Hour long timeout to ssh/telnet/ftp to down host?

Hour long timeout to ssh/telnet/ftp to down host?

[Rob Landley]

I have scripts that ssh into large numbers of boxes, which are sometimes 
down.  The timeout for figuring out the box is down is over an hour.  This is 
just insane.

Telnet and ftp behave similarly, or at least tthey lasted the 5 minutes I was 
willing to wait, anyway.  Basically anything that calls connect().  If the 
box doesn't respond in 15 seconds, I want to give up.

Is this a problem with the kernel or with glibc?  If it's the kernel, I'd 
expect a /proc entry where I can set this, but I can't seem to find one.  Is 
there one?  What would be involved in writing one?

If it's glibc I'm probably better off writing a wrapper to ping the 
destination before trying to connect, or killing the connection after a 
timeout with no traffic.  But both of those are really ugly solutions.

Anybody have any light to shed on the situation?

Rob

Re: Hour long timeout to ssh/telnet/ftp to down host?

[Ben Greear]

Rob Landley wrote:
> 
> I have scripts that ssh into large numbers of boxes, which are sometimes
> down.  The timeout for figuring out the box is down is over an hour.  This is
> just insane.
> 
> Telnet and ftp behave similarly, or at least tthey lasted the 5 minutes I was
> willing to wait, anyway.  Basically anything that calls connect().  If the
> box doesn't respond in 15 seconds, I want to give up.
> 
> Is this a problem with the kernel or with glibc?  If it's the kernel, I'd
> expect a /proc entry where I can set this, but I can't seem to find one.  Is
> there one?  What would be involved in writing one?
> 

You can tune things by setting the tcp-timeout probably..I don't
know exactly where to set this..

You probably don't want all tcp to time out at 15 seconds anyway, so
I'd suggest either using non-blocking connect (if you have the code
that does the connect), or just set a timer (or use sigalarm) when you
start the attempt, and fail the attempt if the timer or alarm signal
goes off.

> If it's glibc I'm probably better off writing a wrapper to ping the
> destination before trying to connect, or killing the connection after a
> timeout with no traffic.  But both of those are really ugly solutions.

Ugly is relative, and don't use ping because there is still a race condition
(ping worked, but by the time you try tcp, the box is down.)

> 
> Anybody have any light to shed on the situation?
> 
> Rob
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-- 
Ben Greear <greearb@candelatech.com>          <Ben_Greear@excite.com>
President of Candela Technologies Inc      http://www.candelatech.com
ScryMUD:  http://scry.wanfear.com     http://scry.wanfear.com/~greear

Re: Hour long timeout to ssh/telnet/ftp to down host?

[Rob Landley]

>You can tune things by setting the tcp-timeout probably..I don't
>know exactly where to set this..

Aha, found it.  /proc/sys/net/ipv4/tcp_syn_retries

I am a victim of the exponential retry falloff, it would seem.  syn_retries 
of 1 takes a few seconds, 3 takes less than half a minute, and 5 takes 
several minutes.  The default value of 10 is what's giving me the problem 
(something like 20 minutes to time out, according to my earlier tests.)

Then the fact that ssh then re-attempts the connection four times before 
actually failing is where I got my hour and change timeout.  ("ssh -v -v -v" 
comes in handy...)

Fun.

Can we change the default value for this to something more sane, like 5?  
Exponential falloff is not good when your order of magnitude hits double 
digits.

> You probably don't want all tcp to time out at 15 seconds anyway, so

Just connection initiation.  (If their ip stack hasn't replied to me by then, 
I doubt it's going to.)

> I'd suggest either using non-blocking connect (if you have the code
> that does the connect), or just set a timer (or use sigalarm) when you
> start the attempt, and fail the attempt if the timer or alarm signal
> goes off.

Except I'm using off-the-shelf ssh.  (I asked them about this problem a month 
ago, and there was some discussion of a workaround on their mailing list, but 
2.9 came out and still had the same behavior.  Apparently, if it's not a 
problem in OpenBSD, it's not a problem in OpenSSH...)

> > If it's glibc I'm probably better off writing a wrapper to ping the
> > destination before trying to connect, or killing the connection after a
> > timeout with no traffic.  But both of those are really ugly solutions.
>
> Ugly is relative, and don't use ping because there is still a race
> condition (ping worked, but by the time you try tcp, the box is down.)

Yeah, but it would eventually time out and recover, I've got ten threads out 
querying boxes, that's a really rare race condition.  And I already 
acknowledged it was ugly. :)

So the problem is just that tcp_syn_retries' default value of 10 is way too 
high due to the exponentially increasing gap between each retry.

Rob

Re: Hour long timeout to ssh/telnet/ftp to down host?

[Luigi Genoni]

On Tue, 12 Jun 2001, Ben Greear wrote:

> Rob Landley wrote:
> >
> > I have scripts that ssh into large numbers of boxes, which are sometimes
> > down.  The timeout for figuring out the box is down is over an hour.  This is
> > just insane.
> >
> > Telnet and ftp behave similarly, or at least tthey lasted the 5 minutes I was
> > willing to wait, anyway.  Basically anything that calls connect().  If the
> > box doesn't respond in 15 seconds, I want to give up.
> >
> > Is this a problem with the kernel or with glibc?  If it's the kernel, I'd
> > expect a /proc entry where I can set this, but I can't seem to find one.  Is
> > there one?  What would be involved in writing one?
> >
>
> You can tune things by setting the tcp-timeout probably..I don't
> know exactly where to set this..

/proc/sys/net/ipv4/tcp_fin_timeout

default is 60.

Re: Hour long timeout to ssh/telnet/ftp to down host?

[Rob Landley]

On Wednesday 13 June 2001 05:40, Luigi Genoni wrote:
> On Tue, 12 Jun 2001, Ben Greear wrote:

> > You can tune things by setting the tcp-timeout probably..I don't
> > know exactly where to set this..
>
> /proc/sys/net/ipv4/tcp_fin_timeout
>
> default is 60.

Never got that far.  My problem was actually tcp_syn_retries. Remember, I was 
talking to a host that was unplugged.  (I wasn't even getting "host 
unreachable" messages, the packets were just disappearing.)  The default 
timeout in that case is rediculous do to the exponentially increasing delays 
between retries.  10 retries wound up being something like 20 minutes.

I set it to 5 and everything works beautifully now.  ssh (which retries the 
connection 4 times, and used to take over an hour to time out) now takes just 
over 3 minutes, which I can live with.

Rob