Whonow DNS Server – Dynamic DNS Rebinding

A malicious DNS server for executing DNS Rebinding attacks on the fly. whonow lets you specify DNS responses and rebind rules dynamically using domain requests themselves.

# respond to DNS queries for this domain with the first time
# it is requested and then every time after that

# respond first with, then the next five times,
# and then start all over again (1, then 5, forever...)

What’s great about dynamic DNS Rebinding rules is that you don’t have to spin up your own malicious DNS server to start exploiting the browser’s Same-origin policy. Instead, everyone can share the same public whonow server running on port 53 of


Subdomains = Rebind Rules

The beauty of whonow is that you can define the behavior of DNS responses via subdomains in the domain name itself. Using only a few simple keywords: A, (n)times, forever, and repeat, you can define complex and powerful DNS behavior.


Anatomy of a whonow request

  • A: The type of DNS request. Currently only A records are supported, but AAAA should be coming soon.
  • <ip-address>: an ipv4 (ipv6 coming soon) address with each octet seprated by a period (e.g.
  • <rule>: One of three rules
    • (n)time[s]: The number of times the DNS server should reply with the previous IP address. Accepts both plural and singular strings (e.g. 1time, 3times, 5000times)
    • forever: Respond with the previous IP address forever.
    • repeat: Repeat the entire set of rules starting from the beginning.
  • [uuid/random-string]: A random string to keep DNS Rebind attacks against the same IP addresses separate from each other. See Gotchas for more info.
  • A domain name you have pointing to a whonow nameserver, like the publicly available whonow instance.

Rules can be chained together to form complex response behavior.



# always respond with This isn't really DNS rebinding
# but it still works

# alternate between localhost and forever

# first respond with then Now respond forever.

# respond with the first time, then whatever `whonow --default-address`
# is set to forever after that (default:



Each label [subdomain] may contain zero to 63 characters… The full domain name may not exceed the length of 253 characters in its textual representation. (from the DNS Wikipedia page)

Additionally, there may not be more than 127 labels/subdomains.



Use Unique Domain Names

Each unique domain name request to whonow creates a small state-saving program in the server’s RAM. The next time that domain name is requested the program counter increments and the state may be mutated. All unique domain names are their own unique program instances. To avoid clashing with other users or having your domain name program’s state inadvertently incremented you should add a UUID subdomain after your rule definitions. That UUID should never be reused.

# this

# not this



The program state associated with each unique domain name is stored by whonow in RAM. To avoid running out of RAM an upper-bound is placed on the number of unique domains who’s program state can be managed at the same time. By default, this value is set to 10,000,000, but can be configured with the --max-ram-domains. Once this limit is reached, domain names and their saved program state will be removed in the order they were added (FIFO).


Running your own whonow server

To run your own whonow server in the cloud use your domain name provider’s admin panel to configure a custom nameserver pointing to your VPS. Then install whonow on that VPS and make sure it’s running on port 53 (the default DNS port) and that port 53 is accessible to the Internet.

# install
npm install --cli -g whonow@latest

# run it!
whonow --port 53




$ whonow --help
usage: whonow [-h] [-v] [-p PORT] [-d DEFAULT_ANSWER] [-b MAX_RAM_DOMAINS]

A malicious DNS server for executing DNS Rebinding attacks on the fly.

Optional arguments:
  -h, --help            Show this help message and exit.
  -v, --version         Show program's version number and exit.
  -p PORT, --port PORT  What port to run the DNS server on (default: 53).
  -d DEFAULT_ANSWER, --default-answer DEFAULT_ANSWER
                        The default IP address to respond with if no rule is
                        found (default: "").
  -b MAX_RAM_DOMAINS, --max-ram-domains MAX_RAM_DOMAINS
                        The number of domain name records to store in RAM at
                        once. Once the number of unique domain names queried
                        surpasses this number domains will be removed from
                        memory in the order they were requested. Domains that
                        have been removed in this way will have their program
                        state reset the next time they are queried (default:


Dynamic DNS Rebinding: Whonow DNS Server Download


Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: