Linux sshd config hardening

=From unixhelp=
 * source page - http://unixhelp.ed.ac.uk/CGI/man-cgi?sshd_config

SSHD_CONFIG(5)		   BSD File Formats Manual		SSHD_CONFIG(5)

NAME

sshd_config - OpenSSH SSH daemon configuration file

SYNOPSIS

/etc/ssh/sshd_config

DESCRIPTION

sshd reads configuration data from /etc/ssh/sshd_config (or the file    specified with -f on the command line). The file contains keyword-argu- ment pairs, one per line. Lines starting with '#' and empty lines are interpreted as comments.

The possible keywords and their meanings are as follows (note that key-    words are case-insensitive and arguments are case-sensitive):

AcceptEnv Specifies what environment variables sent by the client will be copied into the session's environ(7). See SendEnv in ssh_config(5) for how to configure the client. Note that envi- ronment passing is only supported for protocol 2. Variables are specified by name, which may contain the wildcard characters '*' and '?'. Multiple environment variables may be separated by whitespace or spread across multiple AcceptEnv directives. Be	    warned that some environment variables could be used to bypass restricted user environments. For this reason, care should be taken in the use of this directive. The default is not to accept any environment variables.

AddressFamily Specifies which address family should be used by sshd. Valid arguments are "any", "inet" (use IPv4 only) or "inet6" (use IPv6	    only). The default is "any".

AllowGroups This keyword can be followed by a list of group name patterns, separated by spaces. If specified, login is allowed only for users whose primary group or supplementary group list matches one of the patterns. '*' and '?' can be used as wildcards in the patterns. Only group names are valid; a numerical group ID is not recognized. By default, login is allowed for all groups.

AllowTcpForwarding Specifies whether TCP forwarding is permitted. The default is "yes". Note that disabling TCP forwarding does not improve secu- rity unless users are also denied shell access, as they can always install their own forwarders.

AllowUsers This keyword can be followed by a list of user name patterns, separated by spaces. If specified, login is allowed only for user names that match one of the patterns. '*' and '?' can be used as wildcards in the patterns. Only user names are valid; a	    numerical user ID is not recognized. By default, login is allowed for all users. If the pattern takes the form USER@HOST then USER and HOST are separately checked, restricting logins to particular users from particular hosts.

AuthorizedKeysFile Specifies the file that contains the public keys that can be used for user authentication. AuthorizedKeysFile may contain tokens of the form %T which are substituted during connection set-up. The following tokens are defined: %% is replaced by a literal '%', %h is replaced by the home directory of the user being authenticated and %u is replaced by the username of that user. After expansion, AuthorizedKeysFile is taken to be an absolute path or one relative to the user's home directory. The default is ".ssh/authorized_keys".

Banner In some jurisdictions, sending a warning message before authenti- cation may be relevant for getting legal protection. The con- tents of the specified file are sent to the remote user before authentication is allowed. This option is only available for protocol version 2. By default, no banner is displayed.

ChallengeResponseAuthentication Specifies whether challenge response authentication is allowed. All authentication styles from login.conf(5) are supported. The default is "yes".

ChrootDirectory Specifies a path to chroot(2) to after authentication. This path, and all its components, must be root-owned directories that are not writable by any other user or group.

The path may contain the following tokens that are expanded at	    runtime once the connecting user has been authenticated: %% is	     replaced by a literal '%', %h is replaced by the home directory of the user being authenticated, and %u is replaced by the user- name of that user.

The ChrootDirectory must contain the necessary files and directo- ries to support the users' session. For an interactive session this requires at least a shell, typically sh(1), and basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4), arandom(4) and tty(4) devices. For file transfer sessions using "sftp", no additional configuration of the environment is neces- sary if the in-process sftp server is used (see "internal-sftp"	    will force the use of an in-process sftp server that requires no	     support files when used with ChrootDirectory.  Subsystem for	     details).

The default is not to chroot(2).

