fehQlibs

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.