Cacti – Using SSH tunnels to graph a remote Unix server

# Using SSH tunnels to graph a remote Unix server

Howto created by `fmangeant` at Cacti Forum

This HowTo will explain how to use SSH tunnels to graph a Unix server that isn't directly reachable by your Cacti server :

In this example, the Cacti server can reach the Gateway, which can reach the target server.

## Configuring the SSH tunnel

On Gateway, create a "cactiuser" user :

    # useradd -d /home/cactiuser -m cactiuser

Then you have to generate SSH keys (without passphrase) :

    # su - cactiuser
    $ ssh-keygen -t rsa
    Generating public/private rsa key pair.
    Enter file in which to save the key (/home/cactiuser/.ssh/id_rsa):
    Enter passphrase (empty for no passphrase):
    Enter same passphrase again:
    Your identification has been saved in /home/cactiuser/.ssh/id_rsa.
    Your public key has been saved in /home/cactiuser/.ssh/id_rsa.pub.
    The key fingerprint is:
    40:f6:91:a1:2d:d1:46:d4:76:e3:d8:c6:3f:c2:cf:f2 cactiuser@gateway
    ```console

    The public key of the "cactiuser" user has then to be authorized as an input key :

    ```console
    $ cd $HOME/.ssh
    $ cp -p id_rsa.pub authorized_keys

Finally, create the SSH tunnel :

    # su - cactiuser -c "ssh -f -N -g -L 192.168.0.2:10000:192.168.1.2:161 cactiuser@localhost"

This SSH tunnel will forward all packets sent to 192.168.0.2 on TCP port 10000, to 192.168.1.2 on port TCP 161.

The options that are used are the following :

    -f : requests SSH to go to background just before command execution
    -N : do not execute a remote command
    -g : allows remote hosts to connect to local forwarded ports
    -L : specifies that the given port on the local (client) host is to be forwarded to the given host and port on the remote side.

You can add this command to `/etc/rc.local`, so it will be executed at boot time.

## Configuring Net-SNMP

By default, the Net-SNMP agent listens on UDP port 161; we'll modify its configuration to listen on TCP port 161.

To do so, use that in `snmpd.conf` on the target server :

    agentaddress tcp:161
    rocommunity mycommunity

For a more detailed configuration of snmpd.conf, follow the Net-SNMP guide in the manual

### Testing the SNMP connectivity

From the Gateway host, run this command :

    $ snmpwalk -v 1 -c mycommunity tcp:192.168.1.2 sysname
    SNMPv2-MIB::sysName.0 = STRING: target_server

From the Cacti server, run this command :

    $ snmpwalk -v 1 -c mycommunity tcp:192.168.0.2:10000 sysname
    SNMPv2-MIB::sysName.0 = STRING: target_server

If everything works fine, your host is then ready to be added to Cacti.

If not, review your network configuration (firewall rules, especially).

## Adding the device to Cacti

In Cacti, create a new device like that :

Voila ! Your target server is graphed by Cacti :)

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