Ciphers Specifies the ciphers allowed for protocol version 2. Multiple ciphers must be comma-separated. The supported ciphers are "3des-cbc", "aes128-cbc", "aes192-cbc", "aes256-cbc", "aes128-ctr", "aes192-ctr", "aes256-ctr", "arcfour128", "arcfour256", "arcfour", "blowfish-cbc", and "cast128-cbc". The default is

''aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128, aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc, aes256-cbc,arcfour''

ClientAliveCountMax Sets the number of client alive messages (see below) which may be sent without sshd receiving any messages back from the client. If this threshold is reached while client alive messages are being sent, sshd will disconnect the client, terminating the ses- sion. It is important to note that the use of client alive mes- sages is very different from TCPKeepAlive (below). The client alive messages are sent through the encrypted channel and there- fore will not be spoofable. The TCP keepalive option enabled by TCPKeepAlive is spoofable. The client alive mechanism is valu- able when the client or server depend on knowing when a connec- tion has become inactive.

The default value is 3. If ClientAliveInterval (see below) is	    set to 15, and ClientAliveCountMax is left at the default, unre- sponsive ssh clients will be disconnected after approximately 45 seconds.

ClientAliveInterval Sets a timeout interval in seconds after which if no data has been received from the client, sshd will send a message through the encrypted channel to request a response from the client. The default is 0, indicating that these messages will not be sent to the client. This option applies to protocol version 2 only.

Compression Specifies whether compression is allowed, or delayed until the user has authenticated successfully. The argument must be "yes", "delayed", or "no". The default is "delayed".

DenyGroups This keyword can be followed by a list of group name patterns, separated by spaces. Login is disallowed for users whose primary group or supplementary group list matches one of the patterns. '*' and '?' can be used as wildcards in the patterns. Only group names are valid; a numerical group ID is not recognized. By default, login is allowed for all groups.

DenyUsers This keyword can be followed by a list of user name patterns, separated by spaces. Login is disallowed for user names that match one of the patterns. '*' and '?' can be used as wildcards in the patterns. Only user names are valid; a numerical user ID is not recognized. By default, login is allowed for all users. If the pattern takes the form USER@HOST then USER and HOST are separately checked, restricting logins to particular users from particular hosts. Specifying a command of "internal-sftp" will force the use of an in-process sftp server that requires no sup- port files when used with ChrootDirectory.

ForceCommand Forces the execution of the command specified by ForceCommand, ignoring any command supplied by the client. The command is invoked by using the user's login shell with the -c option. This applies to shell, command, or subsystem execution. The command originally supplied by the client is available in the SSH_ORIGINAL_COMMAND environment variable.

GatewayPorts Specifies whether remote hosts are allowed to connect to ports forwarded for the client. By default, sshd binds remote port forwardings to the loopback address. This prevents other remote hosts from connecting to forwarded ports. GatewayPorts can be	    used to specify that sshd should allow remote port forwardings to	     bind to non-loopback addresses, thus allowing other hosts to con- nect. The argument may be "no" to force remote port forwardings to be available to the local host only, "yes" to force remote port forwardings to bind to the wildcard address, or	    "clientspecified" to allow the client to select the address to which the forwarding is bound. The default is "no".

GSSAPIAuthentication Specifies whether user authentication based on GSSAPI is allowed. The default is "no". Note that this option applies to protocol version 2 only.

GSSAPICleanupCredentials Specifies whether to automatically destroy the user's credentials cache on logout. The default is "yes". Note that this option applies to protocol version 2 only.

HostbasedAuthentication Specifies whether rhosts or /etc/hosts.equiv authentication together with successful public key client host authentication is allowed (hostbased authentication). This option is similar to RhostsRSAAuthentication and applies to protocol version 2 only. The default is "no".

