**Nuclei** is based on the concepts of `YAML` based template files that define how the requests will be sent and processed. This allows easy extensibility capabilities to nuclei.
Next important piece of information about a template is the <u>**info**</u> block. Info block provides more context on the purpose of the template and the **author**. It can also contain a **severity** field which indicates the severity of the template.
Actual requests and corresponding matchers are placed below the info block and they perform the task of making requests to target servers and finding if the template request was succesful.
Each template file can contain multiple requests to be made. The template is iterated and one by one the desired HTTP/DNS requests are made to the target sites.
Requests can be fine tuned to perform the exact tasks as desired. Nuclei requests are fully configurable meaning you can configure and define each and every single thing about the requests that will be sent to the target servers.
Redirection conditions can be specified per each template. By default, redirects are not followed. However, if desired, they can be enabled with `redirects: true` in request details. 10 Redirects are followed at max by default which should be good enough for most use cases. More fine grained control can be excercised over number of redirects followed by using `max-redirects` field.
The next part of the requests is the **path** of the request path. Dynamic variables can be placed in the path to modify its behaviour on runtime. Variables start with `{{` and end with `}}` and are case-sensitive.
1.**BaseURL** - Placing BaseURL as a variable in the path will lead to it being replaced on runtime in the request by the original URL as specified in the target file.
2.**Hostname** - Hostname variable is replaced by the hostname of the target on runtime.
Matchers are the core of nuclei. They are what make the tool so powerful. Multiple type of combinations and checks can be added to ensure that the results you get are free from false positives.
To match size, similar structure can be followed. If the status code of response from the site matches any single one specified in the matcher, the request is marked as successful.
1.**AND** - Using AND conditions allows matching of all the words from the list of words for the matcher. Only then will the request be marked as successful when all the words have been matched.
2.**OR** - Using OR conditions allows matching of a single word from the list of matcher. The request will be marked as successful when even one of the word is matched for the matcher.
While using multiple matchers the default condition is to follow OR operation in between all the matchers, AND operation can be used to make sure return the result if all matchers returns true.
Extractors are another important feature of nuclei. Extractors can be used to extract and display in results a match from the response body or headers based on a regular expression.
Requests start with a dns block which specifies the start of the requests for the template.
```yaml
# start the requests for the template right here
dns:
```
DNS requests can be fine tuned to perform the exact tasks as desired. Nuclei requests are fully configurable meaning you can configure and define each and every single thing about the requests that will be sent to the target servers.
#### **Type**
First thing in the request is <u>**type**</u>. Request type can be **A**, **NS**, **CNAME**, **SOA**, **PTR**, **MX**, **TXT**, **AAAA**.
```yaml
# type is the type for the dns request
type: A
```
#### **Name**
The next part of the requests is the **name** of the request path. Dynamic variables can be placed in the path to modify its value on runtime. Variables start with `{{` and end with `}}` and are case-sensitive.
1.**FQDN** - variable is replaced by the hostname/fqdn of the target on runtime.
Some sample dynamic variable replacement examples -
```yaml
name: {{FQDN}}.com
# this value will be replaced on execution with FQDN
# When FQDN = https://this.is.an.example
# name will get replaced to the following -
this.is.an.example.com
```
As of now the tool supports only one question per request.
#### **Class**
Class type can be **INET**, **CSNET**, **CHAOS**, **HESIOD**, **NONE**, **ANY**. Usually it's enough to just leave it as **INET**
```yaml
# method is the class for the dns request
class: inet
```
#### **Recursion**
Recursion is a boolean value, and determines if the resolver should only return cached results, or traverse the whole dns root tree to retrieve fresh results. Generally it's better to leave it as **true**
```yaml
# recursion is a boolean determining if the request is recursive
recursion: true
```
#### **Retries**
Retries is the number of attempts a dns query is retried before giving up among different resolvers. It's recommended a reasonable value, like **3**.
```yaml
# retries is a number of retries before giving up on dns resolution
Matchers are just equal to HTTP, but the search is performed on the whole dns response, therefore it's not necessary to specify the **part**. Multiple type of combinations and checks can be added to ensure that the results you get are free from false positives. The complex **dsl** matcher type allows to build complex queries as described in the HTTP section.
