Linux admin notes - users and permissions

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...particularly in the context of filesystems

Filesystem permission basics

Most people know the below to some degree, and should.


'nix file permissions are a simpler system ACLs.

Which sometimes makes them easier to deal with, and sometimes requires serious contortion - which is one reason ACLs slowly being introduced into unices here and there.


Each filesystem entry (files, directories, other) has a set of properties. The most interesting are:

  • the user id (ususally shown as the name, as it is looked up on the current computer)
  • the group id (ususally shown as the name, as it is looked up on the current computer)
  • read, write, execute permissions bits for the owning user
  • read, write, execute permissions bits for the owning group
  • read, write, execute permissions bits for other - things that apply for everyone.

You can also get and set these numerically, as explained further down.


Permission bits are shown in various ways. The form form is the output used by
ls -l
. Its output for a regular file could look like:
-rw-r--r--  1 sarah users 2414 Nov 21  2004 afile.txt

The first bit shows permission bits (and more, explained later). You should read this as a special flag/filetype field, followed by three group of rwx, one for each of user, group, and other. For example:

- rw- r-- r--

The permission is present if the letter is present. In this case, sarah can read and write, anyone in the users group can read, and anyone else can also read this file.

'Other' is used for minimal permissions, since it always applies (regardless of what user or group id a visiting user comes along with), so -rw----r-- has a very similar effect in to -rw-r--r--.


Details explained later:

  • setgid and setuid are also be indicated in those letters
  • rights work slightly differently on directories


Notes: Seeing a plus sign, e.g.

-rw-r--r--+ 1 sarah users 2414 Nov 21  2004 afile.txt

...means that there are ACLs on this listing. See them with things like getfacl

Changing permissions

When changing permissions on a file there are various ways of describing permissions. One common distinction is whether you specify what should change or whether you specify exactly what the new permission set would be.

There are also different ways of describing each permission, and different operations to the permission bits. Choose the one or two you like.


Common usage:

chmod o-rwx afile.txt            # takes all permissions away from other
chmod +x afile.txt               # gives everyone execute permission
                                 # (short for a+x or the equivalent ugo+x)
chmod ug+w -R dataDir            # Gives user and group write permission


It is technically also possible to combine operations:

chmod a-rwx,u+wr,g+r -R dataDir  # A much-at-once style (not seen used much, but useful)
chmod u=rwx,go=rx dataDir        # Set-to-exactly-this style, in this case rwxr-xr-x

The numeric way sets permission bits as octal numbers, and is the shortest way to set all bits in one go. Where r=4, w=2, x=1:

chmod 0777 somefile              # gives full permission to all, like a=rwx.
chmod 0620 somefile              # sets rw- r-- ---
chmod 0750 somefile              # sets rwx r-x ---

...and any other combination. Most people remember just a few cases, such as 0777 as 'allow all', 600 (owner readwrite) or 644 (owner readwrite, others read-only) for files, 755 or 700 for directories, or such.


The leading zero is usually not necessary for command line utilities, since they know to interpret numbers as octals. In code you will usually need to use them, as otherwise they won't be understood as octal numbers.


Note that execute permissions on a file will include it in command autocompletion in shells. It will also allow execution as a binary, according to the hashbang, or using /bin/sh (as a fallback).

Changing ownership

You can change the user, group and each of these permissions, with chown and chmod respectively:

chown frank:goats afile.txt  # sets user to frank, group to goats
chown frank       afile.txt  # sets user to frank, leave group unchanged
chown frank:      afile.txt  # sets user to frank, and group to frank's login group
chown :goats      afile.txt  # sets group to goats
 
chown -R frank dataDir      #recursive: changes entire subtree (files, directories)

For more details on groups, see #Groups.


chown: changing ownership of `/path': Operation not permitted

Only the file/directorory's owning user (and root) can alter user or group membership.

You may expect that the file/dir's owning group can do so too, if it has write access to the file/dir. It can't. The reason seems to be practical one - it makes it easy to give away files that aren't yours, and/or that you can't get back. (is there some reason based in quota systems?(verify))


If you are the owning user (or root) and still can't, it's probably that the immutable attribute is set. See lsattr and chattr.

It may be that SELinux is prohibiting you (even if you're root).


You may be using a non-*nix filesystem, one which does not store file permissions (e.g. FAT).


Filesystem guts you can usually avoid

mode

Programmers may find it interesting that the permission bits mentioned are a subset of the mode.

