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 UID and 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_PID and 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, where 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 to the IP address a.b.c.d. 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.


walldns was originally part of Daniel J. Bernstein's djbdns toolset in 2000.


Original code and documentation by Daniel J. Bernstein. Documentation modernizations by Jonathan de Boyne Pollard.