Linux utils

Linux utils

Table of Contents

• 1. ======= useful command ==========
• 2. ======= git ==========
• 3. ======= remote develop ==========
• 4. FORWARD and NAT Rules
  • 4.1. DMZs and iptables
• 5. Utility tools
  • 5.1. Check Disk Space with NCurses Disk Usage

1. ======= useful command ==========

• fzf is a general-purpose command-line fuzzy finder

• find all files containing specific text

  • Basic

    grep -rnw '/path/to/somewhere/' -e 'pattern'

  • This will only search through those files which have .c or .h extensions:

    grep --include=\*.{c,h} -rnw '/path/to/somewhere/' -e "pattern"

  • This will exclude searching all the files ending with .o extension:

    grep --exclude=\*.o -rnw '/path/to/somewhere/' -e "pattern"

  • For directories it’s possible to exclude one or more directories using the --exclude-dir parameter. For example, this will exclude the dirs dir1/, dir2/ and all of them matching *.dst/:

    grep --exclude-dir={dir1,dir2,*.dst} -rnw '/path/to/somewhere/' -e "pattern"

2. ======= git ==========

$ git clean -fd
git clean -f
git apply --whitespace=nowarn  ~/br_lge_extensions.patch
git reset --hard HEAD
git clone --recursive

3. ======= remote develop ==========

• Use ssh without password

  $ssh-keygen -t rsa
  $ssh-copy-id -i ~/.ssh/id_rsa.pub {user}@{host}

• Download a single file from a remote ftp server to your machine:

  $sftp {user}@{host}:{remoteFileName} {localFileName}

• Upload a single file from your machine to a remote ftp server:

  $sftp {user}@{host}:{remote_dir} <<< $'put {local_file_path}'

4. FORWARD and NAT Rules

The FORWARD policy allows an administrator to control where packets can be routed within a LAN. For example, to allow forwarding for the entire LAN (assuming the firewall/gateway is assigned an internal IP address on eth1), the following rules can be set:

$ iptables -A FORWARD -i eth1 -j ACCEPT
$ iptables -A FORWARD -o eth1 -j ACCEPT

ℹ️

By default, the IPv4 policy in Linux kernels disables support for IP forwarding, which prevents boxes running Linux from functioning as dedicated edge routers. To enable IP forwarding, run the following command: $ sysctl -w net.ipv4.ip_forward=1 If this command is run via shell prompt, then the setting is not remembered after a reboot. You can permanently set forwarding by editing the /etc/sysctl.conf file. Find and edit the following line, replacing 0 with 1: net.ipv4.ip_forward = 0 Execute the following command to enable the change to the sysctl.conf file: $ sysctl -p /etc/sysctl.conf

Accepting forwarded packets via the firewall’s internal IP device allows LAN nodes to communicate with each other; however they still are not allowed to communicate externally to the Internet. To allow LAN nodes with private IP addresses to communicate with external public networks, configure the firewall for IP masquerading, which masks requests from LAN nodes with the IP address of the firewall’s external device (in this case, eth0):

 $ iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE 

The rule uses the NAT packet matching table (-t nat) and specifies the built-in POSTROUTING chain for NAT (-A POSTROUTING) on the firewall’s external networking device (-o eth0). POSTROUTING allows packets to be altered as they are leaving the firewall’s external device. The -j MASQUERADE target is specified to mask the private IP address of a node with the external IP address of the firewall/gateway.

If you have a server on your internal network that you want make available externally, you can use the -j DNAT target of the PREROUTING chain in NAT to specify a destination IP address and port where incoming packets requesting a connection to your internal service can be forwarded. For example, if you wanted to forward incoming HTTP requests to your dedicated Apache HTTP Server server system at 172.31.0.23, run the following command:

$ iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j DNAT \
      --to 172.31.0.23:80

This rule specifies that the NAT table use the built-in PREROUTING chain to forward incoming HTTP requests exclusively to the listed destination IP address of 172.31.0.23.

ℹ️	Note If you have a default policy of DROP in your FORWARD chain, you must append a rule to allow forwarding of incoming HTTP requests so that destination NAT routing can be possible. To do this, run the following command: iptables -A FORWARD -i eth0 -p tcp --dport 80 -d 172.31.0.23 -j ACCEPT

This rule allows forwarding of incoming HTTP requests from the firewall to its intended destination of the Apache HTTP Server server behind the firewall.

4.1. DMZs and iptables

iptables rules can be set to route traffic to certain machines, such as a dedicated HTTP or FTP server, in a demilitarized zone (DMZ) — a special local subnetwork dedicated to providing services on a public carrier such as the Internet. For example, to set a rule for routing incoming HTTP requests to a dedicated HTTP server at 10.0.4.2 (outside of the 192.168.1.0/24 range of the LAN), NAT calls a PREROUTING table to forward the packets to their proper destination:

iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j DNAT \ --to-destination 10.0.4.2:80

With this command, all HTTP connections to port 80 from the outside of the LAN are routed to the HTTP server on a separate network from the rest of the internal network. This form of network segmentation can prove safer than allowing HTTP connections to a machine on the network. If the HTTP server is configured to accept secure connections, then port 443 must be forwarded as well.

5. Utility tools

5.1. Check Disk Space with NCurses Disk Usage

• Homepage: https://dev.yorhel.nl/ncdu

• Install

  apt-get install ncdu

• Run

  ncdu -x /