RSP Probe - Multiple Connection Refused Alarms from RHEL 6 Servers

Products

• DX Unified Infrastructure Management (Nimsoft / UIM)

Issue/Introduction

Multiple "Connection Refused" alarms are being triggered by the RSP (Remote System Probe) monitoring Red Hat Enterprise Linux 6 (RHEL 6) servers.  

RSP log entries:  

rsp: Connection failed for host <targetservername> kex error: no match for method server host key algo: server [ssh-rsa,sshss], client [ssh-ed25519,ecdsa-sha2-nistp521,ecdsa-sha2-nistp384,ecdsa-sha2-nistp256,[e-mail],[e-mail],rsa-sha2-512,rsa-sha2-256]  
rsp: Retry connecting with ssh1  
rsp: connect_ssh: user <username>  
rsp: connect_ssh: host <targetservername>  
rsp: connect_ssh: timeout 30  
rsp: connect_ssh: pass_less_ssh 1  
rsp: connect_ssh: do ssh_connect for host <targetservername>  
rsp: authenticate: authentication methods: 54  
rsp: authenticate: attempting public key authentication

Note: While manual SSH connections from the probe server may succeed, the RSP probe fails to authenticate or times out when falling back to SSH1. 

Environment

Product: DX Unified Infrastructure Management (Nimsoft / UIM)
Probe: RSP (Remote System Probe) version 5.54 and above
OS: RHEL 6.x (Target systems)

Resolution

To restore connectivity, you must explicitly enable the required legacy algorithms in the SSH configuration of the probe host server.

1. Log in to the server where the RSP probe is installed (the probe host).

2. Open the SSH configuration file (typically /etc/ssh/ssh_config).
3. Append the following lines to the end of the file to allow compatibility with RHEL 6:

KexAlgorithms +diffie-hellman-group14-sha1
HostKeyAlgorithms +ssh-rsa

4. Save the file.

5. In the UIM Infrastructure Manager or Admin Console, restart the RSP probe.

6. Verify the connection by checking the rsp.log. The probe should now successfully complete the fallback handshake and authenticate with the RHEL 6 servers.