full socket buffers

Chris Marget chris at marget.com
Wed Feb 9 12:52:59 UTC 2011

A little background on the OS tunables, which may help the OP get his
problem straightened out.

Or not.  Frankly, the "it starts to fail after a few days" business makes it
sound like it's not directly a buffer problem, but rather a buffer servicing

There are usually two OS settings that are relevant.

One of them is a ceiling.  Applications are not allowed to configure their
socket buffer larger than the ceiling.

The other is the default size.  Applications my re-size their buffers
(probably with setsockopt) from the default, within the constraints of the

To get big buffers without application involvement usually requires tweaking
both of these.
Getting big buffers within the application usually requires tweaking only
the ceiling, because the default is irrelevant.

On some OSes, when the application requests a value larger than the ceiling,
the syscall returns a failure, and the buffer is unchanged.

On other OSes, when the application requests a value larger than the
ceiling, the syscall returns a success, but the buffer is actually
configured to the ceiling-limited size.  It's up to the application to know
about this possibility and test the size with getsockopt.

On Wed, Feb 9, 2011 at 7:38 AM, Mirek Lauš <mirek at admino.cz> wrote:

> According to this: http://www.29west.com/docs/THPM/udp-buffer-sizing.html
> the kern.ipc.maxsockbuf is the right oid. The problem only occurs after few
> days
> of dhcpd process running then it begins do drop UDP randomly forcing us to
> restart dhcpd process. Then everything goes well for few days again. Now
> I really don't know where to look for help.
> Regards,
> Miroslav
> On Sun, Feb 6, 2011 at 11:54 AM,  <sthaug at nethelp.no> wrote:
> >> > This is an operating system setting. Try google for your OS and
> something
> >> > like "set udp buffer size".
> >> >
> >>
> >> This was of course the first thing we did before upgrading the RAID
> controller:
> >>
> >> [root at dhcp1 ~]# sysctl kern.ipc.maxsockbuf
> >> kern.ipc.maxsockbuf: 8388608
> >
> > You might also want to investigate net.inet.udp.recvspace.
> >
> > Steinar Haug, Nethelp consulting, sthaug at nethelp.no
> >
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