networking commands in linux/unix

Networking Commands Example in Unix and Linux 

These are most useful commands in my list while working on Linux server , this enables you to quickly troubleshoot connection issues e.g. whether other system is connected or not , whether other host is responding or not and while working for FIX connectivity for advanced trading system this tools saves quite a lot of time .

• finding host/domain name and IP address - hostname
• test network connection – ping
• getting network configuration – ifconfig
• Network connections, routing tables, interface statistics – netstat
• query DNS lookup name – nslookup
• communicate with other hostname – telnet
• outing steps that packets take to get to network host – traceroute
• view user information – finger
• checking status of destination host - telnet

Example of Networking commands in Unix

let's see some example of various networking command in Unix and Linux. Some of them are quite basic  e.g. ping and telnet and some are more powerful e.g. nslookup and netstat. When you used these commands in combination of find and grep you can get anything you are looking for e.g. hostname, connection end points, connection status etc.


hostname

hostname with no options displays the machines host name
hostname –d displays the domain name the machine belongs to
hostname –f displays the fully qualified host and domain name
hostname –i displays the IP address for the current machine


ping
It sends packets of information to the user-defined source. If the packets are received, the destination device sends packets back. Ping can be used for two purposes

1. To ensure that a network connection can be established.
2. Timing information as to the speed of the connection.

If you do ping www.yahoo.com it will display its IP address. Use ctrl+C to stop the test.

ifconfig
View network configuration, it displays the current network adapter configuration. It is handy to determine if you are getting transmit (TX) or receive (RX) errors.


netstat
Most useful and very versatile for finding connection to and from the host. You can find out all the multicast groups (network) subscribed by this host by issuing "netstat -g"

netstat -nap | grep port will display process id of application which is using that port
netstat -a  or netstat –all will display all connections including TCP  and UDP
netstat --tcp  or netstat –t will display only TCP  connection
netstat --udp or netstat –u will display only UDP  connection
netstat -g will display all multicast network subscribed by this host.

nslookup
If you know the IP address it will display hostname. To find all the IP addresses for a given domain name, the command nslookup is used. You must have a connection to the internet for this utility to be useful.
E.g. nslookup blogger.com

You can also use nslookup to convert hostname to IP Address and from IP Address from hostname.

traceroute
A handy utility to view the number of hops and response time to get to a remote system or web site is traceroute. Again you need an internet connection to make use of this tool.


finger
View user information, displays a user’s login name, real name, terminal name and write status. this is pretty old unix command and rarely used now days.

telnet
Connects destination host via telnet protocol, if telnet connection establish on any port means connectivity between two hosts is working fine.
telnet hostname port   will telnet hostname with the port specified. Normally it is used to see whether host is alive and network connection is fine or not.


whois

This command allows you to check the Internic database for proper hostnames. This is very handy when you are trying to trace back an IP address to a specific hostname, or the reverse. I often use it when troubleshooting connectivity between hosts and checking whether the problem is a host configuration error or an actual physical connectivity error. The most commonly used version of the command is: 

whois -f 10.1.1.1
(replace the 10.1.1.1 with a specific server IP) 

The -f option forces the command to skip any cache that may have stored the host state, and instead goes to the actual server to perform a lookup and verify its hostname.
Another useful variation of the command, especially for trying to identify port problems is:

whois --port=8102 10.1.1.1
This command forces a test on the specific host's port 8102. You can benefit from force checking specific ports in many ways, and I highly recommend testing ports with this technique.


findsmb

   findsmb is used to list info about machines that respond to SMB name queries (for example windows based machines sharing their hard disk's).
Command syntax:

   


 findsmb 

This would find all machines possible, you may need to specify a particular subnet to query those machines only...

I. How Routing is Done? 

Route command is used to show/manipulate the IP routing table. It is primarily used to setup static routes to specific host or networks via an interface.

In this article we will see how to manipulate the routing tables in Linux using route command.
We’ll first explain how routing is done with some basic route command examples, and then we’ll explain using a sample network architecture about how to setup routes in your network.

I. How Routing is Done?

1. Display Existing Routes

route command by default will show the details of the kernel routing table entries. In this example, the ip-address of the system where the route command is being executed is 192.168.1.157

$ route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.1.0     *               255.255.255.0   U     0      0        0 eth0

The above command shows that if the destination is within the network range 192.168.1.0 – 192.168.1.255, then the gateway is *, which is 0.0.0.0.
When packets are sent within this IP range, then the MAC address of the destination is found through ARP Protocol and the packet will be sent to the MAC address.

If you don’t know what ARP is, you should first understand how ARP protocol works.
In order to send packets to destination which is not within this ip range, the packets will be forwarded to a default gateway, which decides further routing for that packet. We will see this shortly.
By default route command displays the host name in its output. We can request it to display the numerical IP address using -n option as shown below.

$ route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.1.0     0.0.0.0         255.255.255.0   U     0      0        0 eth0
0.0.0.0         192.168.1.10    0.0.0.0         UG    0      0        0 eth0

2. Adding a Default Gateway

We can specify that the packets that are not within the network has to be forwarded to a Gateway address.
The following route add command will set the default gateway as 192.168.1.10.

$ route add default gw 192.168.1.10

Now the route command will display the following entries.

$ route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.1.0     *               255.255.255.0   U     0      0        0 eth0
default         gateway.co.in   0.0.0.0         UG    0      0        0 eth0

Now we have just added a default gateway to our machine. To verify whether it is working properly, ping some external host (for example, google.com) to send ICMP packet.

$ ping www.google.com

The following is the sequences of evets that happens when the above ping command is executed.

