Listing of important Linux log files and their formats. These logs play a vital role in troubleshooting and every sysadmin should be aware of them.
Troubleshooting any issues with your system needs proper knowledge of log file structure and their locations. As a sysadmin you should know for which log file should be checked for a particular service issue. In this article we will walk through several hand-picked log files which are pretty much helpful for conducting preliminary analysis during troubleshooting. 10 log files we selected to discuss here are :
/var/log/messages: General message and system related stuff
/var/log/boot.log: Services success/failures at boot
/var/log/auth.log: Authentication log
/var/log/wtmp: Login records
/var/log/btmp: Failed login records
/var/log/cron: Scheduler log file
/var/log/maillog: Mail logs
/var/log/xferlog: File transfer logs
/var/log/lastlog: Last login details
dmesg: Device driver messages
/var/crash logs: System crash dump
Lets see each log file one by one.
System consolidated log file : /var/log/messages
All system services which do not have their own special log file, normally write to this log file. Most of the system activity can be seen here hence its also called Syslog (system’s log). Every sysadmin first opens up this log when he starts troubleshooting! Sample log looks like below :
May 22 02:00:29 server1 rsyslogd: [origin software="rsyslogd" swVersion="5.8.10" x-pid="999" x-info="http://www.rsyslog.com"] exiting on signal 15. May 22 02:00:29 server1 kernel: imklog 5.8.10, log source = /proc/kmsg started. May 22 02:00:29 server1 rsyslogd: [origin software="rsyslogd" swVersion="5.8.10" x-pid="1698" x-info="http://www.rsyslog.com"] start May 22 02:17:43 server1 dhclient: DHCPREQUEST on eth0 to 172.31.0.1 port 67 (xid=0x445faedb)
File can be read from left as:
- System hostname
- Service name (and sometimes PID as well)
- Message text
System hostname stands value when you have a centralized Syslog server. Message text and service name help you narrow your search. You can directly grep on this file to find specific service-related logs omitting other clutter. All file operations like
less, etc can be done on this file since its plain text file.
Since this log fills very fast on a busy system (as it logs almost everything), you should consider configuring logrotate for it so that it won’t full your mount point.
Service success/failures at boot : /var/log/boot.log
At the time of booting Linux server, you can see services being started and their success or failure status is displayed on local console. The same logs can be obtained from the boot log post-boot. This file lists all service’s success/failure status at boot time so that it can be referred later to troubleshoot any service-related issues.
$ cat /var/log/boot.log growroot: FAILED: GPT partition found but no sgdisk Welcome to Red Hat Enterprise Linux Server Starting udev: udevd: can not read 'https://z5.kerneltalks.com/etc/udev/rules.d/75-persistent-net-generator.rules' udevd: can not read 'https://z5.kerneltalks.com/etc/udev/rules.d/75-persistent-net-generator.rules' [ OK ] Setting hostname ip-172-31-1-120.ap-south-1.compute.interna[ OK ] Setting up Logical Volume Management: [ OK ] Checking filesystems /dev/xvda1: clean, 77980/393216 files, 811583/1572864 blocks [ OK ] Remounting root filesystem in read-write mode: [ OK ] Mounting local filesystems: [ OK ] Enabling local filesystem quotas: [ OK ] Enabling /etc/fstab swaps: [ OK ] Entering non-interactive startup
The above sample, file shows services, their status (on right), and any error/warning messages written to console (by service daemons).
Authentication logs : /var/log/secure, /var/log/auth.log
This log file is crucial to check user access logs. Log files stores information on user logins along with authentication used. It also stores sudo logs.
Jun 14 23:41:00 server1 sshd: Accepted publickey for ec2-user from 18.104.22.168 port 51265 ssh2 Jun 14 23:41:01 server1 sshd: pam_unix(sshd:session): session opened for user ec2-user by (uid=0) Jun 14 23:41:04 server1 sudo: ec2-user : TTY=pts/0 ; PWD=/home/ec2-user ; USER=root ; COMMAND=/bin/su - Jun 14 23:41:04 server1 su: pam_unix(su-l:session): session opened for user root by ec2-user(uid=0) Jun 14 23:43:45 server1 su: pam_unix(su-l:session): session closed for user root
Log file can be read from left to right as :
- Server hostname
- Authentication service or daemon (sometimes along with PID)
Login records : /var/log/utmp, /var/log/wtmp
These wtmp or utmp file stores user login details. utmp stores login details like id, time, duration, the terminal used, system reboot details, etc. wtmp provides historical utmp data. These files are not plain text files and hence need to be parsed to
last command to read data within. You can use
last -f <path> to read file.
ec2-user pts/1 22.214.171.124 Mon Jun 19 07:24 still logged in ec2-user pts/0 126.96.36.199 Mon Jun 19 07:21 - 07:24 (00:02) reboot system boot 2.6.32-696.el6.x Mon Jun 19 07:20 - 07:39 (00:18) ec2-user pts/0 188.8.131.52 Wed Jun 14 23:41 - 00:09 (00:28) reboot system boot 2.6.32-696.el6.x Wed Jun 14 23:40 - 00:09 (00:29)
In above output of
last -f /var/log/wtmp you can see from left to right :
- User or event (reboot)
- Terminal used
- IP from which user was connected
- Date/time of login
- Date /time of log out
- Duration of session
Failed login records : /var/log/btmp
Its file dedicated to logging only failed login attempts. This too can be read using
last command as mentioned above.
user1 ssh:notty 184.108.40.206 Tue Jun 13 01:18 gone - no logout user ssh:notty 220.127.116.11 Tue Jun 13 01:18 - 01:18 (00:00) ubnt ssh:notty 18.104.22.168 Tue Jun 13 01:18 - 01:18 (00:00)
It also shows user id, terminal, source IP, and time who tried to log into the system but failed to do so.
