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Diagnosing DNS Problems
Digging until you find the root
Time: 08/21/08 2:41:48 am
Tags: DNS [6], Networking [6], Operating Systems [9], Diagnosing [13], Web [15], Servers [31], Examples [74]
Most relevent 4 of 108 posts with shared tags:

Yesterday I wrote a bit about the DNS system being rather fussy, so I thought today I’d go a bit more into how DNS works, and some good tools for problem solving in this area.


First, some technical background on the subject is required.
  • A network is simply a group of computers hooked together to communicate with each other. In the old days, all networking was done through physical wires (called the medium), but nowadays much of it is done through wireless connections. Wired networking is still required for the fastest communications, and is especially important for major backbones (the super highly utilized lines that connect networks together across the world).
  • A LAN is a local network of all computers connected together in one physical location, whether it be a single room, a building, or a city. Technically, a LAN doesn’t have to be localized in one area, but it is preferred, and we will just assume it is so for arguments sake :-).
  • A WAN is a Wide (Area) Network that connects multiple LANs together. This is what the Internet is.
  • The way one computer finds another computer on a network is through its IP Address [hereby referred to as IPs in this post only]. There are other protocols, but this (TCP/IP) is by far the most widely utilized and is the true backbone of the Internet. IPs are like a house’s address (123 Fake Street, Theoretical City, Made Up Country). To explain it in a very simplified manner (this isn’t even remotely accurate, as networking is a complicated topic, but this is a good generalization), IPs have 4 sections of numbers ranging from 0-255 (1 byte). For example, 67.45.32.28 is a (class 4) IP. Each number in that address is a broader location, so the “28” is like a street address, “32” is the street, “45” is the city, and “67” is the country. When you send a packet from your computer, it goes to your local (street) router which then passes it to the city router and so on until it reaches its destination. If you are in the same city as the final destination of the packet, then it wouldn’t have to go to the country level.
  • The final important part of networking (for this post) is the domain system (DNS) itself. A domain is a label for an IP Address, like calling “1600 Pennsylvania Avenue” as “The White House”. As an example, “www.castledragmire.com” just maps to my web server at “209.85.115.128” (this is the current IP, it will change if the site is ever moved to a new server).

Next is a brief lesson on how DNS itself works:
  • The root DNS servers (a.root-servers.net through m.root-servers.net) point to the servers that hold top-level-domain information (.com, .org., .net, .jp, etc)
    Examples of these servers are as follows:
    auns1.audns.net.au
    bizE.GTLD.biz
    caCA04.CIRA.ca
    cnA.DNS.cn
    com&netA.GTLD-SERVERS.NET
    deZ.NIC.de
    euU.NIC.eu
    infoB9.INFO.AFILIAS-NST.ORG
    orgTLD1.ULTRADNS.NET
    tvC5.NSTLD.COM
  • Next, these root name servers (like A.GTLD-SERVERS.NET through M.GTLD-SERVERS.NET for .com) hold two main pieces of information for ALL domains under their top-level-domain jurisdiction:
    • The registrar where the domain was registered
    • The name server(s) that are responsible for the domain
    Only registrars can talk to these root servers, so you have to go through the registrar to change the name server information.
  • The final lowest rung in the DNS hierarchy is name servers. Name servers hold all the actual addressing information for a domain and can be run by anyone. The 2 most important (or maybe relevant is a better word...) types of DNS records are:
    • A: There should be many of these, each pointing a domain or subdomain (castledragmire.com, www.castledragmire.com, info.castledragmire.com, ...) to a specific IP address (version 4)
    • SOA: Start of Authority - There is only one of these records per domain, and it specifies authoritative information including the primary name server, the domain administrator’s email, the domain serial number, and several timeout values relating to refreshing domain information.

Now that we have all the basics down, on to the actual reason for this post. It’s really a nuisance trying to explain to people why their domain isn’t working, or is pointing to the wrong place. So here’s why it happens!

