FakeDnsProxy is a small python tool that can be used to setup a fake DNS server. FakeDnsProxy can be quickly configured to

• proxy a set of DNS requests to a legitimate DNS Server
• answer a set of DNS requests with pre-defined DNS messages
• suppress answers to DNS requests

FakeDnsProxy can be configured to have a default behavior that defines how FakeDnsProxy reacts to unknown Domains. It can be configured to respond with a static IP, to proxy requests to a real DNS server, or to reply with NXDOMAIN. Furthermore, it is possible to define special behavior for individual domains.

You need to install the following pre-requisites (debian packages):

• python3
• python3-yaml
• python3-dnspython
• python3-twisted

The configuration file is a YAML file that supports the following configuration entries:

Defines the IP and port that fakednsproxy should listen on. listening_info takes two sub-configs ip and port.

listening_info:
  ip: 127.0.0.1
  port: 53

Defines the DNS server that should be used to proxy DNS requests to. dns_server takes two sub_configs ip and port:

dns_server:
  ip: 8.8.8.8
  port: 53

Defines the default behavior for fakednsproxy for queries. The following policies are supported:

• nxdomain If nxdomain is configured, then FakeDnsProxy will send a NXDOMAIN response to every DNS query that does not have a separately defined behavior in the config section domain_config.
• forward This policy will forward every DNS query to the server configured in dns_server if no special configuration exists for this domain in the config section domain_config.
• default_value Will send a default value to every DNS query if no special behavior is configured for this domain in the domain_config section. It is required to set the config defautl_dns_value with the value if this policy is selected

Example:

default_dns_policy: forward

Specifies which value should be returned for the default_value policy. Valid values can either be

• an IP, e.g. '127.0.0.1',

• or a list of IPs, e.g. [ '127.0.0.1', '127.0.0.2' ]

• or a dictionary of the form <record_type>:

  default_value: '127.0.0.1'

or

default_value: [ '127.0.0.1', '127.0.0.2' ]

or

default_value: 
  A: 127.0.0.1
  MX: 127.0.0.2
  NS: 127.0.0.3

Defines a list of special responses for individual domains. This configuraiton overwrites the behavior of the defined default_dns_policy for the configured domains.

Each entry is of the form:

domain: <behavior>

where domain is the domain in the DNS query (e.g. www.github.com), and the behavior is one of:

• nxdomain
• forward

Nxdomain will make FakeDnsProxy return a NXDOMAIN DNS response, forward will make FakeDnsProxy to forward the request to the server defined in dns_server. A value is the value returned in the DNS response. Valid values can either be

• an IP, e.g. '127.0.0.1',
• or a list of IPs, e.g. [ '127.0.0.1', '127.0.0.2' ]
• or a dictionary of the form <record_type>:

Domain can also use '*' as a wildcard character.

domain_config:
  a.com: nxdomain
  foobar.com: forward
  f.com: 127.0.0.1
  b.com: [ '127.0.0.1', '127.0.0.2' ]
  *.com: 127.0.0.1
  *.foobar.com: 1.2.3.4
  c.com:
    A: 1.2.3.5
MX: 1.2.3.4

The supported record types are dependend on the DNS types supported by twisted. Currently, the following values are defined. FakeDnsProxy will try to take the value defined by the configuration for a given record. If it receives a query for such a record, it will take the defined value and tries to generate a appropriate response record.

It is the responsibility of the writer of the configuration to ensure that the content provided for the record can be converted to a valid DNS Response record.

• A,
• A6
• AAAA
• AFSDB
• CNAME
• DNAME
• HINFO
• MB
• MD
• MF
• MG
• MINFO
• MR
• MX
• NAPTR
• NS
• NULL
• OPT
• PTR
• RP
• SOA
• SPF
• SRV
• TXT
• WKS

listening_info:
  ip: 127.0.0.1
  port: 2000
dns_server:
  ip: 8.8.8.8
  port: 53
default_dns_policy: default_value
default_dns_value: 1.2.3.4
domain_config:
  a.com: nxdomain
  foobar.com: forward
  f.com: 127.0.0.1
  b.com: [ '127.0.0.1', '127.0.0.2' ]
  c.com:
    A: 1.2.3.5
MX: 1.2.3.4

Start up fakednsproxy (using the example config above):

python3 fakednsproxy.py myconfig_file.yaml

Then you can test the functionality using DNS tools such das dig:

dig -p 2000 @127.0.0.1 github.com
dig -p 2000 @127.0.0.1 a.com
...