SOS: Sporadic Internet access

[HD101]

We are having problems with accessing the internet. Sometimes we can reach any website we want without any problems, and a minute later we can't get to any site. Can access some sites but not others. Then when trying to access a previous site that we could reach, we can't. Problem may last five minutes or five hours. Happens at different times of the day. I'm on now but I don't know when I'll be back on.

What we get is the standard website can't be displayed message. We've checked our own DNS and nothing has changed on it. ISP says they aren't having any internet problems.

Any ideas on what to check?

Windows NT 4.0, with NT 4.0, 2000 and XP clients. IE 6.0 with latest updates.

 

[dsimpaired]

What kind of router / firewall do you have? Also, what type of connection to ISP? T1/dsl/cable? Are your DNS servers nt4? DHCP?

 

[HD101]

Thanks dsimparied for the quick reply, hope this info will help.

Cisco PIX firewall - no changes made to it.
Fiber optic connection to ISP.
DNS are NT 4.0
DHCP is 2000

Should my primary DNS have a forward to the ISP DNS?
We have recently added some new zones for some new subnets we created for our network expansion, but they have been working for several weeks without this problem.

 

[GlenJohnson]

When you can't get on, can you ping anybody? How about doing an ipconfig /all on a client that can't get on. I'd be curios to see the ip address of a client that can't get on. Are you using fixed ip addresses or dhcp? Good luck.

Glen A. Johnson
If you're from Northern Illinois/Southern Wisconsin feel free to join the Tek-Tips in Chicago, Illinois Forum.
TTinChicago
Johnson Computers

 

[HD101]

Thanks Glen for the reply,

I can ping anyone on our network, I don't remember trying to ping anyone outside the network. I guess that should have been a given huh? The next time it happens I will get the ipconfig information and post it.

Most computers are on DHCP, we have several subnets so the ip address depends on what subnet they are on. We're using Cisco routers with helpers to find the DHCP to get their correct addresses.

 

[HD101]

Our network is divided into subnets. 192.168.1.x, 192.168.2.x, 192.168.3.x, etc.

In DNS do each of these subnets need to be a seperate zone?
This is a sample of current settings.

1.168.192.in-addr.arpa
1.168.192.in-addr.arpa NS dc1.company.org
1.168.192.in-addr.arpa SOA dc1.company.org
192.168.1.6 PTR prt01.company.org

Basically the same for each subnet.

Under properties, should we use forwarders to forward to our ISP DNS?

 

[GlenJohnson]

I usually just point the dns servers to the isp, and point all clients toward local servers. This way if the local servers don't have an address in cache, they contact the isp. Once it's got the address in cache, then the clients will get there faster the next time. Good luck and keep us informed. (Server for each subnet?)

Glen A. Johnson
If you're from Northern Illinois/Southern Wisconsin feel free to join the Tek-Tips in Chicago, Illinois Forum.
TTinChicago
Johnson Computers

 

[MzKitty]

Can you ping your internet provider? You might have a bad switch. We run wireless and had our connection slow way down; it turned out that we had a bad switch. Contact your internet provider and work with them.
Cathy

 

[HD101]

First, thanks to everyone for their input. We finally found out from the ISP that the problem is a DoS attack and that they are working on it. Why they didn't tell us that from the beginning I don't know. Apparently some our experiencing the problem more than others.

Again thanks for your help.