Collector fails to connect to Sumo: The target server failed to respond or HTTP 504 errors

Problem

I'm not receiving any data from my new Collectors located in AWS EC2. Why do I see the following error in my collector.log files?

2015-01-12 17:29:48,216 [HTTP Sender - 15] WARN com.sumologic.scala.collector.CommonsHTTPTransmitter - ConnectException for receiver url: 'https://collectors.sumologic.com:443'; on attempt: '2' 
org.apache.http.NoHttpResponseException: The target server failed to respond

Resolution

This warning indicates that a brief lapse in network connectivity between your Collector and Sumo Logic has occurred. Normally it can be safely ignored because the Collector will automatically retry once per minute until a successful connection is established. However if this error persists a couple checks should be performed to validate the host systems connectivity to the Sumo Logic service.

Note: The following steps will need to be performed against the collection endpoint for your account deployment. For more information on determining the endpoint needed for your account please see the following documentation.

First verify DNS resolution and connectivity to the Sumo service from your host system:
```
curl -i https://collectors.sumologic.com/
```
// you should see the word "Tweep" returned
Next check whether there is a significant delay in performing the DNS lookup:
```
time nslookup collectors.sumologic.com
```

If connectivity to the service can be verified next check your local firewalls to ensure they allow inbound ICMP traffic from the internet. One of the more common causes of a 504 message when registering a Collector is due to an intermediate hop between your network and Sumo Logic not supporting Jumbo Frames. If a firewall within the environment does not allow incoming ICMP Type 3 messages Path MTU Discovery may fail and the client (Collector) may timeout the request to the server resulting in a 504 message.

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