SSH allows the creation of tunnels through which data of any type of service will be sent. The syntax for opening a tunnel is: ssh -L Port_Local: Destination_Direction: Destination_port

The .xsession-errors file is where the X Window system logs all errors that occur in the Linux graphical environment. Logs are logged in each user‘s $ HOME, so ~/.xsession-errors.

The X Display Manager Control Protocol (XDMCP), which is implemented by the display manager. The protocol is simple, with no compression or encryption capabilities.

The default shell is indicated in the last field of each record in the /etc/passwd file. In general, /bin/bash is used, in addition to other shells, another option is to use / bin / false to prevent a user from logging into the system.

CMUSphinx, Simon (from the KDE project) and Julius are some of the many open source speech recognition solutions that are in development.

A series of environment variables initiated by LC_ define the location settings for various data types, including LC_TIME for date and time, LC_MESSAGES for system informational messages, LC_MONETARY for currency units, and so on.

The /etc/cron.allow file is the first one to read, if a user is listed in cron.allow its usage is freed, regardless of whether it is also listed in cron.deny. The empty cron.allow file indicates that no user can use the service. The empty cron.deny file indicates that all users can use the service. If both files are blank, no user can use the feature. If neither file exists, all users can schedule tasks with cron.

The root user must use the crontab command together with the -u option to enter the user name and the -r option to remove.

In the /var/spool/cron directory there will be a text file with the name of each user containing the schedules. In the case of Debian-based systems, the directory will be more specifically /var/spool/cron /crontab/

The at command allows a time to be reported as 1600 or 16:00. The at also allows the use of words like now, noon, midnight and teatime (4p.m.).

The groupmod command can change both the GID and the name of an existing group. On Linux, every user must be associated with at least one group.

By default the userdel does not remove the user‘s default directory, in order to do so the -r option must be used.

The -l option of the chage command displays various information about the user‘s login properties, extracted from the /etc/shadow file. The command “passwd -S“ also returns the information.

All files and sub-directories present in the /etc/skell/ directory will be copied to the new user‘s home directory.

The gpg-agent is started automatically by the gpg processes, the first execution access to the private keys is requested, but after that the information is kept in memory.

The Desktop Environment groups a set of applications, including the window manager, the display manager, and the graphical interface or GUI.

The basis of SSH communication via public and private key is to understand that the public key is shared with another user who will use it to encrypt the data before sending it. The private key must never be shared and it will be used to decrypt the data that was encrypted with the equivalent public key.

The above configuration must be performed on the SSH Server, ie on sshd, so it must be done in the /etc/ssh/sshd_config file. SSH client-related settings must be made in the /etc/ssh/ssh_config file.

When the /etc/nologin file exists, no user, except for root, can log in to the system, when the attempt is made the message written to the file is returned to the user.

The ListenStream option specifies an address to listen to a stream of data, usually a port, but the IP can also be specified.

Because the /etc/shadow file has user password information, read access is restricted to the root user only.

The ulimit command defines in the current session limits for use of the machine resources such as maximum size of files, open files, process usage, CPU utilization, memory, and so on.

The above configuration must be done by the sudo tool, so the /etc/sudoers file must be configured by vi, or simply use the “visudo“ command.

Permission code 4000 is equivalent to SUID and 2000 to SGID. In the exam it is common questions about searching the files that do not have any associated user or group, in this case the find must be used as follows: find / \( -nouser -o -nogroup \)

In the host command, the IP of the DNS server to be used must be informed after the address, according to the syntax of the “c“ option. In the command dig, the @ should be used to indicate the IP of the DNS server, according to the syntax of option “b“.

This setting indicates that the information contained in the /etc/hosts file is queried before the search is sent to the DNS server.

When using “ip address add“ the IP will be added in the interface, keeping the previous IPs if they exist.

In both ss and netstat, the “l“ option displays only “listen“ connections, the “t“ option restricts the TCP protocol, and “n“ prevents name resolution. The “a“ option displays all types of connection states.

The netcat or nc commands (the same command) can perform a variety of activities, including connecting to a specific IP and port. The telnet command can also make this connection, but the syntax entered is incorrect, it should be: telnet 172.16.12.1 1234

The route command is dedicated to viewing and changing machine routes, the -n option disables name resolution. The command has several functions, among them also to view and manipulate the routing table. The main function of the netstat command is to analyze the established connections, however the -r option displays the routes of the machine in a very similar way to the route -n.