1. First it will query the DNS server to obtain the ip-address of google.com ( for example: 74.125.236.34 )
2. The destination address ( 74.125.236.34 ) is not within the network range.
3. So, in Layer-3 (IP header) the DESTINATION IP will be set as “74.125.236.34″.
4. In Layer-2, the DESTINATION MAC address will be the filled in as the MAC address of the default gateway ( 192.168.1.10′s MAC ). The MAC will be found by using ARP as described earlier.
5. When the packet is sent out, the network switch ( which works on Layer-2 ), send the packet to the default gateway since the destination MAC is that of the gateway.
6. Once the gateway receives the packet, based on its routing table, it will forward the packets further.

The above 2 examples would have given a good idea about how routing is done within a network. Now we will see other command line options available with route command.

3. List Kernel’s Routing Cache Information

Kernel maintains the routing cache information to route the packets faster. We can list the kernel’s routing cache information by using the -C flag.

$ route -Cn
Kernel IP routing cache
Source          Destination     Gateway         Flags Metric Ref    Use Iface
192.168.1.157   192.168.1.51    192.168.1.51          0      0        1 eth0
192.168.1.157   74.125.236.69   192.168.1.10          0      0        0 eth0
.
.
.

4. Reject Routing to a Particular Host or Network

Sometimes we may want to reject routing the packets to a particular host/network. To do that, add the following entry.

$ route add -host 192.168.1.51 reject

As you see below, we cannot access that particular host (i.e .51 host that we just rejected).

$ ping 192.168.1.51
connect: Network is unreachable

However we can still access other hosts in the network (for example, .52 host is still accessible).

$ ping 192.168.1.53
PING 192.168.1.53 (192.168.1.53) 56(84) bytes of data.
64 bytes from 192.168.1.53: icmp_seq=1 ttl=64 time=7.77 ms

If you want to reject an entire network ( 192.168.1.1 – 192.168.1.255 ), then add the following entry.

$ route add -net 192.168.1.0 netmask 255.255.255.0 reject

Now, you cannot access any of the host in that network (for example: .51, .52, .53, etc.)

$ ping 192.168.1.51
connect: Network is unreachable

$ ping 192.168.1.52
connect: Network is unreachable

$ ping 192.168.1.53
connect: Network is unreachable

II. A Sample Network Architecture (to understand routing)

Let us use the following sample network architecture for the rest of the examples.
In the diagram below, we have 2 individual networks ( 192.168.1.0 and 192.168.3.0, with subnet mask of 255.255.255.0 ).
We also have a “GATEWAY” machine with 3 network cards. 1st card is connected to 192.168.1.0, 2nd card is connected to 192.168.3.0, and the 3rd card is connected to the external world.

5. Make 192.168.3.* Accessible from 192.168.1.*

Now we need to add a routing entry such that we are able to ping 192.168.3. series ip-addresses from 192.168.1. series. The common point we have is the GATEWAY machine.
So, on each machine in 192.168.1.* network a default gateway will be added as shown below.

$ route add default gw 192.168.1.10

Now when 192.168.1.1 pings 192.168.3.1, it will go to the GATEWAY via 192.168.1.10.
In GATEWAY, add the following routing entry.

$ route add -net 192.168.3.0 netmask 255.255.255.0 gw 192.168.3.10

Now all the packets addressed to 192.168.3.* network will be forwarded via the 192.168.3.10 interface, which then delivers the packets to the addressed machine.

6. Make 192.168.1.* Accessible from 192.168.3.*

It is very similar to what we did earlier.
So, on each machine in 192.168.3.* network a default gateway will be added as shown below.

$ route add default gw 192.168.3.10

In GATEWAY, add the following routing entry.

$ route add -net 192.168.1.0 netmask 255.255.255.0 gw 192.168.1.10

Now 192.168.3.* machines can ping 192.168.1.* machines.

7. Allow Internet Access ( External World )

In the previous two example, we have interconnected the 2 different networks.
Now we need to access the internet from these 2 different networks. For that, we can add a default routing ( when no routing rule matches ) to the 125.250.60.59 which is connected to the external world as follows.

$ route add default gw 125.250.60.59

This is how it works:

1. Now when you try to access the internet (for example: ping google.com) from any of these machines (for example, from 192.168.3.2), the following is the sequence of events that happens.
2. Since the destination (google.com) is not within 3.* series, it will be forwarded to GATEWAY via 3.10 interface
3. In GATEWAY, it checks whether the destination is within 1.* range. In this example, it is not.
4. It then checks whether the destination is within 2.* range. IN this example, it is not
5. Finally, it takes the default route to forward the packets (i.e using the 125.250.60.59 interface, which is connected to the external world). 

Start and Stop the Network Interface Card 

The ifconfig command can be used to start and stop network interface cards:

# ifconfig eth0 up

# ifconfig eth0 down

 

The ifup & ifdown command can also be used to start and stop network interface cards:

# ifup eth0

# ifdown eth0

 

The systemctl commands can also be used to enable, start, stop, restart and check the status of the network interface services -

# systemctl enable network.service

# systemctl start network.service

# systemctl stop network.service

# systemctl restart network.service

# systemctl status network.service

Displaying and Changing your System's Hostname

The command hostname displays the current hostname of the computer, which is 'Gateway':

# hostnameGateway

You can change the hostname by giving the new name at the end of the command -

# hostname Firewall-cx

 

This will change to the new hostname once you have logged out and logged in again. In fact, for any change in the interfaces, the change is implemented only after the user logs in the next time after a log-out.