LookupServer+LibC: Add support for reverse DNS lookups via gethostbyaddr().

LookupServer can now take two types of requests:

* L: Lookup
* R: Reverse lookup

The /bin/host program now does a reverse lookup if the input string is a
valid IPv4 address. :^)

1 files changed, 19 insertions, 1 deletions

diff --git a/Userland/host.cpp b/Userland/host.cpp
index 179e5a649e..52e307ad9b 100644
--- a/Userland/host.cpp
+++ b/Userland/host.cpp
@@ -1,6 +1,7 @@
 #include <netdb.h>
 #include <arpa/inet.h>
 #include <netinet/in.h>
+#include <string.h>
 #include <stdio.h>
 
 int main(int argc, char** argv)
@@ -10,9 +11,26 @@ int main(int argc, char** argv)
         return 0;
     }
 
+    // If input looks like an IPv4 address, we should do a reverse lookup.
+    struct sockaddr_in addr;
+    memset(&addr, 0, sizeof(addr));
+    addr.sin_family = AF_INET;
+    addr.sin_port = htons(53);
+    int rc = inet_pton(AF_INET, argv[1], &addr.sin_addr);
+    if (rc == 1) {
+        // Okay, let's do a reverse lookup.
+        auto* hostent = gethostbyaddr(&addr.sin_addr, sizeof(in_addr), AF_INET);
+        if (!hostent) {
+            fprintf(stderr, "Reverse lookup failed for '%s'\n", argv[1]);
+            return 1;
+        }
+        printf("%s is %s\n", argv[1], hostent->h_name);
+        return 0;
+    }
+
     auto* hostent = gethostbyname(argv[1]);
     if (!hostent) {
-        printf("Lookup failed for '%s'\n", argv[1]);
+        fprintf(stderr, "Lookup failed for '%s'\n", argv[1]);
         return 1;
     }