Solved

Bad TCP checksums

Posted on 2004-09-18
3
5,824 Views
Last Modified: 2013-11-29
I've been noticing some of these in my sniffer logs. A few questions

1. Do packets received with bad checksum errors, let the other side know the data got corrupted/changed on the way there? Or does the receiving end simply just drop the packet?

2. What causes bad checksum errors?  Misconfigured hosts?  Faulty hardware?

3.  I've noticed that I was getting tcp checksum errors when a default gateway wasnt specified on my linux box. When I specified one however, I stopped seeing them.  Did specifying a default gateway correct this?


Also, can someone tell me whats going on here?

08:23:16.729062 00:80:c6:fa:e3:49 > 00:b0:d0:c6:57:11, ethertype IPv4 (0x0800), length 70: IP (tos 0x0, ttl  31, id 8573, offset 0, flags [none], length: 56) 192.168.1.1 > 192.168.1.12: icmp 36: redirect 192.168.4.3 to host 192.168.1.50 for IP (tos 0x0, ttl  63, id 0, offset 0, flags [DF], length: 48) 192.168.1.12.5901 > 192.168.4.3.1201: [|tcp]

It's obviously an ICMP redirect message from my cable router (192.168.1.1) destined for my linux box (192.168.1.12).  But what is this packet telling 192.168.1.12?

0
Comment
Question by:dissolved
3 Comments
 
LVL 4

Accepted Solution

by:
bfarmer earned 250 total points
ID: 12093481
Bad checksums are typically caused by cabling issues, hardware problems, and or driver problems.  

I can't see a gateway setting having much to do with this, but who knows.

The ICMP redirect is telling 192.168.1.12 to use 192.168.1.50 as the gateway for 192.168.4.3.
0
 
LVL 4

Assisted Solution

by:complexymetron
complexymetron earned 250 total points
ID: 12093534
1. TCP provides "secured" data transfer. This means it assures, that data is in the right order and transmitted correctly. So bad checksums would be signalled to the sender, so the packet can be retransmitted by the sender. Look for sequence number and TCP.
(BTW: UDP wouldn't inform the sender. UDP works connectionless - if a packet is wrong it will be dropped)

2. A number of reasons could lead to malformed IP packets. Starts from bad driver on the sender system over faulty hardware (broken cables, etc.) or electromagnetic interference to misconfigured Ethernet frame setting (there are different ones, look for "Ethernet SNAP", "Ethernet 802.2", "Ethernet 802.3", etc. - both stations have to use the same or communication won't work)

3. Normally not. TCP routing works on layer 3 of the OSI networking model, while TCP works on layer 4. Setting a gateway is just setting routing information (e.g. what to do with packets not destined for the local net) - it shouldn't affect reliability of transmission (if connection can be established at all).


Your ethernet trace means:

> 08:23:16.729062 00:80:c6:fa:e3:49 > 00:b0:d0:c6:57:11, ethertype IPv4 (0x0800), length 70: IP (tos 0x0, ttl  31, id 8573, offset 0, flags [none], length: 56) 192.168.1.1 > 192.168.1.12:

= Ethernet frame from MAC1 to MAC2 containing an IP packet from IP1 to IP2

> icmp 36: redirect 192.168.4.3 to host 192.168.1.50

= cleary an ICMP redirect message which informs the host (sender of the original packet) that the gateway found a better route for the packet via the local network and wants to let the host know about that. In this case: "Hey Dude, if you want to reach 192.168.4.3 try 192.168.1.50 as gateway"

> for IP (tos 0x0, ttl  63, id 0, offset 0, flags [DF], length: 48) 192.168.1.12.5901 > 192.168.4.3.1201: [|tcp]

That seems to be a transcript from the original packet the gateway received (this data is included for the host to find the original transmission). The original packet seemed to go from 192.168.1.12 port 5901 to 192.168.4.3 port 1201 using tcp.

For more indepth information, look for RFC 792 (http://www.faqs.org/rfcs/rfc792.html) under "Redirect Message". If you think this behaviour as to be not what you want the gateway to do, then I'd recommend to check the gateway configuration again.
0
 

Author Comment

by:dissolved
ID: 12093560
you guys rock
0

Featured Post

Microsoft Certification Exam 74-409

Veeam® is happy to provide the Microsoft community with a study guide prepared by MVP and MCT, Orin Thomas. This guide will take you through each of the exam objectives, helping you to prepare for and pass the examination.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

#Citrix #Citrix Netscaler #HTTP Compression #Load Balance
Load balancing is the method of dividing the total amount of work performed by one computer between two or more computers. Its aim is to get more work done in the same amount of time, ensuring that all the users get served faster.
Viewers will learn how to connect to a wireless network using the network security key. They will also learn how to access the IP address and DNS server for connections that must be done manually. After setting up a router, find the network security…
Get a first impression of how PRTG looks and learn how it works.   This video is a short introduction to PRTG, as an initial overview or as a quick start for new PRTG users.

791 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question