I'm trying to write a sniffer using pcap -- but that's probably not so relevent to my question(s). I'm learning about network layers at the same time.
My first question is: will the "total length" field of an ip header usually be larger than than the packet size because the packets are split up on a lower level?
If so then I'm sure I've got my read on the ip header aligned correctly. I'm still not sure about the tcp header tho. Here's some sample output:
The tcp stuff is the third line. It looks like there's two machines (26 & 27) in the sample. But the port numbers look possibly wrong to me; like I said, I know almost nothing about networking. Anyone with some experience and an opinion?
Destination: 00:1f:90:01:0e:8c Source: 00:e0:4d:89:b5:73 (type 8)
IP from: 192.168.1.26 to: 188.8.131.52 length=13312 (protocol 6)
<port #15835, >port #20480, FLAGS: ACK (tcp header is 32 bytes)
Packet Size 66
Destination: ff:ff:ff:ff:ff:ff Source: 00:18:de:0b:f1:93 (type 8)
IP from: 192.168.1.27 to: 192.168.1.255 length=60672 (protocol 17)
<port #35328, >port #35328, FLAGS: PUSH URG (tcp header is 48 bytes)
Packet Size 251
ps. what are ethernet types 9728 and 1544?
later...okay I have this right. I was just hoping these ports would align with IANA assignments so I could, eg. pick out SMTP or HTTP