Tracks reserved for Private IPs are: 10.0.0.0 – 10.255.255.255, 172.16.0.0 – 172.31.255.255, 192.168.0.0 – 192.168.255.255 e 169.254.0.0 – 169.254.255.255

In the /usr/share/zoneinfo/directory, the time zone and daylight saving time settings files for all possible locations are found. The /etc/localtime file is usually a link to some of these files.

The hwclock command allows the administrator to configure system time and hardware time (BIOS). You can also set a time based on the other time.

The /etc/ntp.conf file contains the main parameters of the ntpd process, including the servers that will be used as the source of the time data.

By using the “ntpdate pool.ntp.org“ command the system date and time will be immediately updated based on the pool.ntp.org server/pool data.

Rsyslog provides facilities from local0 to local7 to be used at the discretion of the system administrator.

Logrotate is a crontab program that enables you to control logs generated by the system, with rotation, compression and removal of old logs.

The systemd service has a specific log record called the journal, and the journalctl command is the tool specifically designed to query these logs.

SystemMaxUse sets the maximum usage limit for persistent journal logs on the partition, normally stored in the /var/log/journal directory. The default value is 10%.

The main MTA (Mail Transfer Agents) or simply mail servers used in Linux are: Sendmail, Postfix, Exim and qmail. Postgree is a relational database, Procmail is a utility used in conjunction with an MTA to filter emails, make checksums, antispam, and so on.

Every user can create and configure the .forward file in their default directory and indicate other email to which the emails they receive will be sent to their local account.

When you end a bash session, all settings and commands entered in the .bash_logout file will be executed, for example, removing a temporary file or a log record.

The /etc/cups/printers.conf file contains the printers added to the cups and their properties. The /etc/cups/cupsd.conf file is the main and general CUPS configuration file.

The lpinfo command is used to display the devices and drivers supported by the internal base of the cups. The -v option shows all currently supported devices and protocols and the -m option for all supported drivers.

The mask 255.255.255.192 is represented as /26 in the CIDR format and the binary representation is 11111111.11111111.11111111.11000000. The mask 255.255.255.128 uses CIDR /25 and 255.255.255.248 is equal to /29.

In the /etc/aliases file it is possible to create “nicknames“ for internal accounts, that is, to indicate other names that will be interpreted by the local mail server as internal accounts and that in practice will be sent to any of the existing users. After any changes to this file the command “newaliases“ must be executed.

Simple Network Management Protocol (SNMP) uses port 161. 25 = SMTP, 123 = NTP and 465 = SMTPS.

The TCP protocol is connection oriented and the protocol itself ensures that packets will only be delivered if they are integers and in the correct order. The UDP protocol does not do this control. The ICMP protocol is used to control networks and machines and does not transmit data. FTP is a file transfer protocol.

The /etc/hosts file will have a mapping between IPs and names that are normally used internally. The nsswitch.conf file will indicate whether the /etc/hosts file should be queried before or after the DNS service.

The hostnamectl command with the set-hostname option will change the contents of the /etc/hostname file, which defines the hostname of the machine. The hostname command currently changes the machine name, but the /etc/hostname file does not change.

In the nmcli tool, every operation based on the interfaces must be performed with the option “device“, or “dev“ or “d“. The show option displays the details of all or just the specified interfaces.

In the /etc/NetworkManager/system-connections/ directory there is a file for each connection already established in the system, so that you can “remember“ the properties and characteristics already used in that network.

The “locale“ command will display all current values for LANG, LANGUAGE and LC_*

The “source“ command allows the user to indicate a file/script that will be executed in the current shell session, not in a sub-session, using “. script.sh“ has the same effect.

When exporting a variable with the “export“ command, all sub-shells begin to see this variable, then it is said to be a global variable. Note that in the use of export the variable should not be entered with $.

In the declaration of a function the word “function“ can be used or not, it is optional. After the function name there should be () and after that the command sequence between {}, each command ending with ;

The first line of a shell script should tell which shell to use to interpret the script, and this is done by the #! Indicator, also known as shebang.

$# returns the number of parameters sent by the user at runtime. $? is the return code of the last executed command, $$ the PID of the current script/process, and $* returns all the arguments given.

The if is terminated by the statement “fi“, The case is terminated by “esac“. while, for, and until are closed by a “done“

The “seq“ syntax is: “seq “, or “seq “, or “seq “. Will always be generated one number per line.