![]() ![]() And for good reason too - Wireshark provides an excellent GUI that not only displays the contents of individual packets, but also analysis and statistics tools that allow you to, for example, track individual TCP conversations within a pcap, and pull up related metrics. Writing code to properly analyze raw network packets from a capture file - whether pcap, pcapng, Sniffer, snoop, Microsoft Network Monitor, or another packet analyzer - isn't a simple process.For most situations involving analysis of packet captures, Wireshark is the tool of choice. Wireshark's dissection is even more complicated than that. The tcpdump source file with the Ethernet dissector has ~600 lines of source code the tcpdump source files with the IPv4 dissector and with the IPv6 dissector both have ~500 lines of source code. If your network is an Ethernet network with VLANs, then the type/length field may have the value 0x8100, in which case the Ethernet header will be followed by a 2-byte VLAN tag, which is followed by another 2-byte type/length field. If the packet has an Ethernet header, and the type/length field has the value 0x86dd, then an IPv6 header, not an IPv4 header, comes after the Ethernet header. Eventually, there will be a last "extension header", the Next Header field of which will contain a value specifying a protocol such as TCP or UDP. The "extension header" will contain its own Next Header field, which could be for a protocol such as TCP or UDP or for another "extension header". However, there's no guarantee that this will be a value for a protocol such as TCP or UDP it might be a value for an "extension header". ![]() The IPv6 header has a Next Header field, which contains an Internet Protocol Number field. UDP - and TCP, and so on - can also run on top of IPv6. If the packet does not begin with an Ethernet header, then, unless the link-layer header is also 14 bytes long and is immediately followed by the payload, byte 23 is not the IPv4 Protocol field.įurthermore, if the packet is not an IPv4 packet, even if the packet does begin with an Ethernet header, byte 23 is not the IPv4 Protocol field. The Protocol field in the IPv4 header is an an offset of 9 from the beginning of the IPv4 header, so, in an IPv4-over-Ethernet packet, the Protocol field is an an offset of 14+9 = 23 bytes from the beginning of the packet. In a packet with an Ethernet header that has a type/length field value of 0x0800, the first 14 bytes are an Ethernet header, and the next 20 bytes are the fixed portion of an IPv4 header. A value of 6 means TCP a value of 17 means UDP. IPv4 is described by RFC 791 the header of an IPv4 packet has a 1- byte protocol field, which contains an Internet Protocol Number value. If the type/length field has the value 0x0800, then the packet is an IPv4 packet. If the field has the value 1, then the packet begins with an Ethernet header, which begins with a 6-byte Ethernet destination address, followed by a 6-byte Ethernet source address, followed by a 2-byte type/length field. ![]() The list of link-layer header types shows what different values in that field mean. When it is UDP, byte 23 in the packet is set to 17, however in non-UDP, byte 23 doesn't have the same meaningĪ pcap file has, in the file header, a field that indicates the type of link-layer header that the packets in the file have. ![]()
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