HostKey Specifies a file containing a private host key used by SSH. The default is /etc/ssh/ssh_host_key for protocol version 1, and /etc/ssh/ssh_host_rsa_key and /etc/ssh/ssh_host_dsa_key for pro- tocol version 2. Note that sshd will refuse to use a file if it is group/world-accessible. It is possible to have multiple host key files. "rsa1" keys are used for version 1 and "dsa" or "rsa" are used for version 2 of the SSH protocol.

IgnoreRhosts Specifies that .rhosts and .shosts files will not be used in RhostsRSAAuthentication or HostbasedAuthentication.

/etc/hosts.equiv and /etc/ssh/shosts.equiv are still used. The default is "yes".

IgnoreUserKnownHosts Specifies whether sshd should ignore the user's	    ~/.ssh/known_hosts during RhostsRSAAuthentication or HostbasedAuthentication. The default is "no".

KerberosAuthentication Specifies whether the password provided by the user for PasswordAuthentication will be validated through the Kerberos KDC. To use this option, the server needs a Kerberos servtab which allows the verification of the KDC's identity. Default is "no".

KerberosGetAFSToken If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire an AFS token before accessing the user's home directory. Default is "no".

KerberosOrLocalPasswd If set then if password authentication through Kerberos fails then the password will be validated via any additional local mechanism such as /etc/passwd. Default is "yes".

KerberosTicketCleanup Specifies whether to automatically destroy the user's ticket cache file on logout. Default is "yes".

KeyRegenerationInterval In protocol version 1, the ephemeral server key is automatically regenerated after this many seconds (if it has been used). The purpose of regeneration is to prevent decrypting captured ses- sions by later breaking into the machine and stealing the keys. The key is never stored anywhere. If the value is 0, the key is never regenerated. The default is 3600 (seconds).

ListenAddress Specifies the local addresses sshd should listen on. The follow- ing forms may be used:

ListenAddress host|IPv4_addr|IPv6_addr ListenAddress host|IPv4_addr:port ListenAddress [host|IPv6_addr]:port

If port is not specified, sshd will listen on the address and all prior Port options specified. The default is to listen on all local addresses. Multiple ListenAddress options are permitted. Additionally, any Port options must precede this option for non port qualified addresses.

LoginGraceTime The server disconnects after this time if the user has not suc- cessfully logged in. If the value is 0, there is no time limit. The default is 120 seconds.

LogLevel Gives the verbosity level that is used when logging messages from sshd. The possible values are: QUIET, FATAL, ERROR, INFO, VER- BOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3. The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify higher levels of debugging output. Logging with a DEBUG level violates the privacy of users and is not recommended.

MACs   Specifies the available MAC (message authentication code) algo- rithms. The MAC algorithm is used in protocol version 2 for data integrity protection. Multiple algorithms must be comma-sepa- rated. The default is "hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96".

MaxAuthTries Specifies the maximum number of authentication attempts permitted per connection. Once the number of failures reaches half this value, additional failures are logged. The default is 6.

MaxStartups Specifies the maximum number of concurrent unauthenticated con- nections to the sshd daemon. Additional connections will be	    dropped until authentication succeeds or the LoginGraceTime expires for a connection. The default is 10.

Alternatively, random early drop can be enabled by specifying the three colon separated values "start:rate:full" (e.g.,	    "10:30:60"). sshd will refuse connection attempts with a proba- bility of "rate/100" (30%) if there are currently "start" (10) unauthenticated connections. The probability increases linearly and all connection attempts are refused if the number of unau- thenticated connections reaches "full" (60).

PasswordAuthentication Specifies whether password authentication is allowed. The default is "yes".

PermitEmptyPasswords When password authentication is allowed, it specifies whether the server allows login to accounts with empty password strings. The default is "no".

PermitRootLogin Specifies whether root can log in using ssh(1). The argument must be "yes", "without-password", "forced-commands-only" or "no". The default is "yes".

If this option is set to "without-password" password authentica- tion is disabled for root.

If this option is set to "forced-commands-only" root login with public key authentication will be allowed, but only if the command option has been specified (which may be useful for taking	    remote backups even if root login is normally not allowed). All other authentication methods are disabled for root.

