• Usage:

  # list current network interface configs:
  $ ifconfig
  
  # manually set IP address:
  # ifconfig wlan0 192.168.0.80
  # ifconfig wlan0 192.168.0.80  netmask 255.255.252.0
  
  # automatic configuration:
  # dhclient eth0
  
  # print list of network interfaces:
  $ ifconfig | cut -c-10 | tr -d ' ' | tr -s '\n'
  
  # spoof hardware address:
  # ifconfig eth0 hw ether 00:1c:bf:87:25:d5

• We can edit /etc/resolv.conf to add server IP addresses:

  $ cat /etc/resolv.conf
  nameserver 8.8.8.8
  
  # echo nameserver IP_ADDRESS >> /etc/resolv.conf
  
  # To retrieve an IP address we can use ping:
  $ ping google.com
  PING google.com (64.233.181.106) 56(84) bytes of data.
  
  # To list all the Ip addresses we use:
  $ host google.com
  google.com has address 64.233.181.105
  google.com has address 64.233.181.99
  google.com has address 64.233.181.147
  google.com has address 64.233.181.106
  google.com has address 64.233.181.103
  google.com has address 64.233.181.104
  
  # or:
  $ nslookup google.com
  Server:    8.8.8.8
  Address:  8.8.8.8#53
  Non-authoritative answer:
  Name:  google.com
  Address: 64.233.181.105
  Name:  google.com
  Address: 64.233.181.99
  Name:  google.com
  Address: 64.233.181.147
  Name:  google.com
  Address: 64.233.181.106
  Name:  google.com
  Address: 64.233.181.103
  Name:  google.com
  Address: 64.233.181.104
  Server:    8.8.8.8
  
  # we can add symbolic names for a given IP:
  # echo 192.168.0.9 backupserver >> /etc/hosts
  
  # to display rout information:
  $ route -n
  Kernel IP routing table
  Destination   Gateway      Genmask       Flags Metric Ref  Use   Iface
  192.168.0.0   0.0.0.0      255.255.252.0   U     2     0     0   wlan0
  169.254.0.0   0.0.0.0      255.255.0.0     U     1000  0     0   wlan0
  0.0.0.0       192.168.0.4  0.0.0.0         UG    0     0     0   wlan0
  
  # setup a default gateway:
  # route add default gw 192.168.0.1 wlan0

• Ping provides the Round Trip Time (time needed to get to host and come back):

  --- google.com ping statistics ---
  5 packets transmitted, 5 received, 0% packet loss, time 4000ms
  rtt min/avg/max/mdev = 118.012/206.630/347.186/77.713 ms

• Limiting the number of packets to send:

  $ ping 192.168.0.1 -c 2

• Return status of ping command:

  $ ping domain -c2
  if [ $? -eq 0 ];
  then
    echo Successful ;
  else
    echo Failure
  fi

• Trace a packet route:

  $ traceroute google.com
  traceroute to google.com (74.125.77.104), 30 hops max, 60 byte packets
  1  gw-c6509.lxb.as5577.net (195.26.4.1)  0.313 ms  0.371 ms  0.457 ms
  2  40g.lxb-fra.as5577.net (83.243.12.2)  4.684 ms  4.754 ms  4.823 ms
  3  de-cix10.net.google.com (80.81.192.108)  5.312 ms  5.348 ms  5.327 ms
  4  209.85.255.170 (209.85.255.170)  5.816 ms  5.791 ms 209.85.255.172 
  (209.85.255.172)  5.678 ms
  5  209.85.250.140 (209.85.250.140)  10.126 ms  9.867 ms  10.754 ms
  6  64.233.175.246 (64.233.175.246)  12.940 ms 72.14.233.114 
  (72.14.233.114)  13.736 ms  13.803 ms
  7  72.14.239.199 (72.14.239.199)  14.618 ms 209.85.255.166 
  (209.85.255.166)  12.755 ms 209.85.255.143 (209.85.255.143)  13.803 ms
  8  209.85.255.98 (209.85.255.98)  22.625 ms 209.85.255.110 
  (209.85.255.110)  14.122 ms
  * 
  9  ew-in-f104.1e100.net (74.125.77.104)  13.061 ms  13.256 ms  13.484 ms
  
  => one could also use mtr instead to get real time data.

