==== IP FORWARDING ====

<code bash>
sysctl net.ipv4.ip_forward # check if enabled
sysctl -w net.ipv4.ip_forward=1 # turning on
echo "net.ipv4.ip_forward = 1" >> /etc/sysctl.conf
</code>

==== SCP ====


<code bash>
$ scp username@remotehost:file_name.txt /local/directory     # from remote to local
$ scp file_name.txt username@remotehost:/remote/directory    # from local to remote
$ scp -r dir_name username@remotehost:/remote/directory      # copy the local directory "dir_name" to
                                                             # remote host
$ scp -6 username@\[2001:db8:0:1\]:/home/username/test.file ./test.file # going via ipv6
</code>

==== CURL ====

Uploading data to http server using PUT:
<code bash>
curl -u username -T file.conf http://192.168.0.1
</code>

==== NC ====

Basic TCP test:
<code bash>
nc -vz 10.10.0.1 80     # v - verbose; z - just scan for listening daemon,
                        # without sending any data to them
</code>

==== PROXY ====

Set proxy and show current config:
<code bash>
export http_proxy=http://proxyusername:proxypassword@proxyaddress:proxyport
echo $http_proxy
</code>

To make it permanent, add to //**/etc/environment**//
<code bash>
http_proxy=http://10.0.0.1:3128
</code>

Make it work with "sudo", add following to ///etc/sudoers// after //Defaults env_reset//:
<code>
Defaults env_keep = "http_proxy ftp_proxy"
</code>
==== DIG ====

Sample usage:
<code>
dig @10.0.0.1 ipv6.google.com AAAA +short
</code>

==== IPv4/6 DNS RESOLVER PRECEDENCE  ====

  * Check /etc/gai.conf 

==== SSH PORT FORWARDING ====

Dynamic forwarding, **ssh** will act as a SOCKS server:
<code>
ssh -D 1080 victor@172.16.0.1
</code>

==== IPTABLES ====

Port forwarding:
<code>
sudo iptables -A PREROUTING -t nat -i ens160 -p tcp --dport 1005 -j DNAT --to 192.168.0.5:22
sudo iptables -A FORWARD -p tcp -d 192.168.0.2 --dport 22 -j ACCEPT
</code>

Install //iptables-services// for Centos7:
<code bash>
sudo yum install -y iptables-services
sudo systemctl enable iptables.service
sudo /usr/libexec/iptables/iptables.init save
</code>

==== IP ROUTE ====

<code>
ip route get 10.0.0.1
</code>

==== TCP OFFLOADING ====

  * TCO - TCP checksum offloading
  * TSO - TCP segmentation offloading (requires TCO)

Check current situation:
<code bash>
$ sudo ethtool --show-offload eth0
$ sudo ethtool -k ens3 # same as the previous command
</code>

Disabling TSO:
<code bash>
$ sudo ethtool -K ens3 tso off
</code>


==== CONNECTIONS ====

Pinpoint the service that's using a port:
<code bash>
$ lsof -i # "-i" option selects the listing of all network files
$ lsof -i:22 # filter out only port 22
$ sudo ls -la /proc/58198/exe
lrwxrwxrwx 1 user user 0 Aug 17 11:48 /proc/58198/exe -> /opt/python/3.6/bin/python3.6
</code>