connect() - tcpdump output question

hi,

I am working on a large C/C++ program, and I've narrowed down one of the
errors to a connect() call, ie, connect() returns negative number.

I use the loopback, eg, ::1 for testing and the following lines are the
output of my tcpdump. (127.0.0.1 gives me the same output)

04:48:10.549398 IP6 ::1.60782 > ::1.ntp: S 3853331357:3853331357(0) win
32752 <mss 16376,sackOK,timestamp 3414555 0,nop,wscale 7>
04:48:10.549424 IP6 ::1.ntp > ::1.60782: R 0:0(0) ack 3853331358 win 0

I am not sure how I am supposed to interpret these lines. The first line
tells me that the client sends a packet with a seq number and a win size,
and the second line ack's the seq but win 0? Connection rejected?

Does the tcpdump output tell me that the client side connect() is working,
at least trying to establish a connection, but the server side
listen() or accept() fail?

Regards,
Ural

On Mon, 24 Jul 2006 05:26:13 +0100, Ural Mutlu wrote:

> hi,
>
> I am working on a large C/C++ program, and I've narrowed down one of the
> errors to a connect() call, ie, connect() returns negative number.
>
> I use the loopback, eg, ::1 for testing and the following lines are the
> output of my tcpdump. (127.0.0.1 gives me the same output)
>
> 04:48:10.549398 IP6 ::1.60782 > ::1.ntp: S 3853331357:3853331357(0) win
> 32752 <mss 16376,sackOK,timestamp 3414555 0,nop,wscale 7>
> 04:48:10.549424 IP6 ::1.ntp > ::1.60782: R 0:0(0) ack 3853331358 win 0
>
> I am not sure how I am supposed to interpret these lines. The first line
> tells me that the client sends a packet with a seq number and a win size,
> and the second line ack's the seq but win 0? Connection rejected?

Yes. 'S' is for syn. 'R' is for reset, which is what is used to reject a
connection. Could be a firewall, or there is no service listening on that
port on the server.

> Does the tcpdump output tell me that the client side connect() is
> working, at least trying to establish a connection, but the server side
> listen() or accept() fail?

Yes. I think most likely the listen(). Try "netstat --inet -l" on the
server to check if there is a indeed something listening on the right port.

Regards,
Mark.

>> 04:48:10.549398 IP6 ::1.60782 > ::1.ntp: S 3853331357:3853331357(0) win
>> 32752 <mss 16376,sackOK,timestamp 3414555 0,nop,wscale 7>
>> 04:48:10.549424 IP6 ::1.ntp > ::1.60782: R 0:0(0) ack 3853331358 win 0

The ntp port? Is your program indeed meant to talk to a Network Time
Protocol server, or have you simply selected a "well known" port
number than happens to be the one assigned to NTP?

rick jones
--
The glass is neither half-empty nor half-full. The glass has a leak.
The real question is "Can it be patched?"
these opinions are mine, all mine; HP might not want them anyway...
feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...