How to change MTU size on SCO Unix

How do I change MTU size on SCO OpenServer Release 5?
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MashaCPAAsked:
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mikelfritzCommented:
use "ifconfig -a" to find the interface name :

# ifconfig -a
net0: flags=4043<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
        inet 192.168.10.4 netmask ffffff00 broadcast 192.168.10.255
        perf. params: recv size: 24576; send size: 24576; full-size frames: 1
        ether 00:b0:d0:fc:b7:49
lo0: flags=4049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 8232
        inet 127.0.0.1 netmask ff000000
        perf. params: recv size: 57344; send size: 57344; full-size frames: 1
atl0: flags=404a<BROADCAST,LOOPBACK,RUNNING,MULTICAST> mtu 1000
        inet 0.0.0.0 netmask 0
        perf. params: recv size: 4096; send size: 8192; full-size frames: 1

In this case my adapter is "net0"
So, now I can run:

ifconfig net0 mtu 1460

I, of course will not do that to my server...
I will do it to atl0 as a demo though,

# ifconfig atl0 mtu 1460

# ifconfig -a
net0: flags=4043<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
        inet 192.168.10.4 netmask ffffff00 broadcast 192.168.10.255
        perf. params: recv size: 24576; send size: 24576; full-size frames: 1
        ether 00:b0:d0:fc:b7:49
lo0: flags=4049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 8232
        inet 127.0.0.1 netmask ff000000
        perf. params: recv size: 57344; send size: 57344; full-size frames: 1
atl0: flags=404a<BROADCAST,LOOPBACK,RUNNING,MULTICAST> mtu 1460
        inet 0.0.0.0 netmask 0
        perf. params: recv size: 4096; send size: 8192; full-size frames: 1
#

You see that the mtu changed...

Be careful though as SCO does not checking on the validity of the mtu, so you may bust the connection.  Do this from the console so you can change it without a network connection active.  This will only change it for this uptime.  to add it on reboot you will need a startup script to force it after the daemons come up.

I would need to know what version of SCO you are running to go much further, but a common thing would be to create a file "/etc/rc2.d/S99mtu" containing the proper ifconfig command.  rc2.d is the dir for run level 2 startup scripts which is the default on most Openserver systems.


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mikelfritzCommented:
On some releases of Openserver 5 there is a /etc/rc2.d/S85tcp file.  That is where it sets up the connection initially, so once you find the mtu you want you could modify that file and reboot. Mine reads:

S85tcp:         ifconfig -p net0 192.168.10.4  netmask 255.255.255.0 broadcast 1

on one of the lines of the file, you could change it to:

S85tcp:         ifconfig -p net0 192.168.10.4  netmask 255.255.255.0 broadcast 1 mtu 1460

That would do it at the NIC startup and would be cleaner if you have a file "/etc/rc2.d/S85tcp"

The 85 is not really that important - The "S" says it's a startup script and the following number is the order in which it executes.  S15xxx will execute before S25xxx .  Hope that's clear.

Mike
 
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MashaCPAAuthor Commented:
What if I cannot do it from the console?  I am 50  miles away and usually telnet to this server.  Can I do it by telnet?
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mikelfritzCommented:
The problem is if you use the wrong mtu the network connection could stop working, the telnet session would die and you would be blocked out.  It most likely would be fine but you never know.  Can you have an onsite user available to change it back if things go awry?
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MashaCPAAuthor Commented:
I actually did it via telnet with the ifconfig command, and it worked fine.  Does it mean that it will also work if done via telnet in the startup script?
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mikelfritzCommented:
Yes.  The command line did it until reboot.  Changing the script will make it do it again when the system reboots.
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MashaCPAAuthor Commented:
Great, thank you!  I have one last question - when I do ifconfig -a at some locations, I get three interfaces (net0, lo0, and atl0), but at one location I do not get atl0.    The adapter on which I am making the MTU change is net0, but at the location where atl0 is missing, MTU change does not resolve the fragmenting problem.

What is atl0?
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mikelfritzCommented:
atl0 has something to do with the loopback interface.  What does "netstat -i" show?
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MashaCPAAuthor Commented:
Name  Mtu   Network     Address         Ipkts    Ierrs Opkts    Oerrs  Coll
net0  1180  10.1.9      hc15            133140   0     154200   0      0
lo0   8232  loopback    localhost       3672     0     3672     0      0
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MashaCPAAuthor Commented:
Actually, I'll add it as a separate question.
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