Re: [RFC] EADDRINUSE from bind() on application restart after killing

From: Paul Gofman
Date: Fri Oct 14 2022 - 12:39:40 EST

Sorry if I was unclear, to reformulate my question, is blocking listening port (not the accept one) this way a IETF requirement? I am asking because I could not find where such a requirement stems from there. Sorry if I am missing the obvious.

On 10/14/22 11:34, Eric Dumazet wrote:
My question is if the behaviour of blocking listen socket port
while the accepted port (which, as I understand, does not have any
direct relation to listen port anymore from TCP standpoint) is still in
TIME_ or other wait is stipulated by TCP requirements which I am
missing? Or, if not, maybe that can be changed?

Please raise these questions at IETF, this is where major TCP changes
need to be approved.

There are multiple ways to avoid TIME_WAIT, if you really need to.


Thanks,
Paul.


On 10/14/22 11:20, Eric Dumazet wrote:
On Fri, Oct 14, 2022 at 8:52 AM Paul Gofman <pgofman@xxxxxxxxxxxxxxx> wrote:
Hello Eric,

our problem is actually not with the accept socket / port for which
those timeouts apply, we don't care for that temporary port number. The
problem is that the listen port (to which apps bind explicitly) is also
busy until the accept socket waits through all the necessary timeouts
and is fully closed. From my reading of TCP specs I don't understand why
it should be this way. The TCP hazards stipulating those timeouts seem
to apply to accept (connection) socket / port only. Shouldn't listen
socket's port (the only one we care about) be available for bind
immediately after the app stops listening on it (either due to closing
the listen socket or process force kill), or maybe have some other
timeouts not related to connected accept socket / port hazards? Or am I
missing something why it should be the way it is done now?

To quote your initial message :

<quote>
We are able to avoid this error by adding SO_REUSEADDR attribute to the
socket in a hack. But this hack cannot be added to the application
process as we don't own it.
</quote>

Essentially you are complaining of the linux kernel being unable to
run a buggy application.

We are not going to change the linux kernel because you can not
fix/recompile an application.

Note that you could use LD_PRELOAD, or maybe eBPF to automatically
turn SO_REUSEADDR before bind()


Thanks,
Paul.


On 9/30/22 10:16, Eric Dumazet wrote:
On Fri, Sep 30, 2022 at 6:24 AM Muhammad Usama Anjum
<usama.anjum@xxxxxxxxxxxxx> wrote:
Hi Eric,

RFC 1337 describes the TIME-WAIT Assassination Hazards in TCP. Because
of this hazard we have 60 seconds timeout in TIME_WAIT state if
connection isn't closed properly. From RFC 1337:
The TIME-WAIT delay allows all old duplicate segments time
enough to die in the Internet before the connection is reopened.

As on localhost there is virtually no delay. I think the TIME-WAIT delay
must be zero for localhost connections. I'm no expert here. On localhost
there is no delay. So why should we wait for 60 seconds to mitigate a
hazard which isn't there?
Because we do not specialize TCP stack for loopback.

It is easy to force delays even for loopback (tc qdisc add dev lo root
netem ...)

You can avoid TCP complexity (cpu costs) over loopback using AF_UNIX instead.

TIME_WAIT sockets are optional.
If you do not like them, simply set /proc/sys/net/ipv4/tcp_max_tw_buckets to 0 ?

Zapping the sockets in TIME_WAIT and FIN_WAIT_2 does removes them. But
zap is required from privileged (CAP_NET_ADMIN) process. We are having
hard time finding a privileged process to do this.
Really, we are not going to add kludges in TCP stacks because of this reason.

Thanks,
Usama


On 5/24/22 1:18 PM, Muhammad Usama Anjum wrote:
Hello,

We have a set of processes which talk with each other through a local
TCP socket. If the process(es) are killed (through SIGKILL) and
restarted at once, the bind() fails with EADDRINUSE error. This error
only appears if application is restarted at once without waiting for 60
seconds or more. It seems that there is some timeout of 60 seconds for
which the previous TCP connection remains alive waiting to get closed
completely. In that duration if we try to connect again, we get the error.

We are able to avoid this error by adding SO_REUSEADDR attribute to the
socket in a hack. But this hack cannot be added to the application
process as we don't own it.

I've looked at the TCP connection states after killing processes in
different ways. The TCP connection ends up in 2 different states with
timeouts:

(1) Timeout associated with FIN_WAIT_1 state which is set through
`tcp_fin_timeout` in procfs (60 seconds by default)

(2) Timeout associated with TIME_WAIT state which cannot be changed. It
seems like this timeout has come from RFC 1337.

The timeout in (1) can be changed. Timeout in (2) cannot be changed. It
also doesn't seem feasible to change the timeout of TIME_WAIT state as
the RFC mentions several hazards. But we are talking about a local TCP
connection where maybe those hazards aren't applicable directly? Is it
possible to change timeout for TIME_WAIT state for only local
connections without any hazards?

We have tested a hack where we replace timeout of TIME_WAIT state from a
value in procfs for local connections. This solves our problem and
application starts to work without any modifications to it.

The question is that what can be the best possible solution here? Any
thoughts will be very helpful.

Regards,

--
Muhammad Usama Anjum