OpenSSH deny or restrict access to users and groups

by on July 30, 2006 · 24 comments· LAST UPDATED November 2, 2006

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OpenSSH has two directives for allowing and denying ssh user access.

DenyUsers user1 user2 user3

Use to block user login. You can use wild cards as well as user1@somedomain.com (user1 is not allowed to login from somedomain.com host) pattern.

DenyGroups group1 group2
A list of group names, if user is part of primary of supplementary group login access is denied. You can use wildcards.

Please note that you cannot use a numeric group or username ID. If these directives are not used, default is to allow everyone.

AllowUsers user1 user2
This directive is opposite of DenyUsers directive.

AllowGroups group1 group2
This directive is opposite of DenyGroups directive.

You should always block access to root user/group:
Open /etc/ssh/sshd_config file:

# vi /etc/ssh/sshd_config

Append following names (directives):

DenyUsers root finadmin
DenyGroups root finadmin

Make sure at least one user is allowed to use 'su -' command.

Save the file and restart the sshd.

This is a secure setup and you are restricting the users allowed to access the system via SSH with four above directives.

Please note that if you want to deny or allow access to large number of users consider SSH PAM configuration (ideal for ISPs and Web hosting service providers). PAM allows you to store usernames using text files (you do not have to mess up with ssh configuration file).

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{ 24 comments… read them below or add one }

1 eMBee August 1, 2006 at 11:09 pm

i disagree with blocking root entirely.

i like to use ssh public+private keys for root access, giving each root user their own key and password, and thus avoiding the hassle of a shared password. shared passwords are a great security risk, and hard to change, as you need to get all root users together when the password is to be changed.

unfortunately, i have not yet figured out how to restrict root to only be able to use ssh from localhost (other than using a seperate process)

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2 nixCraft August 1, 2006 at 11:50 pm

eMBee,

Root is not allowed to login ssh. However, user embee can login and run su – to become a superuser. This way root is blocked over network login but normal sys admin can login over ssh account can become a super user on demand.

restrict root to only be able to use ssh from localhost

Try out following entry:
AllowUsers root@localhost

If the pattern takes the form USER@HOST then USER and HOST are separately checked, restricting logins to particular users from particular hosts.

I hope this helps.

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3 Planet Malaysia August 2, 2006 at 6:48 am

How about if let said you want to do rsync? Shall you use username: root? beside root, how can I rsync so many folder with different permission.

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4 eMBee August 2, 2006 at 1:36 pm

using su forces the use of a shared password, which is what i want to avoid.

AllowUsers says in the documentation:
“If specified, login is allowed only for user names that match one of the patterns.”

that means ALL OTHER users will be blocked, which is not desired either. i need something that allows me to say: allow any users except root from anywhere, and root only from localhost.

there may be a way to combine AllowUsers and AllowGroups, but the docs say nothing about how those two would interact.

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5 eMBee August 2, 2006 at 1:38 pm

Planet Malaysia: for rsync you would want to allow root specificly for the host that you are synchronizing with.

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6 nixCraft August 2, 2006 at 2:56 pm

Hmm…

Have you evaluated or considered sudo option? I use sudo extensively here. Although it is not 100% perfect but better than sharing root password. It also log down all failed (or command access) messages. Sudo is your best option, IMPO.

Let me know if you are aware of any other tools or methods….

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7 nixCraft August 2, 2006 at 2:58 pm

Planet Malaysia, follow emBee\’s suggestion or add user to group grant necessary file level permissions

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8 nixCraft August 2, 2006 at 3:04 pm

You need to configure PAM login access control tables. These tables will give you desired effect i.e. allow any users except root from anywhere, and root only from localhost (just like iptables).

There is also a iptables module which attempts to match various characteristics of the packet creator (both INPUT and OUTPUT chain) but it is badly broken and it may not work on SMP system at all.

I will post more info about login access control tables soon. See URL: http://www.cyberciti.biz/tips/openssh-root-user-account-restriction-revisited.html

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9 eMBee August 2, 2006 at 11:48 pm

i did a few quick tests on the interaction of AllowUsers and AllowGroups and it seems that they can’t really be used together. when i set:

AllowUsers: root@localhost
AllowGroups: users

the result was that noone could login, even adding root to the group users didn’t help.

