будет крутится база данных, веб сервер, разнообразные приложения. Каждое приложения или служба может иметь свой собственный файл или каталог журналов событий и ошибок. Всех их естественно невозможно перечислить, лишь некоторые.

  • /var/log/mysql/ — Лог базы данных MySQL.
  • /var/log/httpd/ или /var/log/apache2/ — Лог веб сервера Apache, журнал доступа находится в access_log , а ошибки — в error_log .
  • /var/log/lighthttpd/ — Лог веб сервера lighttpd.

В домашнем каталоге пользователя могут находится журналы графических приложений, DE.

/.xsession-errors — Вывод stderr графических приложений X11.

/.xfce4-session.verbose-log — Сообщения рабочего стола XFCE4.

Чем просматривать — lnav

Почти все знают об утилите less и команде tail -f . Также для этих целей сгодится редактор vim и файловый менеджер Midnight Commander. У всех есть свои недостатки: less неважно обрабатывает журналы с длинными строками, принимая их за бинарники. Midnight Commander годится только для беглого просмотра, когда нет необходимости искать по сложному шаблону и переходить помногу взад и вперед между совпадениями. Редактор vim понимает и подсвечивает синтаксис множества форматов, но если журнал часто обновляется, то появляются отвлекающие внимания сообщения об изменениях в файле. Впрочем это легко можно обойти с помощью .

Недавно я обнаружил еще одну годную и многообещающую, но слегка еще сыроватую, утилиту — lnav, в расшифровке Log File Navigator.

Установка пакета как обычно одной командой.

Навигатор журналов lnav понимает ряд форматов файлов.

  • Access_log веб сервера.
  • CUPS page_log
  • Syslog
  • glog
  • dpkg.log
  • strace
  • Произвольные записи с временными отметками
  • gzip, bzip
  • Журнал VMWare ESXi/vCenter

Что в данном случае означает понимание форматов файлов? Фокус в том, что lnav больше чем утилита для просмотра текстовых файлов. Программа умеет кое что еще. Можно открывать несколько файлов сразу и переключаться между ними.

Программа умеет напрямую открывать архивный файл.

Показывает гистограмму информативных сообщений, предупреждений и ошибок, если нажать клавишу . Это с моего syslog-а.

Кроме этого поддерживается подсветка синтаксиса, дополнение по табу и разные полезности в статусной строке. К недостаткам можно отнести нестабильность поведения и зависания. Надеюсь lnav будет активно развиваться, очень полезная программа на мой взгляд.

Источник

Ubuntu Documentation

Needs Expansion
This article is incomplete, and needs to be expanded. More info.

Introduction

One of the things which makes GNU/Linux a great operating system is that virtually anything and everything happening on and to the system may be logged in some manner. This information is invaluable for using the system in an informed manner, and should be one of the first resources you use to trouble-shoot system and application issues. The logs can tell you almost anything you need to know, as long as you have an idea where to look first.

Your Ubuntu system provides vital information using various system log files. These log files are typically plain ASCII text in a standard log file format, and most of them sit in the traditional system log subdirectory /var/log. Many are generated by the system log daemon, syslogd on behalf of the system and certain applications, while some applications generate their own logs by writing directly to files in /var/log.

This guide talks about how to read and use several of these system log files, how to use and configure the system logging daemon, syslogd, and how log rotation works. See the Resources section for additional information.

Target Audience

This guide will be simple enough to use if you have any experience using the console and editing text files using a text editor. See the end of this document for some essential commands that may help you find your way around these files if you’re relatively new to the command line.

System Logs

System logs deal primarily with the functioning of the Ubuntu system, not necessarily with additional applications added by users. Examples include authorization mechanisms, system daemons, system messages, and the all-encompassing system log itself, syslog.

Authorization Log

The Authorization Log tracks usage of authorization systems, the mechanisms for authorizing users which prompt for user passwords, such as the Pluggable Authentication Module (PAM) system, the sudo command, remote logins to sshd and so on. The Authorization Log file may be accessed at /var/log/auth.log. This log is useful for learning about user logins and usage of the sudo command.

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Use grep to cut down on the volume. For example, to see only information in the Authorization Log pertaining to sshd logins, use this:

Daemon Log

A daemon is a program that runs in the background, generally without human intervention, performing some operation important to the proper running of your system. The daemon log at /var/log/daemon.log and contains information about running system and application daemons such as the Gnome Display Manager daemon gdm, the Bluetooth HCI daemon hcid, or the MySQL database daemon mysqld. This can help you trouble-shoot problems with a particular daemon.

