Dynamic routes in TCP/IP Services (335 Views)
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Trusted Contributor
Jeremy Begg
Posts: 380
Registered: ‎01-20-2005
Message 1 of 6 (335 Views)

Dynamic routes in TCP/IP Services

This is a variation on my posting of 18-Aug-2011 where I asked about disabling Path MTU Discovery.

Having done that, the problem still occurs.  So let me try again.

 

The system is running OpenVMS VAX V7.3 with TCP/IP Services V5.3 ECO 4.

 

There are many printer queues sending to various HP laserjet printers around the country and most of them work fine.  However one queue in particular is prone to stalling each evening at about 1:30am when the first job of the day is queued to it.

 

After some investigation I determined that the stall was due to an incorrect TCP/IP route being present on the system.

 

TCP/IP Services is configured with static routes (only), no dynamic routing and no Path MTU Discovery.

The static routing table looks like this:

 

Sys_T1> tcpip show route/perm                         
 
                             PERMANENT
 
Type           Destination                           Gateway
 
PN    0.0.0.0                               192.168.50.253
PN    192.160.0.0/16                        192.168.50.252
PN    192.168.0.0/16                        192.168.50.252

 

For some reason the following route is popping up when the TCPIP$TELNETSYM symbiont attempts to contact the printer at IP address 192.160.24.103

 

                             DYNAMIC
 
Type           Destination                           Gateway
 

DH    192.160.24.103                        192.168.50.254

 

If I manually delete the route it usually goes away and the print job runs to completion.  However that's provided I delete the route after 9am (more or less); it seems that someone insists on having this route on the system between 1am and 9am!  (The gateway in question is the Internet firewall, but the printer is not reachable through it.)

 

Can someone tell me, what TCP/IP component or config setting would cause a route to be dynamically added?  As I said, I've not enabled dynamic routing and I've disabled Path MTU Discovery. Those are the only two mechanisms I know of which could automatically add routes to the system, but maybe there's one I don't know about.

 

Thanks,

Jeremy Begg

 

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Honored Contributor
Hoff
Posts: 4,920
Registered: ‎01-29-2006
Message 2 of 6 (323 Views)

Re: Dynamic routes in TCP/IP Services

Watch your IP traffic for routing information; see if you can spot where this route is arising.

 

And is this VAX environment emulated?

 

 

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Honored Contributor
John Gillings
Posts: 2,992
Registered: ‎07-31-2003
Message 3 of 6 (298 Views)

Re: Dynamic routes in TCP/IP Services

Jeremy,

 

   I've had trouble with dynamic routes before. Not sure I understand exactly how they work, but the blame is possibly outside the OpenVMS system. A router will occasionally give feedback saying "this is a better route", which somehow gets into the dynamic route table. Unfortunately, it can be wrong! In one case, my own workstation would get a subnet route pointing back to itself, breaking all inward and outward network connections.

 

  I gave up trying to figure out where it was coming from, and treated the symptom with a job which checked for and deleted any dynamic routes every 30 minutes or so (I may have been more selective, don't remember the exact details). If you want to be more selective, maybe have a job which pings your target address, if that every fails, delete the dynamic route.

 

  Deleting a dynamic route should always be "safe", as they're an optimisation. Worst case is you will take a longer path than necessary.

 

  In a well designed network, dynamic routes shouldn't be necessary, and as you've discovered, can break things if they're incorrect.

A crucible of informative mistakes
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Honored Contributor
Andy Bustamante
Posts: 975
Registered: ‎10-24-2003
Message 4 of 6 (291 Views)

Re: Dynamic routes in TCP/IP Services

This is probably coming from an icmp redirect message.  A gateway can provide an updated "better" route to a target host back to the originating node.  This a legitmate behavior, but if something on your network is generating these messages in error, that will cause problems. 

 

Kick this back to your networking team.  Run a packet capture.  There may be a misconifgured router or a duplicate address somewhere on the network. 

If you don't have time to do it right, when will you have time to do it over? Reach me at first_name + "." + last_name at sysmanager net
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Trusted Contributor
Jeremy Begg
Posts: 380
Registered: ‎01-20-2005
Message 5 of 6 (279 Views)

Re: Dynamic routes in TCP/IP Services

Hoff, yes, this is an emulated environment but I'm not sure how that's relevant.

 

Andy, your suggestion that it's ICMP redirects seems like a good one.  I've thought it rather odd that the erroneous route is added at roughly the same time every day and maybe this remote site is on a link which goes down regularly (I don't know, but I'll try to find out).

 

I'll have another go at pushing this back to the networking team.


Thanks,

Jeremy Begg

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Honored Contributor
Hoff
Posts: 4,920
Registered: ‎01-29-2006
Message 6 of 6 (269 Views)

Re: Dynamic routes in TCP/IP Services

You're potentially tussling with a network emulation layer that can classically be problematic in various hardware emulators, and you're also involving another operating system platform as a network gateway.  These components can and do introduce IP routes, can mess up network connections, can encounter address collisions, and can implement the evil that is NAT.  

 

Would you rule out networking hardware and a switch and a gateway router that's the primary connection for a physical VAX out to a larger network?  That's what the emulator is providing you with here, after all.

 

The ability to view what the emulator and the host and the network emulation layer is doing is dicy here, too; you can't tap into that nearly as easily as you can mirror a switch port to see what's going on.

 

If you can mirror the switch port, you can see if the routing update is external.   If not, then it's probably the morass that can be the emulator, host and network layer that's "getting helpful".

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