![Packets Packets](http://www.poftut.com/wp-content/uploads/2017/01/img_586f903a8c753.png)
Here a few options you can use when using tcpdump. Using this options, we will try to build some simple usecases. Options-i any: Listen on all interfaces just to see if you’re seeing any traffic.-i eth0: Listen on the eth0 interface.-D: Show the list of available interfaces-n: Don’t resolve hostnames.-nn: Don’t resolve hostnames or port names.
What are Ethernet, IP and TCP Headers in Wireshark CapturesIf I could go back in time when I was a n00b kid wanting to go from zero to a million in networking, the one thing I would change would be spending about 6 months on the fundamentals of networking headers and framing before ever touching a single peice of vendor gear. Ethernet, IP and Transport headers (L2-L4) are the past present and future of networking. Protocols will come and go, Ethernet and IP will undoubtedly be with us for the rest of our careers. I know a lot of good engineers, Ops and architects that have learned and forgotten fundamental details five times over, me included as we fill our heads with timers of IGPs and framing encapsulations of data center interconnects.I tend to try and go back and refresh the basics on the wikis as much as possible. Example, 802.1Q encapsulation is not actually encapsulating the original frame but inserting a 32-bit field with the TPID, VID etc.Layered architectures are great (in theory but it requires understanding how they interact with one another.
I think the encapsulation of the layers can be tough to wrap ones head around as they are entering the field. Once they do they become rock stars, as the beauty of decoupling the layers allow for comprehension of enormous scale.When working with interns at work we tend to start by breaking out Wireshark capture. This doesn’t necessarily always help, as that can be even more confusing than looking at abstracted theoretical layers for a greenhorn. I tend to break a Wireshark capture down and try to correlate that to the three most relevant layers and their headers L2-L4. Ethernet II – Layer 2. IP Header – Layer 3.
TCP Header -Layer 4. I left out UDP since connectionless headers are quite simpler, e.g. Source Port, Destination Port, Length and Checksum.
TCP IP Header Explained. Source port (16 bits) – identifies the sending port. Destination port (16 bits) – identifies the receiving port.
Sequence number (32 bits) – has a dual role:. If the SYN flag is set (1), then this is the initial sequence number. The sequence number of the actual first data byte and the acknowledged number in the corresponding ACK are then this sequence number plus 1.
If the SYN flag is clear (0), then this is the accumulated sequence number of the first data byte of this packet for the current session. Acknowledgment number (32 bits) – if the ACK flag is set then the value of this field is the next sequence number that the receiver is expecting. This acknowledges receipt of all prior bytes (if any). The first ACK sent by each end acknowledges the other end’s initial sequence number itself, but no data. Data offset (4 bits) – specifies the size of the TCP header in 32-bit words. The minimum size header is 5 words and the maximum is 15 words thus giving the minimum size of 20 bytes and maximum of 60 bytes, allowing for up to 40 bytes of options in the header. Hi Joe, Thanks for the nice comment.
I agree but hey its pretty complicated stuff even if we have learned it a few times over again. Have totally been meaning to do it for MPLS packets. Great point on ARP and ICMP. Especially the different types and codes. I really want to do a write up on PMTUD. It is pretty amazing it all works as well as it does.
![Tcpdump Http Headers Tcpdump Http Headers](/uploads/1/2/4/1/124122959/495861405.png)
I just read this in an article “Your average 5-minute YouTube video is about 10 million bits” Seriously 10million bursts of electricity all being managed and run up and down the abstraction stack. Throw your twitter handle on here if you have one so i can follow.cyas!