Mailing List Archive

[Bug 595] lots of simultaneous ssh's cause sporadic failures

Summary: lots of simultaneous ssh's cause sporadic failures
Product: Portable OpenSSH
Version: -current
Platform: All
OS/Version: All
Status: NEW
Severity: normal
Priority: P2
Component: ssh

Openssh seems to fail sporadically if you issue lots of simultaneous
ssh commands. Take the following program:


for NUM in 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15; do
ssh echo $NUM &

So, we're running 16 ssh commands at once, each of which just prints out a
different number.

When I run this program, several of the ssh commands fail with

ssh_exchange_identification: Connection closed by remote host

Interestingly, when I run 10 or fewer ssh commands, they all work OK, at
least on my linux box (I'm using OpenSSH 3.5p1-6 on Redhat Linux 9). On
some other platforms the number is different: OpenSSH 3.2.3p1 on an IBM
SP doesn't like more than 8 simultaneous ssh's in the background, while
OpenSSH_3.6.1p1 on Tru64 does around 9 max.

There doesn't seem to be any pattern in terms of which ssh's get
killed--the first, second and third jobs (ie, those that print 0, 1, and
2) generally run OK, but which of the following ones die seems to be

This smells like some kind of race condition.

Why on earth would I want to run a dozen ssh jobs simultaneously? I'm
writing a compiler that needs to ship some files and run some commands
on a remote server as part of the compilation process. The latency for
doing this is rather high, so I want to allow users to do a 'make -j' to
parallelize the build, in order to hide the network latency.

I guess for now I'll tell users to run 'make -j N' with N < 6 or so
(which is probably not a bad idea anyway). But I can't imagine I'll be
the last person to pound on ssh like this...

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[Bug 595] lots of simultaneous ssh's cause sporadic failures [ In reply to ] changed:

What |Removed |Added
Resolution| |INVALID

------- Additional Comments From 2003-06-17 10:08 -------
Oops. This is not a bug...

Sounds like someone is not reading the manpages. <smile>

man sshd_config
Specifies the maximum number of concurrent unauthenticated con-
nections to the sshd daemon. Additional connections will be
dropped until authentication succeeds or the LoginGraceTime ex-
pires for a connection. The default is 10.

Alternatively, random early drop can be enabled by specifying the
three colon separated values ``start:rate:full'' (e.g.,
"10:30:60"). sshd will refuse connection attempts with a proba-
bility of ``rate/100'' (30%) if there are currently ``start''
(10) unauthenticated connections. The probability increases lin-
early and all connection attempts are refused if the number of
unauthenticated connections reaches ``full'' (60).

Default is '10'.. I bump it up to 20 and your script works fine.

- Ben

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