[ILUG] Bizarre networking problem

[Niall O Broin]

As regulars on #linux will know, I've been having my share of  
networking fun this week. One of the sources of fun was an ISDN point  
to point link to a customer site using two old Elsa routers. This was  
eventually traced to the router on our side having forgotten its ISDN  
protocol settings and having reset that value, the connection was  
back up - but not quite ;-)

Now we get to the part where it gets interesting. We can no longer  
establish connections from a Solaris 8 box on our site to a target  
also running Solaris 8 on the customer site. We CAN establish  
connections from boxes running Linux, and Solaris 2.5.1 .  Sadly, the  
machine which most needs to establish connections is, of course, the  
Solaris 8 box.

For ease of description:

s8A = Solaris 8 box on our site
s8B = Solaris 8 box on customer site
s251 = Solaris 2.5.1 box on our site
tiger = Macbook Pro running tiger
linux = linux box on our site


Running snoop on s8B, I see repeated syn packets come in from s8A on  
attempting an ssh connection, but no acks go back. If I repeat this  
experiment with tiger on the customer site, using tcpdump, I see the  
same thing i.e. syn packets a-plenty, but no acks. However, in  
tcpdumps option list, I see bad-opt reported, which definitely looks  
like a dark skinned person in the fuel store to me, having RTFM of  
tcpdump. If I make ssh connections from linux or s251 to s8B snoop  
output looks as it should - incoming syn, then ack, and the ssh  
connection is made.

So, WHAT can be at play here? Clearly the ISDN router is a huge  
suspect, but why would it decide to bugger up only connections from  
s8A? There has been NO recent OS changes done on s8A BTW.  I can make  
ssh conenctions from s8A to other machines in the LAN here (including  
tiger) without problems.

Any ideas welcomed, because I'm stumped.



Niall