Mode is a larger integer (now often 32-bit), which stores:

  • permission bits (lowest 9 bits)
  • setgid (S_ISGID, usually 2000). In text representation it's in the group-execute field, an s if the group execute bit is set, or S if the execute bit is missing
  • setuid (S_ISUID, usually 4000). In text representation, see setgid, but for the user execute field/bit.
  • sticky (S_ISVTX, usually 1000). In text presentation, it's in the other-execute field. It's t if the execute bit is present, T if it is missing.
  • file type (see below, and these are read-only)

Most command line tools only show (and allow change of) permission, sticky, setuid, setgid,

The mode may also store OS-specific bits, which you can only get at via OS-specific libraries.


File types:

what constant (octal; may differ(verify)) in text representation of mode
regular file S_IFREG, 0100000 -
directory S_IFDIR, 0040000 d
named pipe, fifo S_IFIFO, 0010000 p
character special device S_IFCHR, 0020000 c
block special device S_IFBLK, 0060000 b
symbolic link S_IFLNK, 0120000 l
socket S_IFSOCK, 0140000 s

You usually only need to care about regular files, directories, and the occasional symlink.


You can get the full mode with a stat(), which is what library functions that test 'is this a directory' use, but even the command-line stat command masks out everything but the permissions bits unless you specifically ask for all of the mode bits.

It also shows it in hex for some reason, so to view the full mode in octal, you'ld have to do something like the following:

$ stat --format %f /tmp | tr [a-f] [A-F] | xargs --replace=@ echo "ibase=16; obase=8; @" | bc
41777


Extended file attributes

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)


directory permissions, read and execute

tl;dr: In general, r and x are only useful in combination.


In more detail:

  • directory read rights
allows listing the directory's entry names
  • directory write rights allow: create, rename, remove, and alter entries in it (including changing permissions)
(...so o+w is scary)
  • directory execute rights
allow cd into the directory
allow looking up (basically stat()ing) entry's details within this directory (verify)

(If that seems weird, keen in mind that directories are just another entry within their parent directory - just a specific kind due to being marked as a directory in its entry's mode)



For the examples below

mkdir -p a/b c/d e/f
chmod 0111 a
chmod 0444 c
chmod 0000 e

To have:

d--x--x--x 3 user user 4096 Mar 22 18:30 a
dr--r--r-- 3 user user 4096 Mar 22 18:30 c
d--------- 3 user user 4096 Mar 22 18:30 e

(b, d, f have typical permissions, according to your umask. You will typically have rwx. These contents don't need to be directories, but it can help your mental model of how this works in trees a bit)


x without r

You can go there (because x), but you can't listdir() (because no r)

~$ ls a
ls: cannot open directory a: Permission denied
 
~$ cd a
~a/$ ls
ls: cannot open directory .: Permission denied
Since you can
cd
to it there, and because you may read specific entries,

if you can guess their names you can read them fine. If you can guess their name and they are directories (with x), change to them fine.

To continue the above example:

~a/$ ls b   # says nothing only because our example gave b no contents
~a/$ stat b 
      # gives full output, omitted here 
~a/$ cd b
~a/b$ ls    # says nothing because no contents


r without x
You can list entry names (because r), but not
cd
to it or stat its contents (because no x)
~$ ls c
ls: cannot access c/d: Permission denied
d

When you can't stat entries in a directory (due to -x on its parent), you also can't do descending into them (because you can't even do the permission check)

~$ cd c
cd: c: Permission denied
~$ cd c/d
cd: c/d: Permission denied


neither r or x

Can't list its entries, can't go there or under it, for the reasons explained above.



in a tree
Mount points and permissions

directory permissions, write

o+w permission is scary since it means that anyone can alter directory entries, even if those entries don't list the user as their owner.

The same applies to g+w if the group is something like 'users' or 'students' and that includes people you may not want to trust implicitly.


Sometimes you may want a directory which can be used to give each other files, or a generally shared directory.


directory permissions, sticky

Without sticky, you can share a directory only if you are in the same group, or allow write permissions to world.


When a directory has the sticky bit set, then this parent directory's owner and each directory entry's actual owner can rename and remove entries (anyone can create files/directories).

With sticky, people can work in the same directory, but can shield each other from accidentally (or purposefully) messing with files and directories they don't own.

/tmp is often a stickied directory for this reason.


