Router / DSL, one of the PCs can't access Internet :(

[DigitAlex]

Hello all

I have a very nasty problem with my home LAN and I thought I solved it many times, but it always comes back, so now I'm out of ideas ...

The setup of my home LAN is very simple :

DSL Internet > DSL Modem / Router (ASUS AAM6000EV) > Ethernet hub > 2 PCs

Both PCs are set to static IPs (because they both run servers) even if the modem/router has a DHCP server.

The other thing to know is that normally my ISP does not allow more than one PC / contract, so I really have no clue about that, but I don't think they can find a way to block a second PC accessing the Internet from behind a router with NAT.

So, the problem is the main PC has full Internet access, but the second one does not. Well it's actually partial. I can ping any web server and I get a normal response without any packet loss, but when I try to reach the same website with my browser, it never works. The common IMs don't work either.

I already tried to disconnect my PCs from the router and connect only the second one. It sometimes makes it work, sometimes not. Sometimes it works for a while and then stops working again. Another thing to consider is that I had another machine before at the same place with the same settings, and the problem was about the same ...

As ping works and browsing does not, either it's my ISP who found a way to block that (doubtfully), either I have some kind of problem with the network layers....

As I said, I'm really out of ideas Please help, mighty Network Admins

[NuxPowa]

Hello all

I have a very nasty problem with my home LAN and I thought I solved it many times, but it always comes back, so now I'm out of ideas ...

The setup of my home LAN is very simple :

DSL Internet > DSL Modem / Router (ASUS AAM6000EV) > Ethernet hub > 2 PCs

Both PCs are set to static IPs (because they both run servers) even if the modem/router has a DHCP server.

The other thing to know is that normally my ISP does not allow more than one PC / contract, so I really have no clue about that, but I don't think they can find a way to block a second PC accessing the Internet from behind a router with NAT.

So, the problem is the main PC has full Internet access, but the second one does not. Well it's actually partial. I can ping any web server and I get a normal response without any packet loss, but when I try to reach the same website with my browser, it never works. The common IMs don't work either.

I already tried to disconnect my PCs from the router and connect only the second one. It sometimes makes it work, sometimes not. Sometimes it works for a while and then stops working again. Another thing to consider is that I had another machine before at the same place with the same settings, and the problem was about the same ...

As ping works and browsing does not, either it's my ISP who found a way to block that (doubtfully), either I have some kind of problem with the network layers....

As I said, I'm really out of ideas Please help, mighty Network Admins

Hi Alex
I'm actually thinking it might be a problem of dns, check in your networking options that you do have the right dns's installed (the dns of your internet provider)

Don't worry about your provider, btw, wich one is it?
Hope this might help

kind regards

[Invité]

Well, i really don't think it's the DNS settings, they are exactly the same. Plus I said that when I ping www.something.com I get a normal response without any packet losses. It means that the name is resolved correctly. Thank you anyway

[NuxPowa]

Both PCs are set to static IPs (because they both run servers) even if the modem/router has a DHCP server

Hi again,
Just being curious : is the dhcp server desactivated?

One other thought : what happens if you plug your second putr using the wire of your first putr on the same port of your hub? (i'm thinking about a wire or hardware problem)

Looking forward to hearing from you

[DigitAlex]

The dhcp server is on, but anyway both comps have static IPs & DNS settings set, so as far as I know, it's not a problem of having the DHCP up.
Already tried changing ports on the hub, it sometimes works and sometimes not. I mean, it already forked for a while, then stopped again. But only for the second PC. The first never has any problems of that kind. I can hardly imagine the port on the hub would let pings (ICMP) pass and block HTTP requests...

Thanks for your ideas I have that problem for a while now, so I already tried lots of stuff.

[NuxPowa]

The dhcp server is on, but anyway both comps have static IPs & DNS settings set, so as far as I know, it's not a problem of having the DHCP up.

