Hi Musl team,
Recently we encountered an issue in requesting name resolution in Alpine Linux with Musl.
Environment:
* Alpine Linux 3.14.2
* IPv6 module disabled
* musl-libc 1.2.2
* CoreDNS with "rewrite stop type AAAA A" configured
Reproduce steps:
1. getent hosts <any-name-only-has-ipv4-record>
2. Check output
Expected: Get correct IPv4 address, e.g. 10.96.36.74
Actual: Return an invalid IPv6 address, e.g. a60:244a::, which is the actual IPv4 32-bit address followed by zeros.
Debugging:
2. With requested family AF_INET6, gethostbyname2_r -> __lookup_name -> name_from_dns_search sends a DNS request with RR_AAAA to CoreDNS.
3. In CoreDNS, we configured "rewrite stop type AAAA A", which results in the RR_AAAA request type being rewritten to RR_A. So the response from CoreDNS is: AF_INET (2) and 32-bit IP 10.96.36.74 (0x0a60244a below).
(gdb) p addrs
$55 = {{family = 2, scopeid = 0, addr = "\n`$J", '\000' <repeats 11 times>, sortkey = 0}
(gdb) x/4xb addrs[0]->addr
0x7fffffffe0d8: 0x0a 0x60 0x24 0x4a
4. In gethostbyname2_r, the result address family is assigned to the request's family, regardless of the family in the response.
h->h_addrtype = af;
h->h_length = af==AF_INET6 ? 16 : 4;
This results in the IPv4 address replied from CoreDNS is wrongly copied to the result as IPv6 address which is a60:244a:: .
5. getent gets an IPv4 address, so it won't try AF_INET, rather directly return the invalid IPv6 address a60:244a:: .
Suggested fix:
In gethostbyname2_r when adding answered addresses into the result, we need to filter out the addresses with mismatching address family.
for (i=0; i<cnt; i++) {
// if (addrs[i].family != h->h_addrtype) continue; // need to fix the index of h->h_addr_list too
h->h_addr_list[i] = (void *)buf;
buf += h->h_length;
memcpy(h->h_addr_list[i], addrs[i].addr, h->h_length);
}
Could you review this issue? Thanks in advance!
Best regards!
Tom Shen