c-ipfs/dnslink/dnslink.c

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/*
Package dnslink implements a dns link resolver. dnslink is a basic
standard for placing traversable links in dns itself. See dnslink.info
A dnslink is a path link in a dns TXT record, like this:
dnslink=/ipfs/QmR7tiySn6vFHcEjBeZNtYGAFh735PJHfEMdVEycj9jAPy
For example:
> dig TXT ipfs.io
ipfs.io. 120 IN TXT dnslink=/ipfs/QmR7tiySn6vFHcEjBeZNtYGAFh735PJHfEMdVEycj9jAPy
This package eases resolving and working with thse dns links. For example:
import (
dnslink "github.com/jbenet/go-dnslink"
)
link, err := dnslink.Resolve("ipfs.io")
// link = "/ipfs/QmR7tiySn6vFHcEjBeZNtYGAFh735PJHfEMdVEycj9jAPy"
It even supports recursive resolution. Suppose you have three domains with
dnslink records like these:
> dig TXT foo.com
foo.com. 120 IN TXT dnslink=/dns/bar.com/f/o/o
> dig TXT bar.com
bar.com. 120 IN TXT dnslink=/dns/long.test.baz.it/b/a/r
> dig TXT long.test.baz.it
long.test.baz.it. 120 IN TXT dnslink=/b/a/z
Expect these resolutions:
dnslink.ResolveN("long.test.baz.it", 0) // "/dns/long.test.baz.it"
dnslink.Resolve("long.test.baz.it") // "/b/a/z"
dnslink.ResolveN("bar.com", 1) // "/dns/long.test.baz.it/b/a/r"
dnslink.Resolve("bar.com") // "/b/a/z/b/a/r"
dnslink.ResolveN("foo.com", 1) // "/dns/bar.com/f/o/o/"
dnslink.ResolveN("foo.com", 2) // "/dns/long.test.baz.it/b/a/r/f/o/o/"
dnslink.Resolve("foo.com") // "/b/a/z/b/a/r/f/o/o"
*/
#include <stdlib.h>
#include <string.h>
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>
#include "ipfs/namesys/namesys.h"
#define IPFS_DNSLINK_C
#include "ipfs/dnslink/dnslink.h"
#include "ipfs/cid/cid.h"
#include "ipfs/path/path.h"
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int ipfs_dns (int argc, char **argv)
{
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int err, r=0, i;
char **txt, *path, *param;
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if (argc == 4 && strcmp ("-r", argv[2])==0) {
r = 1;
argc--; argv++;
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}
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if (argc != 3) {
fprintf (stderr, "usage: ipfs dns [-r] dns.name.com\n");
return -1;
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}
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param = malloc (strlen (argv[2]) + 1);
if (!param) {
fprintf (stderr, "memory allocation failed.\n");
return 1;
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}
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strcpy (param, argv[2]);
for (i = 0 ; i < DefaultDepthLimit ; i++) {
if (memcmp(param, "/ipns/", 6) == 0) {
err = ipfs_dnslink_resolv_lookupTXT (&txt, param+6);
} else {
err = ipfs_dnslink_resolv_lookupTXT (&txt, param);
}
if (err) {
fprintf (stderr, "dns lookupTXT: %s\n", Err[err]);
return err;
}
err = ipfs_dnslink_parse_txt(&path, *txt);
if (err) {
free (*txt);
free (txt);
fprintf (stderr, "dns parse_txt: %s\n", Err[err]);
return err;
}
free (*txt);
free (txt);
free (param);
if (! r) {
// not recursive.
break;
}
if (memcmp(path, "/ipfs/", 6) == 0) {
break;
}
param = path;
} while (--r);
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fprintf (stdout, "%s\n", path);
free (path);
return 0;
}
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// ipfs_dnslink_resolve resolves the dnslink at a particular domain. It will
// recursively keep resolving until reaching the defaultDepth of Resolver. If
// the depth is reached, ipfs_dnslink_resolve will return the last value
// retrieved, and ErrResolveLimit. If TXT records are found but are not valid
// dnslink records, ipfs_dnslink_resolve will return ErrInvalidDNSLink.
