If you would like to perform DNS queries from the command line, I recommend using the PowerShell cmdlet, Resolve-DnsName which does use the native Windows DNS Client resolver. NSlookup is a self-contained executable that does not leverage the Windows DNS client resolver. While this tool does perform DNS queries, it is not representative of how Windows resolves DNS queries. If you've done any DNS work in the past you may have leveraged the tool nslookup. After the TTL has elapsed or the DNS cache is cleared out, the client will need to query the network for that record. Meaning that if another query for the same resource were to occur within that TTL limit, the client OS should use its DNS cache instead of sending that query out onto the network. Not to be confused with the TCP TTL which dictates the number of hops a packet can take before it expires, the DNS TTL is the maximum amount of time that a DNS record should be cached for. One of the key things to call out within the Answers section is the Time to Live (TTL). Notice how the transaction ID is the same between the query and response meaning that if we wanted to easily follow DNS query in a packet capture, we could filter using that as our identifier. User Datagram Protocol, Src Port: 53, Dst Port: 36187įlags: 0x8180 Standard query response, No errorīing.com: type A, class IN, addr 13.107.21.200īing.com: type A, class IN, addr 204.79.197.200 Class: This will nearly always be IN for internetĪnd the response follows much of the same format:.Type: The record type we are looking for.Queries: This contains the specifics of the Host A record we are looking for:.Questions: The number of queries we are performing in this request.This includes things like do we want recursion, what the operation is, etc.Flags: Additional options for the DNS query.The numbers are often reused but not in quick succession.Transaction Id: A semi-unique identifier for the DNS query.Starting from top to bottom within the DNS section: User Datagram Protocol, Src Port: 36187, Dst Port: 53 Here is an example of an outbound DNS query for. Due to this lack of protocol overhead this protocol can be fast but relies upon upper layer network protocols for all the reliability of the information. If a UDP packet is dropped, there is no attempt to retransmit the dropped packet. Similar to TCP it uses a source and destination port pair, but UDP is an unreliable protocol. We have talked about TCP previously but we haven't talked about the User Datagram Protocol (UDP). And this traffic will typically take place over UDP port 53 or TCP port 53. It is important to note that the DNS server MUST always respond to the DNS query even if it just responds with "I dunno go ask someone else". One of the large benefits of DNS is that a DNS server can recursively search the DNS zones that it is authoritative for when it receives a query. A client sends out a DNS query for a specific record and the DNS server responds to that query. You could use something like Windows Internet Name Service (WINS) (please don't use WINS) or NetBIOS name resolution to resolve the name using just the contoso single label but let's stick with DNS for now.ĭNS operates in a query response format. So, for example a machine in my lab may be: I want to take a second to dig (see what I did there?) a bit deeper into the Host A record as that is the primary record type we will be talking about in this post.Ī Host A record is a key containing the Fully Qualified Domain Name (FQDN) for a resource. That response may be "Hey I don't know" but it is still a response. And the best part is that a DNS server must return a response. I have so-and-so’s name, but I want their address or their phone number, and through DNS you can get that information. The general idea is that a DNS server will act as a phone book. This is by no means an exhaustive list of the types or records and in this post, we will be focusing on Host A and records as those are the bread and butter of DNS issues. Or even just to some text! (TXT records).IP addresses to computer names (PTR or pointer records).Who is authoritative for a DNS zone (Nameserver or NS records).Computer names to IPv6 addresses (AAAA, often called Quad A records).Computer names to IPv4 addresses (A records).You ask a DNS server for information with a key, and it provides the value. The core of DNS is that it is used like a key-value pair. The concept of DNS is simple enough, but it can lead to some confusing situations if you don't keep its function in mind. This will be our first foray into an application layer protocol. Howdy everyone! I'm back to talk about one of my favorite causes of heartache, the domain name system (DNS).
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