int dns_name4(stralloc* out,const char* ip);
DESCRIPTIONdns_name4 looks up the domain name for the 4-byte IP address in ip. It puts the (first) domain name into out and returns 0. If the relevant in-addr.arpa domain does not exist in DNS, or has no PTR records, out will be empty.
If dns_name4 has trouble with the DNS lookup or runs out of memory, it returns -1, setting errno appropriately. It may or may not change out.