FAQ
Hello List,

Due to a problem with CentOS 5 and one of our firewalls I am having to
disable tcp window scaling to stop network connections grinding to a
halt. I am also trying to Kickstart CentOS 5 machines through the
firewall so need to disable window scaling at install time.

I have added to the kickstart file

%pre
echo 0 > /proc/sys/net/ipv4/tcp_window_scaling

But the install still hangs while transferring the stage2.img

Should the above %pre work or do I need to do something else to disable
window scaling during the install.

In case anyone's interested CentOS 5 is the first time we have suffered
this problem both CentOS 3&4 worked without issue.

Thanks

Dean

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  • Ray Van Dolson at May 10, 2007 at 5:38 pm

    On Thu, May 10, 2007 at 06:34:54PM +0100, Plant, Dean wrote:
    Hello List,

    Due to a problem with CentOS 5 and one of our firewalls I am having to
    disable tcp window scaling to stop network connections grinding to a
    halt. I am also trying to Kickstart CentOS 5 machines through the
    firewall so need to disable window scaling at install time.

    I have added to the kickstart file

    %pre
    echo 0 > /proc/sys/net/ipv4/tcp_window_scaling

    But the install still hangs while transferring the stage2.img

    Should the above %pre work or do I need to do something else to disable
    window scaling during the install.

    In case anyone's interested CentOS 5 is the first time we have suffered
    this problem both CentOS 3&4 worked without issue.
    Weird issue. Haven't seen anyone else have it (at least on this list).

    Can you switch over to your shell console when the hang happens and
    examine the value of tcp_window_scaling to see if maybe it gets reset
    somehow? I believe %pre should work....

    Ray
  • Plant, Dean at May 10, 2007 at 6:04 pm

    Ray Van Dolson wrote:
    On Thu, May 10, 2007 at 06:34:54PM +0100, Plant, Dean wrote:
    Hello List,

    Due to a problem with CentOS 5 and one of our firewalls I am having
    to disable tcp window scaling to stop network connections grinding
    to a halt. I am also trying to Kickstart CentOS 5 machines through
    the firewall so need to disable window scaling at install time.

    I have added to the kickstart file

    %pre
    echo 0 > /proc/sys/net/ipv4/tcp_window_scaling

    But the install still hangs while transferring the stage2.img

    Should the above %pre work or do I need to do something else to
    disable window scaling during the install.

    In case anyone's interested CentOS 5 is the first time we have
    suffered this problem both CentOS 3&4 worked without issue.
    Weird issue. Haven't seen anyone else have it (at least on this
    list).

    Can you switch over to your shell console when the hang happens and
    examine the value of tcp_window_scaling to see if maybe it gets reset
    somehow? I believe %pre should work....
    Weird... Tell me about it, I spent a lot of time head scratching trying
    to figure out why network connections would just mysteriously hang.
    Eventually found reference to a problem with tcp_window_scaling and
    firewalls and after disabling tcp_window_scaling on my test CentOS 5
    box, everything started working.

    The install does not get far enough for the shell console to be
    available, it hangs at the transferring stage2.img.

    Dean
  • Ray Van Dolson at May 10, 2007 at 6:09 pm

    On Thu, May 10, 2007 at 07:04:51PM +0100, Plant, Dean wrote:
    Weird... Tell me about it, I spent a lot of time head scratching trying
    to figure out why network connections would just mysteriously hang.
    Eventually found reference to a problem with tcp_window_scaling and
    firewalls and after disabling tcp_window_scaling on my test CentOS 5
    box, everything started working.

    The install does not get far enough for the shell console to be
    available, it hangs at the transferring stage2.img.

    Dean
    Ah, probably before %pre even has run (?) Well you could rebuild the
    kernel on the install CD to disable window scaling by default. :) Ok,
    not a particularly useful option... maybe someone else will have a
    suggestion.

    In HP-UX you can pass kernel parameters at boot-time, I wonder if
    anything similar could be done? Probably not.

    Have you captured a tcpdump of what happens when the "freeze" occurs
    with an intermediary system? Maybe a setting could be changed on your
    switch? Although I would doubt the switch would be a factor. Are your
    packets going through an intermediary router?

    I plan to do some network installs of CentOS 5 soon, will be
    interesting to see if this issue occurs.

    Ray
  • Plant, Dean at May 11, 2007 at 8:09 am

    Ray Van Dolson wrote:
    On Thu, May 10, 2007 at 07:04:51PM +0100, Plant, Dean wrote:
    Weird... Tell me about it, I spent a lot of time head scratching
    trying to figure out why network connections would just mysteriously
    hang. Eventually found reference to a problem with
    tcp_window_scaling and firewalls and after disabling
    tcp_window_scaling on my test CentOS 5 box, everything started
    working.

    The install does not get far enough for the shell console to be
    available, it hangs at the transferring stage2.img.
    Ah, probably before %pre even has run (?) Well you could rebuild the
    kernel on the install CD to disable window scaling by default. :) Ok,
    not a particularly useful option... maybe someone else will have a
    suggestion.

    In HP-UX you can pass kernel parameters at boot-time, I wonder if
    anything similar could be done? Probably not.

    Have you captured a tcpdump of what happens when the "freeze" occurs
    with an intermediary system? Maybe a setting could be changed on your
    switch? Although I would doubt the switch would be a factor. Are
    your packets going through an intermediary router?

    I plan to do some network installs of CentOS 5 soon, will be
    interesting to see if this issue occurs.
    I don't think this is a issue with CentOS/RHEL. The kickstart in this
    instance is passing through a firewall appliance, which I think is
    Open/FreeBSD based and we now believe is breaking tcp window scaling. It
    seems that CentOS3&4 was unaffected by this but with v5 network
    connections hang after transferring about 500k. On a network that does
    not pass through this firewall v5 works as expected.

    Dean
  • Ray Van Dolson at May 11, 2007 at 1:10 pm

    On Fri, May 11, 2007 at 09:09:32AM +0100, Plant, Dean wrote:
    I don't think this is a issue with CentOS/RHEL. The kickstart in this
    instance is passing through a firewall appliance, which I think is
    Open/FreeBSD based and we now believe is breaking tcp window scaling. It
    seems that CentOS3&4 was unaffected by this but with v5 network
    connections hang after transferring about 500k. On a network that does
    not pass through this firewall v5 works as expected.
    Ah. Well, to partially confirm; did a CentOS 5 install via NFS
    yesterday. Would have passed through Cisco and Checkpoint routers /
    firewalls. No issues.

    Ray

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postedMay 10, '07 at 5:34p
activeMay 11, '07 at 1:10p
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Ray Van Dolson: 3 posts Plant, Dean: 3 posts

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