(The sticky bit was named for its historical use on regular files, where it was a signal to the OS to keep this around in memory (or in local rather than remote storage), as it would probably be used regularly. Modern caching and drive speed has made this mostly obsolete. (I'm not sure whether modern kernels still listen to the sticky bit(verify))

setuid, setgid, and sticky on files

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

tl;dr:

  • the SUID and SGID bits in a file's mode will trigger seteuid() / setegid() syscalls at launch time, with the file's current user/group
  • "SUID root" is the typical case: chown as root, set suid bit, to have root permissions without need for sudo
can only be set by root / through sudo (for obvious security reasons)
often for daemons. Many of which will, after basic startup, use the same syscalls to give away root permissions again (switch to a normal user)
(the point may e.g. be binding to a port under 1024, which normal users can't do)


These bits are stored in the mode, on top of basic rwx permissions, are:

  • setuid (SUID), shown in the user's execute position, 4000 in octal, add with
    chmod u+s
  • setgid (SGID), shown in the group's execute position, 2000 in octal, add with
    chmod g+s
  • sticky bit, shown in other's execute position, 1000 in octal, add with
    chmod o+t


In
ls -l
they show up instead of the execute field field, e.g.
-rwsrwsr-t 1 root root  1191834 Nov 23  2013 thingie

(When they have these bits without execute, the S and T are shown in uppercase)



Some details

Every program runs with an effective user and group.

Under regular circumstances, they are the user (and current group(verify)) that executes the executable, which means the program can do exactly what the user can.


Which securitywise is sensible default behaviour, but doesn't serve all cases. Some things need to get at something more restricted to work (some just while initializing, some throughout their execution).

Some system commands need this, including passwd, mount, umount, ping, and su.

For another sort of example, svgalib needs root access to access graphics hardware directly, which would mean anything that needs svgalib is only useful when root runs it, or you're allowed to run it via sudo.


With SUID and/or SGID, once a program starts executing, it reads the user and/or group ID from the file's directory entry, and hands those to setuid and/or setgid, and the related seteuid and setegid.

Which are system calls which you can always use, it's just that the file permission flag is a handy way to set this up outside of changing the executable's code.


Since you can't really give away ownership (e.g. set a file to be owned by root, unless you are root), this isn't a security problem. However, you and particularly root should be careful of what they take ownership of; whatever files are owned by root, generally accessible, o+x and setuid or setgid will effectively run as root when executed by anyone, and therefore be able to do anything. There are a few fairly simple ways around this, such as to disallow directory access, which can be done by moving files you take ownership of to /root, when it has something like 0700 permissions - like it usually does.


UID and EUID

In general, the Effective UID (EUID) is checked when checking whether a process has permissions for an operation, while the real UID typically describes which user started the process, basically who asked for the operation.


For many programs this For typical programs they are the same. They can differ because this can be useful around elevated privileges, at which point they may wish to change their EUID. (Keep in mind this is mostly a bit of carefulness, but not a strong security feature, as it does not require authentication, and there is more than one way to get elevated rights)(verify)

It's also possible for a progam to change its UID (setuid()), but relatively few do, because in most cases it is doesn't matter to permissions, being more of a display, double-check, or better-impersonation thing.


For example, consider
passwd
and its root-owned password file.

While running it, the UID describes the user which ran it to alter their password, while the EUID will have to be root to write to the password file. For this reason, passwd is usually owned by root and has its setuid bit on in the filesystem mode, which means the OS will do the seteuid() call for it (and the effective authentication is the fact that only root could change the file ownership).

For another example, a webserver process started as root may wish to set its EUID before it does any filesystem accesses to satisfy a request.


The relevant underlying system calls:

  • geteuid()
  • getuid()
  • seteuid(uid_t euid)
  • setuid(uid_t ruid)

Also:

  • setreuid(uid_t ruid, uid_t euid) - can set both real and effective UID at once, or just one


There is analogous logic for the real and effective group (TODO: more explanation). The relevant system calls for it:

  • getegid()
  • getgid()
  • setegid(gid_t egid)
  • setgid(gid_t rgid)

Also:

  • setregid(gid_t rgid, gid_t egid) - can set both at once

umask

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

The umask removes permission bits from files and directories (programs running as) you create, from mkdir to whatever fanciness you are running.

You can always change them later - the point here is what protection you get by default.

Example: If the umask is 0022:

  • 0022 itself means --- -w- -w-
  • ...which are the things that are removed from permissions
  • in this case meaning that on newly created files, write permissions will never appear for group or other. Read and execute is not masked away, so other people can go into your directories and read files.


When people work in groups -- (but you still want at least processes to have a clear owner, so sharing a single account is not so useful), you might want to not mask anything from the group so that you can work with each others' data by default. Example: 0002 (or so),

If your group is something like 'students' or 'staff', you probably don't want group writability. You might use 0027 (no write on group, nothing on other), or perhaps 0077 if paranoid.


You can make this persistent by adding something like

umask 0002

to your ~/.bashrc (or whatever applies for your shell).

POSIX ACLs

See also:

Changing UIDs and GIDs

Accounts

By default, account information is backed by /etc/passwd (user list) and /etc/group (group list), and usually also the shadow files.

