openssh unix dev - Jul 2021 - strange failure

There is a Centos 7 machine where this falls after 80 loops

target="same host"
while [ $i -ne 500 ];do
echo -e "$i"
ssh -p 22 root@${target} "ls / 1>/dev/null && exit;"
((i++))
done
The loop does not fail against Ubuntu boxes.
kindly let me know if I am doing something wrong.

This is my sshd_config
sshd -T
port 22
addressfamily any
listenaddress [::]:22
listenaddress 0.0.0.0:22
usepam no
logingracetime 120
x11displayoffset 10
maxauthtries 20
maxsessions 100
clientaliveinterval 0
clientalivecountmax 3
streamlocalbindmask 0177
permitrootlogin without-password
ignorerhosts yes
ignoreuserknownhosts no
hostbasedauthentication no
hostbasedusesnamefrompacketonly no
pubkeyauthentication yes
kerberosauthentication no
kerberosorlocalpasswd yes
kerberosticketcleanup yes
gssapiauthentication no
gssapicleanupcredentials yes
passwordauthentication no
kbdinteractiveauthentication yes
challengeresponseauthentication yes
printmotd yes
printlastlog yes
x11forwarding no
x11uselocalhost yes
permittty yes
permituserrc yes
strictmodes no
tcpkeepalive yes
permitemptypasswords no
compression yes
gatewayports no
usedns no
allowtcpforwarding yes
allowagentforwarding yes
disableforwarding no
allowstreamlocalforwarding yes
streamlocalbindunlink no
fingerprinthash SHA256
exposeauthinfo no
pidfile /var/run/sshd.pid
xauthlocation /usr/bin/xauth
ciphers chacha20-poly1305 at openssh.com,aes128-ctr,aes192-ctr,aes256-ctr,
aes128-gcm at openssh.com,aes256-gcm at openssh.com
macs umac-64-etm at openssh.com,umac-128-etm at openssh.com,
hmac-sha2-256-etm at openssh.com,hmac-sha2-512-etm at openssh.com,
hmac-sha1-etm at openssh.com,umac-64 at openssh.com,umac-128 at openssh.com
,hmac-sha2-256,hmac-sha2-512,hmac-sha1
banner none
forcecommand none
chrootdirectory none
trustedusercakeys none
revokedkeys none
authorizedprincipalsfile none
versionaddendum none
authorizedkeyscommand none
authorizedkeyscommanduser none
authorizedprincipalscommand none
authorizedprincipalscommanduser none
hostkeyagent none
kexalgorithms curve25519-sha256,curve25519-sha256 at libssh.org
,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group14-sha256,diffie-hellman-group14-sha1
casignaturealgorithms
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
hostbasedacceptedkeytypes ecdsa-sha2-nistp256-cert-v01 at openssh.com,
ecdsa-sha2-nistp384-cert-v01 at openssh.com,
ecdsa-sha2-nistp521-cert-v01 at openssh.com,ssh-ed25519-cert-v01 at openssh.com,
rsa-sha2-512-cert-v01 at openssh.com,rsa-sha2-256-cert-v01 at openssh.com,
ssh-rsa-cert-v01 at openssh.com
,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
hostkeyalgorithms ecdsa-sha2-nistp256-cert-v01 at openssh.com,
ecdsa-sha2-nistp384-cert-v01 at openssh.com,
ecdsa-sha2-nistp521-cert-v01 at openssh.com,ssh-ed25519-cert-v01 at openssh.com,
rsa-sha2-512-cert-v01 at openssh.com,rsa-sha2-256-cert-v01 at openssh.com,
ssh-rsa-cert-v01 at openssh.com
,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
pubkeyacceptedkeytypes ecdsa-sha2-nistp256-cert-v01 at openssh.com,
ecdsa-sha2-nistp384-cert-v01 at openssh.com,
ecdsa-sha2-nistp521-cert-v01 at openssh.com,ssh-ed25519-cert-v01 at openssh.com,
rsa-sha2-512-cert-v01 at openssh.com,rsa-sha2-256-cert-v01 at openssh.com,
ssh-rsa-cert-v01 at openssh.com
,ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
rdomain none
loglevel INFO
syslogfacility AUTH
authorizedkeysfile .ssh/authorized_keys
hostkey /etc/ssh/ssh_host_rsa_key
hostkey /etc/ssh/ssh_host_ecdsa_key
hostkey /etc/ssh/ssh_host_ed25519_key
allowusers root
authenticationmethods any
subsystem sftp /usr/libexec/sftp-server
maxstartups 500:30:500
permittunnel no
ipqos af21 cs1
rekeylimit 0 0
permitopen any
permitlisten any
permituserenvironment no

Hi,

Saint Michael wrote:
> There is a Centos 7 machine where this falls after 80 loops
> 
> target="same host"
> while [ $i -ne 500 ];do
> echo -e "$i"
> ssh -p 22 root@${target} "ls / 1>/dev/null && exit;"
> ((i++))
> done
> The loop does not fail against Ubuntu boxes.
Can you describe the failure? Is there any error message?

> kindly let me know if I am doing something wrong.
There's nothing wrong with the above snippet - that should always
result in 500 times ls output.

> This is my sshd_config
That looks fine to me, nothing in there strikes me as a cause for
intermittent issues.


One thing to keep in mind is that nearly all Linux distributions make
modifications to lots of packages, including OpenSSH, and I've experienced
such distributions changes to cause their OpenSSH packages to malfunction
while the upstream version (as published by this community) doesn't have
those same issues. It can be worthwhile to build the corresponding
upstream version yourself and retest.


Kind regards

//Peter

On Sun, 25 Jul 2021, Saint Michael wrote:
> There is a Centos 7 machine where this falls after 80 loops
> 
> target="same host"
> while [ $i -ne 500 ];do
> echo -e "$i"
> ssh -p 22 root@${target} "ls / 1>/dev/null && exit;"
> ((i++))
> done
> The loop does not fail against Ubuntu boxes.
> kindly let me know if I am doing something wrong.
It's very hard to figure out what is wrong without logs (both client
and server), but I'd start by checking your PAM configuration.

-d