If this option is set to "no" root is not allowed to log in.

PermitTunnel Specifies whether tun(4) device forwarding is allowed. The argu- ment must be "yes", "point-to-point", "ethernet" or "no". The default is "no".

PermitUserEnvironment Specifies whether ~/.ssh/environment and environment= options in ~/.ssh/authorized_keys are processed by sshd. The default is "no". Enabling environment processing may enable users to bypass access restrictions in some configurations using mechanisms such as LD_PRELOAD.

PidFile Specifies the file that contains the process ID of the sshd dae- mon. The default is /var/run/sshd.pid.

Port   Specifies the port number that sshd listens on. The default is 22. Multiple options of this type are permitted. See also ListenAddress.

PrintLastLog Specifies whether sshd should print the date and time of the last user login when a user logs in interactively. The default is "yes".

PrintMotd Specifies whether sshd should print /etc/motd when a user logs in interactively. (On some systems it is also printed by the shell,	    /etc/profile, or equivalent.)  The default is "yes".

Protocol Specifies the protocol versions sshd supports. The possible val- ues are "1" and "2". Multiple versions must be comma-separated. The default is "2,1". Note that the order of the protocol list does not indicate preference, because the client selects among multiple protocol versions offered by the server. Specifying "2,1" is identical to "1,2".

PubkeyAuthentication Specifies whether public key authentication is allowed. The default is "yes". Note that this option applies to protocol ver- sion 2 only.

RhostsRSAAuthentication Specifies whether rhosts or /etc/hosts.equiv authentication together with successful RSA host authentication is allowed. The default is "no". This option applies to protocol version 1 only.

RSAAuthentication Specifies whether pure RSA authentication is allowed. The default is "yes". This option applies to protocol version 1 only.

ServerKeyBits Defines the number of bits in the ephemeral protocol version 1 server key. The minimum value is 512, and the default is 768.

ShowPatchLevel Specifies whether sshd will display the patch level of the binary in the identification string. The patch level is set at compile- time. The default is "no". This option applies to protocol ver- sion 1 only.

StrictModes Specifies whether sshd should check file modes and ownership of the user's files and home directory before accepting login. This is normally desirable because novices sometimes accidentally leave their directory or files world-writable. The default is "yes".

Subsystem Configures an external subsystem (e.g., file transfer daemon). Arguments should be a subsystem name and a command (with optional	    arguments) to execute upon subsystem request.

The command sftp-server(8) implements the "sftp" file transfer subsystem.

Alternately the name "internal-sftp" implements an in-process "sftp" server. This may simplify configurations using ChrootDirectory to force a different filesystem root on clients.

By default no subsystems are defined. Note that this option applies to protocol version 2 only.

SyslogFacility Gives the facility code that is used when logging messages from sshd. The possible values are: DAEMON, USER, AUTH, AUTHPRIV, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is AUTH.

TCPKeepAlive Specifies whether the system should send TCP keepalive messages to the other side. If they are sent, death of the connection or crash of one of the machines will be properly noticed. However, this means that connections will die if the route is down tem- porarily, and some people find it annoying. On the other hand, if TCP keepalives are not sent, sessions may hang indefinitely on the server, leaving "ghost" users and consuming server resources.

The default is "yes" (to send TCP keepalive messages), and the server will notice if the network goes down or the client host crashes. This avoids infinitely hanging sessions.

To disable TCP keepalive messages, the value should be set to "no".

UseDNS Specifies whether sshd should look up the remote host name and check that the resolved host name for the remote IP address maps back to the very same IP address. The default is "yes".

UseLogin Specifies whether login(1) is used for interactive login ses- sions. The default is "no". Note that login(1) is never used for remote command execution. Note also, that if this is	    enabled, X11Forwarding will be disabled because login(1) does not know how to handle xauth(1) cookies. If UsePrivilegeSeparation is specified, it will be disabled after authentication.

