Tuesday, August 16, 2011

OpenSSH Security - Client Configuration

OpenSSH provides a suite of tools for encrypting traffic between endpoints, port forwarding, IP tunneling, and authentication. Part I of this guide will outline the client side OpenSSH configuration. The OpenSSH client is running on OS X Lion. The built in firewall, ipfw, is enabled on the client to restrict outbound and inbound traffic. Part II (currently on hold) of this guide will cover the configuration of OpenSSH on the server along with the available options and alternatives for authentication, authorization, and traffic encryption. The configuration will force AES 256 in Counter Mode and will restrict the available Message Authentication Algorithms that may be used between endpoints. Most of the options in the ssh configuration file on the server will be disabled, public key authentication will be used, password authentication will be disabled, and the ssh daemon will bind to a high number port. Multiple SSH sessions will use the same connection via the ControlMaster and ControlPath client configuration directive. Also, a server certificate will be generated and used to sign user public keys. The CA signed user public keys constitute a user certificate which the server will in turn use for client authentication. PF will be used on the server for stateful packet filtering, connection blocking, and connection throttling. The below configuration will also detail

First and foremost, the client has ipfw enabled and the firewall ruleset is configured in /etc/ipfw.conf. ipfw has been configured to block all inbound traffic and block all outbound traffic except for the ports and IP addresses that are necessary for connecting to the OpenSSH server. The server is running FreeBSD 8.2.

FreeBSD 8.2 - sshd on a.b.c.d:21465 pf |  <--------Internet---------->  | ipfw  OS X Lion - ssh client
To start with, you will need to install coreutils and apg on the client. coreutils and apg can be obtained from Mac ports and can be installed as follows:

client: $ sudo port install coreutils 
client: $ sudo port install apg 

Before generating your public/private keypair, You will need to generate a strong passphrase for your private key. It is important to store this passphrase in a secure location, not on your computer.

client: $ openssl rand -base64 1000 | shasum-5.12 -a 512 | apg -M SNCL -a 1 -m 20 -x 20

Depending on your version of OpenSSH (should be using latest stable for your OS), ECDSA may be used in addition to DSA and RSA. Certificates may also be used for user and host authentication. See the ssh-keygen man page for details.
You can generate your keypair using the following command. When prompted for the passphrase, use the output from the above command.

client: $ ssh-keygen -b 4096 -t rsa -C"$(id -un)@$hostname)-$(gdate --rfc-3339=date)"

Here is an example of how to use ssh-keygen to generate a public/private keypair using the Eliptic Curve Digital Signature Algorithm. Both the client and server must be running a version of OpenSSH >= 5.7.

client: $ ssh-keygen -b 521 -t ecdsa -C"$(id -un)@$hostname)-$(gdate --rfc-3339=date)"

Now, we need to push the public key to the server and place it in the authorized_keys file of the user that we are going to log in as over ssh.
The ssh-copy-id command can be used to automate this process. On the OS X client, the ssh-copy-id command does not come preinstalled with SSH. The ssh-copy-id command can be obtained from http://www.freebsd.org/cgi/cvsweb.cgi/~checkout~/ports/security/ssh-copy-id/files/ssh-copy-id?rev=1.1;content-type=text%2Fplain. After downloading the script, change its permissions and place it in your path.
At this point, you should already have a server that is running OpenSSH on port 22 with the default configuration. Thus, you can transfer your public key with the following command:

client: $ ssh-copy-id -i ~/.ssh/id_xxxyy.pub bryan@a.b.c.d \

It is time to setup connection sharing. Create the following file if it does not currently exist.

client: $ ls -l ~/.ssh/config -rw------- 1 bryan scclp 104 Aug 13 10:55 config

The file should contain these lines.

