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After I read List all human users I noticed that there is a user account named 'nobody' in my Ubuntu system.

Also I noticed that I can login in this account from terminal using the following command and my password:

sudo su nobody

su nobody

It doesn't mind me at all, but I want to know what is the purpose of this user? Is it created by default on a fresh install of Ubuntu or is created by installing a particular package?

  • 8
    Note that when you log in using your password, you're using your password for the sudo step, not for the nobody account (and that the reason it works is because the superuser can su to anyone without needing to enter their password (although as mentioned below, I believe at least on RH-derivatives, if nobody's shell is set to /sbin/nologin, you still wouldn't be able to log in even using superuser (aka root) – Foon Aug 8 '13 at 14:48
  • That's the case by default now (18.04+?). sudo su nobody return This account is currently not available. because the shell for the user nobody is set to /usr/sbin/nologin (getent passwd nobody). – Pablo Bianchi Jan 6 at 19:00
  • @sarnold - please see my comment on the answer I believe you're alluding at. It's a rather poor answer, as it doesn't reason or cite sources. It furthermore goes counter to all I know about the nobody account and how NFS works: with root_squash on it will map root to nobody on remote systems. This is more or less exactly the opposite of what this answer states – vidarlo May 13 at 19:12
82

It's there to run things that don't need any special permissions. It's usually reserved for vulnerable services (httpd, etc) so that if they get hacked, they'll have minimal damage on the rest of the system.

Contrast this with running something as a real user, if that service were compromised (web servers are occasionally exploited to run arbitrary code), it would run as that user and have access to everything that user had. In most cases, this is as bad as getting root.

You can read a little bit more about the nobody user on the Ubuntu Wiki:

To answer your follow-ups:

Why I can't access this account with su nobody?

sudo grep nobody /etc/shadow will show you that nobody doesn't have a password and you can't su without an account password. The cleanest way is to sudo su nobody instead. That'll leave you in a pretty desolate sh shell.

Can you give a particular example when is indicated to use this account?

When permissions aren't required for a program's operations. This is most notable when there isn't ever going to be any disk activity.

A real world example of this is memcached (a key-value in-memory cache/database/thing), sitting on my computer and my server running under the nobody account. Why? Because it just doesn't need any permissions and to give it an account that did have write access to files would just be a needless risk.

  • Just two more things if you can explain: 1) why I can't access this account with su nobody and 2) can you give a particular example when is indicated to use this account? – Radu Rădeanu Aug 7 '13 at 19:34
  • @RaduRădeanu 1) I'm guessing that's because it doesn't have a password set, and when you su as an ordinary user, you must give the target user's password. Try sudo -i then su nobody from the root shell (which won't require a password). – a CVn Aug 7 '13 at 20:40
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    Network File System maps root to nobody so local root cannot access everything like the remote root can. – Sylwester Aug 7 '13 at 21:36
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    @RaduRădeanu Please note the edit history. When I tested the original command (not what's there) this originally I was ending up in a dash (/bin/sh) shell but I can't replicate that now. Your original edit was fine. It wasn't me who changed it. – Oli Mar 29 '14 at 18:48
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    I always thought nobody is actually/primarily used by NFS as linuxstandardbase states. – dgonzalez May 14 at 11:38
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In many Unix variants, "nobody" is the conventional name of a user account which owns no files, is in no privileged groups, and has no abilities except those which every other user has.

It is common to run daemons as nobody, especially servers, in order to limit the damage that could be done by a malicious user who gained control of them. However, the usefulness of this technique is reduced if more than one daemon is run like this, because then gaining control of one daemon would provide control of them all. The reason is that nobody-owned processes have the ability to send signals to each other and even debug each other, allowing them to read or even modify each other's memory.

Information taken from http://en.wikipedia.org/wiki/Nobody_(username).

  • -1: nobody is strictly for NFS and should not be used by other services and certainly not by system administrators. Thanks. – sarnold May 15 at 1:29
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+500

The anwers above are rather wrong, because they assume that nobody is a "generic" anonymous/guest style user id.