• Using ping:

  #!/bin/bash
  #Filename: ping.sh
  # Change base address 192.168.0 according to your network.
  for ip in 192.168.0.{1..255} ;
  do
    ping $ip -c 2 &> /dev/null ;
    
    if [ $? -eq 0 ];
    then
      echo $ip is alive
    fi
  done

• Using fping:

  $ fping -a 192.160.1/24 -g 2> /dev/null 
  192.168.0.1 
  192.168.0.90
  
  # or :
  $ fping -a 192.168.0.1 192.168.0.255 -g
  
  # Parallel pings:
  #!/bin/bash
  #Filename: fast_ping.sh
  # Change base address 192.168.0 according to your network.
  for ip in 192.168.0.{1..255} ;
  do
     (
        ping $ip -c2 &> /dev/null ;
    
        if [ $? -eq 0 ];
        then
         echo $ip is alive
        fi
     )&
    done
  wait

• To run a command on a remote we use:

  $ ssh user@host 'COMMANDS'
  
  # multiple commands:
  $ ssh user@host "command1 ; command2 ; command3"
  
  # ssh with compression:
  $ ssh -C user@hostname COMMANDS
  
  # Redirect stdin data to remote shell:
  $ echo 'text' | ssh user@remote_host 'echo'
  text
  # or Redirect data from file as:
  $ ssh user@remote_host 'echo'  < file
  
  # Running graphical commands:
  ssh user@host "export DISPLAY=:0 ; command1; command2"
  
  # or to get the display on the local computer (using ssh X11 forwarding)
  ssh -X user@host "command1; command2"

• Usage:

  # To connect to FTP server
  $ lftp username@ftphost
  
  # Using SCP to copy to remote:
  $ scp filename user@remotehost:/home/path
  
  # recursive copy with scp:
  $ scp -r /home/slynux user@remotehost:/home/backups

⇒ Recipe providing script for wireless connection with WEP key. See source directly.

• Setup:

  # Create key on the machine that requires login to remote:
  $ ssh-keygen -t rsa
  
  # Append the key on the host:
  $ ssh USER@REMOTE_HOST "cat >> ~/.ssh/authorized_keys" < ~/.ssh/id_rsa.pub
  Password:
  
  # We can use ssh-copy-id to register our public key instead:
  ssh-copy-id USER@REMOTE_HOST

• To forward port 8000 on the local machine to port 80 of www.kernel.org:

  ssh -L 8000:www.kernel.org:80 user@localhost
  
  # here we just replace user with the actual user name.

• To forward port 8000 on a remote machine to port 80 of www.kernel.org:

  ssh -R 8000:www.kernel.org:80 user@REMOTE_MACHINE
  
  # Here we replace the user and the remote machine name.

• Non-interactive port forward:

  ssh -fR 8000:www.kernel.org:80 user@localhost -N
  
  # -f : fork to background before executing command
  # -R : use login name on the remote
  # -N : there is no command to execute.

• Reverse port forwarding:

  ssh -R 8000:localhost:80 user@REMOTE_MACHINE

• To mount a remote folder we use:

  # sshfs -o allow_other user@remotehost:/home/path /mnt/mountpoint
  
  # umount /mnt/mountpoint

• List all open ports with:

  $ lsof -i
  COMMAND    PID   USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
  firefox-b 2261 slynux   78u  IPv4  63729      0t0  TCP localhost:47797-
  >localhost:42486 (ESTABLISHED)
  firefox-b 2261 slynux   80u  IPv4  68270      0t0  TCP slynux-laptop.
  local:41204->192.168.0.2:3128 (CLOSE_WAIT)
  firefox-b 2261 slynux   82u  IPv4  68195      0t0  TCP slynux-laptop.
  local:41197->192.168.0.2:3128 (ESTABLISHED)
  ssh       3570 slynux    3u  IPv6  30025      0t0  TCP localhost:39263-
  >localhost:ssh (ESTABLISHED)

• To list open ports from current machine:

  $ lsof -i | grep ":[0-9]\+->" -o | grep "[0-9]\+" -o  | sort | uniq

• List open ports with netstat:

  $ netstat -tnp

• Setup listening socket :

  # listen on port 1234 on local machine.
  nc -l 1234

• Connect to socket using:

  nc HOST 1234
  # HOST could be "localhost" or remote IP

• Then to send messages we jsut type them and press Enter.
• Quickly copy files over the network:

  # On receiving computer:
  nc -l 1234 > destination_filename
  
  # Then on sender computer:
  nc HOST 1234 < source_filename

⇒ How to use linux and iptables for sharing internet connection. See source document.

• Block traffic to a specific address:

  # iptables -A OUTPUT -d 8.8.8.8 -j DROP

• Block traffic to a specific port:

  # iptables -A OUTPUT -p tcp -dport 21 -j DROP

• Clear the changes made to the iptables chains:

  # iptables --flush