walldns — an opaque content DNS server using the UDP protocol
walldns is a content DNS server that speaks the DNS/UDP protocol. It accepts DNS queries from hosts around the Internet, and responds with fixed information. The queries ask about various IP addresses, taking the form of a reverse lookup, to which it supplies generic responses that avoid revealing local host information.
When it starts walldns changes its root to the directory specified by the
ROOT environment variable, and drops privileges to run as the user ID and group ID specified by the
GID environment variables.
The latter can be set up with envuidgid(1).
Normally walldns is run via a server program such as udp-socket-listen to listen for DNS/UDP queries from hosts around the Internet.
It understands the
LISTEN_FDS environment variable convention for having an already-listening socket passed to it by such a program, and uses the last open file descriptor in the list that refers to a UDP/IPv4 socket.
If no such open file descriptor is provided it falls back to opening its own UDP/IPv4 socket, bound to port 53 of the IP address given by the value of the
IP environment variable.
It does not handle DNS/TCP.
walldns rejects zone-transfer requests, inverse queries, non-Internet-class queries, truncated packets, packets that contain anything other than a single query, query types other than A, PTR, or ANY, and queries for domain names not beneath
walldns answers queries in a fixed form.
The reverse lookup domain names take the form
a.b.c.d is the IP address being looked up.
It publishes PTR responses mapping the IP address
a.b.c.d back to the name
It publishes A responses mapping the name
d.c.b.a.in-addr.arpa to the IP address
This creates a bidirectional mapping that maps IP addresses to domain names that map back to those same IP addresses.
walldns does not include any NS or SOA resource records in its responses; and uses TTLs of one week.