// ipfs_dnslink_resolve will check every TXT record returned. If resolution
// fails otherwise, ipfs_dnslink_resolve will return ErrResolveFailed
int ipfs_dnslink_resolve (char **p, char *domain)
{
return ipfs_dnslink_resolve_n (p, domain, DefaultDepthLimit);
}
// ipfs_dnslink_lookup_txt is a function that looks up a TXT record in some dns resolver.
// This is useful for testing or passing your own dns resolution process, which
// could take into account non-standard TLDs like .bit, .onion, .ipfs, etc.
int (*ipfs_dnslink_lookup_txt)(char ***txt, char *name) = NULL;
// ipfs_dnslink_resolve_n is just like Resolve, with the option to specify a
// maximum resolution depth.
int ipfs_dnslink_resolve_n (char **p, char *d, int depth)
{
int err, i, l;
char *rest, **link, tail[500], buf[500], domain[500];
char dns_prefix[] = "/dns/";
domain[sizeof(domain)-1] = '\0';
strncpy (domain, d, sizeof(domain) - 1);
for (i=0 ; i < depth ; i++) {
err = ipfs_dnslink_resolve_once (&link, domain);
if (err) {
return err;
}
// if does not have /dns/ as a prefix, done.
if (memcmp (*link, dns_prefix, sizeof(dns_prefix) - 1)!=0) {
l = strlen(*link) + strlen(tail);
*p = malloc(l + 1);
if (!*p) {
free(*link);
free(link);
return ErrAllocFailed;
}
*p[l] = '\0';
strncpy(*p, *link, l);
free(*link);
free(link);
strncat(*p, tail, l - strlen(*p));
return 0; // done
}
// keep resolving
err = ipfs_dnslink_parse_link_domain (&d, &rest, *link);
free (*link);
free (link);
if (err) {
*p = NULL;
return err;
}
strncpy (domain, d, sizeof(domain) - 1);
free (d);
strncpy (buf, tail, sizeof(buf) - 1);
strncpy (tail, rest, sizeof(tail) - 1);
strncat (tail, buf, sizeof(tail) - 1 - strlen(tail));
}
strncpy (buf, tail, sizeof(buf) - 1);
strncpy (tail, dns_prefix, sizeof(tail) - 1);
strncat (tail, domain, sizeof(tail) - 1 - strlen(tail));
strncat (tail, buf, sizeof(tail) - 1 - strlen(tail));
return ErrResolveLimit;
}
// lookup using libresolv -lresolv
int ipfs_dnslink_resolv_lookupTXT(char ***txt, char *domain)
{
char buf[4096], *p;
int responseLength;
int i, l, n = 0;
ns_msg query_parse_msg;
ns_rr query_parse_rr;
u_char responseByte[4096];
// Use res_query from libresolv to retrieve TXT record from DNS server.
if ((responseLength = res_query(domain,C_IN,T_TXT,responseByte,sizeof(responseByte))) < 0 ||
ns_initparse(responseByte,responseLength,&query_parse_msg) < 0) {
return ErrResolveFailed;
} else {
l = sizeof (buf);
buf[--l] = '\0';
p = buf;
// save every TXT record to buffer separating with a \0
for (i=0 ; i < ns_msg_count(query_parse_msg,ns_s_an) ; i++) {
if (ns_parserr(&query_parse_msg,ns_s_an,i,&query_parse_rr)) {
return ErrResolveFailed;
} else {
char *rdata = ns_rr_rdata(query_parse_rr);
memcpy(p, rdata+1, *rdata); // first byte is record length
p += *rdata; // update pointer
*p++ = '\0'; // mark end-of-record and update pointer to next record.
n++; // update record count
}
}
// allocate array for all records + NULL pointer terminator.
