More Secure SSH Connections
If you need remote access to a machine, you'll probably use SSH, and for a good reason. The secure shell protocol uses modern cryptography methods to provide privacy and confidentiality, even over an unsecured, unsafe network, such as the Internet. However, its very availability also makes it an appealing target for attackers, so you should consider hardening its standard setup to provide more resilient, difficult-to-break-into connections. In this article, I cover several methods to provide such extra protections, starting with simple configuration changes, then limiting access with PAM and finishing with restricted, public key certificates for passwordless restricted logins.
Knock for SSH
Trying to attack your machine will be harder if the would-be invader cannot
even find a possible SSH door. The methods shown in this article are compatible with
the port-knocking technique I wrote about in a previous article ("Implement
Port-Knocking Security with knockd", January 2010), so I won't
knockd configuration here. By using all techniques together,
attackers will have an even harder time getting to your machine (where all the other
measures shown in this article will be waiting), because they won't even be able to
start trying to attack your box.
Where Is SSH?
As defined in the standard, SSH uses port 22 by default. This implies
that with the standard SSH configuration, your machine already has a nice
target to attack. The first method to consider is quite simple—just
change the port to an unused, nonstandard port, such as 22022. (Numbers
above 1024 are usually free and safe, but check the Resources at the end
of this article just to avoid possible clashes.) This change won't affect
your remote users much. They will just need to add an extra parameter to
their connection, as in
ssh -p 22022
yes, this kind of change lies fully in what's called "security through
obscurity"—doing things obscurely, hoping that no one will get wise
to your methods—which usually is just asking for problems. However,
it will help at least against script kiddies, whose scripts just try
to get in via port 22 instead of being thorough enough to try to scan
your machine for all open ports.
In order to implement this change, you need to change the
/etc/ssh/sshd_config file. Working as root, open it with an editor,
look for a line that reads "Port 22", and change the 22 to whatever
number you chose. If the line starts with a hash sign (#), then
remove it, because otherwise the line will be considered a comment. Save
the file, and then restart SSH with
With some distributions, that could be
instead. Finally, also remember to close port 22 in your firewall and to
open the chosen port so remote users will be able to access your server.
While you are at this, for an extra bit of security, you also could add or
edit some other lines in the SSH configuration file (Listing 1). The
Protocol line avoids a weaker, older version of the SSH protocol. The
LoginGraceTime gives the user 30 seconds to accomplish a login. The
MaxAuthTries limits users to three wrong attempts at entering
the password before they are rejected. And finally,
forbids a user from logging in remotely as root (any attacker who managed
to get into your machine still would have to be able to break into the
root account; an extra hurdle), so would-be attackers will have a harder
time at getting privileges on your machine.
Listing 1. These little SSH configuration changes can add a bit of security
Port 22022 Protocol 2 LoginGraceTime 30 MaxAuthTries 3 PermitRootLogin no
Be sure to restart the SSH service dæmon after these changes
/etc/init.d/sshd restart does it) and, for now, you already
to add a bit of extra safety (but not much really), so let's get down to
adding more restrictions.
Who Can Use SSH?
Your machine may have several servers, but you might want to limit
remote access to only a few. You can tweak the sshd_config file a
bit more, and use the
DenyGroups parameters. The first one,
AllowUsers, can be
followed by a list of user names (or even patterns, using the common *
and ? wild cards) or user@host pairs, further restricting access to
the user only from the given host. Similarly,
a list of group name patterns, and login is allowed only for members of
those groups. Finally,
DenyGroups work likewise,
but prohibit access to specific users and groups. Note: the priority order
for rules is
DenyUsers first, then
AllowGroups, so if you explicitly
disallow users from
DenyUsers, no other rules will allow
them to connect.
For example, a common rule is that from the internal network, everybody
should be able to access the machine. (This sounds reasonable; attacks
usually come from outside the network.) Then, you could say
that only two users, fkereki and eguerrero, should be able
to connect from the outside, and nobody else should be able to connect. You
can enable these restrictions by adding a single line
*:192.168.1.*,fkereki,eguerrero to the SSH configuration file and
restarting the service. If you wanted to forbid jandrews from remote
connections, an extra
DenyUsers jandrews would be needed. More
specific rules could be added (say, maybe eguerrero should
be able to log in only from home), but if things start getting out of hand
with too many rules, the idea of editing the ssh configuration files and
restarting the server begins to look less attractive, and there's a better
solution through PAM, which uses separate files for security rules.
The PAM Way
If you google for meanings of PAM, you can find several definitions, ranging from a cooking oil spray to several acronyms (such as Power Amplitude Modulation or Positive Active Mass), but in this case, you are interested in Pluggable Authentication Modules, a way to provide extra authentication rules and harden access to your server. Let's use PAM as an alternative solution to specify which users can access your server.
