getaddrinfo dns lookup: use larger answer buffer to handle long CNAMEs

the size of 512 is not sufficient to get at least one address in the
worst case where the name is at or near max length and resolves to a
CNAME at or near max length. prior to tcp fallback, there was nothing
we could do about this case anyway, but now it's fixable.

the new limit 768 is chosen so as to admit roughly the number of
addresses with a worst-case CNAME as could fit for a worst-case name
that's not a CNAME in the old 512-byte limit. outside of this
worst-case, the number of addresses that might be obtained is
increased.

MAXADDRS (48) was originally chosen as an upper bound on the combined
number of A and AAAA records that could fit in 512-byte packets (31
and 17, respectively). it is not increased at this time.

so as to prevent a situation where the A records consume almost all of
these slots (at 768 bytes, a "best-case" name can fit almost 47 A
records), the order of parsing is swapped to process AAAA first. this
ensures roughly half of the slots are available to each address
family.

src/network/lookup_name.c

@@ -108,6 +108,8 @@ struct dpc_ctx {
 #define RR_CNAME 5
 #define RR_AAAA 28
 
+#define ABUF_SIZE 768
+
 static int dns_parse_callback(void *c, int rr, const void *data, int len, const void *packet)
 {
 	char tmp[256];
@@ -127,7 +129,7 @@ static int dns_parse_callback(void *c, int rr, const void *data, int len, const
 		memcpy(ctx->addrs[ctx->cnt++].addr, data, 16);
 		break;
 	case RR_CNAME:
-		if (__dn_expand(packet, (const unsigned char *)packet + 512,
+		if (__dn_expand(packet, (const unsigned char *)packet + ABUF_SIZE,
 		    data, tmp, sizeof tmp) > 0 && is_valid_hostname(tmp))
 			strcpy(ctx->canon, tmp);
 		break;
@@ -137,7 +139,7 @@ static int dns_parse_callback(void *c, int rr, const void *data, int len, const
 
 static int name_from_dns(struct address buf[static MAXADDRS], char canon[static 256], const char *name, int family, const struct resolvconf *conf)
 {
-	unsigned char qbuf[2][280], abuf[2][512];
+	unsigned char qbuf[2][280], abuf[2][ABUF_SIZE];
 	const unsigned char *qp[2] = { qbuf[0], qbuf[1] };
 	unsigned char *ap[2] = { abuf[0], abuf[1] };
 	int qlens[2], alens[2];
@@ -171,7 +173,7 @@ static int name_from_dns(struct address buf[static MAXADDRS], char canon[static
 		if ((abuf[i][3] & 15) != 0) return EAI_FAIL;
 	}
 
-	for (i=0; i<nq; i++)
+	for (i=nq-1; i>=0; i--)
 		__dns_parse(abuf[i], alens[i], dns_parse_callback, &ctx);
 
 	if (ctx.cnt) return ctx.cnt;