Scheduler logs : /var/log/cron
Cron (Linux scheduler) logs are saved under
/var/log/cron. Job run status, daemon logs are saved here. It helps in understanding job execution and troubleshooting of scheduled jobs. Its plain text file and supports all normal file operations.
Jun 15 00:09:51 server1 crond: (CRON) INFO (Shutting down) Jun 19 07:21:18 server1 crond: (CRON) STARTUP (1.4.4) Jun 19 07:21:18 server1 crond: (CRON) INFO (RANDOM_DELAY will be scaled with factor 26% if used.) Jun 19 07:21:20 server1 crond: (CRON) INFO (running with inotify support) Jun 19 07:30:01 server1 CROND: (root) CMD (/usr/lib64/sa/sa1 1 1) Jun 19 07:40:01 server1 CROND: (root) CMD (/usr/lib64/sa/sa1 1 1)
Since the server was shut on 15 Jun and started on 19 Jun you can see cron daemon wrote down and up logs in this log file. For job executed, log code CMD is used. Daemon messages are tagged with INFO.
Mail logs : /var/log/maillog
Mail related logs saved here. It logs date-time, hostname, mail protocol (with PID sometimes), and messages.
Jun 15 00:09:51 server1 postfix/postfix-script: stopping the Postfix mail system Jun 15 00:09:51 server1 postfix/master: terminating on signal 15 Jun 19 07:21:17 server1 postfix/postfix-script: starting the Postfix mail system Jun 19 07:21:17 server1 postfix/master: daemon started -- version 2.6.6, configuration /etc/postfix
Since our server doesn’t have an SMTP configuration, there is nothing much in our file shown above.
File transfer logs : /var/log/xferlog
This log contains information from file transfer program/service daemons.
Mon Jun 19 04:44:43 2017 1 10.4.1.22 20 /CSV/test1.csv a _ o r testuser ftp 0 * c Mon Jun 19 04:51:33 2017 1 10.4.1.22 432 /CSV/test4.csv a _ o r testuser ftp 0 * c Mon Jun 19 04:57:15 2017 1 10.4.1.22 110 /CSV/test14.csv a _ o r testuser ftp 0 * c Mon Jun 19 04:57:19 2017 1 10.4.1.22 2505 /CSV/master.csv a _ o r testuser ftp 0 * c
Sample log files hows date, time, source IP, file path copied, options used, the user who copied files, the protocol used details.
Last login details : /var/log/lastlog
System’s all user’s recent login details are saved to this log file. Its not plain text file and hence you need to use
lastlog command which reads data from this log file and print on the terminal in a human-readable format. It sorts users with their order in /etc/passwd file.
# lastlog Username Port From Latest root **Never logged in** bin **Never logged in** oprofile **Never logged in** tcpdump **Never logged in** ec2-user pts/1 22.214.171.124 Mon Jun 19 07:24:14 -0400 2017 apache **Never logged in**
Above output is pretty self explanatory.
Device driver messages : dmesg
Boot time device/driver writes messages on the console. Those messages can be viewed as post boot as well using
dmesg command. These messages help in troubleshooting devices or driver initialization issues.
# dmesg Initializing cgroup subsys cpuset Initializing cgroup subsys cpu Linux version 2.6.32-696.el6.x86_64 (email@example.com) (gcc version 4.4.7 20120313 (Red Hat 4.4.7-18) (GCC) ) #1 SMP Tue Feb 21 00:53:17 EST 2017 Command line: console=ttyS0 ro root=UUID=1b7ea291-67b4-48da-802a-be4b2bcb64d5 rd_NO_LUKS KEYBOARDTYPE=pc KEYTABLE=us LANG=en_US.UTF-8 xen_blkfront.sda_is_xvda=1 console=ttyS0,115200n8 console=tty0 rd_NO_MD SYSFONT=latarcyrheb-sun16 crashkernel=auto rd_NO_LVM rd_NO_DM KERNEL supported cpus: Intel GenuineIntel AMD AuthenticAMD Centaur CentaurHauls BIOS-provided physical RAM map: BIOS-e820: 0000000000000000 - 000000000009e000 (usable) BIOS-e820: 000000000009e000 - 00000000000a0000 (reserved) ----- output trimmed -----
Those are all boot time messages just before /var/log/boot.log
System crash dump : /var/crash
This is the directory under which the core-dump is saved in case of a system crash. Obviously you need to configure dump configs on the system for it. This information is very important to get the root cause of the system crash. This is almost the image of the current state of the system at the time of the crash. Most of the vendor asks this dump files to analyze when we log a case with them for a system crash.
Very informative article. Thanks for taking your time to write it.
Shrikant Lavhate says