strange routes appearing after boot on AiX 5.2

[trifo]

Hi!

I have some strange routes coming up on my AiX 5.2 boxes, as the example below:

root@c51 / netstat -rn
Routing tables
Destination      Gateway           Flags   Refs     Use  If   PMTU Exp Groups

Route Tree for Protocol Family 2 (Internet):
default          10.128.0.1        UG        4      805  en0     -   -      -
10.128.0.0       10.128.2.51       UHSb      0        0  en0     -   -      -   =>
10.128.0.0       10.128.2.73       UHSb      0        0  en3     -   -      -   =>
10.128/22        10.128.2.51       U         4     1631  en0     -   -      -   =>
10.128/22        10.128.2.73       U         2      463  en3     -   -      -
10.128.2.51      127.0.0.1         UGHS      9      723  lo0     -   -      -
10.128.2.72      127.0.0.1         UGHS      0       18  lo0     -   -      -
10.128.2.73      127.0.0.1         UGHS      1       97  lo0     -   -      -
10.128.3.255     10.128.2.51       UHSb      0        4  en0     -   -      -   =>
10.128.3.255     10.128.2.73       UHSb      0        0  en3     -   -      -
127/8            127.0.0.1         U        26     3656  lo0     -   -      -
192.168.0.0      192.168.0.51      UHSb      0        0  en4     -   -      -   =>
192.168.0/24     192.168.0.51      U        24     8096  en4     -   -      -
192.168.0.51     127.0.0.1         UGHS     34      433  lo0     -   -      -
192.168.0.255    192.168.0.51      UHSb      0        5  en4     -   -      -

Route Tree for Protocol Family 24 (Internet v6):
::1              ::1               UH        0        7  lo0 16896   -      -
root@c51 /

The AiX develops routes for all three network interfaces which are host routes but looks like class B network routes:

10.128.0.0       10.128.2.51       UHSb      0        0  en0     -   -      -   =>
10.128.0.0       10.128.2.73       UHSb      0        0  en3     -   -      -   =>
192.168.0.0      192.168.0.51      UHSb      0        0  en4     -   -      -   =>

These routes sometimes deny packages to flow to and from the boxes. They appear during every boot and sometimes during uptime also. Have you seen similar things to happen? How to set network parameters to avoid these flase routes to appear?

--Trifo

 

[mart1]

Hi,

I already noticed that AIX add lots of route in his routing tables. I think that it keep in the routing table all the hosts that have connected to it.
The strange thing is that you have some route that appear during boot (but may be some host tries to connect ot the box before you can even login).
What you can try is :
netstat -n -f ; netstat -rn
The first one will clear the route that the machine have added. Then you should see only the default OS routes.

 

[trifo]

mart1: I think you mean path MTU discovery. If it is turned on, AIX determines maximum MTU for every host it communicates with, and sets a host route using the discovered MTU value. These routes have a "W" flag thus you can easily see which routes were added. Also path MTU recovery can be turned off using "no" command.

no -o tcp_pmtu_discover=0
no -o udp_pmtu_discover=0

My problem is different. I have only one faulty route entry per adapter. These are host routes looking like class B network addresses.

--Trifo

 

[Breslau]

i'm thinking this a 'feature' of 5.2. if you look up the docs on netstat for aix 5.2 the 'b' designates a broadcast address. it may just be that the netstat command is giving more information about the routing table than it did in previous releases.

just a guess on my part tho, haven't dug around enough to be sure yet.

 

[hirschaj]

The "S" means that the route was manually added. Check your startup scripts for route commands. Also, please provide output from lsattr -El inet0.


Jim Hirschauer

http://www.aixexpert.com

 

[trifo]

Well, all routes referring to local interfaces have the "S" flag. It is also given to route entries which appear on setting up an IP alias using ifconfig or even chdev.

Here I provide the output of lsattr -El inet0

root@c51 / lsattr -El inet0
authm         65536                         Authentication Methods              True
bootup_option no                            Use BSD-style Network Configuration True
gateway                                     Gateway                             True
hostname      c51                           Host Name                           True
rout6                                       IPv6 Route                          True
route         net,-hopcount,0,,0,10.128.0.1 Route                               True
root@c51 /

 

[hirschaj]

Your output from inet0 looks fine. I would take a close look at any startup scripts listed in your inittab. I had a similar issue once and it was due to someone adding a route command in a startup script.


Jim Hirschauer

http://www.aixexpert.com

 

[trifo]

There is NO route command in our startup scripts. We all here are aware of ODM knowledge related to routing entries. And yet there are no extra routing entries in the ODM, just the only one for the "default gw".

Some network options maybe? Like the path MTU discovery?

--Trifo

 

[hirschaj]

I always turn Path MTU discovery off, but like you mentioned earlier, those routes should have a "W" next to them if pmtu was the culprit.


Jim Hirschauer

http://www.aixexpert.com

 

[hirschaj]

One other thing I wanted to mention was that it looks like the system has not tried to use those routes (from your initial post). How did you come to the conclusion that those routes are a problem?


Jim Hirschauer

http://www.aixexpert.com

 

[trifo]

Well, I know that my case is not the PMTU discovery thing. I meant if it is some slimilar flag to turn off or stuff.

--Trifo

 

[hirschaj]

Looks like those routes are added anytime an interface is configured. This seems to be a new feature.....

http://www-1.ibm.com/support/docview.wss?uid=isg1pTechnote1456

Jim Hirschauer

http://www.aixexpert.com

 

[trifo]

Yes, I know this ThechNote. It mentions 3 routes added during interface configuration. But my problematic routes are extra fourth routing entries per adapter, above the three mentioned in the TechNote. I have checked that before.

--Trifo