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10 eMBee August 3, 2006 at 12:03 am

LinuxTitli: using the users password for root access is not really good either. if some users account is compromized (because someone managed to get at the users password), then root is automaticly compromized as well.

i look forward to try the suggestions in nixcrafts new article soon.

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11 steve July 20, 2007 at 6:42 am

Dear all,

how can i restrict a user to view others folder except his own folder after login?

thanks

steve

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12 sobas September 17, 2007 at 7:23 am

/etc/ssh/sshd_config
PermitRootLogon no

Above denies root to login remotely but is able to login locally on the console.

Use sudo to give access to sys admins.

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13 ramsam July 16, 2008 at 4:45 pm

OS: uBuntu 7.10

I tried adding AllowUsers and restarting the sshd server but it still allows other users to ssh.
It does not restrict users too… I even tried DenyUsers nothing works…. Need help.

Thank you.
Regards,
Ram

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14 jose antonio November 16, 2008 at 12:05 am

Thank you very much!

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15 milegrin August 14, 2009 at 8:34 am

Hi

Like eMBee, I am trying to deny root login from everywhere EXCEPT a specific host which is used to run automated remote tasks as root through ssh using keys.

I have tried combinations AllowUser, DenyUsers to no avail.
sshd_config takes preference over ssh_config so host definitions get overridden by the sshd_config entry “PermitRootLogin no”

Using PAM restrictions is not really an option as this is an AIX box.

Any idea as too how to achieve this would be greatly appreciated!
– Michael

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16 LinuxLuser August 27, 2009 at 4:24 am

eMBee / milegrin,

Your best option may well just be a seperate sshd process that listens on a different port, that only allows root access. I would combine that with iptables (or other firewall) to only accept incoming connections to your second sshd port from your known/allowed hosts. Then, just update your automated scrips to connect to your non-standard port. Not too bad, considering that once you get it set up, it should run forever. Any localhost root users should be smart enough to accomodate the non-standart port assignment.

Cheers,
LinuxLuser

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17 Daniel Reinhardt December 1, 2009 at 12:42 pm

For those of you who want to enable root access to multiple people then setup SUDO and give people sudo access via sudoers file. This way you can limit what each sudo user does.

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18 milegrin December 1, 2009 at 9:16 pm

sudo is already in place for standard OS & application admins. Direct root SSH is required for AIX’s CSM or “Cluster Server Manager” which essentially allows me to run a command fromt eh CSM server on all servers or a specific server and I use it extensively for various reporting scripts, security and other functions but it requires root ssh.

I have yet to find a decent working solution that does not rely on multiple instances or external dependencies (eg PAM) but I have it can be done however how is another animal completely.

Thanx again for the advice

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19 sahab December 30, 2009 at 5:22 pm

Hi All
How can I limit the user one time ssh login in freebsd 7.2, I have added below entry in @username – maxlogins 1

This method works with ubuntu linux. Not in Freebsd,

Any idea for resolving

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20 Andreas June 22, 2010 at 9:27 am

put into sshd_config following line, than you are able to root login with certs, but not with password:

PermitRootLogin without-password

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21 DarkFader July 20, 2010 at 11:18 pm

Match Address 127.0.0.1
PermitRootLogin without-password

Yes, you can do that. It seems to work somewhat although it still asks for a password from elsewhere.
Now I can have my “alias sshdo=’ssh root@localhost'” combined with ssh-agent :)

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22 suzuki September 26, 2010 at 9:29 am

I tried editing sshd_config with AllowUsers root. but all of users can ssh to configured host. I tried with DenyUsers too, but that is not working too. why?
what i missed?

thanks any help

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23 Rashid May 14, 2011 at 10:06 am

thanks very helpful. i block icmp on my server through this.

thanks

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24 Lhavanya February 6, 2013 at 6:55 am

whether possible to add the same user name in allow group as well as deny group?

if yes which takes more preference ? And how it work?

Anybody know please help.

Reply

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