I have seen what the text representation of an HTTP request is, but what does a DNS request look like? Where in the data is the location of the URL you are trying to locate? Also, how is the response formatted?
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3Your best bet would be to download wireshark and capture the packets you that you can get a full picture of what happens when a DNS request is made.– mrdennyAug 22, 2010 at 6:51
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1seems like a homework question– JimsmithkkaAug 22, 2010 at 7:35
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2@mydenny - no, the best bet would be to do that with a copy of RFC1035 to hand. Wireshark won't give you the "full" picture, it can only ever show you what the particular packets you've seen look like.– AlnitakAug 23, 2010 at 9:00
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@Jim-It's not @mrdenny-I have used Wireshark and don't know how the raw request translates into what I am sending.– AMWJAug 31, 2010 at 20:58
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4 Answers
This is a raw dump from Wireshark of a DNS query.
The DNS part starts with 24 1a:
0000 00 00 00 00 00 00 00 00 00 00 00 00 08 00 45 00 ........ ......E.
0010 00 3c 51 e3 40 00 40 11 ea cb 7f 00 00 01 7f 00 .<Q.@.@. ........
0020 00 01 ec ed 00 35 00 28 fe 3b 24 1a 01 00 00 01 .....5.( .;$.....
0030 00 00 00 00 00 00 03 77 77 77 06 67 6f 6f 67 6c .......w ww.googl
0040 65 03 63 6f 6d 00 00 01 00 01 e.com... ..
And here is the breakdown:
Domain Name System (query)
[Response In: 1852]
Transaction ID: 0x241a
Flags: 0x0100 (Standard query)
0... .... .... .... = Response: Message is a query
.000 0... .... .... = Opcode: Standard query (0)
.... ..0. .... .... = Truncated: Message is not truncated
.... ...1 .... .... = Recursion desired: Do query recursively
.... .... .0.. .... = Z: reserved (0)
.... .... ...0 .... = Non-authenticated data OK: Non-authenticated data is unacceptable
Questions: 1
Answer RRs: 0
Authority RRs: 0
Additional RRs: 0
Queries
www.google.com: type A, class IN
Name: www.google.com
Type: A (Host address)
Class: IN (0x0001)
And the response, again starting at 24 1a:
0000 00 00 00 00 00 00 00 00 00 00 00 00 08 00 45 00 ........ ......E.
0010 00 7a 00 00 40 00 40 11 3c 71 7f 00 00 01 7f 00 .z..@.@. <q......
0020 00 01 00 35 ec ed 00 66 fe 79 24 1a 81 80 00 01 ...5...f .y$.....
0030 00 03 00 00 00 00 03 77 77 77 06 67 6f 6f 67 6c .......w ww.googl
0040 65 03 63 6f 6d 00 00 01 00 01 c0 0c 00 05 00 01 e.com... ........
0050 00 05 28 39 00 12 03 77 77 77 01 6c 06 67 6f 6f ..(9...w ww.l.goo
0060 67 6c 65 03 63 6f 6d 00 c0 2c 00 01 00 01 00 00 gle.com. .,......
0070 00 e3 00 04 42 f9 59 63 c0 2c 00 01 00 01 00 00 ....B.Yc .,......
0080 00 e3 00 04 42 f9 59 68 ....B.Yh
Breakdown:
Domain Name System (response)
[Request In: 1851]
[Time: 0.000125000 seconds]
Transaction ID: 0x241a
Flags: 0x8180 (Standard query response, No error)
1... .... .... .... = Response: Message is a response
.000 0... .... .... = Opcode: Standard query (0)
.... .0.. .... .... = Authoritative: Server is not an authority for domain
.... ..0. .... .... = Truncated: Message is not truncated
.... ...1 .... .... = Recursion desired: Do query recursively
.... .... 1... .... = Recursion available: Server can do recursive queries
.... .... .0.. .... = Z: reserved (0)
.... .... ..0. .... = Answer authenticated: Answer/authority portion was not authenticated by the server
.... .... .... 0000 = Reply code: No error (0)
Questions: 1
Answer RRs: 3
Authority RRs: 0
Additional RRs: 0
Queries
www.google.com: type A, class IN
Name: www.google.com
Type: A (Host address)
Class: IN (0x0001)
Answers
www.google.com: type CNAME, class IN, cname www.l.google.com
Name: www.google.com
Type: CNAME (Canonical name for an alias)
Class: IN (0x0001)
Time to live: 3 days, 21 hours, 52 minutes, 57 seconds
Data length: 18
Primary name: www.l.google.com
www.l.google.com: type A, class IN, addr 66.249.89.99
Name: www.l.google.com
Type: A (Host address)
Class: IN (0x0001)
Time to live: 3 minutes, 47 seconds
Data length: 4
Addr: 66.249.89.99
www.l.google.com: type A, class IN, addr 66.249.89.104
Name: www.l.google.com
Type: A (Host address)
Class: IN (0x0001)
Time to live: 3 minutes, 47 seconds
Data length: 4
Addr: 66.249.89.104
Edit:
Note that if your real question is "how do I write a DNS server?", then there are two appropriate answers:
Edit(2):
The request was sent using host on a linux box:
host www.google.com
If you are on Windows, you can use nslookup
nslookup www.google.com
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2Not upvoting a wire trace without a reference to the real specification. Far too many DNS interop problems have been caused by people believing they can reverse engineer from a wire trace, and missing subtle nuances. In any case this dump is insufficient - it doesn't demonstrate how labels and RRs are encoded.– AlnitakAug 23, 2010 at 8:58
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7@Alnitak: I read the question as "what is an example of a DNS request/response", rather than what do all possible requests look like. Updated in case that is incorrect.– ngoozeffAug 23, 2010 at 12:48
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1I would like to know if URL parameters as used in browsers are sent over as well, or is it just the domain name? So for this question, is
/questions/173187/what-does-a-dns-request-look-likeincluded in the request?– SPRBRNApr 23, 2014 at 7:47 -
2
DNS request data layout is described in RFC 1035. I think it's a bit pointless to copy the text here...
DNS queries and responses are best looked at using a protocol analyzer - Wireshark is a good cross platform tool that can capture and deconstruct the requests and responses into their various parts. There is a nice introduction to the structure of DNS Requests and Responses at Firewall.cx here.
DNS Requests contain questions that specify a name (or maybe a somewhat arbitrary text field) and a record type - the content of the response will vary depending on the type. Most requests are simple direct lookups of a server name looking for an ip-address in response (Type A) but some will be looking for more information on name servers themselves (Type NS), mail records (Type MX) and other services (Type SRV that will return names, ports, weights and priorities). DNS responses contain answers to these questions, possibly more than one if the request requires that and are not always just ip-addresses.
One other clarification - DNS doesn't resolve URLs - in most scenarios involving URLs DNS is only used to enable the client side system to find the ip-address of the server part of the URL, everything else is handled by other protocols.
If you can get onto a Linux machine, you can run the dig command to perform a DNS lookup. This utility performs a lookup and returns exactly what the name server responds with. For example:
; <<>> DiG 9.6.1-P2 <<>> serverfault.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 32383
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;serverfault.com. IN A
;; ANSWER SECTION:
serverfault.com. 55961 IN A 69.59.196.211
;; Query time: 21 msec
;; SERVER: 68.87.64.150#53(68.87.64.150)
;; WHEN: Sun Aug 22 09:21:35 2010
;; MSG SIZE rcvd: 49
Everything starting with the "HEADER" section are what gets returned from the name server. I'm assuming this is what you're referring to as the text format because this isn't the format of the actual packet, but it is the text that gets returned.
answered Aug 22, 2010 at 13:24