In the UNIX/Linux access control model anonymous/guest style user ids don't exist and these are bad suggestions:

  • "common to run daemons as nobody, especially servers, in order to limit the damage that could be done by a malicious user who gained control of them." because of the that follows: "However, the usefulness of this technique is reduced if more than one daemon is run like this, because then gaining control of one daemon would provide control of them all".
  • "A real world example of this is memcached (a key-value in-memory cache/database/thing), sitting on my computer and my server running under the nobody account. Why? Because it just doesn't need any permissions and to give it an account that did have write access to files would just be a needless risk."

The nobody user name with user id 65534 was created and reserved for a specific purpose and should be used only for that purpose: as a placeholder for "unmapped" users and user ids in NFS tree exports.

That is, unless user/id mapping is setup for NFS tree exports, all files in the export will appear owned by nobody. The purpose of this is to prevent all users on the importing system from accessing those files (unless they have "other" permissions), as none of them (except root) can be/become nobody.

Therefore it is a very bad idea to use nobody for any other purpose, because its purpose is to be a user name/user id for files that must not be accessible to anybody.

The Wiki entry is very wrong too.

The UNIX/Linux practice is to create a new account for each "application" or application area that needs a separate access control domain, and to never reuse nobody outside NFS.

  • This answer does not cite any sources, and explicitly contradicts several of the other answers, which do cite sources. The current bounty indicates this answer is particularly good, in which case it should cite some sources, or at list have some reasoning. – vidarlo May 13 at 19:05
  • @mook765 That part is fine. The paragraph about NFS however, I do not grok. For instance with root_squash on, root is mapped to the user nobody, so files having owner nobody would make absolutely no sense. In addition the statement that files owned by nobody is meant to be inaccessible to anyone makes little sense, as file permissions are separate from ownership in UNIX. I'm not saying that everything in the answer is wrong, just that elements of it makes little or no sense to me :) – vidarlo May 13 at 20:12
  • @vidarlo, this answer isn't suggesting that you should set files to be owned by nobody. It's telling you that nobody is for NFS to use when mapping permissions, and that's the most important point to me. How NFS uses nobody is less interesting than the fact that it does use nobody. Thanks. – sarnold May 15 at 1:28
  • @sarnold the answer is still plain wrong in this matter in my opinion. If a user reads the answer and man exports, he may be very confused. – vidarlo May 15 at 3:31
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The nobody user is created by default on a fresh install (checked on Ubuntu Desktop 13.04).

In many *nix variants, nobody is the conventional name of a user account which owns no files, is in no privileged groups, and has no abilities except those which every other user has (the nobody user and group do not have any entry in the /etc/sudoers file).

It is common to run daemons as nobody, especially servers, in order to limit the damage that could be done by a malicious user who gained control of them. However, the usefulness of this technique is reduced if more than one daemon is run like this, because then gaining control of one daemon would provide control of them all. The reason is that nobody-owned processes have the ability to send signals to each other and even debug each other, allowing them to read or even modify each other's memory.

Source: Wikipedia - Nobody (username)


The nobody-owned processes are able to send signals to each others and even ptrace each other in Linux, meaning that a nobody-owned process can read and write the memory of another nobody-owned process.

This is a sample entry of the nobody user in the /etc/passwd file:

alaa@aa-lu:~$ grep nobody /etc/passwd
nobody:x:65534:65534:nobody:/nonexistent:/bin/sh

As you may notice, the nobody user has /bin/sh as a login shell and /nonexistent as the home directory. As the name suggests, the /nonexistent directory does not exist, by default.

If you are paranoid, you can set nobody’s default shell as /usr/sbin/nologin and so, deny the ssh login for the nobody user.

Source: LinuxG.net - The Linux and Unix Nobody User

  • This answer would deserve a +1 if the incorrect paragraph from Wikipedia were removed. Thanks. – sarnold May 15 at 1:30
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nobody is a special user and group account. Because it is an actual username (and groupname) and can be used by processes and even users, it is not literally nobody. For example, some Apache configurations have nobody as the user/group that owns the website files and directories. The problem comes when multiple processes might use the nobody user, such as NFS directories and the webserver.

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