[Snort-users] simple question on packet sizes

Martin Roesch roesch at ...1935...
Mon May 28 00:43:25 EDT 2001

"James R. Hendrick" wrote:
> Hi,
> When looking at the output of snort, there are several (basic) fields that I
> need some help understanding:
> IpLen

IP header length from the p->iph->ip_hlen field of the IP header.  This
is normally set to "5" (indicated in the decode as 20-bytes) except when
IP options are present.

> TcpLen

Value of the p->tcph->th_off field, indicates the length of TCP
headers.  This is normally "5" (20-bytes in the output again) except
when TCP options are available.

> DgmLen

This is the datagram length from the p->iph->ip_len field, it indicates
the length of the packet (including the IP header).

> Len

UDP length from the UDP header decode, length of the payload + UDP


> I am trying to determine the number of bytes in packets (of various types,
> mostly TCP) that make up "payload" vs. "header" for various services and
> must simply be missing something obvious (need more coffffeeeeee).
> The part that confuses me is that I see DgmLen in TCP packets as well as UDP
> and TcpLen in UDP packets as well as TCP. Can someone help me understand
> which fields I will see for which protocols and what they represent?

That's not right, you shouldn't be seeing TcpLen in UDP packets, the
printout code is mutually exclusive so this is (should be) impossible. 
Can you show me an example?


> Thanks!
> Jim
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Martin Roesch
roesch at ...1935...
http://www.sourcefire.com - http://www.snort.org

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