[lwip-devel] [task #7017] Implement DNS client

[Jonathan Larmour]

Follow-up Comment #82, task #7017 (project lwip):

Strictly it's incorrect: the name localhost is not special, and the
configured DNS server should map it to 127.0.0.1 anyway. In practice it makes
sense though :).

But once we consider this, it doesn't take much to take one little extra step
to provide a general hard-coded hosts lookup functionality, configured by the
user. For example in the following, the user can enable LWIP_HOSTS_LOOKUP in
lwipopts.h, and then define their own host mappings, e.g.:

#define LWIP_HOSTS_MAP 
  { "server1", htonl((192<<24)|(168<<16)|(1<<8)|(1<<0)) }, 
  { "server2", htonl((192<<24)|(168<<16)|(1<<8)|(2<<0)) }

And here's the code for dns.c:

#if LWIP_HOSTS_LOOKUP
struct hosts_map_s {
  const char *name;
  u32_t addr;
};

struct hosts_map_s hosts_map[] = {
#if LWIP_HAVE_LOOPIF 
   { "localhost", INADDR_LOOPBACK }
#endif
#ifdef LWIP_HOSTS_MAP
   ,
   LWIP_HOSTS_MAP
#endif
};

In dns_gethostbyname:

#if LWIP_HOSTS_LOOKUP
{
  u16_t i;
  for (i=0;i<(sizeof(hosts_map)/sizeof(hosts_map[0]));i++) {
    if (!strcmp(hostname, host_maps[i].name)) {
      addr->addr=hosts_maps[i].addr;
      return DNS_COMPLETE;
    }
  }
}
#endif


It's just a thought, take it or leave it.


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