DNS over TCP

MaraDNS has full support for DNS over TCP. However, this setup is not automatically done; a little configuration needs to be set up and the "zoneserver" daemon has to be running in addition to the MaraDNS daemon in order for DNS records to be served over TCP.

The first mararc variable that needs to be set is tcp_convert_acl. This is a list of IPs that are allowed to make DNS-over-TCP queries. In the case of using MaraDNS as an authoritative nameserver, this should have a value of "0.0.0.0/0" (anyone on the internet can make TCP DNS connections).

The second mararc variable that needs to be set is tcp_convert_server. This is the IP of the UDP DNS server that we will connect whenever we get a DNS-over-TCP request. The way we perform DNS over TCP is as follows:

The resolver or end user connects via TCP to the zoneserver daemon.
The resolver sends a DNS query over TCP.
The zoneserver program converts the query in to a UDP query.
zoneserver sends the UDP query to the ip specified in the tcp_convert_server mararc variable.
zoneserver waits for a UDP reply
After zoneserver gets a UDP reply, it converts this reply to a TCP reply to give to the resolver/end user.

This is what a mararc file which serves the domain example.com and will provide both DNS over UDP and TCP will look like:

ipv4_bind_addresses = "10.1.2.3"
chroot_dir = "/etc/maradns"
csv2 = {}
csv2["example.com."] = "db.example.com"
tcp_convert_acl = "0.0.0.0/0"
tcp_convert_server = "10.1.2.3"

With this mararc file, and the csv2 zone file "db.example.com" in the directory /etc/maradns, the program maradns will process UDP DNS queries, and zoneserver will process TCP DNS queries. maradns is a standalone UDP DNS server; zoneserver is a TCP proxy that converts TCP requests in to UDP requests. Both daemons must be running to process both UDP and TCP DNS queries.

Long packets

maradns, the UDP DNS server, in compliance with RFC1035 section 2.3.4, will not output a packet longer than 512 bytes long. This is sufficient packet size for well over 99% of the DNS traffic out there.

However, some DNS packets may need to be longer than this. MaraDNS has support for outputting DNS packets up to 4096 bytes long. RFC1035 only allows such packets to be sent over TCP; the work around is to have the UDP server only send long RFC-violating DNS packets to the TCP server; which then converts the long UDP packet in to a RFC-compliant long TCP packet.

One uses the long_packet_ipv4 mararc variable to tell maradns which IPs we will send long UDP packets to. This value is usually the same value as is set for ipv4_bind_addresses.

long_packet_ipv4 is a list of IPs maradns will send RFC-violating long UDP packets to. When zoneserver listens for a UDP reply from maradns, the zoneserver program will be able to process long UDP packets, converting them in to RFC-compliant TCP DNS packets.

This is what a mararc file which provides authoritative DNS over TCP and UDP, sending long UDP packets to the TCP server to process, will look like:

ipv4_bind_addresses = "10.1.2.3"
chroot_dir = "/etc/maradns"
csv2 = {}
csv2["example.com."] = "db.example.com"
tcp_convert_acl = "0.0.0.0/0"
tcp_convert_server = "10.1.2.3"
long_packet_ipv4 = "10.1.2.3"

The zoneserver program can also be used to serve DNS zones; see the file dnsmaster for details.