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I'm setting up a load balancer for my network. I have three options to select as a verification method if WAN is still alive:

  1. Link Up (whether a modem is alive). This doesn't work for me because modem is alive even if a line is down.

  2. IP Address for Ping. I would like to use this but I don't know what server should I choose.

  3. DNS Lookup. This also may be an option.

Shall I select Ping or DNS? In both cases - what IP address should I use? (I cannot use a domain name - I need an IP address)

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What is the scenario ? Do you have redundant WAN links and want to stop using one when it goes down ? That is not load balancing, but failover. Also, using DNS lookups without names makes no sense :) –  adaptr Oct 18 '11 at 15:19
    
Ping a host that you also have control or regular communications with. Pinging random hosts on a regular basis is (was?) considered rude. Is the failover/balancing for an internal application? If so ping or lookup IPs from other locations which are expected to regularly communicate with the service you're managing. –  Freiheit Oct 18 '11 at 18:37
    
@adaprt It have a load balancing scenario. But the question also holds for fail-over –  agsamek Oct 20 '11 at 13:50

2 Answers 2

up vote 4 down vote accepted

I would go with Option 2: Ping 8.8.8.8 (Google Public DNS "A")
This breaks if 8.8.8.8 ever goes away or Google blocks ICMP Echo requests/replies -- Not likely at the moment. It has the benefit of also testing your routing to a host that should be reachable from anywhere.

Using DNS to test connectivity has "other issues" (ISPs that take over your DNS and send their own shit instead of NXDOMAIN like God and Mockapetris intended ; Your target domain expiring and causing your network link to appear "down" ; etc.)

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this would be good but I have 40% drops on this target. Seems to be overloaded. –  agsamek Oct 18 '11 at 15:42
2  
I respectfully submit that your network may in fact be the one that's overloaded (or otherwise hosed): 101 packets transmitted, 101 packets received, 0% packet loss & round-trip min/avg/max/stddev = 14.775/20.403/37.046/5.278 ms, and I get excellent results on a flood ping (6456 out of 6490) -- Granted we may not be talking to the same 8.8.8.8, but that should be an extremely reliable host to ping. You can try others to verify though... –  voretaq7 Oct 18 '11 at 15:47
    
You are right - today it works perfectly. –  agsamek Oct 20 '11 at 13:51

Do a traceroute to a remote destination and try to ping one of the first hops after your modem.

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