Tools Index


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

User commands;


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 ::
        # The default requires explicit activation of protocol 1
        Protocol 2
        # Ciphers and keying
        #RekeyLimit default none
        Ciphers aes256-ctr,aes192-ctr,aes128-ctr

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


Example of

        $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/   : public key

Default uses id_rsa and 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
        $ chmod 600 ~/.ssh/id_rsa

Add identity to ssh agent;

        $ ssh-add ~/.ssh/id_rsa

2.1. Install Public Keys

Example how to give ssh access to bob user to admin account using public key authentication. Is necessary to make user public key available in the server, this can be done by several ways;

        ssh-copy-id -i ~/.ssh/

Or doing manually using scp;

        $ scp /home/bob/.ssh/'s password:                              100%  390     0.4KB/s   00:00

Login on remote as admin and add bob public key to authorized keys;

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

Now bob can login as admin on remote server using publik key athentication;

        $ ssh -P 2222

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 core
            IdentityFile ~/.ssh/id_rsa
            Port 2222
            User admin

        Host git
            IdentityFile ~/.ssh/id_rsa
            Port 2222
            User gitolite

        Host git-admin
            IdentityFile ~/.ssh/gitolite
            Port 2222
            User gitolite

        Host *
              IdentitiesOnly yes

Now you can just type ssh core to connect on port 2222 with ~/.ssh/id_rsa as identity, or to connect to git server as gitolite admin;

        $ ssh git-admin

To take advantage of tmux first login on remote and start tmux, detach from the session with ctrl + b d. Change ~/.bashrc and add follow alias;

        alias core-server="ssh core -t tmux a"

Source it and attach to remote;

        $ source ~/.profile
        $ core-server

To logout just detach from tmux 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.

        chmod +x ./
        sudo ./
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