UsePAM Enables the Pluggable Authentication Module interface. If set to	    "yes" this will enable PAM authentication using ChallengeResponseAuthentication and PAM account and session mod- ule processing for all authentication types.

Because PAM challenge-response authentication usually serves an	    equivalent role to password authentication, you should disable either PasswordAuthentication or ChallengeResponseAuthentication.

If UsePAM is enabled, you will not be able to run sshd(8) as a	    non-root user. The default is "no".

UsePrivilegeSeparation Specifies whether sshd separates privileges by creating an unprivileged child process to deal with incoming network traffic. After successful authentication, another process will be created that has the privilege of the authenticated user. The goal of	    privilege separation is to prevent privilege escalation by con- taining any corruption within the unprivileged processes. The default is "yes".

X11DisplayOffset Specifies the first display number available for sshd's X11 for- warding. This prevents sshd from interfering with real X11 servers. The default is 10.

X11Forwarding Specifies whether X11 forwarding is permitted. The argument must be "yes" or "no". The default is "no".

When X11 forwarding is enabled, there may be additional exposure to the server and to client displays if the sshd proxy display is	    configured to listen on the wildcard address (see X11UseLocalhost	     below), however this is not the default. Additionally, the authentication spoofing and authentication data verification and substitution occur on the client side. The security risk of	    using X11 forwarding is that the client's X11 display server may be exposed to attack when the ssh client requests forwarding (see	    the warnings for ForwardX11 in ssh_config(5)). A system adminis- trator may have a stance in which they want to protect clients that may expose themselves to attack by unwittingly requesting X11 forwarding, which can warrant a "no" setting.

Note that disabling X11 forwarding does not prevent users from forwarding X11 traffic, as users can always install their own forwarders. X11 forwarding is automatically disabled if UseLogin is enabled.

X11UseLocalhost Specifies whether sshd should bind the X11 forwarding server to the loopback address or to the wildcard address. By default, sshd binds the forwarding server to the loopback address and sets the hostname part of the DISPLAY environment variable to "localhost". This prevents remote hosts from connecting to the proxy display. However, some older X11 clients may not function with this configuration. X11UseLocalhost may be set to "no" to	    specify that the forwarding server should be bound to the wild- card address. The argument must be "yes" or "no". The default is "yes".

XAuthLocation Specifies the full pathname of the xauth(1) program. The default is /usr/bin/xauth.

Time Formats sshd command-line arguments and configuration file options that specify time may be expressed using a sequence of the form: time[qualifier], where time is a positive integer value and qualifier is one of the fol- lowing:

seconds s | S  seconds m | M  minutes h | H  hours d | D  days w | W  weeks

Each member of the sequence is added together to calculate the total time value.

Time format examples:

600	  600 seconds (10 minutes) 10m	  10 minutes 1h30m  1 hour 30 minutes (90 minutes)

FILES

/etc/ssh/sshd_config Contains configuration data for sshd. This file should be	    writable by root only, but it is recommended (though not neces-	     sary) that it be world-readable.

SEE ALSO

sshd(8)

AUTHORS

OpenSSH is a derivative of the original and free ssh 1.2.12 release by    Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-added newer features and cre- ated OpenSSH. Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support for privilege separation.

BSD			     September 25, 1999			   BSD

1994 Man-cgi 1.15, Panagiotis Christias 

=from teknoteknik=
 * source page - http://teknoteknik.wordpress.com/2010/07/06/best-practices-to-secure-a-opensshssh-server/

What are you using for remote connection? For linux users there are two protocols which regulate remote connection. One is telnet and the other is ssh. People use telnet to connect to remote host but problem with telnet is the whole communication between the local host and the remote host is sent in clear text which any one can see if he will put a packet sniffer and start capture the packets. Hence Telnet is not secure. To avoid this security risk we are using OpenSSH protocol. OpenSSH uses SSH (Secure Shell) protocol which is fully secure as it uses strong encryption like 3DES, Blowfish, AES and also it uses public/private key pair for authentication. N:B-Whatever steps given below has to be done by following the steps given below.

a.Open two terminals, One for experiment and other for safety if sshd show some problem

b.Take the backup of configuration files

c.After each change reload SSH service only after testing the configuration. You can test it by executing

/usr/sbin/sshd –t

d.If you are finding some error in configuration file after executing the above command you should revert back to original config file in order to prevent yourself from getting disconnected.

When configuring SSH the default files we need to edit are

i. /etc/ssh/sshd_config –configuration file for SSH server side.

ii. etc/ssh/ssh_config – Configuration file for client side.

iii.~/.ssh/ -This is the directory where all ssh keypair and authorization file stays

iii./etc/nologin – If this file exists then sshd protocol refuses all login except root login

Now we will come to our topic on configuring a secure SSH server. To make your server secure you can follow the following points. Before changing anything in the default configuration file always make a backup copy of config files you wish to edit. 1.Change the default port number

By default SSHD protocol runs on port 22. The vast majority of ssh attacks are directed by compromised zombie machines against ssh servers listening on the default port of “22″.Your first task is to change the port for ssh. To do this follow steps below

a. open /etc/ssh/sshd_config file

b. Search for line containing Port 22

c. Edit that line as Port 2222  //here 2222 port is my choice you can use yours

d. Save

2.Listen to specific IP Address

In most case you need to connect to your server from your office network or from some specific machine. If you can make your SSHD protocol to listen to those specific IP address and reject others then you will minimize risk of getting attacked from outside world. To do this

a. open /etc/ssh/sshd_config file

b. Search for the line which says

ListenAddress *

c. Now comment the above line and make entry as per your criteria. Lets say if I will give access to 192.168.1.1 and 192.168.1.2 to access then I have to add following two line

ListenAddress 192.168.1.1

ListenAddress 192.168.1.2

3. Now you need to update IP table rule to allow your allowed IP’s (192.168.1.1,192.168.1.2) to          communicate with the custom port (port 2222 in my case).

4. You can also use IP table throttling feature to throttle the incoming connections

5.   Limit the users(allow or deny) who can access by username We have already seen how we can limit the number of IP address which can connect to SSH server. We may have multiple users in single system from which some users do not need to access. So we have to restrict that user’s access. To do sowe can follow the steps

a. Open /etc/ssh/sshd_config file

b. Add the following line to allow specific user.For example allow users root ,hari,prit

AllowUsers root hari prit

c.       Also we can allow specific groups. To allow a group called ssh access add the line

AllowGroups sshaccess

d. similarly if we want to deny some specific users or groups to use ssh we can add the deny lines .Ex:-If I want to deny users navin,amiya and group kopex then I have to add lines

DenyUsers navin amiya

DenyGroups kopex

It all depend upon you how you want to put user access control

6.   Do not allow root login As we know the power of root we should not allow root users to log in rather we should create some sudo users and we can limit them to doing specific task. Also if someone needs to work as root they can use su command. To disable root login

a. Open /etc/ssh/sshd_config file

b.Uncomment the following line

PermitRootLogin no

7.Disable Empty Passwords You should not allow remote login from accounts with empty passwords. If you do this there is chance that some bots will try to log in continuously and damage your system.To do this

a.Open /etc/ssh/sshd_config file

b.Edit the line containing PermitEmptyPasswords to no

PermitEmptyPasswords no

8. Configure client idle Timeout Interval You can set idle time interval for the users who are logging through SSH so that if someone has forgotten to logout from a session the system will log him out immediately. To do this

a. Open /etc/ssh/sshd_config file

b. Set these two lines as per your requirement

ClientAliveInterval 180        //180 is in seconds ClientAliveCountMax 0

9.Disable Host-Based Authentication Avoid using this method as if you add one host it does not ask for any kind of password to login.To disable host based authentication you can do the following steps

a. Open /etc/ssh/sshd_config file

b. Edit the entry HostbasedAuthentication in the file as below

HostbasedAuthentication  no

10. Always use Latest version of SSH as always latest version will be having patches and more security features which makes your job easy or else you have to patch with the latest patch from the vendor.

