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I have 2 AWS EC2 instances with RHEL 6 and 8 respectively. I followed this procedure to migrate all the users from the oldest server to the new one. In the process I had to delete some lines from all the .bak files created from the procedure (because they would conflict with default existing users on the new server). Another thing I had to do was changing the UID and GID for the ssm-user and ec2-user on the new server, since they conflicted with existing users on the old server and I was afraid on making changes on the old one and risk damaging something.

After all of this I rebooted the new server but couldn't access it from SSH anymore. After many tries the only difference I've found is that 22 port seems to be not open.

Results of the lsof command can be seen below

Server before migrating users

# lsof -i -P -n
COMMAND     PID   USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
chronyd     719 chrony    6u  IPv4  20809      0t0  UDP 127.0.0.1:323
chronyd     719 chrony    7u  IPv6  20810      0t0  UDP [::1]:323
NetworkMa   766   root   27u  IPv4  22927      0t0  UDP <serverip>:68-><ip>>67
ssm-agent  1302   root   10u  IPv4  51028      0t0  TCP <serverip>:41646-><ip>>443 (ESTABLISHED)
ssm-agent  1302   root   15u  IPv4  28155      0t0  TCP <serverip>:47220-><ip>>443 (ESTABLISHED)
sshd      10532   root    5u  IPv4  48231      0t0  TCP *:22 (LISTEN)
sshd      10532   root    7u  IPv6  48233      0t0  TCP *:22 (LISTEN)

Server after migrating users

# lsof -i -P -n
COMMAND    PID   USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
chronyd    717 chrony    6u  IPv4  20118      0t0  UDP 127.0.0.1:323
chronyd    717 chrony    7u  IPv6  20119      0t0  UDP [::1]:323
NetworkMa  764   root   27u  IPv4  22164      0t0  UDP <serverip>:68-><ip>>67
ssm-agent 1324   root   10u  IPv4  29098      0t0  TCP <serverip>:36630-><ip>>443 (ESTABLISHED)
ssm-agent 1324   root   15u  IPv4  27154      0t0  TCP <serverip>:58756-><ip>>443 (ESTABLISHED)
ssm-sessi 1449   root   16u  IPv4  27463      0t0  TCP <serverip>:33702-><ip>>443 (ESTABLISHED)

I'm accessing the servers without a key and I need to repeat this process at least on 8 more servers, so I would like how to solve this and how to avoid it when repeating the process on another server.

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  • What is in the error log for sshd on the target server that's failed to start sshd? It will tell you why it's failed to start
    – Chris Davies
    Commented Oct 4, 2022 at 21:36
  • 2
    Changing the UID/GID for the ssm-user is very likely the cause of your access trouble. (perhaps also the ec2-user). The AWS SSM mangement application software was installed on the EC2 server to run under that UID/GID, and any security-related subsystems (such as SELinux) were configured to expect it to run that way. It's likely the SSM application could not start after the change to the UID/GID and the application is what provides your ssh access to the server.
    – Sotto Voce
    Commented Oct 4, 2022 at 22:00

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