diff --git a/lib/irc/msg.ml b/lib/irc/msg.ml
index fd029eb..4957005 100644
--- a/lib/irc/msg.ml
+++ b/lib/irc/msg.ml
@@ -3,7 +3,7 @@ open Types
 type prefix =
   | No_prefix
   | Server_prefix of string
-  | User_prefix of nick * userinfo option * string option
+  | User_prefix of name * userinfo option * string option
 
 let pp_userinfo_opt ppf = function
   | None -> ()
diff --git a/lib/irc/msg.mli b/lib/irc/msg.mli
index 40c8390..f4ead93 100644
--- a/lib/irc/msg.mli
+++ b/lib/irc/msg.mli
@@ -3,7 +3,7 @@ open Types
 type prefix =
   | No_prefix
   | Server_prefix of string
-  | User_prefix of nick * userinfo option * string option
+  | User_prefix of name * userinfo option * string option
 
 val prefix_string : prefix -> string
 
diff --git a/lib/irc/types.ml b/lib/irc/types.ml
index 930f3c7..297ee95 100644
--- a/lib/irc/types.ml
+++ b/lib/irc/types.ml
@@ -1,2 +1,17 @@
-type nick = string
+type name = string
 type userinfo = { username : string; realname : string }
+
+let name_type s =
+  let rec valid i =
+    if i >= String.length s then true
+    else
+      match s.[i] with
+      | ' ' | '@' | '+' | ',' -> false
+      | _ -> valid (i + 1)
+  in
+  if s = "" then `invalid
+  else
+    match s.[0] with
+    | ':' -> `invalid
+    | '#' -> if valid 1 then `chan else `invalid
+    | _ -> if valid 0 then `nick else `invalid
diff --git a/lib/server/connection.ml b/lib/server/connection.ml
index dc7f050..a3dcc0f 100644
--- a/lib/server/connection.ml
+++ b/lib/server/connection.ml
@@ -51,11 +51,14 @@ let attempt_to_register t =
   | _, _ -> `ok
 
 let on_msg_nick t nick =
-  (* TODO: validate nick *)
-  if User.is_registered t.user then
+  if Irc.name_type nick <> `nick then
+    (if nick = "" then `nonicknamegiven else `erroneusnickname nick)
+  else if User.is_registered t.user then
     match User.set_nick t.user nick with
     | `nick_in_use -> `nicknameinuse nick
-    | `nick_set -> `ok
+    | `nick_set ->
+      ((* TODO: relay NICK message *));
+      `ok
   else begin
     t.pending_nick <- Some nick;
     attempt_to_register t
@@ -141,14 +144,16 @@ let rpl_names t chan_prefix chan users =
     rpl t "366" [chan; "End of NAMES list"]
   end
 
-let err_alreadyregistered t = rpl t "462" ["Unauthorized command (already registered)"]
-let err_needmoreparams t cmd = rpl t "461" [cmd; "Not enough parameters"]
-let err_nicknameinuse t nick = rpl t "433" [nick; "Nickname is already in use"]
-let err_norecipient t cmd = rpl t "411" [Fmt.str "No recipient given (%s)" cmd]
 let err_nosuchnick t tgt = rpl t "401" [tgt; "No such nick/channel"]
+let err_norecipient t cmd = rpl t "411" [Fmt.str "No recipient given (%s)" cmd]
 let err_notexttosend t = rpl t "412" ["No text to send"]
-let err_notregistered t = rpl t "451" ["You have not registered"]
 let err_unknowncommand t cmd = rpl t "421" [cmd; "Unknown command"]
+let err_nonicknamegiven t = rpl t "431" ["No nickname given"]
+let err_erroneousnickname t nick = rpl t "432" [nick; "Erroneus nickname"]
+let err_nicknameinuse t nick = rpl t "433" [nick; "Nickname is already in use"]
+let err_notregistered t = rpl t "451" ["You have not registered"]
+let err_needmoreparams t cmd = rpl t "461" [cmd; "Not enough parameters"]
+let err_alreadyregistered t = rpl t "462" ["Unauthorized command (already registered)"]
 
 (* message parsing *)
 
@@ -158,6 +163,7 @@ let on_msg t (msg : Irc.Msg.t) : unit =
     match msg.command, msg.params with
     | "NICK", new_nick :: _ ->
       on_msg_nick t new_nick
+    | "NICK", [] -> `nonicknamegiven
     | "USER", username :: modestr :: _host :: realname :: _ ->
       on_msg_user t username modestr realname
     | "QUIT", why ->
@@ -168,7 +174,7 @@ let on_msg t (msg : Irc.Msg.t) : unit =
     | "PRIVMSG", [_] -> `notexttosend | "PRIVMSG", [] -> `norecipient
     | "JOIN", tgt :: _ ->
       on_msg_join t tgt
-    | "NICK", _ | "USER", _ | "JOIN", _ -> `needmoreparams
+    | "USER", _ | "JOIN", _ -> `needmoreparams
     | _, _ -> `unknowncommand
   in
   match result with
@@ -186,3 +192,5 @@ let on_msg t (msg : Irc.Msg.t) : unit =
   | `notexttosend -> err_notexttosend t
   | `notregistered -> err_notregistered t
   | `unknowncommand -> err_unknowncommand t msg.command
+  | `nonicknamegiven -> err_nonicknamegiven t
+  | `erroneusnickname n -> err_erroneousnickname t n
diff --git a/lib/server/router.ml b/lib/server/router.ml
index aa5c51c..8cad975 100644
--- a/lib/server/router.ml
+++ b/lib/server/router.ml
@@ -10,7 +10,7 @@ and user = {
   hostname : string;
   outbox : Outbox.t;
   mutable key : string_ci;
-  mutable nick : Irc.nick;
+  mutable nick : Irc.name;
   mutable userinfo : Irc.userinfo option;
   mutable mode : Irc.Mode.t;
 }