SSH problem - unable to login at times
 

Hi All,

I gave a interview yesterday. I did well on all files / server configuration etc.

I could not answer few questions, one of them is :-


Quest We are trying to login to server B from A. Sometimes we are able to login and sometimes we are not. For e.g. I logged on to this server, and after 5 minutes when I tried logging in again, I was unable to login.
Note: Login prompt is coming every time, there is no network problem.
How will you trouble shoot this problem.



Pls help me with this.

Thanks in advance.

You would check the log files on the system from the console.

Well in answer to the actual question, I would check the messages and secure and establish how the authentication back end is working. If it's more complex than local files, e.g. an ldap server are there issues in querying those. Clearly the network is ok if the prompt is coming up. I would clarify what unable means. there is no mention of error messages or precise behaviour.

TBH I can't think of a good reason likely to be behind this myself... That's not actually what they've asked though.

Even, I told them I would check configuration and log files, but I guess they were looking for some other answer.

MaxStartups too low? There's no comment on how many other people use ssh to this box...

Server load could be an issue, but that's probably too accidentally to be part of a quiz.

A more esoteric explanation would be that some firewall rule restricts SSH access from the same IP address within a give time range. This is usually done to avoid automated attacks, though it's unusual to set this time range to 5 minutes or more in such a context.

A rather theoretical concept would consider a server setting that allows only one login for any user at a given time. I wouldn't know how to do something like that on a server, but it's not a bad idea for a security-obsessed system administrator.

linux

Well, to summarize I can now think of these reasons:

- Maximum no. remote logins reached
- Server Load
- Authentication files