IP Routing in C

Did you know that you can find the details of the internet connection of a system by using the C programming language? Well if you didn't know, now you know. Before we delve into how this is done, let us first briefly define the terms related to IP routing:
  • IP Address: This is the internet protocol address. It is a fixed numerical number that is associated with and used to identify each device. IP addresses are used by devices to communicate over the internet.
  • Subnet Mask: The subnet mask is a 32-bit component of the IP address. It is used to differentiate the network component of the IP address into two parts, one identifies the host computer while the other part identifies the network to which it belongs.
  • The Default Gateway: Devices in one network uses the default gateway to communicate with devices in another network. You can think of it as an intermediate device between the internet and the local network. It is responsible for transferring internal data to the internet and back again.
  • IP routing: IP routing refers to a set of protocols that determine the path followed by data when traveling across multiple networks, from its source to its destination. A series of routers are used to route data from its source to its final destination across multiple networks. Routers use the IP routing protocols to build up a forwarding table that correlates the final destination with next-hop addresses.
You can use the following two methods in C programming to check the IP’s of a system:

• The System Command

The system function in the stdlib library provided by C programming language can be used to access the IP configuration of the system. This can be done by using Ipconfig. You have to pass in the call of the function, the full address of the Ipconfig file that needs to be extracted. Here is an example:
The System Command

• The Excel Command

The excel() function can also be used to fetch the IP details of the system. However, you should remember that this function needs more than one parameter as shown in the code example below:
The Excel Command
The type of system that you are using will determine the output of the codes in the figures above. You can run the codes and your system will display the details. Get IP routing in C homework help from us when you need professional help with your assignment.
The source IP in the IP packets sent out from a system is known as the Local IP. In most cases, the local IP is always something like 192.168.*.* when the system is part of a local area network. An IP address of that range is usually assigned to the network interface (eth0). However, the ISP will directly allot an IP address to the system when it is connected to the internet through some PPP connection like a dial-up modem.

Getting the local IP on Linux using the C programming language.

Kernel Routing Tables

The routing tables maintained by the kernel are used to determine the default gateway, its interface and the local IP configured for that interface. You can find more information on this via the /proc/net/route. It normally looks like this:
Kernel Routing Tables
In the above list, the interface of the default gateway is the Iface whose destination is 000000000. The route command output is as shown below:
interface of the default gateway
In the snip above, the default gateway is the gateway for the destination 0.0.0.0. This line is of interest from the proc/net/route:
gateway for the destination
Its gateway is 0100A8C0 and destination is 00000000. The gateway is written in hex format in reverse order (little-endian). It can be decoded as 01.00.A8.C0 which should be 1.0.168.192. To find the default gateway and its interface, we can read that line in a C program as shown:
hex format in reverse order
The code above will display the default interface as eth0. Next, we need to find the IP address of eth0. We can use the getnameinfo function for this and combine it with our previous code. The output will give us both the default interface and the address.
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