[CentOS] ssh failure from CentOS7 to Centos6

Mon Jan 13 18:00:05 UTC 2020
Stephen John Smoogen <smooge at gmail.com>

On Mon, 13 Jan 2020 at 12:23, Patrick Bégou
<Patrick.Begou at legi.grenoble-inp.fr> wrote:
> Hi,
> I have a strange problem with a freshly installed Centos7 desktop
> (most8pc25). I can't ssh to 2 CentOS6 servers, even with firewall
> disabled on the client and on the server. But I can connect from the
> server to the client, all in the same VLAN. I can also ssh from this
> desktop to centos7 servers in the same VLAN or in another VLAN.
> No idea about this problem.
> On the server kareline (client is most8pc25), tcpdump says:

So the client is able to talk to the server and the server is responding.

> On the Centos7 client, with "-v -v" ssh says:
> [tec21 at most8pc25 ~]$ssh -v -v kareline
> OpenSSH_7.4p1, OpenSSL 1.0.2k-fips  26 Jan 2017
> debug1: Reading configuration data /etc/ssh/ssh_config
> debug1: /etc/ssh/ssh_config line 58: Applying options for *
> debug1: Connecting to kareline [] port 22.
> debug1: Connection established.

^^ this says the first part started working.

> debug1: Local version string SSH-2.0-OpenSSH_7.4
> debug1: Remote protocol version 2.0, remote software version OpenSSH_5.3
> debug1: match: OpenSSH_5.3 pat OpenSSH_5* compat 0x0c000000
> debug1: Authenticating to kareline:22 as 'tec21'
> debug1: SSH2_MSG_KEXINIT sent
> debug1: SSH2_MSG_KEXINIT received

It got items and says it is going to use the user tec21 to login

> debug2: local client KEXINIT proposal

> debug1: kex: client->server cipher: aes128-ctr MAC: umac-64 at openssh.com
> debug1: kex: diffie-hellman-group-exchange-sha256 need=16 dh_need=16
> debug1: kex: diffie-hellman-group-exchange-sha256 need=16 dh_need=16
> debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<3072<8192) sent
> Connection closed by port 22

The server then stops the connection. I would then go through the
following on the host:
1. Is fail2ban or something else dropping the connection for some reason?
2. Is there a log in /var/log/secure to say something is going on?
3. Does running the server on port 2222 in debug mode and connecting
from the client give a reason for it dieing?
4. On the client and server are /etc/ssh/*_config changed from
defaults and what changes are there. Sometimes saying you want XYZ
algo in one and not having it in another causes dropped connections
but I thought it gave an error.

> I'm stuck....
> Patrick
> _______________________________________________
> CentOS mailing list
> CentOS at centos.org
> https://lists.centos.org/mailman/listinfo/centos

Stephen J Smoogen.