This is a simple DNS server which can be used to delay A or AAAA responses. This is useful for quickly exploiting DNS rebinding in Safari, as described in the accompanying research blog.

Ensure that you have golang properly installed and setup, then install this tool with:

go get github.com/intruder-io/dns-delay-server@latest

To use this server you will need a domain, and you will need to setup a corresponding NS record for that domain to point to this server. For example, if you own example.com and wanted this DNS server to handle all queries for *.r.example.com, you would need to:

1. Run this server on a VPS
2. Create a hostname for that VPS, such as vps.example.com, with corresponding A and AAAA records in your DNS provider
3. Create an NS record for *.r.example.com with a valu of vps.example.com

To have this server respond to all A queries with 1.2.3.4 and all AAAA queries with ::1, and delay all AAAA responses by 200ms, you would run the following as root:

dns-delay-server -p 53 -a 1.2.3.4 -6 ::1 -D 200ms

It is common to already have a DNS server listening on a local adapter. In this case, you can avoid clashes by specifying the IP address to listen on with -l.

The full help text can be seen with dns-delay-server -h.

The server will respond to cname.<your-domain> with a CNAME record to <your-domain>. If you want these responses to return different IP addresses to A/AAAA queries, you can use the -c and -C flags to specify the IP addresses to include in CNAME responses, or prevent them being included at all.