ServerAliveInterval 60 Host a.b.c.d ControlMaster auto ControlPath ~/.ssh/sockets/%r@%h:%p

The goal is to only allow connections to the server in AES 256 Counter mode, with umac-64 or hmac-ripemd160 MACs, and compression, on a non-standard SSH port from a designated IP range using public key authentication. Connections will also be throttled and SSHGuard along with a few custom PF rules on the server will be used to block and log attackers. The commands that the client will use to connect to the server will look like this:client: 

$ alias sshconnect="ssh -l bryan a.b.c.d -p 21465 -C -c aes256-ctr -m umac-64@openssh.com,hmac-ripemd160 client: 
$ alias sshtunnel="ssh -v -ND 8090 bryan@a.b.c.d -p 21465 -C -c aes256-ctr -m umac-64@openssh.com,hmac-ripemd160 client:
$ alias sshmonitor="yes | pv | ssh -l bryan a.b.c.d -p 21465 -C -c aes256-ctr -m umac-64@openssh.com,hmac-ripemd160 \"cat > /dev/null\"" client: 
$ alias sshportforward="ssh -f bryan@a.b.c.d -p 21465 -C -c aes256-ctr -m umac-64@openssh.com,hmac-ripemd160 -L 15478:localhost:15479 -N" client: 
$ alias sshportforward2="ssh -f bryan@a.b.c.d -p 21465 -C -c aes256-ctr -m umac-64@openssh.com,hmac-ripemd160 -L 17293:localhost:17294 -N"

Alternatively, Ciphers, MACs, and compression can be specified in the user config file as follows:

ServerAliveInterval 60 
Host host.name.com 
    ControlMaster auto 
    ControlPath ~/.ssh/sockets/%r@%h:%p 
    Port 21465 
    User bryan 
    Ciphers aes256-ctr 
    Compression yes 
    MACs umac-64@openssh.com,hmac-ripemd160 
    StrictHostKeyChecking yes

User and Host certificates provide a more convenient method of authentication for multiple clients (users) and servers (hosts). Certificate revocation can also provide an easier method of quickly invalidating user access.A certificate authority key pair is first generated as follows. The ca is then placed in the /etc/ssh directory on the host.

ca $ ssh-keygen -t ecdsa -b 521 -f user_ca server $ sudo mv user_ca* /etc/ssh/

On the client, generate a public/private key pair and then copy the public key to the server so that it can be signed with the ca. Make sure to set the validity period of the certificate. Alternatively, a host key may be signed with a ca key that is stored in a PKCS11 token. OpenSSH supports ca keys stored PCKS11 tokens. Check your version of SSH and see ssh-keygen for more information.client 

client $ ssh-keygen -t ends -b 521 -f ~/.ssh/id_ecdsa 
client $ scp .ssh/id_ecdsa.pub bryan@server-ca:~/user_public_keys 
server-ca $ ssh-keygen -s /etc/ssh/user_ca \ 
      -O source-address=clientip 
      -O permit-pty 
      -O no-port-forwarding       
      -O no-user-rc 
      -O no-x11-forwarding \ -V -1d:+52w1d -z 6739301351 -I "bryan"       -n bryan,clienthostname id_ecdsa.pub
id "bryan" serial 6739301351 for bryan,clienthostname valid from 2011-08-18T15:05:24 to 2012-08-17T15:05:24 

Copy the signed user cert back to the client.

client $ scp bryan@server:~/user_public_keys/id_ecdsa-cert.pub ~/.ssh/ 

Setup TrustedUserCAKeys and AuthorizedPrincipalsFile files. Subsequently, set appropriate options in /etc/ssh/sshd_config on the server.

server-ca $ sudo cat /etc/ssh/user_ca.pub > /etc/ssh/trusted_user_ca_keys 

Modify /etc/ssh/authorized_principals to include the following lines.bryan from="clientip" bryan
Modify /etc/ssh/sshd_config on the server to include the following lines

TrustedUserCAKeys /etc/ssh/trusted_user_ca_keys 
AuthorizedPrincipalsFile /etc/ssh/authorized_principals 

Now, restart sshd on the server and add an appropriate host configuration for certificate authentication to ~/.ssh/config on the client.

Last of all, if you want to setup a host certificate, you will need to use the -h option with ssh-keygen when signing a host key.

In the second part of this guide (currently on hold), I will cover how to lock down SSH on the server. I will also cover port knocking, Single Packet Authorization (SPA), and other options for authentication including Kerberos 5, PAM, and two/three factor authentication using the Google Authenticator project.

It is important to always keep OpenSSH updated with the latest, stable version that has been released for your operating system.


OpenSSH, http://openssh.org

fwknop, http://cipherdyne.org/fwknop/

OpenBSD, http://openbsd.org

FreeBSD, http://freebsd.org