Tools Index

OpenSSH

OpenBSD Secure Shell, is a suite of security-related network-level utilities based on the SSH protocol, which help to secure network communications via the encryption of network traffic over multiple authentication methods and by providing secure tunneling capabilities.

1. Server

Crux openssh port install this files to etc;

        $ pkginfo -l openssh
        etc/rc.d/sshd
        etc/ssh/moduli
        etc/ssh/ssh_config
        etc/ssh/sshd_config

User commands;

        usr/bin/scp
        usr/bin/sftp
        usr/bin/slogin
        usr/bin/ssh
        usr/bin/ssh-add
        usr/bin/ssh-agent
        usr/bin/ssh-keygen
        usr/bin/ssh-keyscan

More information about sshd in man;

        $ man sshd

1.1. Configure Server

Read OpenSSH server Best Security Practices, This example uses 2222 port to avoid "default" port, edit /etc/ssh/sshd_config;

        #Port 22
        Port 2222

By default ssh will listen on all local addresses, to restrict to a specific ip edit;

        #AddressFamily any
        AddressFamily inet
        #ListenAddress 0.0.0.0
        #ListenAddress 192.168.1.254
        #ListenAddress ::

        # The default requires explicit activation of protocol 1
        Protocol 2

Authentication settings;

        # Authentication:

        #LoginGraceTime 2m
        LoginGraceTime 1m
        #PermitRootLogin prohibit-password
        PermitRootLogin no
        #StrictModes yes
        #MaxAuthTries 6
        MaxAuthTries 3
        #MaxSessions 10

Restrict AllowUsers, AllowGroups that can login;

        #RSAAuthentication yes
        #PubkeyAuthentication yes

        AllowGroups admin users gitolite

Disable interactive-keyboard and password login;

        # To disable tunneled clear text passwords, change to no here!
        #PasswordAuthentication yes
        PasswordAuthentication no
        #PermitEmptyPasswords no

        # Change to no to disable s/key passwords
        #ChallengeResponseAuthentication yes
        ChallengeResponseAuthentication no

Make sure PAM is disable or above settings can be overridden. Set banner;

        # no default banner path
        #Banner none
        Banner /etc/issue

1.2. Configure iptables

Iptables;

Example of system-iptables.sh

        $IPT -A INPUT  -i ${PUB_IF} -p tcp --dport 2222 --sport 1024:65535 -m state --state ESTABLISHED -j ACCEPT
        $IPT -A INPUT  -i ${PUB_IF} -p tcp --dport 2222 --sport 1024:65535 -m state --state NEW -m limit --limit 3/min --limit-burst 3 -j ACCEPT
        $IPT -A OUTPUT -o ${PUB_IF} -p tcp --sport 2222 --dport 1024:65535 -m state --state ESTABLISHED -j ACCEPT

1.3. Configure Syslog-ng

Change SyslogFacility in accordance with syslog-ng configuration;

        # Logging
        # obsoletes QuietMode and FascistLogging
        #SyslogFacility AUTH
        SyslogFacility LOCAL1
        #LogLevel INFO
        LogLevel VERBOSE

Example rule for syslog-ng;

        destination d_sshd { file("/var/log/sshd"); };
        filter f_sshd { facility(local1); };
        log { source(s_log); filter(f_sshd); destination(d_sshd); };

Deny login for root, limit max sessions to 3 if you have limited resources and only allow 3 failed logins;

Start sshd server;

        # sh /etc/rc.d/sshd start
        # ss -f inet -l -p | grep ssh

2. Client

To create new key;

        $ ssh-keygen -t rsa

By default this creates two files;

        ~/.ssh/id_rsa       : identification (private) key
        ~/.ssh/id_rsa.pub   : public key

Default uses id_rsa and id_rsa.pub as output files in this example we will create keys for gitolite admin so we name output as gitolte;

        $ ssh-keygen -t rsa -f ~/.ssh/gitolite

Set correct permissions;

        $ chmod 700  ~/.ssh
        $ touch ~/.ssh/authorized_keys
        $ chmod 600 ~/.ssh/authorized_keys
        $ chmod 600 ~/.ssh/gitolite

2.1. Install Public Keys

Send gitolite.pub public key to server. In this example bob (administrator of gitolite) is on same host, first copy is public key to admin home directory;

        # install -o admin -g admin /home/bob/.ssh/gitolite.pub /home/admin/.ssh/gitolite.pub

If the server is on remote a remote machine;

        $ scp /home/bob/.ssh/gitolite.pub admin@core.privat-network.net:/home/admin/.ssh/
        bob@core.privat-network.net's password:
        gitolite.pub                              100%  390     0.4KB/s   00:00

In case of bob public key for normal ssh login, admin can add his public key to authorized keys;

        $ cat bob_rsa.pub >> ~/.ssh/authorized_keys

        $ ssh -P 2222 bob@remote.org

2.2. Configure Identities

When you have multiple accounts/identities you can configure ssh client so you dont need to give -i flag. Create or edit ~/.ssh/config

        Host admin
            Hostname core.privat-network.net
            IdentityFile ~/.ssh/id_rsa
            Port 2222
            User admin

        Host gitolite
            Hostname core.privat-network.net
            IdentityFile ~/.ssh/gitolite
            Port 2222
            User gitolite

        Host box
            Hostname core.privat-network.net
            IdentityFile ~/.ssh/id_rsa
            Port 2222
            User bob

        Host devbox
            Hostname core.privat-network.net
            IdentityFile ~/.ssh/id_rsa
            Port 2222
            User gitolite

Now you can just type;

        $ ssh box

On remote start tmux and detach from the session with ctrl + b d

Create alias on ~/.profile;

        alias boxtmux="ssh servername -t tmux a"

Source it and attach to remote;

        $ boxtmux

Logout just detach from session with ctrl + b d

3. Reverse connection

This information is inspired by Reverse SSH connections and implement the update from SSH all the time,

Simple way, run this command on the machine you want to access (server);

        $ ssh -f -N -R 2222:localhost:22 user@laptop

This creates a connection from server to client, client will listen on 2222 port and forward requests to the server as they are on localhost on port 22.

        wget http://github.com/mikeymckay/reverse_ssh_tunnel/raw/master/setup_reverse_tunnel.sh
        chmod +x ./setup_reverse_tunnel.sh
        sudo ./setup_reverse_tunnel.sh

Tools Index