System - VBrazhnik/init GitHub Wiki

Answer

/etc/debian_version

Answer

hostname new_name

Explanation

man hostname:

Hostname is used to display the system's DNS name, and to display or set its hostname or NIS domain name.

Answer

/etc/hostname

Answer

uptime

Explanation

man uptime:

uptime gives a one line display of the following information. The current time, how long the system has been running, how many users are currently logged on, and the system load averages for the past 1, 5, and 15 minutes.

Answer

service ssh status

Tip

Install SSH server:

apt-get install openssh-server

Answer

service ssh restart

Answer

cat /var/run/sshd.pid

Answer

~/.ssh/authorized_keys

Explanation

The file contains public keys for public key authentication.

Answer

who

Explanation

man who:

Print information about users who are currently logged in.

Answer

fdisk -l

Explanation

man fdisk:

-l, --list — List the partition tables for the specified devices and then exit. If no devices are given, those mentioned in /proc/partitions (if that exists) are used.

Answer

df -h --total

Explanation

man df:

df — report file system disk space usage.

-h, --human-readable — Print sizes in powers of 1024 (e.g., 1023M).

--total — Elide all entries insignificant to available space, and produce a grand total.

Answer

du -sh /var/*

Explanation

du — estimate file space usage.

Summarize disk usage of each FILE, recursively for directories.

-s, --summarize — Display only a total for each argument.

-h, --human-readable — Print sizes in human readable format (e.g., 1K 234M 2G).

Alternative answers

du -hd0 /var/*

du -h --max-depth=0 /var/*

Explanation

-d, --max-depth=N — Print the total for a directory (or file, with --all) only if it is N or fewer levels below the command line argument; --max-depth=0 is the same as --summarize.

Answer

ps aux

Answer

tail -f /var/log/syslog &

Explanation

Running Bash Commands in the Background the Right Way [Linux]:

If you want to push a command into the background, using & at the end is an easy way to do that. It comes with a catch, though. Using & doesn’t disconnect the command away from you; it just pushes it in the background so you can continue using a terminal.

command &

When the terminal session is closed, the command ends. Using & is good if you need to push something off for a bit, but don’t expect it to continue forever.

Answer

pkill tail

Explanation

The pkill command allows to kill a program by specifying the full name or partial name.

Alternative answer

kill %<job number>

Explanation

Instead of <job number> we need to place job number of process, which we want to kill.

Alternative answer

kill <PID>

Explanation

How do I run a Unix process in the background?:

You can kill a background process by entering:

kill PID

Replace PID with the process ID of the job. If that fails, enter the following:

kill -KILL PID

kill(1) - Linux man page:

The command kill sends the specified signal to the specified process or process group. If no signal is specified, the TERM signal is sent. The TERM signal will kill processes which do not catch this signal. For other processes, it may be necessary to use the KILL (9) signal, since this signal cannot be caught.

If we want to do it this way, first of all we need to know PID of process. jobs -l will help us.

Linux 2.6 - man page for jobs (linux section 1):

jobs — display status of jobs in the current session.

-l — Provide more information about each job listed. This information shall include the job number, current job, process group ID, state, and the command that formed the job.

Answer

cron

Explanation

man cron:

cron — daemon to execute scheduled commands.

Answer

iptables -L

Note

You must be root to run this command.

Explanation

iptables(8) - Linux man page:

iptables — administration tool for IPv4 packet filtering and NAT.

-L, --list [chain] — List all rules in the selected chain. If no chain is selected, all chains are listed.

Types of chains

The Beginner’s Guide to iptables, the Linux Firewall:

iptables uses three different chains: input, forward, and output.

Input – This chain is used to control the behavior for incoming connections. For example, if a user attempts to SSH into your PC/server, iptables will attempt to match the IP address and port to a rule in the input chain.

Forward – This chain is used for incoming connections that aren’t actually being delivered locally. Think of a router – data is always being sent to it but rarely actually destined for the router itself; the data is just forwarded to its target. Unless you’re doing some kind of routing, NATing, or something else on your system that requires forwarding, you won’t even use this chain.

Output – This chain is used for outgoing connections. For example, if you try to ping howtogeek.com, iptables will check its output chain to see what the rules are regarding ping and howtogeek.com before making a decision to allow or deny the connection attempt.

Even though pinging an external host seems like something that would only need to traverse the output chain, keep in mind that to return the data, the input chain will be used as well. When using iptables to lock down your system, remember that a lot of protocols will require two-way communication, so both the input and output chains will need to be configured properly. SSH is a common protocol that people forget to allow on both chains.

Policy chain default behavior

More times than not, you’ll want your system to accept connections by default. Unless you’ve changed the policy chain rules previously, this setting should already be configured. Either way, here’s the command to accept connections by default:

iptables --policy INPUT ACCEPT
iptables --policy OUTPUT ACCEPT
iptables --policy FORWARD ACCEPT

If you would rather deny all connections and manually specify which ones you want to allow to connect, you should change the default policy of your chains to drop. Doing this would probably only be useful for servers that contain sensitive information and only ever have the same IP addresses connect to them.

iptables --policy INPUT DROP
iptables --policy OUTPUT DROP
iptables --policy FORWARD DROP

Connection-specific responses

Accept – Allow the connection.

Drop – Drop the connection, act like it never happened. This is best if you don’t want the source to realize your system exists.

Reject – Don’t allow the connection, but send back an error. This is best if you don’t want a particular source to connect to your system, but you want them to know that your firewall blocked them.

Answer

iptables -A INPUT -s 10.0.0.0/8 -j ACCEPT
iptables -A OUTPUT -d 10.0.0.0/8 -j ACCEPT

Explanation

man iptables:

-A, --append chain rule-specification — Append one or more rules to the end of the selected chain. When the source and/or destination names resolve to more than one address, a rule will be added for each possible address combination.

-j, --jump target — This specifies the target of the rule; i.e., what to do if the packet matches it. The target can be a user-defined chain (other than the one this rule is in), one of the special builtin targets which decide the fate of the packet immediately, or an extension (see EXTENSIONS below). If this option is omitted in a rule (and -g is not used), then matching the rule will have no effect on the packet's fate, but the counters on the rule will be incremented.

[!] -s, --source address[/mask][,...] — Source specification. Address can be either a network name, a hostname, a network IP address (with /mask), or a plain IP address. Hostnames will be resolved once only, before the rule is submitted to the kernel. Please note that specifying any name to be resolved with a remote query such as DNS is a really bad idea. The mask can be either an ipv4 network mask (for iptables) or a plain number, specifying the number of 1's at the left side of the network mask. Thus, an iptables mask of 24 is equivalent to 255.255.255.0. A "!" argument before the address specification inverts the sense of the address. The flag --src is an alias for this option. Multiple addresses can be specified, but this will expand to multiple rules (when adding with -A), or will cause multiple rules to be deleted (with -D).

[!] -d, --destination address[/mask][,...] — Destination specification. See the description of the -s(source) flag for a detailed description of the syntax. The flag --dst is an alias for this option.

Answer

iptables -P INPUT DROP
iptables -P OUTPUT DROP

Explanation

man iptables:

-P, --policy chain target — Set the policy for the built-in (non-user-defined) chain to the given target. The policy target must be either ACCEPT or DROP.