Note that filesystems store UIDs and GIDs, not usernames and group names, meaning filesystem permissions are inherently local to a computer unless those IDs are centralized / synchronized. This also matters to network filesystems such as NFS


useradd, usermod

useradd
adds a user
adduser
is a wrapper in Debian and derivatives (e.g. Ubuntu) which adds configurable site-specific configuration, making some things easier.
You may wish to use only adduser to avoid confusion.
(though I can never remember which name is which, but man is your friend)
usermod
changes details.

Most people user them so rarely that we just look at the man page.


On homedir:

Notes:

  • useradd isn't always configured to create the user's home directory.
you'll want
useradd -m
(as it's more convenient than manually copying from /etc/skel/ and fiddling with the permissions)
  • (Similarly, whether userdel deletes a home directory, mail spool and such depends on configuration)
  • not having a home directory means the account cannot log in (as does not having a valid shell).
Not having a password set usually also disallows logins, at least remotely(verify).
(This can be useful to e.g. have users for samba to map to, but to not allow them to shell into the same server)


On groups:

Often, default is to create a group of the same name, and add the user to it.

If you want users to go to a shared group instead, you can control this via arguments and, often, configuration files (see the man page for your system to be sure) like:

/etc/login.defs
/etc/defaults/useradd

Changing passwords

Use
passwd


On systems that integrate with networked systems like NIS, you may need to use another executable (like
yppasswd
). A plain
passwd
may tell you so - or may be symlinked to what you need to use.


To not allow password login, use passwd -l. This prepends something (!) at the start of the hash value, meaning no password will ever work. It also means you can unlock (passwd -u) by removing that character again (avoids having to set/remember the hash).

Keep in mind that -l applies only to this type of password login. SSH logins may have their own rules, and may effectively bypass it depending on how it is set up, and on how a user is trying to authenticate.


You may be able to set the hash of a blank password - as in 'accept pressing enter for a password'. While passwd probably won't allow it, you can generate the appropriate hash value via mkpasswd. This is generally a bad idea, though. If you want passwordless remote login, look at using a passphraseless keypair.

Deleting (passwd -d) seems to (usually? always?) mean a blank password (verify) - if so, you rarely want this.



Keep in mind that anyone with root (/general sudo) rights can su to any user, regardless of whether they are allowed remote or local login.

Groups

Theory: group logic

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)


Some important distinctions:

  • your primary group, a.k.a. initial login group
    • part of your login info
  • your additional group memberships
    • part of your login info, can be queried, but not everything does (verify)
  • a process's current effective group
    • your shell also has a currently effective group - which likely is your primary group, unless it was explicitly forced to be something different.


The easiest command line tool to see a user's currently effective GID(verify), and collection of all group memberships is probably
id
.

For regular users it may be as simple as:

uid=1004(sarah) gid=100(users) groups=100(users)

Root gets around more, and may look like:

uid=0(root) gid=0(root) groups=0(root),1(bin),2(daemon),3(sys),4(adm),
6(disk),10(wheel),11(floppy),18(audio),20(dialout),26(tape),27(video)



Because of the history of how processes work, the mental model and the code is slightly hairy.

Lazy and older programs may test only for the current effective GID and UID. A process can checks for additional memberships, but they have to care, and have to know how to do it.


To change one's own currently effective group (in the shell), use
newgrp groupname
.
  • If you are a member of that group. it will switch without question.
  • If you are not a member, it will either
    • deny you or
    • ask for the group password, ...depending on how the group admin has set up the group.


There is a
sg
[1], analogous to but for groups:

it sets a different group for executing a particular command.

Practice: Group administration

groupadd groupname
groupdel groupname

usermod and gpasswd (see below)

sometimes
groupmod
to change the name, or gid (group ID) [2] Sometimes
gpasswd
:
gpasswd group
to set the group password
gpasswd -r group
to remove the password
gpasswd -R group
disables the ability for non-members to use password login
These can be done by a group admin and root.
Group admins are one or more (non-root) users who are allowed to change group membership.
Group admins are added to groups by root, using
gpasswd -A user group
.)}}


You often have to worry less about creating groups, asadduser may be configured to

add a group with the same name as the user it creates (is this usually /etc/adduser.conf?(verify)), OR
to have a default group to add everyone to.


group membership:

  • Change additional groups:
    • usermod -a -G group1,group2 user
      add additional group membership(s)
      • -a is a relatively recent feature, so you may not have it, in which case you may need to use...
    • usermod -G group1,group2 sets group membership to exactly this list, so be careful
    • gpasswd -d user group1
      to remove a user's group membership
  • set primary group, a.k.a. initial login group
    • Warning:
      usermod -g group username
      works but also removes all other group memberships. Look at -G to re-add them if you accidentally did that.
      • Files in the user's home directory will be chowned from the old to the new GID(verify). The rest of the filesystem is unchanged, so you will have to do that yourself.


