How-To-SSH-Tunnels.md

How To Set Up SSH Tunnels to Graph a Remote Unix Server

Howto created by fmangeant at the Cacti Forum

This guide explains how to use SSH tunnels to graph a Unix server that is not directly reachable by your Cacti server.

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

Important — TCP vs UDP: SSH port forwarding (-L) is TCP-only. Standard SNMP uses UDP port 161 and cannot be tunneled this way. This guide works only when the target's snmpd is explicitly configured to accept TCP transport (as shown below). It is not suitable for most network devices (routers, switches, etc.), which support UDP SNMP only. For UDP SNMP across untrusted networks, use a VPN instead.

Configuring the SSH tunnel

On the Gateway, create a cactiuser account:

# useradd -d /home/cactiuser -m cactiuser

Generate an SSH key pair (no passphrase, so the tunnel can start unattended). The modern recommendation is ed25519:

# su - cactiuser
$ ssh-keygen -t ed25519
Generating public/private ed25519 key pair.
Enter file in which to save the key (/home/cactiuser/.ssh/id_ed25519):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/cactiuser/.ssh/id_ed25519.
Your public key has been saved in /home/cactiuser/.ssh/id_ed25519.pub.

Authorize the public key for login:

$ cd $HOME/.ssh
$ cp -p id_ed25519.pub authorized_keys

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 forwards all TCP traffic sent to 192.168.0.2:10000 on the Gateway to 192.168.1.2:161 on the target server.

Option summary:

-f  Go to background before executing the command
-N  Do not execute a remote command
-g  Allow remote hosts to connect to locally forwarded ports
-L  Forward the given local port to the given host and port on the remote side

Making the tunnel persistent (systemd)

On systemd-based hosts (Ubuntu 16.04+, Debian 9+, most current distros), /etc/rc.local is deprecated and disabled by default. Use a systemd service instead.

Create /etc/systemd/system/cacti-ssh-tunnel.service:

[Unit]
Description=SSH tunnel for Cacti SNMP polling
After=network.target

[Service]
User=cactiuser
ExecStart=/usr/bin/ssh -N -g -L 192.168.0.2:10000:192.168.1.2:161 cactiuser@localhost
Restart=always
RestartSec=10

[Install]
WantedBy=multi-user.target

Enable and start it:

# systemctl daemon-reload
# systemctl enable cacti-ssh-tunnel
# systemctl start cacti-ssh-tunnel

Configuring Net-SNMP on the target server

By default, the Net-SNMP agent listens on UDP port 161. For this SSH tunnel approach you must configure it to listen on TCP port 161 instead.

In snmpd.conf on the target server:

agentaddress tcp:161
rocommunity mycommunity

For a more detailed snmpd.conf reference, see the Net-SNMP snmpd.conf man page.

Testing SNMP connectivity

From the Gateway host:

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

From the Cacti server:

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

If these succeed, the host is ready to be added to Cacti. If not, review your firewall rules and verify snmpd is listening on TCP.

Adding the device to Cacti

In Cacti, create a new device as shown:

Your target server is now graphed by Cacti.

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