helpers: add verification of response code

src/helpers.zig

@@ -215,6 +215,10 @@ const AddressList = struct {
     }
 };
 
+/// A very simple getAddressList that sets up the DNS connection and extracts
+/// the A records.
+///
+/// This function does not implement the "happy eyeballs" algorithm.
 pub fn getAddressList(incoming_name: []const u8, allocator: std.mem.Allocator) !AddressList {
     var name_buffer: [128][]const u8 = undefined;
     const name = try dns.Name.fromString(incoming_name, &name_buffer);
@@ -250,8 +254,17 @@ pub fn getAddressList(incoming_name: []const u8, allocator: std.mem.Allocator) !
 
     const reply_packet = reply.packet;
 
-    std.debug.assert(packet.header.id == reply_packet.header.id);
-    std.debug.assert(reply_packet.header.is_response);
+    if (packet.header.id != reply_packet.header.id) return error.InvalidReply;
+    if (!reply_packet.header.is_response) return error.InvalidResponse;
+
+    switch (reply_packet.header.response_code) {
+        .NoError => {},
+        .FormatError => return error.ServerFormatError, // bug in implementation caught by server?
+        .ServerFailure => return error.ServerFailure,
+        .NameError => return error.ServerNameError,
+        .NotImplemented => return error.ServerNotImplemented,
+        .Refused => return error.ServerRefused,
+    }
 
     var list = std.ArrayList(std.net.Address).init(allocator);
     defer list.deinit();

src/lib.zig

@@ -3,6 +3,8 @@ pub const ResourceClass = @import("enums.zig").ResourceClass;
 pub const Name = @import("name.zig").Name;
 const pkt = @import("packet.zig");
 pub const Packet = pkt.Packet;
+pub const ResponseCode = pkt.ResponseCode;
+pub const OpCode = pkt.OpCode;
 pub const IncomingPacket = pkt.IncomingPacket;
 pub const Question = pkt.Question;
 pub const Resource = pkt.Resource;

src/packet.zig

@@ -9,14 +9,32 @@ const logger = std.log.scoped(.dns_packet);
 
 pub const ResponseCode = enum(u4) {
     NoError = 0,
+
+    /// Format error - The name server was unable to interpret the query.
     FormatError = 1,
+
+    /// Server failure - The name server was unable to process this query
+    /// due to a problem with the name server.
     ServerFailure = 2,
+
+    /// Name Error - Meaningful only for responses from an authoritative name
+    /// server, this code signifies that the domain name referenced in
+    /// the query does not exist.
     NameError = 3,
+
+    /// Not Implemented - The name server does not support the requested
+    /// kind of query.
     NotImplemented = 4,
+
+    /// Refused - The name server refuses to perform the specified
+    /// operation for policy reasons.  For example, a name server may not
+    /// wish to provide the information to the particular requester,
+    /// or a name server may not wish to perform a particular operation
+    /// (e.g., zone transfer) for particular data.
     Refused = 5,
 };
 
-const OpCode = enum(u4) {
+pub const OpCode = enum(u4) {
     Query = 0,
     InverseQuery = 1,
     ServerStatusRequest = 2,