Want to win a PS4? Go Premium and enter to win our High-Tech Treats giveaway. Enter to Win

x
?
Solved

Bad TCP checksums

Posted on 2004-09-18
3
Medium Priority
?
6,317 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
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
3 Comments
 
LVL 4

Accepted Solution

by:
bfarmer earned 1000 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 1000 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

Veeam Disaster Recovery in Microsoft Azure

Veeam PN for Microsoft Azure is a FREE solution designed to simplify and automate the setup of a DR site in Microsoft Azure using lightweight software-defined networking. It reduces the complexity of VPN deployments and is designed for businesses of ALL sizes.

Question has a verified solution.

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

Most of the applications these days are on Cloud. Cloud is ubiquitous with many service providers in the market. Since it has many benefits such as cost reduction, software updates, remote access, disaster recovery and much more.
This article explains the fundamentals of industrial networking which ultimately is the backbone network which is providing communications for process devices like robots and other not so interesting stuff.
Viewers will learn how to properly install and use Secure Shell (SSH) to work on projects or homework remotely. Download Secure Shell: Follow basic installation instructions: Open Secure Shell and use "Quick Connect" to enter credentials includi…
Monitoring a network: why having a policy is the best policy? Michael Kulchisky, MCSE, MCSA, MCP, VTSP, VSP, CCSP outlines the enormous benefits of having a policy-based approach when monitoring medium and large networks. Software utilized in this v…

636 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