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-   -   No SYNACK to port 80? (http://www.linux-archive.org/debian-user/44772-no-synack-port-80-a.html)

"st.john.johnson@gmail.com" 02-01-2008 04:02 PM

No SYNACK to port 80?
 
Hi there. I'm having a strange problem. Sometimes, for short periods
of time, when connecting to my web server from an external IP address,
the connection doesn't complete. But at the same time, I can connect
from a local ip address.

I ran tshark on the machine to monitor traffic when these "short
periods" happen and I noticed that for external connections, my
machine is not replying to the ACK in the three-way handshake.

0.000000 69.120.132.213 -> 10.1.1.202 TCP 52197 > www [SYN] Seq=0
Len=0 MSS=1460 WS=2
3.006786 69.120.132.213 -> 10.1.1.202 TCP 52197 > www [SYN] Seq=0
Len=0 MSS=1460 WS=2
8.973167 69.120.132.213 -> 10.1.1.202 TCP 52197 > www [SYN] Seq=0
Len=0 MSS=1460

I don't have iptables installed, and SELinux is not enabled.


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"st.john.johnson@gmail.com" 02-08-2008 02:34 AM

No SYNACK to port 80?
 
On Feb 1, 12:20 pm, "st.john.john...@gmail.com"
<st.john.john...@gmail.com> wrote:
> Hi there. I'm having a strange problem. Sometimes, for short periods
> of time, when connecting to my web server from an external IP address,
> the connection doesn't complete. But at the same time, I can connect
> from a local ip address.
>
> I ran tshark on the machine to monitor traffic when these "short
> periods" happen and I noticed that for external connections, my
> machine is not replying to the ACK in the three-way handshake.
>
> 0.000000 69.120.132.213 -> 10.1.1.202 TCP 52197 > www [SYN] Seq=0
> Len=0 MSS=1460 WS=2
> 3.006786 69.120.132.213 -> 10.1.1.202 TCP 52197 > www [SYN] Seq=0
> Len=0 MSS=1460 WS=2
> 8.973167 69.120.132.213 -> 10.1.1.202 TCP 52197 > www [SYN] Seq=0
> Len=0 MSS=1460
>
> I don't have iptables installed, and SELinux is not enabled.

It happened again, and I captured the raw packets. There is NO
difference between two packets coming in except one is from an
external IP and one is from an internal one.
The internal one is replied to with SYNACK and the external one is
ignored. I'm not sure how to continue debugging this. I can post my
raw capture.


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