TCP conection problems IBM VM -> FreeBSD

Chuck Swiger cswiger at
Thu Mar 22 17:03:43 UTC 2007

On Mar 21, 2007, at 5:03 PM, Torbjorn Granlund wrote:
> When vm attempts to make a TCP connection (e.g., on port 25) to
> I see the following traffic on the router:
> 22:46:27.015389 IP > S  
> 27523124:27523124(0) win 8192 <mss 1420,wscale  
> 0,nop,nop,nop,timestamp 1888741492 0>
> 22:46:27.015523 IP > S  
> 1745147473:1745147473(0) ack 3530628660 win 57344 <mss 1460>
> 22:46:27.056277 IP > R  
> 3530628660:3530628660(0) win 0
> I.e., the vm box appears to dislike the SYNACK from, and
> sends an RST.  One might ask if it is the fault of vm or of  

The second line should have been SYN+ACK'ing the  
ISN of 27523124.  vm is sending a RST to that because the sequence  
#'s don't match.  It's also odd that the set of options being listed  
don't correspond at all...if you run the tcpdump for several minutes,  
can you track down other SYN requests which do correspond?

Sometimes this kind of re-writing can happen if natd or PF is  
attempting to translate the packets, perhaps when they shouldn't if  
both sides of your router box are using routable IPs....


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