Ubuntu Manpage: dnscache

NAME

       dnscache - a DNS cache.

DESCRIPTION

       This is a reference page.  For tutorial information, see the instructions for
       workstations (http://cr.yp.to/djbdns/run-cache.html),
       home computers (http://cr.yp.to/djbdns/run-cache-home.html),
       external caches (http://cr.yp.to/djbdns/run-cache-x.html), or
       upgrading from BIND (http://cr.yp.to/djbdns/run-cache-bind-1.html).

       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 to save time later.

Configuration

Normally dnscache is set up by the dnscache-conf(8) 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.

dnscache listens for incoming UDP packets and TCP connections addressed to port 53 of $IP. Typically $IP
is 127.0.0.1, but it can also be an externally accessible IP address.

dnscache accepts a packet or connection from IP address 1.2.3.4 if it sees a file named ip/1.2.3.4 or
ip/1.2.3 or ip/1.2 or ip/1.

dnscache sends outgoing packets from high ports of $IPSEND. Typically $IPSEND is 0.0.0.0, meaning the
machine's primary IP address.

dnscache reads a seed, up to 128 bytes, from standard input, and passes the seed to dns_random_init.

dnscache reads a list of dotted-decimal root server IP addresses, one address per line, from servers/@.
It 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 from outside servers such as the
root servers.

Versions 1.03 and above: 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 the same way that a client does,
rather than contacting a chain of servers according to NS records.

Memory use

dnscache uses a fixed-size table, under 256K, to keep track of as many as 200 simultaneous UDP queries
and 20 simultaneous TCP connections. It also dynamically allocates memory, usually just a few bytes but
occasionally much more, for each active query. If it runs out of memory handling a query, it discards
that query.

dnscache asks the operating system to reserve a 128K buffer for bursts of incoming UDP queries. In
versions 1.03 and above, if a new UDP query arrives when dnscache is already handling 200 simultaneous
UDP queries, dnscache drops the oldest query. If a new TCP connection arrives when dnscache is already
handling 20 simultaneous TCP connections, dnscache drops the oldest connection.

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 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 the names.

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 name.

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.

Resolution and caching policies

       dnscache relies on a configured list of root name servers.  In contrast, BIND starts from a ``hint file''
       listing name servers, and asks those name servers where the root name servers are.

       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  server's bailiwick, or glue with TTL 0, or glue that
       violates other caching policies.

       dnscache caches records for at most a week.  It interprets TTLs above 2147483647 as 0.

       dnscache does not cache SOA records.  However, it does use SOA TTLs to determine cache times  (up  to  an
       hour) for zero-record responses and nonexistent domains.

Responses to DNS clients

       dnscache's  responses  are  generally  much smaller than BIND's responses.  They do not include authority
       records (NS records of the source name servers and  SOA  records  for  negative  answers)  or  additional
       records  (A  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.  In versions before 1.03, 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.  (This appears to have been fixed in BIND 9.)

Repeated IP addresses

       If a server sends dnscache a repeated IP address, dnscache passes the repeated IP address  along  to  the
       client.   The server's behavior violates RFC 2181, section 5.5, but there are reasonable uses of repeated
       IP addresses for load balancing, so dnscache does not go out of its way to remove repetitions  when  they
       occur.

       A  widespread  BIND  server  bug  (apparently  fixed in BIND 9.1) can unintentionally produce repeated IP
       addresses.  Here is an example from one of the BIND company's servers (now fixed):

         % dnsq a ns-ext.vix.com ns-ext.vix.com
         1 ns-ext.vix.com:
         117 bytes, 1+1+2+2 records, response, authoritative, noerror
         query: 1 ns-ext.vix.com
         answer: ns-ext.vix.com 3600 A 204.152.184.64
         authority: vix.com 3600 NS ns-ext.vix.com
         authority: vix.com 3600 NS ns1.gnac.com
         additional: ns-ext.vix.com 3600 A 204.152.184.64
         additional: ns1.gnac.com 130768 A 209.182.195.77

       This BIND bug is the most common reason for users to see repeated IP addresses from dnscache.

Special names

       dnscache handles localhost internally, giving it an A record of 127.0.0.1.

       dnscache handles 1.0.0.127.in-addr.arpa internally, giving it a PTR record of localhost.

       dnscache handles dotted-decimal domain names internally, giving (e.g.) the domain name 192.48.96.2  an  A
       record of 192.48.96.2.