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Using SSH tunnels to graph a remote Unix server

Howto created by fmangeant at

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/
The key fingerprint is:
40:f6:91:a1:2d:d1:46:d4:76:e3:d8:c6:3f:c2:cf:f2 cactiuser@gateway

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

$ cd $HOME/.ssh
$ cp -p authorized_keys

Finally, create the SSH tunnel :

# su - cactiuser -c "ssh -f -N -g -L cactiuser@localhost"

This SSH tunnel will forward all packets sent to on TCP port 10000, to 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: sysname
SNMPv2-MIB::sysName.0 = STRING: target_server

From the Cacti server, run this command :

$ snmpwalk -v 1 -c mycommunity tcp: 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 :

Add Device

Voila ! Your target server is graphed by Cacti :)

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