Connection Errors
IP not in DNS
If you are receiving an error that states that your IP address is not
in DNS, you need to talk to the network admin at your work or home ISP.
Ask your network admin to add your host name and IP address to the
DNS forward and reverse maps.
Systems staff cannot fix this for you.
DNS stands for Domain Name Service. DNS allows computers to map domain
names (like exa.cs.umn.edu) to an IP address (like 128.101.35.175). This
is called resolving the host name in the "forward DNS map."
Similarly, DNS allows you to convert IP addresses (like 128.101.35.175)
to domain names (like exa.cs.umn.edu). This is called resolving the ip
address in the "reverse DNS maps."
To determine the mapping of an IP address, you can use the nslookup
command. For example, to determine which ip name is associated with the
ip address 128.101.35.175 do the following:
(~)--> nslookup 128.101.35.175
Server: 128.101.32.201
Address: 128.101.32.201#53
175.35.101.128.in-addr.arpa name = exa.cs.umn.edu.
This states that exa.cs.umn.edu is associated with the IP address 128.101.35.175.
The following example shows the result of running nslookup on an IP
address that is not in the reverse DNS maps.
(~)--> nslookup <your_ip>
Server: localhost
Address: 127.0.0.1
*** localhost can't find <your_ip>: Non-existent host/domain
When you try to access a host in the
CS or IT Labs domain, your computer must have a registered IP DNS name
and address. We need to do various kinds of checks based on your
domain name, and if we can’t get your domain name, the
computer can’t easily discern who manages your Internet
connectivity. We block Internet connectivity from providers who
don’t cooperate or have historically tried to attack our
network. Hence, we disallow access if we can't figure out your
domain name.
It is the ISP’s job to fix this by adding your IP name and IP
address to their DNS servers. Every well-maintained IP on the
Internet has a DNS entry mapping it to some IP name. If yours
doesn’t, that means that your ISP will need to fix it.
If your host name and IP address are registered in DNS and you
still cannot connect, send an e-mail to
with the name of your IP host and we can see if that domain is
being blocked.
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