TCP time_wait and port exhaustion for servers

Mark Andrews marka at
Wed Dec 5 22:35:29 UTC 2012

In message <CAP-guGVSMXgt-xhnqC191-Mfh2-W38Gg5mZf1MegCgwObVKK-Q at>,
 William Herrin writes:
> On Wed, Dec 5, 2012 at 5:01 PM, Mark Andrews <marka at> wrote:
> > In message <CAP-guGW6oXo=UfTfg+SDiFjB4=qxPShO+YfK6vxnLkCC58PvgQ at
> om>,
> >  William Herrin writes:
> >> The thing is, Linux doesn't behave quite that way.
> >>
> >> If you do an anonymous connect(), that is you socket() and then
> >> connect() without a bind() in the middle, then the limit applies *per
> >> destination IP:port pair*. So, you should be able to do 30,000
> >> connections to port 80, another 30,000 connections to
> >> port 80, and so on.
> >
> > The socket api is missing a bind + connect call which restricts the
> > source address when making the connect.  This is needed when you
> > are required to use a fixed source address.
> Hi Mark,
> There are ways around this problem in Linux. For example you can mark
> a packet with iptables based on the uid of the process which created
> it and then you can NAT the source address based on the mark. Little
> messy but the tools are there.

And not available to the ordinary user.  Nameservers potentially run
into this limit.  This is something The OpenGroup need to address when
updating the next revision of the socket api in POSIX.
> Anyway, Ray didn't indicate that he needed a fixed source address
> other than the one the machine would ordinarily choose for itself.

But he didn't say it wasn't required either.

Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at

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