Again, use grep to find specific information, plugging in the name of the daemon you’re interested in.

Debug Log

The debug log at /var/log/debug and provides detailed debug messages from the Ubuntu system and applications which log to syslogd at the DEBUG level.

Kernel Log

The kernel log at /var/log/kern.log provides a detailed log of messages from the Ubuntu Linux kernel. These messages may prove useful for trouble-shooting a new or custom-built kernel, for example.

Kernel Ring Buffer

The kernel ring buffer is not really a log file per se, but rather an area in the running kernel you can query for kernel bootup messages via the dmesg utility. To see the messages, use this:

Or to search for lines that mention the Plug & Play system, for example, use grep like this:

By default, the system initialization script /etc/init.d/bootmisc.sh sends all bootup messages to the file /var/log/dmesg as well. You can view and search this file the usual way.

System Log

The system log typically contains the greatest deal of information by default about your Ubuntu system. It is located at /var/log/syslog, and may contain information other logs do not. Consult the System Log when you can’t locate the desired log information in another log. It also contains everything that used to be in /var/log/messages.

Application Logs

Many applications also create logs in /var/log. If you list the contents of your /var/log subdirectory, you will see familiar names, such as /var/log/apache2 representing the logs for the Apache 2 web server, or /var/log/samba, which contains the logs for the Samba server. This section of the guide introduces some specific examples of application logs, and information contained within them.

Apache HTTP Server Logs

The default installation for Apache2 on Ubuntu creates a log subdirectory: /var/log/apache2. Within this subdirectory are two log files with two distinct purposes:

/var/log/apache2/access.log — records of every page served and every file loaded by the web server.

/var/log/apache2/error.log — records of all error conditions reported by the HTTP server

By default, every time Apache accesses a file or page, the access logs record the IP address, time and date, browser identification string, HTTP result code and the text of the actual query, which will generally be a GET for a page view. Look at the Apache documentation for a complete rundown; quite a lot can be gleaned from this file, and indeed many statistical packages exist that perform analyses of these logs.

Also, every time any error occurs, Apache adds a line to the error log. If you run PHP with error and warning messages disabled, this can be your only way to identify bugs.

CUPS Print System Logs

The Common Unix Printing System (CUPS) uses the default log file /var/log/cups/error_log to store informational and error messages. If you need to solve a printing issue in Ubuntu, this log may be a good place to start.

Rootkit Hunter Log

The Rootkit Hunter utility (rkhunter) checks your Ubuntu system for backdoors, sniffers and rootkits, which are all signs of compromise of your system. The log rkhunter uses is located at /var/log/rkhunter.log.

Samba SMB Server Logs

The Server Message Block Protocol (SMB) server, Samba is popularly used for sharing files between your Ubuntu computer and other computers which support the SMB protocol. Samba keeps three distinct types of logs in the subdirectory /var/log/samba:

log.nmbd — messages related to Samba’s NETBIOS over IP functionality (the network stuff)

log.smbd — messages related to Samba’s SMB/CIFS functionality (the file and print sharing stuff)

log.[IP_ADDRESS] — messages related to requests for services from the IP address contained in the log file name, for example, log.192.168.1.1.

X11 Server Log

The default X11 Windowing Server in use with Ubuntu is the Xorg X11 server, and assuming your computer has only one display defined, it stores log messages in the file /var/log/Xorg.0.log. This log is helpful for diagnosing issues with your X11 environment.

Non-Human-Readable Logs

Some log files found in the /var/log subdirectory are designed to be readable by applications, not necessarily by humans. Some examples of such log files which appear in /var/log follow.

Login Failures Log

The login failures log located at /var/log/faillog is actually designed to be parsed and displayed by the faillog command. For example, to print recent login failures, use this:

Last Logins Log

The last logins log at /var/log/lastlog should not typically be parsed and examined by humans, but rather should be used in conjunction with the lastlog command. For example to see a listing of logins with the lastlog command, displayed one page per screen with the less command, use the following command:

Login Records Log

The file /var/log/wtmp contains login records, but unlike /var/log/lastlog above, /var/log/wtmp is not used to show a list of recent logins, but is instead used by other utilities such as the who command to present a listed of currently logged in users. This command will show the users currently logged in to your machine:

System Logging Daemon (syslogd)

The system logging daemon syslogd, also known as sysklogd, awaits logging messages from numerous sources and routes the messages to the appropriate file or network destination. Messages logged to syslogd usually contain common elements like system hostnames and time-stamps in addition to the specific log information.