From a software engineering viewpoint, it would just be awful if each and every program had to invent and define and implement its own authentication logic. How could you be certain that all applications did implement the very same checks, in the same way, without any differences? PAM provides a way out; if a program needs to, say, authenticate a user, it can call the PAM routines, which will run all the checks you might have specified in its configuration files. With PAM, you even can change authentication rules on the fly by merely updating its configuration. And, even if that's not your main interest here, if you were to include new biometrics security hardware (such as fingerprint readers, iris scanners or face recognition) with an appropriate PAM, your device instantly would be available to all applications.
PAMs can be used for four security concerns: account limitations (what the users are allowed to do), authorization (how the users identify themselves), passwords and sessions. PAM checks can be marked optional (may succeed or fail), required (must succeed), requisite (must succeed, and if it doesn't, stop immediately without trying any more checks) and sufficient (if it succeeds, don't run any more checks), so you can vary your policies. I don't cover all these details here, but rather move on to the specific need of specifying who can (or cannot) log in to your server. See the PAM, PAM Everywhere sidebar for a list of some available modules.
PAM, PAM Everywhere
Although there is no "official" list of PAMs, most distributions are likely to include the following:
pam_access: allows or denies access according to the file /etc/security/access.conf.
pam_cracklib: checks passwords against dictionaries.
pam_debug: used for testing only.
pam_deny: always denies access.
pam_echo: displays the contents of a file.
pam_env: sets or unsets environment variables.
pam_exec: lets you run an external command.
pam_group: grants group memberships to the user.
pam_lastlog: shows the date and time of the user's last log in.
pam_ldap: allows authentication against an LDAP server.
pam_limits: lets you set system resource limits, through the file /etc/security/limits.conf.
pam_listfile: an alternative to pam_access, with some extra options.
pam_mail: checks if the user has pending mail.
makein a given directory.
pam_motd: displays the "message of the day" file, usually /etc/motd.
pam_nologin: blocks all logins should file /etc/nologin exist.
pam_permit: always allows access.
pam_pwcheck: checks passwords for strength.
pam_pwhistory: checks new passwords against recently used ones to avoid repetition.
pam_rootok: usually is included in /etc/pam.d/su as a "sufficient" test so root can act as any other user without providing a password.
pam_selinux: sets the default security context for SELinux.
pam_sepermit: allows or denies login depending on SELinux state.
pam_shells: allows access only if the user's shell is listed in the file /etc/shells.
pam_succeed_if: checks for account characteristics, such as belonging to a given group.
pam_tally: just keeps count of attempted accesses and can deny access if too many attempts fail.
pam_time: restricts access based on rules in the file /etc/security/time.conf.
pam_umask: lets you set the file mode creation mask (think
umask) for newly created files.
pam_unix (or pam_unix2): provides classical UNIX-style authentication per the /etc/passwd and /etc/shadow files.
pam_userdb: authenticates the user against a Berkeley database.
pam_warn: records logs in the system logs.
pam_wheel: provides root access only to members of group wheel.
File locations vary, but you can check /usr/lib/security or
/lib/security (or read lib64 for lib, for 64-bit Linux) to
see what modules you actually have. For more information on each module,
man name.of.the.module, but don't try to execute them from the
command line, for they can't be run that way.
PAM configurations are stored in /etc/pam.d, with a file for each
command to which they apply. As root, edit /etc/pam.d/sshd, and add an
account required pam_access.so line after all the
so it ends up looking like Listing 2. (Your specific version of the
file may have some different options; just add the single line to it,
and that's it.) You'll also have to modify the sshd configuration
file (the same one that you modified earlier) so it uses PAM; add a
line to it, and restart the sshd dæmon.
Listing 2. Adding
pam_access.so to the
account PAM checks lets you specify which users have SSH access to your
account required pam_unix2.so account required pam_access.so auth required pam_env.so auth required pam_unix2.so auth required pam_nologin.so password requisite pam_pwcheck.so nullok cracklib password required pam_unix2.so use_authtok nullok session required pam_limits.so session required pam_unix2.so session optional pam_umask.so
account part is what is important here. After using the standard UNIX
methods for checking your password (usually against the files /etc/passwd
and /etc/shadow), it uses the module
pam_access.so to check if
the user is in a list, such as shown in Listing 3. Both
required, meaning that the user must pass both checks
in order to proceed. For extra restrictions, you might want to
pam_listfile, which is similar to
provides even more options, and
pam_time, which lets you fix time
restrictions. You also would need to add extra
account lines to the
You need to edit /etc/security/access.conf to specify which users
can access the machine (Listing 3). Each line in the list starts
with either a plus sign (login allowed) or a minus sign (login disabled),
followed by a colon, a user name (or ALL), another colon and a host
(or ALL). The
pam_access.so module goes down the list in order, and
depending on the first match for the user, it either allows or forbids
the connection. The order of the rules is important. First, jandrews
is forbidden access, then everybody in the internal network is allowed
to log in to the server. Then, users fkereki and eguerrero
are allowed access from any machine. The final
-:ALL:ALL line is
a catchall that denies access to anybody not specifically allowed to
log in in the previous lines, and it always should be present.