Note: In almost all cases you can specify either the group's name or its GID.

Other notes

Creating homedir after user creation

Duplicate UIDs

Change UID

/etc/passwd, /etc/shadow, /etc/groups

These are plain-text, colon-delimited databases containing all information about users, passwords, and group membership.

Password shadowing describes system that separate out publicly fetchable information, and store the actual password hash in a separate file.


It's generally best to modify these with system utils like useradd, usermod, userdel, groupadd, groupmod, groupdel, chpasswd (changing password), gpasswd (changing group membership), chfn (change finger/CEGOS field), chsh (change login shell), ...although there are a number of situations where you can get away with editing them directly.

There is also vigr and vipw.


/etc/passwd is a colon-delimited file that stores basic user information:

  • username (up to 32 chars?(verify))
  • password - in the old days this stored the password hash.
Nowadays it is typical to put the password hash in /etc/shadow (which general users/programs cannot read), and leave this file readable, because the rest of the fields are public and useful.
...so this field will contain something like an x to indicate the real password hash is stored in /etc/shadow (it seems other values have been used, like !(verify))
  • UID - user's numeric ID
  • GID - primary group ID, refers to entry in /etc/group
  • Sometimes called comment, sometimes called GECOS. (Used by finger, if you use it) May contain comma-separated sub-fields including the following (but there are deviations from this)
    • Real name
    • physical address (building, room)
    • phone number
    • other contact info - pager, fax, email address
  • home directory for user
  • login shell to be invoked


/etc/shadow stores the user's password hashes, and some further password-related information

  • username
  • password hash
    • *, x and ! all(verify) seem to indicate 'no password is valid'
    • nothing seems to mean 'no password is present' - which allows passwordless login (verify)
    • starting with $ indicates use of a non-default hash method, and typically starts by specifying which hash.

Apparenrly $1$ is MD5, $2a$ is Blowfish, $3$ is NT hash $5$ is SHA-256, $6$ is SHA-512. (there may be another $ in the value, splitting a salt from the hash)

  • date of last change (days since Jan 1, 1970)
  • minimum days before a next password change is allowed (0, i.e. no limitations, seems common)
  • days after which a password is expired, and change is forced on the next login (99999 basically means "won't expire")
  • amount of days before the previous entry that you get warnings (7 seems common)
  • number of days after expiry after which the account is actually disabled (rather than just forces password change)
  • absolute date of expiry (days since Jan 1, 1970)
  • reserved field

Notes:

  • 99999 days after 1970 is somewhere in October of 2243.
  • Dates in ~2012 are around 15500
  • passwd
    to change the password
  • chage
    ('change user password expiry information') to change most other things, or list the details in a more friendly way.


/etc/groups

  • group name
  • GID
  • password (if any) - though if you use password shadowing, it will be in /etc/gshadow
  • comma-separated list of group members (is this an exact mirror of /etc/gshadow?(verify))


/etc/gshadow

  • group name
  • password
  • comma-separated list of group admins
  • comma-separated list of group members (is this an exact mirror of /etc/groups?(verify))


su, sudo, sudoers

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)


tl;dr

  • su
    switches active user to another (by default to root)
which requires the password of that user
(Not a complete re-login. When things seem to break because of this, su -l can help, and in practice a ssh to localhost may be easier)
note that root can switch to any account (also relevant to sudo su)


  • sudo
    executes something as another user (by default as root)
root can do this arbitrarily
other users/groups can do so when configured as such
optionally configured to not ask for a password. Which can make sense for mounting - but think about implications.


  • sudo su
    is mostly the simple combination of the above:
"switch to a root, given that your account has the (sudo) rights to do so"
You don't need the other user's password, but yours, because you only need to prove to sudo you are who you say you are
note that in practice it's similar to sudo bash, but without the new shell (due entirely to what su does in general)

su

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

su can be remembered as 'switch user' (its general purpose) or 'superuser' (its default behaviour, and its real name).


When executed as root, su also gives you the ability to switch to accounts that do not have a password - those that are not regular logins and/or cannot be directly logged in as. This can be useful to administer accounts for services (that should run as their own user, but that should not imply ability to log in).


