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I am trying to frame the ICMP packet and send it through raw socket. Looking at the examples, I see that the IP packet length is calculated as :
iphdr.ip_hl = sizeof(struct ip) >> 2
Can you please explain why we need to right shift struct ip by 2 times instead of assigning a constan value ?
The ‘ip_hl’ field of an IP (or ICMP) packet is defined as the length of the IP header, in 32-bit words.
sizeof(struct ip) yields the length of the IP header, in 8-bit bytes. Right shifting this value twice provides the length in 32-bit words, as expected in the ip_hl field.
A good reason not to use a constant for this, is to eliminate magic numbers in source code. (The compiler will generate a constant value anyway for ‘sizeof(struct ip) >> 2’).