Listing 3. The file /etc/security/access.conf specifies which users have access and from which hosts.
-:jandrews:ALL +:ALL:192.168.1. +:fkereki:ALL +:eguerrero:ALL -:ALL:ALL
Note that you could use this configuration for other programs and services (FTP, maybe?), and the same rules could be applied. That's an advantage of PAM. A second advantage is that you can change rules on the fly, without having to restart the SSH service. Not messing with running services is always a good idea! Using PAM adds a bit of hardening to SSH to restrict who can log in. Now, let's look at an even safer way of saying who can access your machine by using certificates.
Passwords can be reasonably secure, but you don't have them written down on a Post-It by your computer, do you? However, if you use a not-too-complex password (so it can be determined by brute force or a dictionary attack), then your site will be compromised for so long as the attacker wishes. There's a safer way, by using public/private key logins, that has the extra advantage of requiring no passwords on the remote site. Rather, you'll have a part of the key (the "private" part) on your remote machine and the other part (the "public" part) on the remote server. Others won't be able to impersonate you unless they have your private key, and it's computationally unfeasible to calculate. Without going into how the key pair is created, let's move on to using it.
First, make sure your sshd configuration file allows for
private key logins. You should have
PubkeyAuthentication yes lines in it. (If not, add them, and restart
the service as described above.) Without those lines, nothing I explain
below will work. Then, use
ssh-keygen to create a public/private key
pair. By directly using it without any more parameters (Listing 4),
you'll be asked in which file to save the key (accept the standard),
whether to use a passphrase for extra security (more on this below, but
you'd better do so), and the key pair will be generated. Pay attention
to the name of the file in which the key was saved. You'll need it in
Listing 4. Generating a public/private key pair with
ssh-keygen is simple.
Opt for using a passphrase for extra security.
$ ssh-keygen Generating public/private rsa key pair. Enter file in which to save the key (/home/fkereki/.ssh/id_rsa): Created directory '/home/fkereki/.ssh'. Enter passphrase (empty for no passphrase): Enter same passphrase again: Your identification has been saved in /home/fkereki/.ssh/id_rsa. Your public key has been saved in /home/fkereki/.ssh/id_rsa.pub. The key fingerprint is: 84:13:e6:07:a3:b1:b4:c6:9f:29:b8:40:58:f5:23:26 fkereki@fedoraxfce The key's randomart image is: +--[ RSA 2048]----+ | ..+ = | |.. o O = | |..E O * o | |. = o B | |. . . + S | | . . . | | . | | | | | +-----------------+
Now, in order to be able to connect to the remote server, you need to
copy it over. If you search the Internet, many sites recommend
directly editing certain files in order to accomplish this, but using
ssh-copy-id is far easier. You just have to type
the.file.where.the.key.was.saved email@example.com specifying the
name of the file in which the public key was saved (as you saw above)
and the remote user and host to which you will be connecting (Listing 5).
And you're done.
Listing 5. After generating your public/private pair, you need to use
copy the public part to the remote server.
$ ssh-copy-id -i /home/fkereki/.ssh/id_rsa.pub firstname.lastname@example.org The authenticity of host '192.168.1.107 (192.168.1.107)' ↪can't be established. RSA key fingerprint is 16:a4:d8:6a:ee:e0:8d:f4:72:a8:af:42:75:1d:28:3b. Are you sure you want to continue connecting (yes/no)? yes Warning: Permanently added '192.168.1.107' (RSA) to the list ↪of known hosts. email@example.com's password:
In order to test your new passwordless connection, just do
firstname.lastname@example.org. If you used a passphrase, you'll be asked
for it now. In either case, the connection will be established, and you
to enter your password for the remote site (Listing 6).
Listing 6. After you've copied the public key over, you can log in to the remote server without a password. You will have to enter your passphrase though, if you used one when generating the public/private pair.