I'm just being curious to see what's gonna happen if you turn dhcp off. On one side, a dhcp server trying to assign ip's to putrs, on the other side, putrs trying to keep their assigned ip's
There might be a kind of conflict, don't you think?
@#

[DigitAlex]

dhcp disabled
no firewall rules, i tried diabling NAT, still the same

as i wrote, 2 dif machines had the same problem

another thing weird is that my second PC sometimes dissapears from the router ARP table, the first never does

and restoring it to the arp table manually is not giving it access to the internet

[NuxPowa]

Sorry Mate,
I'm affraid i can't really help, all i found is this :
There are also some simple diagnostic tools (ping) on the router for help with finding problems on your local LAN.
This link might help you.
Looks like your english is better than mine

[DigitAlex]

Un grand merci Je vois pas pourquoi tu rigole à propos de mon anglais

Je te tiens au courant si j'ai trouvé quelque chose ....

[NuxPowa]

c'est koi ce biiinzz?

[BlCa`]

Looks like the MTU size is set too high on the second PC. U should try lowering it to something less than 1492. It can be done by editing the windows registry. A quick search on google will tell you the exact key to modify. Here's a link where u may find the answer http://www.speedguide.net/articles.php?category=93

regards

[DigitAlex]

Okay, I changed the network card and it works fine now. Either it will continue working and then it means it was just the NIC that was fucked up (weird, because pings were passing through, and 100MBPS LAN was working perfectly), either it will stop working after a while, either it means that ne new card (which is an old 10 MBPS one does not support MTU units over 1492 by default, which solves the problem.

[NuxPowa]

Okay, I changed the network card and it works fine now. Either it will continue working and then it means it was just the NIC that was fucked up (weird, because pings were passing through, and 100MBPS LAN was working perfectly), either it will stop working after a while, either it means that ne new card (which is an old 10 MBPS one does not support MTU units over 1492 by default, which solves the problem.

That's weird
Aniway, good luck

[BlCa`]

My first guess is the NIC swap reset the MTU to its default value. I'd bet the settings for the 100Mb one were modified to 1500 (which isn't suitable if ur router is configured for PPPoE)...

[DigitAlex]

It is configured for PPPoE.
It's messy to open the pc and change NICs back, but I really wanna find out what was going on, so I'll try to look and I will tell you. If they just screw up the whole thing because of 1500 instead of 1492 ....

[DigitAlex]

http://techrepublic.com.com/5100-6268-1061241-1.html

Networks that use Internet Connection Sharing (ICS) or Point-to-Point Protocol over Ethernet (PPPoE) for their broadband connections must have an MTU size of 1,492 or lower to run efficiently.

...

by default, Windows 2000 and Windows XP set the MTU size to 1,500

You were right ! After checking the theory, I'll check the reality ... here goes

[BlCa`]

Then there are two working solutions.

1) IF your router supports PPPoA, reconfigure the connection using PPPoA. This is the cleanest solution as u won't have to modify the clients and PPPoA has less overhead than PPPoE (so it's faster, theoretically, 'cause u won't feel te difference).

2) U leave de router as is. Then find out the suitable MTU for ur network and modify all clients accordingly.

[NuxPowa]

The good news is, following this link, that this router supports PPPoA

[DigitAlex]

Yes !

Ok, I've put the first card again in the computer and tested the max MTU size with

Code : Tout sélectionner

ping -f -l 1492 www.somehost.com

-f > don't fragment blocks
-l > block size
1492 > example block size.

If you set the block size to more than allowed by the network, the ping will time out or tell you that the block has to be fragmented.

By default, Win2000 have NO MTU SET !!!

So I set the MTU to 1490 on the main PC (nForce2 board NIC chip) and 1450 for the second PC (3Com 10/100 NIC). It worked fine for 24 hours now, I will see if it continues working, because the problem was appearing before only after a while.
Looks like the MTU was the problem at the first glance.

Thanks a lot to you guys !

[BlCa`]

Glad to see ur sorted out now

This is a program that will determine the best MTU for ur particular network automaticaly.
The maxMTU part of it seems to work fine, alhtough I haven't tested the rest thoroughly...
I just think it should prove useful