IP FORWARDING

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

SCP

$ 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

CURL

Uploading data to http server using PUT: 

curl -u username -T file.conf http://192.168.0.1

NC

Basic TCP test: 

nc -vz 10.10.0.1 80     # v - verbose; z - just scan for listening daemon,
                        # without sending any data to them

PROXY

Set proxy and show current config: 

export http_proxy=http://proxyusername:proxypassword@proxyaddress:proxyport
echo $http_proxy

To make it permanent, add to /etc/environment 

http_proxy=http://10.0.0.1:3128

Make it work with “sudo”, add following to /etc/sudoers after Defaults env_reset: 

Defaults env_keep = "http_proxy ftp_proxy"

DIG

Sample usage: 

dig @10.0.0.1 ipv6.google.com AAAA +short

IPv4/6 DNS RESOLVER PRECEDENCE

SSH PORT FORWARDING

Dynamic forwarding, ssh will act as a SOCKS server: 

ssh -D 1080 victor@172.16.0.1

IPTABLES

Port forwarding: 

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

Install iptables-services for Centos7: 

sudo yum install -y iptables-services
sudo systemctl enable iptables.service
sudo /usr/libexec/iptables/iptables.init save

IP ROUTE

ip route get 10.0.0.1

TCP OFFLOADING

Check current situation: 

$ sudo ethtool --show-offload eth0
$ sudo ethtool -k ens3 # same as the previous command

Disabling TSO: 

$ sudo ethtool -K ens3 tso off

CONNECTIONS

Pinpoint the service that's using a port: 

$ 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