Back in the old days, it often took days for DNS propagation to happen after you made changes at your registrar or elsewhere, but fortunately, this problem is of the past. The reason for this is that ISPs and/or routers cached domain lookups and only refreshed them according to the metrics in the SOA record mentioned above, as they were supposed to. This was done for network speed reasons, as I believe older OSs might not have cached domains (wild speculation), and ISPs didn’t want to look up the address for a domain every time it was requested. Now, though, I rarely see caching on any level except at the local computer; not only on the OS level, but even some programs cache domains, like FireFox.

So the answer for when a person is getting the wrong address for a domain, and you know it is set correctly, is usually to just reboot. Clearing the DNS cache works too (for the OS level), but explaining how to do that is harder than saying “just reboot” ^_^;.

To clear the DNS cache in XP, enter the following into your “run” menu or in the command prompt: “ipconfig /flushdns”. This does not ALWAYS work, but it should work.


If your domain is still resolving to the wrong address when you ping it after your DNS cache is cleared, the next step is to see what name servers are being used for the information. You can do a whois on your domain to get the information directly form the registrar who controls the domain, but be careful where you do this as you never know what people are doing with the information. For a quick and secure whois, you can use “whois” from your linux command line, which I have patched through to a web script here. This script gives both normal and extended information, FYI.

Whois just tells you the name servers that you SHOULD be contacting, it doesn’t mean these are the ones you are asking, as the root DNS servers may not have updated the information yet. This is where our command line programs come into play.

In XP, you can use “nslookup -query=hinfo DOMAINNAME” and “nslookup -query=soa DOMAINNAME” to get a domain’s name servers, and then “nslookup NAMESERVER DOMAINNAME” to get the IP the name server points too. For example: (Important information in the following examples are bolded and in white)

C:\>nslookup -query=hinfo castledragmire.com
Server:  dns-redirect-lb-01.texas.rr.com
Address:  24.93.41.127

castledragmire.com
        primary name server = ns3.deltaarc.com
        responsible mail addr = admins.deltaarc.net
        serial  = 2007022713
        refresh = 14400 (4 hours)
        retry   = 7200 (2 hours)
        expire  = 3600000 (41 days 16 hours)
        default TTL = 86400 (1 day)

C:\>nslookup -query=soa castledragmire.com
Server:  dns-redirect-lb-01.texas.rr.com
Address:  24.93.41.127

Non-authoritative answer:
castledragmire.com
        primary name server = ns3.deltaarc.com
        responsible mail addr = admins.deltaarc.net
        serial  = 2007022713
        refresh = 14400 (4 hours)
        retry   = 7200 (2 hours)
        expire  = 3600000 (41 days 16 hours)
        default TTL = 86400 (1 day)

castledragmire.com      nameserver = ns4.deltaarc.com
castledragmire.com      nameserver = ns3.deltaarc.com
ns3.deltaarc.com        internet address = 216.127.92.71

C:\>nslookup ns3.deltaarc.com castledragmire.com
Server:  ev1s-209-85-115-128.theplanet.com
Address:  209.85.115.128

Name:    ns3.deltaarc.com
Address:  216.127.92.71

Nslookup is also available in Linux, but Linux has a better tool for this, as nslookup itself doesn’t always seem to give the correct answers, for some reason. So I recommend you use dig if you have it or Linux available to you. So with dig, we just start at the root name servers and work our way up to the SOA name server to get the real information of where the domain is resolving to and why.

root@www [~]# dig @a.root-servers.net castledragmire.com

; <<>> DiG 9.2.4 <<>> @a.root-servers.net castledragmire.com
; (2 servers found)
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 5587
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 13, ADDITIONAL: 14

