Linux – How to ping via an alternate gateway

gatewaylinuxpingrouting

Our network has two modems, each one hidden behind a router. The primary router is on 10.1.1.1, the backup router is on 10.1.1.2, and are both configured to the 10.1.1.0/24 subnet. Both routers have their gateway configured to 192.168.0.1. However, the 192.168.0.1 that you see if your gateway is 10.1.1.1 is a different machine from the 192.168.0.1 that you see if your gateway is 10.1.1.2.

My Nagios server is set up accordingly:

auto eth0
iface eth0 inet static
address 10.1.1.10
netmask 255.255.255.0
gateway 10.1.1.1

So to be clear: there are four gateways here. Router1 (10.1.1.1) Router2 (10.1.1.2) Modem1 (192.168.0.1) Modem2 (192.168.0.1)

Here's an illustration of what I can do manually to check the status of the two modems:

ping -c4 192.168.0.1 # Is Modem1 up? 
ip route add via 10.1.1.2
ip route change default via 10.1.1.2
ping -c4 192.168.0.1 # Is Modem2 up?
wget 192.168.0.1 # yields expected control webpage for Modem2
ip route change default via 10.1.1.1
ip route del default via 10.1.1.2
wget 192.168.0.1 # Connection refused; Modem1 has no web interface
ssh adminuser@192.168.0.1 # I can log in to Modem1 and check status

I would like at a minimum to be able to ping Modem2.

*Edited to correct my abominable misuse of CIDR notation, and provide clear detail.

Best Answer

Neither ping -N nor arping worked for me, I finally settled with the solution from this answer:

You can use the tool nping from the nmap package:

# nping --icmp --dest-mac [gateway mac] [target ip]

You can find your router's mac in your local ARP cache:

$ arp -v [gateway ip]

The tool also supports different ping types, like --tcp or --udp.

Related Solutions

Routing network traffic between 2 subnets using a Raspberry Pi

The full context of the setup is that we have 2 apartments with their own Internet connection and LAN which we have joined together using a RPi via jumper cabling in the garage. The whole point of this exercise was so that I can administer the upstairs apartment's NAS without having the network traffic leave the building (which would also make sharing data a LOT faster), and also retain each apt's Internet/network setup so that they may continue working in case the RPi goes down. Since the commodity routers in each apartment are so lacking in options and features, I was willing to compromise by manually adding routes to the hosts that I would do the administration from on my subnet.

It's true that I need to get a better understanding of networking, and I am continually in the process of doing so as an IT professional. I spoke with a colleague today about my issue and it turns out I didn't need to add any routes, I just needed to change the firewall rules to use SNAT instead of MASQUERADE and the subnets started talking to each other (I did also need to allow IPv4 forwarding as Paul suggested, thanks!):

iptables -t nat -A POSTROUTING -s 192.168.1.0/24 -d 192.168.0.0/24 -o eth0 -j SNAT --to-source 192.168.0.4
iptables -t nat -A POSTROUTING -s 192.168.0.0/24 -d 192.168.1.0/24 -o eth1 -j SNAT --to-source 192.168.1.4

Full iptables list:

# iptables -t nat -n -L -v
Chain PREROUTING (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination

Chain POSTROUTING (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
    0     0 SNAT       all  --  *      eth0    192.168.1.0/24       192.168.0.0/24       to:192.168.0.4
    0     0 SNAT       all  --  *      eth1    192.168.0.0/24       192.168.1.0/24       to:192.168.1.4

As you can see, now the traffic is staying inside the building when testing from host 192.168.0.3:

>tracert 192.168.1.1

Tracing route to 192.168.1.1 over a maximum of 30 hops

  1     1 ms     1 ms    <1 ms  192.168.0.4
  2     1 ms     1 ms     1 ms  192.168.1.1

Trace complete.

Many thanks for your comments.

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