Linux 14 - Network Operations in Linux

By Sheldon L

Published at 2020-04-29

Updated at 2020-04-20

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IP Addresses

• IP (Internet Protocol) address

IPv4 and IPv6

• 2^32, 4.3 * 10^9

• 2^64, 3.4 * 10^38

Classful Network Address

• Class A, B, C, D, E

IP Address Allocation

• DHCP (Dynamic Host Configuration Protocol)

Name Resolution

• DNS

• hostname

cat /etc/hosts
cat /etc/resolv.conf

host linuxfoundation.org
nslookup linuxfoundation.org
dig linuxfoundation.org

Network Configuration and Tools

Network Configuration Files

# Debian
ls -l /etc/network/

# Fedora and SUSE
ls -l /etc/sysconfig/network/

nmtui
nmcli

Network Interfaces

• Physically, network interfaces can proceed through a network interface card (NIC), or can be more abstractly implemented as software.
• Specific interfaces can be brought up (activated) or brought down (de-activated) at any time.
• net-tools package, install it if you want to use it.

ifconfig enp4s0f2          # enp4s0f2 is the network interface
ip -s link show enp4s0f2
ip address show enp4s0f2

The ip Utility

/sbin/ip addr show   # interface config, similar to `ifconfig`
/sbin/ip route show  # routing table, similar to `route`
ip --brief addr show # more brief

ifconfig             # interface config
route -n             # routing table

ping

ping -c [n] [address]  # ping [n] times

route

route -n  # Show current routing table
ip route  # Show current routing table

route add -net [address] # Add static route
ip route add [address]   # Add static route

route del -net [address] # Delete static route
ip route del [address]   # Delete static route

traceroute

traceroute [address]

More Networking Tools

Networking Tools	Description
ethtool	Queries network interfaces and can also set various parameters such as the speed
netstat	Displays all active connections and routing tables. Useful for monitoring performance and troubleshooting
nmap	Scans open ports on a network. Important for security analysis
tcpdump	Dumps network traffic for analysis
iptraf	Monitors network traffic in text mode
mtr	Combines functionality of ping and traceroute and gives a continuously updated display
dig	Tests DNS workings. A good replacement for host and nslookup

sudo ethtool [interface]
netstat -r
nmap -sP 192.168.0.0/24

Browsers, wget and curl

Graphical and Non-Graphical Browsers

• Graphical: Firefox, Google Chrome, Chromium, Epiphany (renamed web), Konqueror, Opera
• Text: linx, lynx, w3m

wget and curl

• wget
  • Large file downloads
  • Recursive downloads, where a web page refers to other web pages and all are downloaded at once
  • Password-required downloads
  • Multiple file downloads.
• curl
  • Read url information
  • Save the contents of a web page to a file, as does wget

wget [url]  # download

curl [url]  # read
curl -o [saved_file_name] [url] # download

Network Troubleshooting

• First make certain your network is properly configured. If your Ethernet device is up and running

/sbin/ifconfig
ip addr show

# if IP address not show
# one of these can restart network
sudo systemctl restart NetworkManager
sudo systemctl restart network
sudo service NetworkManager restart
sudo service network restart
# and try to refresh dhcp address
sudo dhclient [interface]

• If your interface is up and running with an assigned IP address and you still can not reach google.com, we should make sure you have a valid hostname assigned to your machine

hostname

• When you type in a name of a site such as google.com, that name needs to be connected to a known IP address. This is usually done employing the DNS sever

sudo ping -c 3 google.com
host google.com
# nameserver conf: /etc/resolv.conf
# If you wanted to override that you could do:
host google.com 8.8.8.8 # google nameserver
# Note that there is another file, /etc/hosts,
# where you can associate names with IP addresses,
# which is used before the DNS server is consulted.
# This is most useful for specifying nodes on your local network.

dig google.com

• Suppose host or dig fail to connect the name to an IP address. There are many reasons DNS can fail, some of which are:
  • The DNS server is down. In this case try pinging it to see if it is alive (you should have the IP address in /etc/resolv.conf.
  • The server can be up and running, but DNS may not be currently available on the machine.
  • Your route to the DNS server may not be correct.

# Tracing the route to one of the public name server
sudo traceroute 8.8.8.8

# default gate way
ip route show

sudo mtr --report-cycles 3 8.8.8.8  # an enhanced version of traceroute

Transfering Files

• FTP clients enable you to transfer files with remote computers using the FTP protocol. These clients can be either graphical or command line tools.
  • GUI: Filezilla
  • CLI: ftp, sftp, ncftp, yafc (Yet Another FTP Client).

FTP

• ftp has fallen into disfavor on modern systems, as it is intrinsically insecure, since passwords are user credentials that can be transmitted without encryption and are thus prone to interception. So it was removed from rsync and web browser https. Connecting to an FTP server:

ftp [address]  # such as ftp.gnu.org
# Hint: The user name for anonymous FTP site is `anonymous`.
# most anonymous FTP sites require you to supply a password, but it does not matter what you give!
> ls
> get [file]   # such as fwelcome.msg
> quit

ssh and scp

• We move files securely using scp (Secure Copy) between two networked hosts. scp uses the SSH protocol for transferring data.

# ssh
ssh [user]@[address]

# scp
scp [localfile] <user@remotesystem>:/home/user/
scp -r [localfolder] <user@remotesystem>:/home/user/