11. User strong SSH passwords and paraphrases so that it won’t be easy for someone to crack. You can check your password strength using john the ripper tool

12. Use Authentication based on only public/private keys You can also disable the password and use public/private key pairs to login to remote system. But it is advisable to protect your keys by giving strong paraphrases so that if someone takes your key still he needs to know paraphrase to login

To disable password logins, add the following to sshd_config: PasswordAuthentication no

13. Restrict users to their home directories Using some tools or chroot concept you can restrict users to their home directories so that they cannot move or delete the configuration and system files.

14. Allow/Deny using TCP Wrappers As ssh is associated with the library libwrap.so we can implement TCP wrapper concept. So we can allow/deny some specific IP/Host by putting entries into /etc/hosts.allow and /etc/hosts.deny file. For that we need to put an entry at the end of the file like sshd : 192.168.1.5 192.168.1. 8

to allow or deny the IP 192.168.1.5 and 192.168.1.8 to connect the server using SSH.

15. Disable .rhosts Files The rhosts file specifies which remote computer or users can access a local account using rsh or rcp commands. So you can disable this. To disable this

a. Open /etc/ssh/ sshd_config file

b. Search for the line containing IgnoreRhosts and make it to yes

IgnoreRhosts yes

16. Reduce MaxStartups MaxStartup means the ability of SSH server to handle unauthorized access to a server at a given instance. This will help in coordinated attack from different server at same time. To achieve this

a. Open /etc/ssh/ sshd_config file

b. Search for the line containing MaxStartup and replace it as

MaxStartups 4:10:8

Here the 4:10:8 tells the ssh server to, “allow 4 users to attempt logging in at the same time, and to randomly and increasingly drop connection attempts between 4 and the maximum of 8″. Note: this should be increased on servers with substantial numbers of valid ssh users logging in.

17. Hide openssh version This step is not only applicable for SSH but also applicable for all the services running on the system. As no tools/software/packages is free of bugs. This is why people release different versions. If you are running an older version of SSH and hacker knows this he will first try to find out the bugs in this version and using those info he may attack on the particular port. To achive this you have to make changes in source code of ssh and recompile it.

18. Forward X11 only  if needed By default, the X11Forwarding directive is set to yes. If you don’t need this, set it to no. If you don’t need this you should disable this. To disable this

a.Open /etc/ssh/ sshd_config file

b.Search for X11Forwarding and edit as

X11Forwarding  no

For X11 forwarding to work, the ForwardX11 declaration or the ForwardX11Trusted declaration must also be set to yes in /etc/ssh/ssh_config client configuration file.SO to disable you can make it no from this file.

19. Enable a Warning Banner Set a warning banner for ssh connection in consultation with your legal advisor in order to warn authorized user what legel action you can take against them. To do this defines a text file location in sshd_config file which contains the warning messages. To do this

a. Open /etc/ssh/ sshd_config file

b. Search for the line containing Banner and edit as given below

Banner /etc/issue

Now you can edit the /etc/issue file to put your warnings.

20. Use Logwatch Make sure to set LogLevel to INFO or DEBUG in sshd_config. I recommend keeping it INFO as it will give you more details. You can use logwatch to manage the SSH logs. To do this

a.Open /etc/ssh/ sshd_config file

b.Change the line containing LogLevel to info

LogLevel INFO

What else you can do?

–Install some security tools to protect your server from brute force attack

–Patch your operating system

–Remove unnecessary packages / software.

–Harden the kernel against synflood and basic DOS attacks.

–Remove common user access to compilers and fetching software (wget, fetch, lynx, etc.).

– Ensure /tmp is in its own partition with noexec, nosuid.

–Ensure kernel and software is up to date.

–Remove unnecessary users and groups.

–Install chkrootkit and tripwire.