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Like most technical folks, I suppose, I'm my family's primary source of tech support. I'm a developer--not a sysadmin--by trade and tonight I bumped into something I've never seen before. I'm hoping someone here has.

In order to better help my Mom, I have her set up on a home network behind a Linksys router (WRT54G). She's got a Mac, so I have her router set up to forward SSH requests to her laptop's internal IP. I also have her router running DDNS through DynDns. Tonight she called to tell me that she can't access the Internet.

Assuming it was one of the many simple, stupid problems most of us encounter with parents, I logged into the router admin remotely and took a look around. Everything looked normal. Then I SSH'd into her machine to check out her IP, DNS, etc. settings. Everything still looked fine. Then I noticed something weird. When SSH'd into her machine, I can't ping her router.

In other words, I seem to be able to access her computer through her router, but not access her router from her computer. A traceroute dies immediately as well. Any ideas what I might try next? I've bounced her computer and even unplugged her router (it was plugged back in, of course).

Thanks.

UPDATE:

Oddly, this could be a problem at the ISP level. I walked her through bypassing the router all together (plugging her computer directly into her cable modem) and she still can't get out (where "get out" means "access a web page using Safari"). What confuses me is that I'm able to get in. That seems very odd to me.

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migrated from serverfault.com Apr 6 '10 at 23:08

This question came from our site for professional system and network administrators.

    
What IP are you using to access the router from her machine? The public IP will not work from inside the LAN. You need to use the internal IP. –  Chris Nava Apr 6 '10 at 22:21
    
Are you staying at the Hotel California, by any chance? –  Mark Henderson Apr 6 '10 at 23:45
    
Yep, that I'm doing. –  Rob Wilkerson Apr 6 '10 at 23:46
    
@Farseeker - I'm beginning to wonder... –  Rob Wilkerson Apr 6 '10 at 23:46
    
Would make sense, because you can check out, but never leave –  Mark Henderson Apr 7 '10 at 0:21

3 Answers 3

Clearly internal traffic is traversing the network fine and the NAT router part is working OK as you are getting in, and the return traffic is getting back. Your error is mistaking ping and tracert for tools that tell you whether something is up\available - they only tell you information about how the target responds to ICMP traffic.

Ping\tracert use ICMP which may well be just being dropped by her router so if they don't respond that doesn't indicate that anything is broken. It all depends on how the router has been configured. My (home) router drops external ICMP traffic aimed at its external interface so I can (normally) ping the inner (private) address but not the outer (public) address. I can also disable ICMP totally on it and get no response on either side but its still working properly in all cases, I can manage it through it's normal web admin interface and it routes properly.

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Okay, so that may explain why what I'm doing isn't working (although it's always worked for me in the past as a quick test), but what should I be doing? –  Rob Wilkerson Apr 6 '10 at 23:47

Perform a route print in a command prompt to see if there is any funky routing:

route print

Post the output.

Edit: Sorry, Mac. Open a terminal and type "route".

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It looks that a firewall on your mom computer our router is filtering the traffic. You should run on your mom computer:

arp -d <IP_router>
ping -c1 <IP_router>
arp -n <IP_router>

If you see the arp entry with the IP and the MAC address of the router, then you have a issue with the firewall on her computer or on her router.

If traceroute "dies" immediately this means that it has received an ICMP error message (as oposed to timeout). This confirms the firewall issue.

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