Internal IP addresses fail to resolve

June 12, 2007 6:24 AM

Having trouble with resolving internal ip addresses though the external ip address resolves with no problem

I run two 2k3 servers, with one as a primary DNS server and one as a backup. Yesterday, I started having issues with client computers accessing the servers.

Lets say the servers' internal IP's are 172.27.1.100 and 172.27.1.101 with external addresses of 10.0.0.100 and 10.0.0.101, respectively. Intermittently, I can no longer access or ping 172.27.1.100 or 172.27.1.100 (both don't always stop resolving at the same time, and it occurrence varies from computer to computer) but I can still ping the external IP's of 10.0.0.100 and 10.0.0.101 and access the web server hosted on the server.

I also connect through 3 3com switches, though the ports/switch makes no difference for connectivity.
At the same time, the servers never have issues communicating with each other.

I'm assuming that the internal DNS got screwed up yesterday, but I cannot figure out where the problem is.
On the client computers, the current setting's to resolve DNS automatically, and I've manually changed the DNS to resolve at the DNS servers with no success.

I'm not exactly a DNS master, so any help is appreciated.

posted by jmd82 to computers & internet (4 comments total)

Are the servers multihomed, or is the external IP a NAT?

Are you trying to access by IP or by name?

Is it possible some other machine(s) on the network were inadvertently assigned the same IPs as those used by these servers?
posted by jma at 8:24 AM on June 12, 2007

Yes, External IP is a NAT

Trying to access via internal internal IP or name doesn't work (name translates to the internal IP, thus recreating the wheel).

I checked and don't see any inadvertent multi-IP assignments.
posted by jmd82 at 10:07 AM on June 12, 2007

If you cannot access via the IP address, then DNS is not the issue (assuming you are using ping or something else in the TCP/IP stack).

Are these servers on a Windows Domain? Are they Domain Controllers? Do you have multiple NIC cards in the Win2k3 machines? Where is the NAT being handled?

Are the 3Com switches layer 2 or Layer 3 switches? Do you have any VLAN's setup on the switches?

After trying to ping the server from a machine, try running "arp -a" from the command prompt. Is the MAC address registered? Is it different than what you see on a machine that can properly access the server?
posted by stovenator at 12:22 PM on June 12, 2007

It appears that a Netgear router has decided to take DCHP upon itself after running on its own for a few years. It delegated out the same IP addresses as my servers and once I removed the router, all appears well in the world.
posted by jmd82 at 7:30 AM on June 13, 2007