Example:

su              # switch to root
su someuser     # switch to user someuser
By default you get an interactive shell. You can get a login shell by using
-
or
-l
or
--login
, much more like the environment the user might expect. (If you use the - form, it must be the last option)
su -l              # switch to root with login shell
su - someuser     # switch to user someuser with login shell

(You may want to read Bash_and_profiles#login_vs._interactive_shells)


For security reasons, su does not preserve the environment that it was called from. Often enough that's right, but sometimes you do want to take things along. You can get (still selective) evironment copying by using -m or -p or --preserve-environment (equivalent)


X and su; "MIT-MAGIC-COOKIE-1 data did not match"

See X_notes#.22MIT-MAGIC-COOKIE-1_data_did_not_match.22

sudo

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Sudo (super user do ...) is a framework which allows users/groups/everyone to run specific commands as another user -- often as root.

The sudo system can log commands and arguments as an audit trail.


Sudo mostly changes the effective user of the process to be run(verify).

It does not start a new shell for the user. This may be an insignificant detail, a subtle difference, or a significant problem.

By default leaves most of the environment (but not all(verify)) intact. Allowing/blocking environment matters to things like cron jobs (where you use a different effective user)


The /etc/sudoers file is its configuration: it controls what commands can be run, by who, as who, and more.



Examples:

everyone can run a specific command

Say you have a classroom full of workstations, in which you want users to be able to mount CDs (and you do not have an auto-mounter doing that for them).

Since mounting a volume requires root permissions, you could allow users to execute only two commands as root,
mount /cdrom
and
umount /cdrom
- and for no other devices.


a specific user can run a specific command

I wanted a web-facing port-scan, which means my apache needed to run nmap as root.


specific people can run anything

Usually, anyone who is considered an administrator for a host is given either root access or sudo rights. Some people prefer sudo rights, because that way you're only ever intentionally dangerous, when you use sudo before a command.

How this is done depends a little on the exact setup. /etc/sudoers often has a line that allows members of a specific user group sudo rights for everything. This is often sudo or wheel. In sudoers that means

%sudo ALL=(ALL) ALL     
# or   
%wheel ALL=(ALL) ALL     

...and it would mean adding the user to the group.


sudo versus su versus sudo su

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Basically:

  • su
    switches some amount of the shell to another effective user
  • sudo
    runs a single command as another user
when the admin gave you sudo rights on your account
which means you get asked for your password, not root's.

As such:

  • sudo su
su by default switches to root, but would ask for root's password
running su as root (that's sudo's effect here) to switch to root means it doesn't ask for a password
...but the sudo just did, and it asked for yours (based on your rights to sudo)
When you're not root, this is often the only way you may be allowed to use su


More notes:

  • You can be given either the right to use sudo in general (useful for admin accounts that are not root), or sudo specific commands (sometimes useful)
  • sudo su -
    The dash means "don't just make me root, give me root's environment, as as if I've logged in as root"
  • You could do
    sudo bash
    , but it's probably cleaner to do one of:
  • sudo -i
    means 'simulate a login shell as root'
Some argue this is preferable to
sudo su
as your environment will be more predictable (arguably only matters when root is an account that you've ever set up a profile for, not so much when you're just using it for the privileges).
  • sudo -s
    means 'give me an interactive shell as root'
So yes,
su -
is similar to
sudo -s
not all variants/versions of sudo have -i (or -s(verify)
  • There are some further details to whether the environment is kept/cleared. (su - clears, su keeps, -s and -i effectively clear(verify))
  • allowing a user to sudo su or otherwise run a new shell lets them do anything as that user. This is either exactly what you want (e.g. letting a user administer the machine, administer the database as the database user, etc.), or not at all what you want. Also varying with personal trust.
  • Allowing an account to sudo su to root means that someone gaining access to that user account can be close equivalent to gaining root privileges (depending on how they got in. If they brute forced the password, for example). This is why admins don't easily give this out
  • sudo is often configured to allow shell users to do successive commands without giving a password each time (for up to a few minutes at a time). This trades a little security for a bunch of convenience when you're doing admin work

sudoers

editing
To edit the sudo configuration, run
visudo
. It mostly just invokes an editor on /etc/sudoers, but adds some locking and checking.


The default editor it uses is vi, but you can configure this in sudoers itself (the env_editor flag in the Default section), or tell visudo to not clear the environment (which it does for slightly paranoid security reasons) using
sudo -E visudo
).