;; QUESTION SECTION:
;castledragmire.com.            IN      A

;; AUTHORITY SECTION:
com.                    172800  IN      NS      H.GTLD-SERVERS.NET.
com.                    172800  IN      NS      I.GTLD-SERVERS.NET.
com.                    172800  IN      NS      J.GTLD-SERVERS.NET.
com.                    172800  IN      NS      K.GTLD-SERVERS.NET.
com.                    172800  IN      NS      L.GTLD-SERVERS.NET.
com.                    172800  IN      NS      M.GTLD-SERVERS.NET.
com.                    172800  IN      NS      A.GTLD-SERVERS.NET.
com.                    172800  IN      NS      B.GTLD-SERVERS.NET.
com.                    172800  IN      NS      C.GTLD-SERVERS.NET.
com.                    172800  IN      NS      D.GTLD-SERVERS.NET.
com.                    172800  IN      NS      E.GTLD-SERVERS.NET.
com.                    172800  IN      NS      F.GTLD-SERVERS.NET.
com.                    172800  IN      NS      G.GTLD-SERVERS.NET.

;; ADDITIONAL SECTION:
A.GTLD-SERVERS.NET.     172800  IN      A       192.5.6.30
A.GTLD-SERVERS.NET.     172800  IN      AAAA    2001:503:a83e::2:30
B.GTLD-SERVERS.NET.     172800  IN      A       192.33.14.30
B.GTLD-SERVERS.NET.     172800  IN      AAAA    2001:503:231d::2:30
C.GTLD-SERVERS.NET.     172800  IN      A       192.26.92.30
D.GTLD-SERVERS.NET.     172800  IN      A       192.31.80.30
E.GTLD-SERVERS.NET.     172800  IN      A       192.12.94.30
F.GTLD-SERVERS.NET.     172800  IN      A       192.35.51.30
G.GTLD-SERVERS.NET.     172800  IN      A       192.42.93.30
H.GTLD-SERVERS.NET.     172800  IN      A       192.54.112.30
I.GTLD-SERVERS.NET.     172800  IN      A       192.43.172.30
J.GTLD-SERVERS.NET.     172800  IN      A       192.48.79.30
K.GTLD-SERVERS.NET.     172800  IN      A       192.52.178.30
L.GTLD-SERVERS.NET.     172800  IN      A       192.41.162.30

;; Query time: 240 msec
;; SERVER: 198.41.0.4#53(198.41.0.4)
;; WHEN: Sat Aug 23 04:15:28 2008
;; MSG SIZE  rcvd: 508

root@www [~]# dig @a.gtld-servers.net castledragmire.com

; <<>> DiG 9.2.4 <<>> @a.gtld-servers.net castledragmire.com
; (2 servers found)
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 35586
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 2, ADDITIONAL: 2

;; QUESTION SECTION:
;castledragmire.com.            IN      A

;; AUTHORITY SECTION:
castledragmire.com.     172800  IN      NS      ns3.deltaarc.com.
castledragmire.com.     172800  IN      NS      ns4.deltaarc.com.

;; ADDITIONAL SECTION:
ns3.deltaarc.com.       172800  IN      A       216.127.92.71
ns4.deltaarc.com.       172800  IN      A       209.85.115.181

;; Query time: 58 msec
;; SERVER: 192.5.6.30#53(192.5.6.30)
;; WHEN: Sat Aug 23 04:15:42 2008
;; MSG SIZE  rcvd: 113

root@www [~]# dig @ns3.deltaarc.com castledragmire.com

; <<>> DiG 9.2.4 <<>> @ns3.deltaarc.com castledragmire.com
; (1 server found)
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 26198
;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 0

;; QUESTION SECTION:
;castledragmire.com.            IN      A

;; ANSWER SECTION:
castledragmire.com.     14400   IN      A       209.85.115.128

;; AUTHORITY SECTION:
castledragmire.com.     14400   IN      NS      ns4.deltaarc.com.
castledragmire.com.     14400   IN      NS      ns3.deltaarc.com.

;; Query time: 1 msec
;; SERVER: 216.127.92.71#53(216.127.92.71)
;; WHEN: Sat Aug 23 04:15:52 2008
;; MSG SIZE  rcvd: 97

Linux also has the “host” command, but I prefer and recommend “dig”.


And that’s how you diagnose DNS problems! :-). For reference, two common DNS configuration problems are not having your SOA and NS records properly set for the domain on your name server.


I also went ahead and added dig to the “Useful Bash commands and scripts” post.


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