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/* Copyright (C) 2020 C. McEnroe <june@causal.agency>
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <https://www.gnu.org/licenses/>.
*
* Additional permission under GNU GPL version 3 section 7:
*
* If you modify this Program, or any covered work, by linking or
* combining it with OpenSSL (or a modified version of that library),
* containing parts covered by the terms of the OpenSSL License and the
* original SSLeay license, the licensors of this Program grant you
* additional permission to convey the resulting work. Corresponding
* Source for a non-source form of such a combination shall include the
* source code for the parts of OpenSSL used as well as that of the
* covered work.
*/
#include <netdb.h>
#include <netinet/in.h>
#include <resolv.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#ifndef EAI_BADHINTS
#define EAI_BADHINTS EAI_BADFLAGS
#endif
#ifndef EAI_PROTOCOL
#define EAI_PROTOCOL EAI_BADFLAGS
#endif
static uint16_t u16(uint8_t **ptr) {
uint16_t x = *(*ptr)++;
x = x << 8 | *(*ptr)++;
return x;
}
static void nameSkip(uint8_t **ptr) {
for (uint8_t len; (len = *(*ptr)++); *ptr += len) {
if (len & 0xC0) {
(*ptr)++;
break;
}
}
}
static char *nameString(uint8_t **ptr) {
char *name = (char *)(*ptr + 1);
for (uint8_t len; (len = **ptr); *ptr += len) {
*(*ptr)++ = '.';
}
return name;
}
/* A wrapper around getaddrinfo(3) which first performs SRV record (RFC 2782)
* lookup. hints must be provided and hints->ai_protocol must be set. SRV
* lookup is skipped if servname is numerical. If SRV lookup is successful, the
* ai_canonname field of the first addrinfo structure returned is set to the
* target name.
*/
int getservinfo(
const char *hostname, const char *servname,
const struct addrinfo *hints, struct addrinfo **res
) {
if (!hints) return EAI_BADHINTS;
if (!hints->ai_protocol) return EAI_PROTOCOL;
if (hints->ai_flags & AI_NUMERICHOST) return EAI_BADFLAGS;
if (hints->ai_flags & AI_NUMERICSERV) return EAI_BADFLAGS;
char *rest;
strtoul(servname, &rest, 10);
if (!*rest) return getaddrinfo(hostname, servname, hints, res);
struct protoent *proto = getprotobynumber(hints->ai_protocol);
if (!proto) return EAI_PROTOCOL;
char dname[256];
int len = snprintf(
dname, sizeof(dname), "_%s._%s.%s",
servname, proto->p_name, hostname
);
if ((size_t)len >= sizeof(dname)) return EAI_OVERFLOW;
uint8_t msg[512];
len = res_query(dname, 1 /* IN */, 33 /* SRV */, msg, sizeof(msg));
if (len < 0) return getaddrinfo(hostname, servname, hints, res);
uint8_t *ptr = msg;
u16(&ptr); // ID
uint16_t rcode = u16(&ptr) & 0x000F;
uint16_t qdcount = u16(&ptr);
uint16_t ancount = u16(&ptr);
u16(&ptr); // NSCOUNT
u16(&ptr); // ARCOUNT
if (rcode || !ancount) return getaddrinfo(hostname, servname, hints, res);
for (uint16_t q = 0; q < qdcount; ++q) {
nameSkip(&ptr); // QNAME
u16(&ptr); // QTYPE
u16(&ptr); // QCLASS
}
// Read only one answer, ignoring Priority and Weight. Does anyone actually
// use them?
nameSkip(&ptr); // NAME
u16(&ptr); // TYPE
u16(&ptr); // CLASS
u16(&ptr); // TTL
u16(&ptr); // TTL
u16(&ptr); // RDLENGTH
u16(&ptr); // Priority
u16(&ptr); // Weight
uint16_t port = u16(&ptr);
hostname = nameString(&ptr);
if (!hostname[0]) return EAI_NONAME;
char myServ[sizeof("65535")];
snprintf(myServ, sizeof(myServ), "%hu", port);
struct addrinfo myHints = *hints;
#ifdef AI_DEFAULT
if (!myHints.ai_flags) myHints.ai_flags = AI_DEFAULT;
#endif
myHints.ai_flags |= AI_NUMERICSERV;
myHints.ai_flags &= ~AI_CANONNAME;
int error = getaddrinfo(hostname, myServ, &myHints, res);
if (error) return error;
(*res)->ai_canonname = strdup(hostname);
if (!(*res)->ai_canonname) {
freeaddrinfo(*res);
return EAI_MEMORY;
}
return 0;
}
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