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. :^)

LibC/netdb.cpp

@@ -15,6 +15,39 @@ static char __gethostbyname_name_buffer[512];
 static in_addr_t __gethostbyname_address;
 static in_addr_t* __gethostbyname_address_list_buffer[2];
 
+static hostent __gethostbyaddr_buffer;
+static char __gethostbyaddr_name_buffer[512];
+static in_addr_t* __gethostbyaddr_address_list_buffer[2];
+
+static int connect_to_lookup_server()
+{
+    int fd = socket(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0);
+    if (fd < 0) {
+        perror("socket");
+        return -1;
+    }
+
+    sockaddr_un address;
+    address.sun_family = AF_LOCAL;
+    strcpy(address.sun_path, "/tmp/.LookupServer-socket");
+
+    int retries = 3;
+    int rc = 0;
+    while (retries) {
+        rc = connect(fd, (const sockaddr*)&address, sizeof(address));
+        if (rc == 0)
+            break;
+        --retries;
+        sleep(1);
+    }
+
+    if (rc < 0) {
+        close(fd);
+        return -1;
+    }
+    return fd;
+}
+
 hostent* gethostbyname(const char* name)
 {
     unsigned a;
@@ -36,35 +69,14 @@ hostent* gethostbyname(const char* name)
         return &__gethostbyname_buffer;
     }
 
-    int fd = socket(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0);
-    if (fd < 0) {
-        perror("socket");
+    int fd = connect_to_lookup_server();
+    if (fd < 0)
         return nullptr;
-    }
 
-    sockaddr_un address;
-    address.sun_family = AF_LOCAL;
-    strcpy(address.sun_path, "/tmp/.LookupServer-socket");
-
-    int retries = 3;
-    int rc = 0;
-    while (retries) {
-        rc = connect(fd, (const sockaddr*)&address, sizeof(address));
-        if (rc == 0)
-            break;
-        --retries;
-        sleep(1);
-    }
-
-    if (rc < 0) {
-        close(fd);
-        return nullptr;
-    }
-
-    auto line = String::format("%s\n", name);
+    auto line = String::format("L%s\n", name);
     int nsent = write(fd, line.characters(), line.length());
     if (nsent < 0) {
-        perror("send");
+        perror("write");
         close(fd);
         return nullptr;
     }
@@ -84,8 +96,8 @@ hostent* gethostbyname(const char* name)
     if (!memcmp(buffer, "Not found.", sizeof("Not found.") - 1))
         return nullptr;
 
-    rc = inet_pton(AF_INET, buffer, &__gethostbyname_address);
-    if (rc < 0)
+    int rc = inet_pton(AF_INET, buffer, &__gethostbyname_address);
+    if (rc <= 0)
         return nullptr;
 
     strncpy(__gethostbyname_name_buffer, name, strlen(name));
@@ -101,4 +113,67 @@ hostent* gethostbyname(const char* name)
     return &__gethostbyname_buffer;
 }
 
+hostent* gethostbyaddr(const void* addr, socklen_t addr_size, int type)
+{
+    if (type != AF_INET) {
+        errno = EAFNOSUPPORT;
+        return nullptr;
+    }
+
+    if (addr_size < sizeof(in_addr)) {
+        errno = EINVAL;
+        return nullptr;
+    }
+
+    int fd = connect_to_lookup_server();
+    if (fd < 0)
+        return nullptr;
+
+    IPv4Address ipv4_address((const byte*)&((const in_addr*)addr)->s_addr);
+
+    auto line = String::format("R%d.%d.%d.%d.in-addr.arpa\n",
+        ipv4_address[3],
+        ipv4_address[2],
+        ipv4_address[1],
+        ipv4_address[0]
+    );
+    int nsent = write(fd, line.characters(), line.length());
+    if (nsent < 0) {
+        perror("write");
+        close(fd);
+        return nullptr;
+    }
+
+    ASSERT(nsent == line.length());
+
+    char buffer[1024];
+    int nrecv = read(fd, buffer, sizeof(buffer) - 1);
+    if (nrecv < 0) {
+        perror("recv");
+        close(fd);
+        return nullptr;
+    }
+    if (nrecv > 1) {
+        // Strip newline.
+        buffer[nrecv - 1] = '\0';
+    }
+    buffer[nrecv] = '\0';
+    close(fd);
+
+    if (!memcmp(buffer, "Not found.", sizeof("Not found.") - 1))
+        return nullptr;
+
+    strncpy(__gethostbyaddr_name_buffer, buffer, max(sizeof(__gethostbyaddr_name_buffer), (size_t)nrecv));
+
+    __gethostbyaddr_buffer.h_name = __gethostbyaddr_name_buffer;
+    __gethostbyaddr_buffer.h_aliases = nullptr;
+    __gethostbyaddr_buffer.h_addrtype = AF_INET;
+    // FIXME: Should we populate the hostent's address list here with a sockaddr_in for the provided host?
+    __gethostbyaddr_address_list_buffer[0] = nullptr;
+    __gethostbyaddr_buffer.h_addr_list = (char**)__gethostbyaddr_address_list_buffer;
+    __gethostbyaddr_buffer.h_length = 4;
+
+    return &__gethostbyaddr_buffer;
+}
+
 }

LibC/netdb.h

@@ -15,6 +15,7 @@ struct hostent {
 };
 
 struct hostent* gethostbyname(const char*);
+struct hostent* gethostbyaddr(const void* addr, socklen_t len, int type);
 
 struct servent {
     char* s_name;