dnscache - DNS cache server and iterative resolver
dnscache accepts recursive DNS queries from local clients such as web
browsers and mail transfer agents. It collects responses from remote
DNS servers. It caches the responses for faster client lookup.
Upon sending iterative queries dnscache will use the received FQDN of
authoritive name servers to evalute their potential usage for
encryption in case the hostname starts with uz5. Subsequently dnscache
will automatically encrypt queries to those servers identified to be
CurveDNS capable. Now, both queries and responses are encrypted using
either the propriatory CurveDNS stream format, or if the environment
variable $USETEXTFORMAT is set, the less performant TXT format to cope
with deep packet inspecting Firewalls analysing DNS traffic and
expecting a standard DNS header.
In case the name server does not respond to encrypted UDP queries,
dnscache may fall back to unencrypted queries after $UZ5FALLBACK
trials, which usually defaults 2 and can be modified setting this
dnscache maybe used in a forward only mode.
Normally dnscache is set up by the dnscache-conf program.
dnscache runs chrooted in the directory specified by the $ROOT
environment variable, under the uid and gid specified by the $UID and
$GID environment variables. In case $USETEXTFORMAT is set, dnscache
will send queries even for CurveDNS enabled name servers using standard
DNS TXT headers.
dnscache can be adviced to go to unencrypted fallback mode, if hostname
of the name server starts with uz5 but don't respond to encrypted UDP
queries for this lookup. This behavior can be changed using the
environment variable $UZ5FALLBACK=n. The value n=2 is the default
value initally set-up be dnscache-conf. A value like n=1 might impact
correctly behaving CurveDNS name servers which do not respond to the
initial query, while larger values like n=3 delays name resolution for
those name servers significantely. Setting $UZ5FALLBACK=0 disables
fallback mode, which is the default.
dnscache listens for incoming UDP packets and TCP connections addressed
to port 53 for $IP, which could be either an IPv4 or IPv6 scoped
addresss, supporting both public or internal access.
Given a host scope, one typically uses the addresses 127.0.0.1, ::1, or
fe80::1%lo0. In those cases, dnscache serves the own host only.
Setting up dnscache on a private network requires private IPv4
addresses; while for IPv6 ULA and LLU addresses can be used. Examples:
10.10.10.53, fd00::53, fe80::53%eth0.
dnscache is able to serve all existing IP addresses on the host (multi-
homing). For IPv4 specify 0.0.0.0 and for IPv6 set :: within env/IP.
In the case of :: dnscache additionally supports reverse anycasting
for IPv6. Now, dnscache will accept IPv6 packets from every available
interface, even if dynamically allocated.
dnscache forces simultaneous bind to IPv4 and IPv6 addresses in case a
'pseudo' IP address is specified as :0. However, this will not trigger
reverse anycasting support.
dnscache sends outgoing packets from high ports of $IPSEND. Typically
$IPSEND is 0.0.0.0 or :: meaning the machine's primary IP address
covering both IPv4 and IPv6. However, a specific sending IP address
can be used, which might be destinct from the receiving ones.
If $FORWARDONLY is set, dnscache treats servers/@ as a list of IP
addresses for other caches, not root servers. It forwards queries to
those caches. It does not contact the root servers, or any other DNS
dnscache accepts a packet or connection from the IPv4 address 188.8.131.52
if it sees a file named ip/184.108.40.206 or ip/1.2.3 or ip/1.2 or ip/1. For
IPv6 addresses dnscache can be instructed in a similar way:
ip/2001::fefe, ip/2001:a:b:c:d, ip/2001, ip/fe80 (all LLU), ip/fd00
dnscache will reject packets or connections from IP addresses marked
as 'commented out': ip/#2001::fec, ip/#192.168.1. Rejections have
precedence over acceptance.
Note: In any case, the delimiter (either '.' or ':') shall not be used
as last character.
