DJBDNS Stub Resolver ==================== Simple DJBDNS stub-resolver based on 'djbdns-1.05(IPv6)' allowing for each calling application individually to include up to 16 DNSCACHEIP(s) as DNS forwarding/resolving servers to be tried sequentially. IP Addresses ------------ Here, - global IPv6, - IPv6 ULA, and - IPv6 LLU addresses with a given Interface-Id can be specified. The IPv4 format could be either a - legacy dotted-decimal or a - IPv4-mapped IPv6 address. In any case, compactified IPv6 addresses are understood. IPv4/IPv6 addresses in brackets are understood by dns_ip. Resolver Call ------------- If $DNSCACHEIP is not provided as environment variable, the stub-resolver will use the system-wide - /etc/resolv.conf file; however now without the capability for IPv6 LLU addresses. While IPv4-mapped IPv6 addresses are supported here by default as well, care has to taken not to jeopardize other client's usage. Name Qualification ------------------ If provided, the stub-resolver uses either a system-wide configuration file - /etc/dnsrewritefile or assumes this file to available as given in - $DNSREWRITEFILE in order to define persistent mapping-rules of local domain names to public ones (for lookup) or IP addresses (for direct matching). Well-known domain names 'localhost', 'ip4-loopback' and 'ip6-loopback' are handled locally, thus no DNS query is used (RFC 6761). 'localhost' is advertised as '::1' and '::ff:127.0.0.1' in it's native IPv6 format. It is up to the caller to convert the IPv6-mapped IPv4 address to the IPv4 format. Local domain names can be alternatively specified (per application) using the environment variable - $LOCALDOMAIN to be appended to unqualified hostnames dynamically. This is roughly equivalent with the 'search' string in /etc/resolv. Several domains names may be specified within $LOCALDOMAIN separated by blanks. See: https://cr.yp.to/djbdns/qualify.html Specific DNS Record type lookup ------------------------------- * dns_ip (A, AAAAA) * dns_name (PTR) * dns_cname (CNAME) * dns_txt (TXT) -- now considering several 'labels' * dns_mx (MX) Internals --------- * UDP message size: Unlike other implementations, this DNS stub-resolver supports UDP packet sizes up to 1028 byte without the need for (E)DNS0 packet enhancements. * DNS UDP query retrials: In case the NS is not able to initally reply to the query, it is retried again at the intervalls {1, 2, 4, 8, 16} secs. * DNS name qualification (dns_ip_qualify): Well-known domain names are qualified locally without invoking a DNS query while handling IPv4 and IPv6 addresses separately. * NS qualification/sorting for NS replies: NS qualification is not supported (yet), thus we use a randomly sorted list of NS IP addresses. * Query/Reply to/from DNS Cache servers/forwarders: Neither message (CurveDNS) nor transport layer (TLS) encryption is provided; the sub-resolver 'trusts' it's upstream caches/forwarders. We recommend to setup communication on private IPv4/IPv6 addresses; if applicable. * DNS TXT Records: The label substructure is now recognized in the RDATA section; each label may have the size of 255 byte. The length information is excluded from the output. Only printable characters are recognized in the output. * Return Codes: Different from DJB's initial routines, the DNS front-end routines dns_cname*, dns_ip*, dns_mx*, dns_name*, dns dns_txt* return now the number of replies received (not bytes!). Thus, three cases need to be considered: - rc < 0: Problem occured (SOFTFAIL, HARDFAIL) - rc = 0: No answer obtained (but query was successful) = NXDOMAIN - rc > 0: rc answers received; positive reply For return codes < 0, the following conventions have been applied: include/dnsresolv.h \#define DNS_NXD 0 \#define DNS_MEM -1 \#define DNS_ERR -2 /* parsing errors and others */ \#define DNS_COM -3 /* (socket) communication errors */ \#define DNS_INT -4 /* internal errors */ \#define DNS_SOFT -5 /* either -2 or -3 */ \#define DNS_HARD -6 /* CNAME loop problem */ The modification of the return code is typically not problematic, since mostly just rc = -1 is checked. In the future, these return codes are subject of change. Thus, instead of if (dns_XX(...) == -1) one shoud use the more general syntax if (dns_XX(...) < 0) to check for 'negative' results, allowing further actions and refinements given the calling sequence. Environment Variables Read -------------------------- $DNSCACHEPIP The upstream resolver's IP[v4|v6] addresses (up to 32). IPv6 LLU addresses may be suffixed with the interface name. $DNSREWRITEFILE Alternate location for the system-wide /etc/dnsrewrite file $LOCALDOMAIN Additional local domain name appended to unqualified hostnames dynamically. Sample for the file /etc/dnsrewrite: \#annything.local -> me \-.example.com:me \# me -> 127.0.0.1 \=me:127.0.0.1 \# any.name.a -> any.name.af.mil \*.a:.af.mil \# any-name-without-dots -> any-name-without-dots.heaven.af.mil \?:.heaven.af.mil \# remove trailing dot \*.: and DJB's explanations are given here: Instructions are followed in order, each at most once. There are four types of instructions: \=post:new means that the host name post is replaced by new. \*post:new means that any name of the form prepost is replaced by prenew. \?post:new means that any name of the form prepost, where pre does not contain dots or brackets, is replaced by prenew. \-post:new means that any name of the form prepost is replaced by new. Erwin Hoffmann, June 2023.