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I run a website, Today, while visiting my parent's house, I noticed I couldn't access the site while on their internet connection. I got a message saying "Firefox can't establish a connection to the server at". I checked with several other people using different connections and they could all access the site.

Puzzled, I played around with my parent's internet wiring. I found they had it set up so that the internet was piped from cable modem -> vonage phone box -> wireless router. I swapped the position of the vonage box and the router, and that solved the problem.

If their internet configuration had rendered a number of sites useless, then I would shrug and move on, never worrying about this again. But I find it disconcerting that SeatGeek was the only site that had problems--all other sites worked fine.

Can anyone think of any issues with our site's configuration that might cause this problem? I'm not sure whether the solution (swapping the position of the Vonage box and wirless router) provides any clues...

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Assuming you brought your computer with you...with the wiring back to original configuration, can your parent's computer connect to – Scott Lundberg Nov 25 '09 at 20:17

The most common reason for this sort of problem is incorrect MTU setting and/or blocked Path-MTU-Discovery (PMTUD).

The latter in particular requires that no device in the path is blocking the ICMP "fragmentation required" message.

Unfortunately many naive firewall administrators believe that "all ICMP is bad" and block this message.

The result is a server that sends a 1500 byte packet can't get it through the network without fragmenting it, but the message that tells the server that the fragmentation is needed never gets through.

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while what you're saying is correct, i'm don't believe it could be the problem based on the evidence. if it were the mtu on the vonage box the problem would most likely affect all internet traffic, not just traffic to the op's web site. if the problem was the mtu on the web server side, it would most likely affect all visitors to the site, not just the op's parents. i can't see an intermediate hop causing the problem only for the op's parents while leaving all other site visitors unaffected. – joeqwerty Nov 25 '09 at 19:39
@joeqwerty If the MTU problem is on's side it wouldn't affect all users, just ones that were unaware of a lower PMTU between them and This is the sort of thing that would happen if the Vonage box encapsulated the traffic between the wireless router and cable modem (extra encapsulation headers mean a lower MTU). I can't ping, so it looks like they're blocking echo requests and replies at the very least. – Gerald Combs Nov 25 '09 at 20:45
@Alnitak & @Gerald: i know that what you're saying is technically correct, it just seems strange that the problem is apparently only affecting the op's parents and apparently only for his web site. that's why i suggested it might be a dns or routing issue in the vonage box. i get the idea that a lower pmtu might be exacerbated by the vonage doing some type of encapsulation and\or not reporting the need to fragment to the client. – joeqwerty Nov 25 '09 at 22:05

You may have broken their vonage phone service. all the info i've read regarding vonage is that it needs to sit outside of the firewall. also, it may be that other sites are affected as well, unless you tested every web site that exists, in which case you've reached the end of the internet so it's time to turn around and go back. ;)

could it be that the vonage box is performing some dns resolution or routing for the internal computers and that's causing the problem? have you searched the vonage web site for information or an explanation of the issue?

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