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Short version:

Whats is the maximum number of different users that can simultaneously be logged into a single linux-based server? (via telnet or SSH)

Long version:

I'm a CS student in Pakistan and while being taught how to use Solaris i asked the instructor if it would be better to teach us Linux? (since linux is open-er, which is good for a country which is financially weak) He replied something like "on Solaris you can haz 100 or 1000 users logged in simultaneously while linux is limited to 10!" My gut didn't agree so decided to google it up but couldn't find what i was looking for.

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There is simply no hard limit. It is completely dependent on hardware and software configuration. I have been logged into (but not admin'd) systems with ~200 users at the same time. With the right combination of hardware and settings you should be able to handle a huge number of users at the same time on a single piece of hardware.

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  • guess i'll have to look for such information in distro specific documentation then? Commented Feb 23, 2012 at 9:08
  • could you kindly provide a source of the "not being a hard limit" fact? to help me cite you i.e. Commented Feb 23, 2012 at 9:15
  • It is FOSS. If any program/component had a limit, just patch it. Linux is used on some of the largest super computers, I kinda doubt you are going to run into something that hasn't already been addressed at some point. en.wikipedia.org/wiki/Supercomputer#Operating_systems Commented Feb 23, 2012 at 9:19
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linux ssh's sshd_config typically has a (commented out) MaxSessions 10 line, but i'm guessing the real limit would be 64k or so, (as many as there are free ports numbered above 1024)

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  • since i'm using Ubuntu desktop i found my ssh config file to be /etc/ssh/ssh_config but it has no mention of "MaxSessions 10" am i doing something wrong? or is it specific to the server version? Commented Feb 23, 2012 at 8:44
  • might be specific to fedora (and maybe redhat / centos). But it should be an option none-the-less. as i say, its commented even in fedora. Certainly we've a linux system that typically has 600 users concurrently. Commented Feb 23, 2012 at 9:01
  • @OsamaHussain: /etc/ssh/ssh_config is the config file for the ssh client. If you install the "openssh-server" package you get the server, including the /etc/ssh/sshd_config file. Commented Feb 23, 2012 at 9:42
  • Note that MaxSessions, per the manpage, is "maximum number of open sessions permitted per network connection". AFAIK there is no builtin hard limit on the number of total sessions other than available resources (free ports, memory, whatever). Commented Feb 23, 2012 at 9:44
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    yes, a user can login multiple times. Either way, your instructor is very badly wrong. Perhaps even worryingly so. Commented Feb 24, 2012 at 7:42

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