D. J. Bernstein
Internet publication
djbdns

The dnscache program

dnscache is a DNS cache. It 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.

This is a reference page. For tutorial information, see the instructions for workstations, home computers, external caches, or upgrading from BIND.

Configuration

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.

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):

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.