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A simple DNS resolver, similar to nslookup

2018-04-17

Introduction

The project code can be found at https://github.com/melonskin/simple-dns-lookup.

This is a naive implementation of DNS resolver. The function is basically the same as nslookup. It takes two arguments. The first is the lookup string. It can be either a host or a IP address. The second is the DNS server you would like to query. For example:

dns-resolver.exe www.dhs.gov 128.194.135.84
dns-resolver.exe 206.190.39.42 8.8.8.8

The result is shown as below.

PS ...> .\dns-resolver.exe www.dhs.gov 128.194.135.84
Lookup   : www.dhs.gov
Query    : www.dhs.gov, type 1 TXID 0x567A
Server   : 128.194.135.84
*******************************************
Attempt 0 with 29 bytes... response in 29 ms with 442 bytes
        TXID 0x567A flags 0x8180 questions 1 answers 3 authority 8 additional 9
        succeeded with Rcode = 0
        ------------- [questions] -----------
                www.dhs.gov type 1 class 1
        ------------- [answers] -----------
                www.dhs.gov CNAME www.dhs.gov.edgekey.net TTL = 1625
                www.dhs.gov.edgekey.net CNAME e6485.dsca.akamaiedge.net TTL = 174
                e6485.dsca.akamaiedge.net A 23.35.235.8 TTL = 20
        ------------- [authority] -----------
                dsca.akamaiedge.net NS n2dsca.akamaiedge.net TTL = 2025
                dsca.akamaiedge.net NS n0dsca.akamaiedge.net TTL = 2025
                dsca.akamaiedge.net NS n1dsca.akamaiedge.net TTL = 2025
                dsca.akamaiedge.net NS n6dsca.akamaiedge.net TTL = 2025
                dsca.akamaiedge.net NS n4dsca.akamaiedge.net TTL = 2025
                dsca.akamaiedge.net NS n7dsca.akamaiedge.net TTL = 2025
                dsca.akamaiedge.net NS n5dsca.akamaiedge.net TTL = 2025
                dsca.akamaiedge.net NS n3dsca.akamaiedge.net TTL = 2025
        ------------- [additional] -----------
                n0dsca.akamaiedge.net A 88.221.81.192 TTL = 2025
                n1dsca.akamaiedge.net A 66.171.227.43 TTL = 2025
                n2dsca.akamaiedge.net A 66.171.227.44 TTL = 2025
                n3dsca.akamaiedge.net A 66.171.227.45 TTL = 2025
                n4dsca.akamaiedge.net A 66.171.227.242 TTL = 2025
                n5dsca.akamaiedge.net A 66.171.227.246 TTL = 2025
                n6dsca.akamaiedge.net A 69.31.59.75 TTL = 2025
                n7dsca.akamaiedge.net A 128.177.98.250 TTL = 2025

PS ...> .\dns-resolver.exe 206.190.39.42 8.8.8.8
Lookup   : 206.190.39.42
Query    : 42.39.190.206.in-addr.arpa, type 12 TXID 0x35DB
Server   : 8.8.8.8
*******************************************
Attempt 0 with 44 bytes... response in 16 ms with 103 bytes
        TXID 0x35DB flags 0x8180 questions 1 answers 1 authority 0 additional 0
        succeeded with Rcode = 0
        ------------- [questions] -----------
                42.39.190.206.in-addr.arpa type 12 class 1
        ------------- [answers] -----------
                42.39.190.206.in-addr.arpa PTR media-router-fp1.prod.media.vip.gq1.yahoo.com TTL = 454

Implementation

The program takes the lookup string and identify whether it’s a type A or PTR. A query packet is assembled and sent to the server via UDP. Response are checked for safety purpose and parsed into human-readable format.

Multiple error response cases can be recognized and reported, such as:

  • invalid reply: smaller than fixed header
  • invalid section: not enough records
  • invalid record: jump beyond packet boundary
  • invalid record: truncated name
  • invalid record: truncated fixed RR header
  • invalid record: truncated jump offset
  • invalid record: jump into fixed header
  • invalid record: jump loop
  • invalid record: RR value length beyond packet
  • invalid reply: TXID mismatch

Copyright claim: A simple DNS resolver, similar to nslookup is created by melonskin on 2018/04/17. Its copyright belongs to the author. Commercial usage must be authorized by the author. The source should be included for non-commercial purposes.
Link to the article: https://amelon.org/2018/04/17/dns-resolver.html

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