*txt = calloc(n+1, sizeof(void*));
if (!*txt) {
return ErrAllocFailed;
}
l = p - buf; // length of all records in buffer.
p = malloc(l); // allocate memory that will be used as string data at *txt array.
if (!p) {
free(*txt);
*txt = NULL;
return ErrAllocFailed;
}
memcpy(p, buf, l); // transfer from buffer to allocated memory.
for (i = 0 ; i < n ; i++) {
*txt[i] = p; // save position of current record at *txt array.
p = memchr(p, '\0', l - (p - *txt[0])) + 1; // find next record position after next \0
}
}
return 0;
}
// ipfs_dnslink_resolve_once implements resolver.
int ipfs_dnslink_resolve_once (char ***p, char *domain)
{
int err, i;
char **txt;
if (!p || !domain) {
return ErrInvalidParam;
}
*p = NULL;
if (!ipfs_isdomain_is_domain (domain)) {
return ErrInvalidDomain;
}
if (!ipfs_dnslink_lookup_txt) { // if not set
ipfs_dnslink_lookup_txt = ipfs_dnslink_resolv_lookupTXT; // use default libresolv
}
err = ipfs_dnslink_lookup_txt (&txt, domain);
if (err) {
return err;
}
err = ErrResolveFailed;
for (i=0 ; txt[i] ; i++) {
err = ipfs_dnslink_parse_txt(*p, txt[i]);
if (!err) {
break;
}
}
free(*txt);
free(txt);
return err;
}
// ipfs_dnslink_parse_txt parses a TXT record value for a dnslink value.
// The TXT record must follow the dnslink format:
// TXT dnslink=<path>
// TXT dnslink=/foo/bar/baz
// ipfs_dnslink_parse_txt will return ErrInvalidDNSLink if parsing fails.
int ipfs_dnslink_parse_txt (char **path, char *txt)
{
char **parts;
if (!path || !txt) {
return ErrInvalidParam;
}
parts = ipfs_path_split_n (txt, "=", 2);
if (!parts) {
return ErrAllocFailed;
}
if (ipfs_path_segments_length (parts) == 2 && strcmp(parts[0], "dnslink")==0 && memcmp(parts[1], "/", 1)==0) {
*path = ipfs_path_clean_path(parts[1]);
if (path) {
ipfs_path_free_segments (&parts);
return 0;
}
}
ipfs_path_free_segments (&parts);
*path = NULL;
return ErrInvalidDNSLink;
}
// ipfs_dnslink_parse_link_domain parses a domain from a dnslink path.
// The link path must follow the dnslink format:
// /dns/<domain>/<path>
// /dns/ipfs.io
// /dns/ipfs.io/blog/0-hello-worlds
// ipfs_dnslink_parse_link_domain will return ErrInvalidDNSLink if parsing
// fails, and ErrInvalidDomain if the domain is not valid.
int ipfs_dnslink_parse_link_domain (char **domain, char**rest, char *txt)
{
char **parts;
int parts_len;
if (!domain || !rest || !txt) {
return ErrInvalidParam;
}
*domain = *rest = NULL;
parts = ipfs_path_split_n (txt, "/", 4);
parts_len = ipfs_path_segments_length(parts);
if (!parts || parts_len < 3 || parts[0][0]!='\0' || strcmp(parts[1], "dns") != 0) {
return ErrInvalidDNSLink;
}
if (! ipfs_isdomain_is_domain (parts[2])) {
ipfs_path_free_segments (&parts);
return ErrInvalidDomain;
}
*domain = malloc(strlen (parts[2]) + 1);
if (!*domain) {
ipfs_path_free_segments (&parts);
return ErrAllocFailed;
}
strcpy(*domain, parts[2]);
if (parts_len > 3) {
*rest = malloc(strlen (parts[3]) + 1);
if (!*rest) {
ipfs_path_free_segments (&parts);
free (*domain);
*domain = NULL;
return ErrAllocFailed;
}
strcpy(*rest, parts[3]);
}
return 0;
}