Socket Connection Probolem - SYN - RST

I have been working on a connection problem that has me totally baffled and
I would not mind some pointers on where to start to search.

I have a small White Box Linux server up and running with a small server app
that listens to port 3020. I can telnet to that port from my workstation and
connect.
I have a small embedded device (Rabbit) that is supposed to connect to the
server to transfer data. I cannot get a connection between the two systems.
I've taken the code from the rabbit and the server and sent it to another
developer to try. He can get a connection between the two devices.
From the sniffer I can see the Rabbit sending out SYN packets and then RST
packets, but the server never responds. I can ping the Rabbit from the
server with no problem...
There is nothing in the iptables and I'm at a loss as to where to check
next..

any ideas?

thanks in advance

dave

 

Have you tried taking a packet trace on the server to make certain the
SYN's are actually getting to the server? Also, when does the Rabbit
sent RSTs? Just to be pendantic, you know those abort connections
right?

rick jones

 

Are the "Rabbit" and server in the same subnet? Check the ip address of the
"Rabbit" and it's subnet mask. A wrong subnet mask could cause the problem.

Klazmon.

 

Piggy backing on my own post. Another possibilty is the physical
connection. I assume you are connecting the Rabbit via a switch or hub.
Could be a speed mismatch problem. If the Rabbit only does 10mb HDX and
your switch is forcing 100Mb FDX for example. Can you check the switch
configuration?

You need to verify that the hardware layer and also layer 2 is working
before worrying about the tcp/ip stuff.

Klazmon.

 

Thanks for the response....thoese were some of the first thing that I
checked.
Physical connection is fine, I can ping both the Rabbit and the Server from
ny worksation, and the packet sniffer shows the packets being sent from the
Rabbit to the Server. This small network is flat, ie only using a hub for
testing so I know there is no problem with switches.

I'm going to drop a packet sniffer on the linux box to see what is going on
there...

thanks again

dave

 

I'm dropping a packer sniffer on the server as i type this to see what is
going on...
I'm assuming that the packets are getting there, since I can reach both the
server and rabbit from my workstation
and the server can ping the rabbit with no problem....it is a flat network
at the moment..ie only a hub no switch...

thanks for your help

dave

 

Physical connection is fine, I can ping both the Rabbit and the

That's fine - in "general" however, one cannot assume that because A
can reach B and A can reach C that that B can reach C.

Also, a clean ping does not always mean that duplex settings are fine
because a ping on an otherwise idle network does not try to have more
than one system access the network simultaneously.

Excellent.

rick jones

And now for the insomniacs, some boilerplate on duplex:

How 100Base-T Autoneg is supposed to work:

When both sides of the link are set to autoneg, they will "negotiate"
the duplex setting and select full-duplex if both sides can do
full-duplex.

If one side is hardcoded and not using autoneg, the autoneg process
will "fail" and the side trying to autoneg is required by spec to use
half-duplex mode.

If one side is using half-duplex, and the other is using full-duplex,
sorrow and woe is the usual result.

So, the following table shows what will happen given various settings
on each side:

Auto Half Full

Auto Happiness Lucky Sorrow

Half Lucky Happiness Sorrow

Full Sorrow Sorrow Happiness

Happiness means that there is a good shot of everything going well.
Lucky means that things will likely go well, but not because you did
anything correctly Sorrow means that there _will_ be a duplex
mis-match.

When there is a duplex mismatch, on the side running half-duplex you
will see various errors and probably a number of _LATE_ collisions
("normal" collisions don't count here). On the side running
full-duplex you will see things like FCS errors. Note that those
errors are not necessarily conclusive, they are simply indicators.

Further, it is important to keep in mind that a "clean" ping (or the
like - eg "linkloop" or default netperf TCP_RR) test result is
inconclusive here - a duplex mismatch causes lost traffic _only_ when
both sides of the link try to speak at the same time. A typical ping
test, being synchronous, one at a time request/response, never tries
to have both sides talking at the same time.

Finally, when/if you migrate to 1000Base-T, everything has to be set
to auto-neg anyway.