--- name: "dnscache" section: "8" platform: "linux" package: "dnscache" version: "1:1.05-22" release: "Debian 13 (Trixie)" platforms: ["linux"] url: "https://new.carta.tech/dnscache" --- # dnscache(8) a DNS cache. ## 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)](/dnscache-conf/8.md) 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*. ## SEE ALSO [dnscache-conf(8)](/dnscache-conf/8.md) http://cr.yp.to/djbdns.html