Configuration of syslogd

The syslogd daemon’s configuration file is /etc/syslog.conf. Each entry in this file consists of two fields, the selector and the action. The selector field specifies a facility to be logged, such as for example the auth facility which deals with authorization, and a priority level to log such information at, such as info, or warning. The action field consists of a target for the log information, such as a standard log file (i.e. /var/log/syslog), or the hostname of a remote computer to send the log information to.

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Echoing Messages to syslogd With Logger

A neat utility exists in the logger tool, which allows one to place messages into the System Log (i.e. /var/log/syslog) arbitrarily. For example, assume your user name is buddha, and you would like to enter a message into the syslog about a particularly delicious pizza you’re eating, you could use a command such as the following at a terminal prompt:

logger This Pizza from Vinnys Gourmet Rocks

and you would end up with a line in the /var/log/syslog file like this:

You can even specify a tag the messages come from, and redirect the output standard error too.

Executing this script as chkdir.sh on the machine butters where Fred does not have a home directory, /home/fred, gives the following results:

So, as you can see, we received the messages both via standard error, at the terminal prompt, and they also appear in our syslog.

Log Rotation

When viewing directory listings in /var/log or any of its subdirectories, you may encounter log files with names such as daemon.log.0, daemon.log.1.gz, and so on. What are these log files? They are ‘rotated’ log files. That is, they have automatically been renamed after a predefined time-frame, and a new original log started. After even more time the log files are compressed with the gzip utility as in the case of the example daemon.log.1.gz. The purpose of log rotation is to archive and compress old logs so that they consume less disk space, but are still available for inspection as needed. What handles this functionality? Why, the logrotate command of course! Typically, logrotate is called from the system-wide cron script /etc/cron.daily/logrotate, and further defined by the configuration file /etc/logrotate.conf. Individual configuration files can be added into /etc/logrotate.d (where the apache2 and mysql configurations are stored for example).

This guide will not cover the myriad of ways logrotate may be configured to handle the automatic rotation of any log file on your Ubuntu system. For more detail, check the Resources section of this guide.

NOTE: You may also rotate system log files via the cron.daily script /etc/cron.daily/sysklogd instead of using logrotate. Actually, the utility savelog may produce unexpected results on log rotation which configuring logrotate seems to have no effect on. In those cases, you should check the cron.daily sysklogd script in /etc/cron.daily/sysklogd and read the savelog manual page to see if savelog is not in fact doing the rotation in a way that is not what you are specifying with logrotate.

Essential Commands

If you’re new to the console and the Linux command line, these commands will get you up and running to the point where you can work with log files at a basic level.

Getting Started

To change to the log directory, where most of these files sit, use the cd command. This saves having to type out a full path name for every subsequent command:

Editing Files

You can view and edit files in GEdit or Kate, the simple text editors that come with Ubuntu and Kubuntu respectively, but these can be overkill when all you want to do is look at a file or make simple changes. The easiest editor to use from the console is nano, which is less powerful but also less complicated than vim or emacs. The command to edit a particular logfile /var/log/example.log using nano is:

Press Ctrl+X to exit. It will ask if you want to save your changes when you exit, but unless you run it with the sudo command the files won’t be writable. In general, you won’t want to save your changes to log files, of course.

Viewing Files

To simply look at a file, an editor is overkill. Use the less command, which pages through a file one screen at a time:

You don’t need sudo to look at a file. Press h for help, or q to quit. The cursor keys and page up/down keys will work as expected, and the slash key («/») will do a case-sensitive search; the n key repeats the last search.

Viewing the Beginning of Files

To see the first ten lines of a file, use the head command:

To see some other number of lines from the beginning of the file, add the -n switch, thus:

head -n 20 example.log

Viewing the End of Files

To see the final ten lines of a file, the analogous command is tail:

Again, the -n switch gives you control over how many lines it displays:

tail -n 20 example.log

Watching a Changing File

Also, the -f («follow») switch puts tail into a loop, constantly waiting for new additions to the file it’s displaying. This is useful for monitoring files that are being updated in real time:

Press Ctrl+C to quit the loop.

Searching Files

Because log files can be large and unwieldy, it helps to be able to focus. The grep command helps you strip out only the content you care about. To find all the lines in a file containing the word «system», for example, use this:

To find all the lines containing «system» at the beginning of the line, use this:

Note the caret symbol, a regular expression that matches only the start of a line. This is less useful for standard log files, which always start with a date and time, but it can be handy otherwise. Not all files have a standard format.

Any time the result of a grep is still too long, you can pipe it through less:

Resources

Additional information on system and application logs and syslogd is available via the following resources:

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