Remember you can check the effects of the current sudoers file using sudo -l


Various types of rules
This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

The basic form of these rules is:

who    where=(aswho) options: /path/to/what, /path/to/what


  • 'who' is who is allowed to do something, and can be
    • username
    •  %systemgroup
    • ALIAS
    • ALL
  • 'where' describes the hostname. For local rules this is usually usually usually 'ALL' or omitted, unless the sysadmin decided to write one sudoers file for use across many hosts.
  • 'aswho' is the user that should be executed as. Usually root.
  • options
    • NOPASSWD is a common one, meaning you don't have to enter your password, useful for scripted and cronned stuff
  • path to the executable
    • note that since you usually use absolute paths in sudoers, you should also do so on the invocation, or the rule won't match
    • essentially a prefix match, so you can force certain arguments

Examples:

#Everyone can run synergyc (with any options) as root, without having to type their password
ALL     ALL=(root) /usr/bin/synergyc
 
#wilma can do anything as the database user (including starting a shell)
#  without needing a password:
wilma		ALL = (dbuser)  NOPASSWD: ALL
# Note: To use aliases in that field, use Runas_Alias

# Anyone can mount and umount removable devices
ALL            NOPASSWD: /bin/mount /mnt/floppy,  \
                         /bin/umount /mnt/floppy, \
                         /bin/mount /mnt/cdrom,  \
                         /bin/umount /mnt/cdrom,

#(are these globs or EBNF/regex style wildcards?)
# Wildcards can be useful in various ways, for example:
ALL            NOPASSWD: /bin/u?mount /mnt/cd*,  \
                         /bin/u?mount /mnt/dvd*, \
                         /bin/u?mount /mnt/flop*

# john can su to everyone except root (negative match)
john		!/usr/bin/su *root*

# PBIS users can mount some personal shares (should probably be more restricted)
%domain^users   ALL=(root) NOPASSWD:/bin/mount
%domain^users   ALL=(root) NOPASSWD:/bin/umount

#Root can do everything everywhere, as anyone
root		ALL = (ALL) ALL
#same for wheel, except it can't run lilo
%wheel		ALL = (ALL) ALL,!/sbin/lilo


Note that blacklist setups are, as in general, not very secure.

Also, note that it is hard to limit people from changing user.

Setting Defaults
This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)
Setting Aliases
This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

You can add aliases to add your own groups of...

#...users:
User_Alias DEVELOPERS = alice, blob
User_Alias TEMPS      = mike, wendy
# You may prefer using only the system's groups, though.
# ...commands:
Cmnd_Alias PRINT = /usr/sbin/lpc, /usr/bin/lprm
Cmnd_Alias BOOT  = /usr/sbin/shutdown, /usr/sbin/halt, /usr/sbin/fasthalt
Cmnd_Alias SU    = /usr/bin/su
# ...hosts and nets:
Host_Alias WEBHOSTS = www1, www2
Host_Alias OURNET   = 5.5.0.0/255.255.0.0


See also

Debugging visudo

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Perhaps the most useful thing related to figuring out why you can't do what you think you just allowed is:

sudo -l

This will show a list of allowed commands.


You can do stricter checking of the sudoers file to catch bordercases that may be biting you.

visudo -cs

This may also reveal further included sudoers-parsed files that you may not have been directly aware of.


Keep in mind:

  • when there are multiple matches for a given sudo, the first applies.
Generally, that means more broad allow rules should go first
  • the sudo line should contain the absolute path to the executable.


Related errors

error: parse error in /etc/sudoers near line -1

Usually means the user that is trying to sudo is not allowed to.

This can happen when there is no applicable rule for them, or when the user for an applicable rule comes from an external system (say, LDAP) and that system did not respond at the time of the sudo command.


no tty present and no askpass program specified

Late 2009(verify) saw a behaviour change where sudo refuses to ask for a password if no tty is present, because it can't turn echo off. You can make it act like it did before this time by using the 'visiblepw' flag.


You'll get this error when (your configuration means) sudo needs to ask for a password, and the way you run it means there is no terminal on which to do so

...which often means that you wrote a NOPASSWD rule that doesn't apply as you think it would (try using full paths, checking whether the right user applies, and such).

Sorry, user username is not allowed to execute 'something' as root on host

When it mentions bash, in response to su

In my case, I had allowed a user /bin/su as specific other user via sudoers, but not to run anything else as that user.

...most relevantly the shell (that sudo implies when invoked like sudo -i or -l). In this specific case, the easy-enough fix was to allow the shell in the same sudoers line.


Note that in the same situation, sudo su won't complain as it's the already-elevated su that invokes that shell.(verify)


In other cases

You may have written rules that don't apply (can be transient failure, e.g. hostname confusion), or confused sudoers in another way.

gksu, kdesu

These are wrapper for GUI programs that may want root permissions. gksu is from/for GNOME, kdesu for/from KDE.

They do little more than ask for the root login when you are not root, but make it much easier for e.g. menu entries to easily deal with system configuration apps to apply to root and non-root users.