$ ssh email@example.com Enter passphrase for key '/home/fkereki/.ssh/id_rsa': Last login: Mon Jan 10 18:40:11 2011 6.0 Light Final built on March 31, 2009 on Linux 126.96.36.199 You are working as fkereki Frequently used programs: Configuration : vasm File manager : mc (press F2 for useful menu) Editor : mcedit, nano, vi Multimedia : alsamixer, play vector:/~ $ logout Connection to 192.168.1.107 closed.
Now, what about the passphrase? If you create a public/private key pair
without using a passphrase, anybody who gets access to your machine and
the private key immediately will have access to all the remote servers
to which you have access. Using the passphrase adds another level of security
to your log in process. However, having to enter it over and over again
is a bother. So, you would do better by using
ssh-agent, which can
"remember" your passphrase and enter it automatically whenever you try
to log in to a remote server. After running
to add your passphrase. (You could run it several times if you have
many passphrases.) After that, a remote connection won't need a
passphrase any more (Listing 7). If you want to end a session,
ssh-agent -k, and you'll have to re-enter the passphrase if you
want to do a remote login.
Listing 7. Using
ssh-agent frees you from having to
$ ssh-agent SSH_AUTH_SOCK=/tmp/ssh-Rvhhx30943/agent.30943; export SSH_AUTH_SOCK; SSH_AGENT_PID=30944; export SSH_AGENT_PID; echo Agent pid 30944; $ ssh-add Enter passphrase for /home/fkereki/.ssh/id_rsa: Identity added: /home/fkereki/.ssh/id_rsa (/home/fkereki/.ssh/id_rsa) $ ssh firstname.lastname@example.org Last login: Mon Jun 10 18:44:15 2013 from 192.168.1.108 6.0 Light Final built on March 31, 2009 on Linux 188.8.131.52 You are working as fkereki Frequently used programs: Configuration : vasm File manager : mc (press F2 for useful menu) Editor : mcedit, nano, vi Multimedia : alsamixer, play
You also may want to look at
keychain, which allows you to reuse
ssh-agent between logins. (Not all distributions include this
command; you may have to use your package manager to install it.) Just
keychain the.path.to.your.private.key, enter your passphrase
(Figure 1), and until you reboot the server or specifically run
keychain -k all to stop
keychain, your passphrase will be stored,
and you won't have to re-enter it. Note: you even could log out and
log in again, and your key still would be available. If you just want to
clear all cached keys, use
Figure 1. By entering your passphrase once with
keychain, it will be remembered even if you log out.
If you use a passphrase, you could take your private keys with you on a USB stick or the like and use it from any other machine in order to log in to your remote servers. Doing this without using passphrases would just be too dangerous. Losing your USB stick would mean automatically compromising all the remote servers you could log in to. Also, using a passphrase is an extra safety measure. If others got hold of your private key, they wouldn't be able to use it without first determining your passphrase.
Finally, if you are feeling quite confident that all needed users have
their passwordless logins set up, you could go the whole mile and disable
common passwords by editing the sshd configuration file and setting
PasswordAuthentication no and
no, but you'd better be quite
sure everything's working, because otherwise you'll have problems.
Using SSH and PuTTY
You can use SSH public/private pairs with the common
PuTTY program, but not directly, because it requires a
specific, different key file. In order to convert your SSH key,
you need to do
puttygen $HOME/.ssh/your.private.key -o
your.private.key.file.for.putty. Afterward, you simply can open
PuTTY, go to Connection, SSH, Auth and browse for your newly
generated "Private key file for authentication".
There's no definitive set of security measures that can 100% guarantee that no attacker ever will be able to get access to your server, but adding extra layers can harden your setup and make the attacks less likely to succeed. In this article, I described several methods, involving modifying SSH configuration, using PAM for access control and public/private key cryptography for passwordless logins, all of which will enhance your security. However, even if these methods do make your server harder to attack, remember you always need to be on the lookout and set up as many obstacles for attackers as you can manage.
The SSH protocol is defined over a host of RFC (Request for Comments) documents; check http://en.wikipedia.org/wiki/Secure_Shell#Internet_standard_documentation for a list.
Port numbers are assigned by IANA (Internet Assigned Numbers Authority), and you can go to http://www.iana.org/assignments/port-numbers for a list.
Read http://www.funtoo.org/wiki/Keychain for more on
its author, Daniel Robbins.
You can see the RSA original patent at http://www.google.com/patents?vid=4405829 and the RSA Cryptography Standard at http://www.emc.com/emc-plus/rsa-labs/pkcs/files/h11300-wp-pkcs-1v2-2-rsa-cryptography-standard.pdf.
For extra security measures, read "Implement Port-Knocking Security with knockd", in the January 2010 issue of Linux Journal, or check it out on-line at http://www.linuxjournal.com/article/10600.