[nos-bbs] UDP Port Unreachable - problem found

Michael E Fox - N6MEF n6mef at mefox.org
Sat Nov 16 21:20:10 EST 2013

No Bob, I'm talking about two JNOS systems behind the same firewall.   The firewall has to change the source port of at least the second one on the way out.  Otherwise, the destination IP and destination port are all the same on the way back in.  I believe I've already mentioned this a couple of times.  This is basic firewall connection muxing.

Sonicwall happens to change the source port all the time, instead of after the first connection.  And it works just fine for everything but JNOS.  If JNOS behaved like a normal UDP app, it would work fine, too.

The point is that there is simply no reason to require a specific source port.  It's just not the way the UDP world works.  And doing so renders the system unworkable when placed behind 10s of 1000s of commercial firewalls.

The whole point of axudp is so it can be used in situations where axip can't be used.  But with this bizarre restriction, it's defeating that purpose.


Sent from my Verizon Wireless 4G LTE smartphone

-------- Original message --------
From: Bob Tenty <bobtenty at gmail.com> 
Date:11/16/2013  3:38 PM  (GMT-08:00) 
To: TAPR xNOS Mailing List <nos-bbs at tapr.org> 
Subject: Re: [nos-bbs] UDP Port Unreachable - problem found 


You are making a thinking error here.

If I make a link to another jnos system the destination ip number is different.
Also the return route from that second jnos system uses another ip number as the first jnos system so there is no problem it all.
Even with the same port.  It is the combination of ip number + port.

Those consumer router/firewall boxes are cheaply designed and targeted for the average Joe customer user needs.
Install dd-wrt in it if available for it and you will be much happier.



On 13-11-16 12:35 PM, Michael E. Fox - N6MEF wrote:
Yes, Linux leaves the source port alone on the first connection.  But that only works for the first JNOS system.  Even a firewall that initially leave the source port alone will need to change the source port if a second JNOS system exists so it can track connections to two different machines.
From: nos-bbs-bounces at tapr.org [mailto:nos-bbs-bounces at tapr.org] On Behalf Of Bob Tenty
Sent: Friday, November 15, 2013 11:37 PM
To: TAPR xNOS Mailing List
Subject: Re: [nos-bbs] UDP Port Unreachable - problem found
Yes, I have seen             that with those boxes.  That is why I always replace the firmware with linux when possible.


On 13-11-16 01:33 AM, Michael E. Fox - N6MEF wrote:
I found the problem with the UDP port 93 unreachable message: JNOS is (incorrectly) requiring the source port to also be 93 in AXUDP connections.
When I connect outbound from my JNOS system, through my firewall, the firewall is changing the source port when it performs the outbound NAT. But this is normal for a firewall. In fact, it HAS to do this if it’s going to allow for multiple connects of the same protocol from different machines. Many consumer-grade firewalls will leave the source port alone for the first connection (if it’s not already in use) and only change it for subsequent connections. SonicWall is a bit more strict, frequently changing the source port, making it harder for intercepted packets to be tracked to any one machine.
Normally, this doesn’t matter. Applications/services listen on a particular port and respond to whatever incoming connections use that *destination* port. They don’t care what the source port is. Firewalls then use different source ports to track multiple conversations so that when the packets return, all addressed to the same external NAT address, it can direct them to the proper place by the port number.
But when JNOS receives an AXUDP packet, apparently it doesn’t behave like a normal UDP application. JNOS apparently rejects the connection if the *source* port is not 93, even if the destination port is correctly set to 93. This is unusual, to say the least. But even worse, it issues an ICMP “udp port 93 unreachable” message which is completely wrong, since port 93 is definitely reachable.
It seems the following is needed: Remove the source port restriction for AXUDP. JNOS should not care what the source port is. And, just like any other UDP app, when responding it should use whatever source port was specified as the destination port when it constructs the return packet.

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