Quota

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Quota

  • keeps track of, and limits, users's and/or groups's files
    • both their size (units of 1K blocks) and the number of inodes (usually less relevant)
    • There is a soft limit that you can cross with only a warning, for a configured grace period.
    • The hard limit cannot be crossed. Using both is nicer to your users, and usually you only care about order of magnitude anyway
  • applies per ext[234] filesystem
can work through NFS - clients will ask for quota at the server end (what you'ld want)
other filesystems vary:
XFS is slightly different.
ZFS has its own separate system
  • requires kernel support
  • isn't useful without its management tools
  • stores a record of the current use, in what it calls the index
(There is an old variant, but pretty much everyone will have be using v2 by now)



Setting up:

  • Make sure your kernel supports it (on modern *nix you can assume it)
  • Install the quota tools. Usually called quota. (There are additional tools, such as quotatool, that you may not need)
  • edit /etc/fstab to add usrquota,grpquota to the mount options of the filesystems you want a quotum on
    • you can use journaled quota when supported. This basically means you don't need to run
      quotacheck
      after an unclean shutdown. It would involve adding
      usrjquota=aquota.user,grpjquota=aquota.group,jqfmt=vfsv0
  • make sure the files to store the index exist in the root of each relevant filesystem. Either:
touch aquota.user aquota.group
, or:
use -c when you manually run quotacheck the first time
  • remount, to make usrquot,grpquota apply
For filesystems currently in use (e.g. root filesystem), using
mount -vo remount
is nice
Less-subtle variants: umount and mount, or reboot.
  • create the quota index, to account for existing contents:
    quotacheck -vgum mountpoint
    • note: -a on this and other tools means '...for all mountpoints for which we use quota', which can save typing
    • -v for verbose may be nice for feedback to you.
    • -u for users, -g for groups
    • -m without this, the tool will remount the filesystem read-only, which you may not like on a running filesystem (particularly your root filesystem). This option tells it to leave it read-write, but keep in mind that any file currently changing size will be incorrectly indexed.
    • -c for create the indices (alternative is creating empty files beforehand)
    • if it tells you it can't guess format from filename, use -F vfsv0 (or vfsv1 if supported and wanted)
  • Optional: run
    repquota -a
    to get a summary of the index we just made
  • start keeping track of(verify) and enforcing(verify) quota:
quotaon -v filesystem
you'll probably want to set that at bootup, e.g. in your init script


  • You may want to put
    quotacheck -mug mountpoint
    in your crontab to make sure the index doesn't go off much(verify)
There are warnings about using that on mounted filesystem. Not because it can corrupt things, but because if a user was handling a huge file just when the index was re-made, the quota can be off by that file's size until the next rescan
...so try to schedule for time of light use, like 4AM


Using - users:

  • quota
    display (-s makes for human-readable sizes)


Using - admin:

  • edquota user
    or
    edquota -g group
    - interactively alter quota for user or group.
    • edquota -p user1 user2 user3 ...
      - copy quota settings from user1 to user2, user3, etc. Also works on groups. Sometimes quite convenient.
    • soft: max blocks/inodes the user may have before warning is issued and grace countdown begins.
    • hard: max blocks/inodes
    • 0 means no enforced limit
    • the grace period is the same for both

Example:

edquota -t 7 days
edquota user_or_uid
  • setquota
    is a non-interactive alternative, useful when automating things. Example:
    setquota -u -F vfsv0 quotauser 1048576 2097152 0 0 /
  • repquota mountpoint
    - report quota use per user. Use -g for groups. Use -s for human-readable sizes.
  • TODO: you probably want to automate quota setting for new users.



See also:



Quota over NFS

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

NFS supports it, though under some conditions you may "Input/output error" instead of "Disk quota exceeded"(verify) (I got the latter, though, so that seems to work too)


Basically:

  • Server side:
    • Configure quota as usual
    • set up
      rquotad
      to run (...if you want client-side
      quota
      commands to work). This service may be called something else, such as rpcquota (verify)
  • Client side:
    • mount NFS as usual (no quota related mount options, they won't work)
    • add the -r option to edquota to make it contact the relevant NFS server (note: many other quota tools are local-only, so a habit of logging into the server may be easier)


http://chschneider.eu/linux/server/nfs.shtml

Running with different credentials / reduced permissions

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)

Know the permission details above.

See also su, sudo, sudoers


SUID, SGID


chroot jails


The graphical run-as-root wrapper is
gksu
, or
kdesu
(GNOME and KDE variants). May be injected into the command when the system can guess you'll need it (particularly in Alt-F2 style launching).