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I seem to randomly receive "page cannot load, cannot connect to server" errors for sites on one of my servers. when this happens, it seems to only happen on certain IPs or IP ranges at a time. I say this because while I'll get the error from my home laptop I'll be able to access the site fine from my work computer or from an offsite VPS.

DNS records should already be fully propagated as these records were updated months ago.

I have no idea how to diagnose what's going on. Is there a tool in cpanel or outside on the web that can help me figure out what's going on?

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If its a publicly accessible site, why not post the URL and the community will run some tests for you and then provide you with the tools and results for the next time? You've listed a generic problem, but have not provided enough information to troubleshoot a specific problem. –  Brent Pabst Jun 8 '12 at 13:57
    
Check your logs. What are the real error messages you get and do they have HTTP error codes ? –  Iain Jun 8 '12 at 14:00
    
two of the sites are shop.ewebshops.net and onlycattrees.com. to be clear most locations seem to be able to access these sites fine, however, randomly i get cannot find server errors, sometimes i'm at home, sometimes at the airport, starbucks, ect. Also i'm very much out of my league with even server related terms, let alone how to fix a problem, it was just suggested to me to ask this question here. I'll try to provide as much information as possible. How do i check my logs? –  Derek Jun 8 '12 at 14:14
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2 Answers

You can't possibly hope to diagnose a problem if you don't know what the problem actually is. Data data data! If something on a Linux system does not work, the absolute first thing you should always do is look at the logs. If there's nothing in them, turn on debugging and watch the logs.

You don't say what web server you are using, but you do say it's a Linux system, so the chances are good it's Apache. You also don't mention what Linux distro it is, but the chances are good that you'll find the apache config in /etc/httpd or /usr/local/etc/apache2 or something like that. The config file will tell you where the log files are, most probably they 'll be in /var/log/httpd or /var/log/apache or something like that.

Once you have the logs, you'll actually know what is happening, and the chances are, you'll have enough data to figure out what the problem is. Or, at the very least, you'll have something concrete to put into Google to help you find the answer, or, to put in a question here to let others help you find the answer. Without a specific error message you've got nothing to go on.

Until you have data, you are shooting in the dark. I could make guesses at what MIGHT be going on, but really, I'd be no more accurate than a digital astrologer!

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Mostly, you need to figure out if you're struggling to get traffic to the server (i.e. a network connection issue), or if the service is not responding (i.e. some software issue, web server down, etc.)

Tools such as,

ping
traceroute (Unix-like)
tracert (Windows)

can help you work out if the network connection is okay, but they can be blocked by firewalls even if those firewalls let web traffic through, so you need to know how they look when it's working and when it's not working.

Tools such as,

nslookup
dig

allow you to do DNS lookups locally to make sure that's all working.

As well as running those tools locally, some websites allow you run them so you can rule out your own network connection to the server, for example (chosen at random, no affiliation with these sites),

There are also websites which offer free 'uptime' monitoring, where they will check your site every few minutes or hours and allow you to collect data.

If name lookups work and ping/traceroute suggest the network is okay, you need to dig down into the web service. That usually entails looking at the web server logs to see if anything untoward is happening on the specific server in question. Is it stop/starting, is the server running out of memory, etc. That's a huge area, and not suitable to a single answer.

To do a full analysis, as the comments suggest, you would need to post logs from the web server before, during and after the issue, along with the exact error messages you were seeing at the client end. But, you asked about tools, and hopefully I've given you some pointers for those.

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thank you. yes, i asked about tools but after quickly looking at a couple of them and reading your response, I'm not sure that I'll be able to properly interpret the findings. So I'll likely need help fully diagnosing the problem as well. I'll spend some time looking into these tools and gathering results and will report back. also, where exactly can i find server logs? I'm using cpanel. –  Derek Jun 8 '12 at 14:19
    
Can't help with cpanel, don't use it, suggest you write a new question asking that as part of it. –  EightBitTony Jun 8 '12 at 14:27
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