If dnscache recognizes the environment variable $FLAGEDSERVER, name
server listed under ip/ are treated in the following way: Servers
included as ip/%220.127.116.11 or ip/%2001::a:b:c:d and given their dotted-
decimal IPv4 or compactified IPv6 addresses are omitted for name
resolution in case the IP address is prepended with a %. If the IP
addresses is prepended with a -, rather instead of a CurveDNS query a
standard query will be used, irrespectively if the server's FQDN starts
with the magic uz5. Example: ip/-18.104.22.168.
In case $IP4 is set, dnscache will contact only nameserves given their
dnscache reads a seed, up to 128 bytes, from standard input, and passes
the seed to dns_random_init.
dnscache reads a list of root server given as dotted-decimal IPv4
and/or compactified IPv6 addresses one per line from the file
A total of 32 names servers is handled, which are specified in dotted-
decimal IPv4 or compactified IPv6 format. Name severs specified by
their IPv6 LLU addresses need to include the interface name via those
they are reachable.
dnscache also scans the servers directory for server IP addresses for
other domains. If there are addresses listed in servers/moon.af.mil,
for example, then dnscache will send queries for anything.moon.af.mil
to those addresses, and will not cache records for anything.moon.af.mil
dnscache uses a fixed-size cache, as controlled by the $CACHESIZE
environment variable. Roughly 5% of the cache is used for a hash
table. The rest is used for cache entries (including 8-byte
Y2038-compliant expiration times):
o A sets. 22 bytes plus 4 bytes per address plus the length of
the owner name.
o AAAA sets. 22 bytes plus 16 bytes per address plus the length
of the owner name.
o NS sets or PTR sets or CNAME sets. 22 bytes plus the length of
the owner name and all the data names.
o MX sets. 22 bytes plus 2 bytes per MX plus the length of all
o Other record sets. 22 bytes plus 2 bytes per record plus the
length of all the data strings plus the length of the owner
o Nonexistent domain or server failure. 22 bytes plus the length
of the owner name.
Sets larger than 8192 bytes are not cached.
dnscache does not exit when it runs out of space in its cache; it
simply removes the oldest entries to make more space.
dnscache is expecting to be used on IPv6 capabable networks supporting
a 'minimum length' MLMTU size of 1280 byte (RFC 8200) allowing larger
UDP packet sizes than for IPv4 only. Upon start, dnscache shows the
UDP message size supported by default. In addition, dnscache
understands EDNS(0) extensions in DNS messages (RFC 6891), typically
used by DNSSEC.
RESOLUTION AND CACHING POLICIES
dnscache relies on a configured list of root name servers. However,
the IP addresses of the Internet root servers are subject of change.
dnscache does not cache (or pass along) records outside the server's
bailiwick; those records could be poisoned. Records for foo.dom, for
example, are accepted only from the root servers, the dom servers, and
the foo.dom servers.
dnscache does not bypass its cache to obtain glue from the additional
section of a response. In particular, it will not use glue outside the
records relevant to NS or MX records). When the answer section is
truncated by UDP length limits, it is eliminated entirely.
dnscache tries to prevent local users from snooping on other local
users. It discards non-recursive queries; it discards inverse queries;
and it discards zone-transfer requests. If $HIDETTL is set, dnscache
always uses a TTL of 0 in its responses.
According to RFC 1035, the AA bit ``specifies that the responding name
server is an authority for the domain name in question section.''
dnscache is not an authority for any domain names.
dnscache never sets the AA bit (except in NXDOMAIN responses, as
required by RFC 2308, to work around a common client bug). In
contrast, BIND often sets AA for positive responses even when it is not
an authority for the domain name.
dnscache handles localhost internally, giving it an A record of
127.0.0.1. In addition, for the IPv6 address ::1 it considers those as
ipv6-localhost together with the respective AAAA record. dnscache
handles 22.214.171.124.in-addr.arpa and 1.0.0...0.ip6.arpa internally,
giving it a PTR record of 127.0.0.1 and ::1 respectively.
dnscache handles dotted-decimal domain names internally, giving (e.g.)
the domain name 126.96.36.199 an A record of 188.8.131.52.
Man(1) output converted with