Add support for Hidden AVPs and chap-response

[l2tpns.git] / l2tpns.c

diff --git a/l2tpns.c b/l2tpns.c
index eb7783b..c22918a 100644 (file)
--- a/l2tpns.c
+++ b/l2tpns.c
@@ -4,7 +4,7 @@
 // Copyright (c) 2002 FireBrick (Andrews & Arnold Ltd / Watchfront Ltd) - GPL licenced
 // vim: sw=8 ts=8
 
-char const *cvs_id_l2tpns = "$Id: l2tpns.c,v 1.25 2004/09/02 04:18:07 fred_nerk Exp $";
+char const *cvs_id_l2tpns = "$Id: l2tpns.c,v 1.26 2004/09/19 23:19:23 fred_nerk Exp $";
 
 #include <arpa/inet.h>
 #include <assert.h>
@@ -169,6 +169,7 @@ void dump_state();
 void tunnel_clean();
 tunnelidt new_tunnel();
 void update_config();
+int unhide_avp(u8 *avp, tunnelidt t, sessionidt s, u16 length);
 
 static void cache_ipmap(ipt ip, int s);
 static void uncache_ipmap(ipt ip);
@@ -1452,6 +1453,13 @@ void processudp(u8 * buf, int len, struct sockaddr_in *addr)
                                                continue;
                                        }
                                        log(4, ntohl(addr->sin_addr.s_addr), s, t, "Hidden AVP\n");
+                                       // Unhide the AVP
+                                       n = unhide_avp(b, t, s, n);
+                                       if (n == 0)
+                                       {
+                                               fatal = flags;
+                                               continue;
+                                       }
                                }
                                if (*b & 0x3C)
                                {
@@ -1566,6 +1574,11 @@ void processudp(u8 * buf, int len, struct sockaddr_in *addr)
                                                build_chap_response(b, 2, n, &chapresponse);
                                        }
                                        break;
+                               case 13:    // Response
+                                       // Why did they send a response? We never challenge.
+                                       log(2, ntohl(addr->sin_addr.s_addr), s, t, "   received unexpected challenge response\n");
+                               break;
+
                                case 14:    // assigned session
                                        asession = session[s].far = ntohs(*(u16 *) (b));
                                        log(4, ntohl(addr->sin_addr.s_addr), s, t, "   assigned session = %d\n", asession);
@@ -4013,3 +4026,77 @@ int cmd_show_hist_open(struct cli_def *cli, char *command, char **argv, int argc
        cli_print(cli, "%d total sessions open.", count);
        return CLI_OK;
 }
+
+/* Unhide an avp.
+ *
+ * This unencodes the AVP using the L2TP CHAP secret and the
+ * previously stored random vector. It replaces the hidden data with
+ * the cleartext data and returns the length of the cleartext data
+ * (including the AVP "header" of 6 bytes).
+ *
+ * Based on code from rp-l2tpd by Roaring Penguin Software Inc.
+ */
+int unhide_avp(u8 *avp, tunnelidt t, sessionidt s, u16 length)
+{
+       MD5_CTX ctx;
+       u8 *cursor;
+       u8 digest[16];
+       u8 working_vector[16];
+       uint16_t hidden_length;
+       u8 type[2];
+       size_t done, todo;
+       u8 *output;
+
+       // Find the AVP type.
+       type[0] = *(avp + 4);
+       type[1] = *(avp + 5);
+
+       // Line up with the hidden data
+       cursor = output = avp + 6;
+
+       // Compute initial pad
+       MD5Init(&ctx);
+       MD5Update(&ctx, type, 2);
+       MD5Update(&ctx, config->l2tpsecret, strlen(config->l2tpsecret));
+       MD5Update(&ctx, session[s].random_vector, session[s].random_vector_length);
+       MD5Final(digest, &ctx);
+
+       // Get hidden length
+       hidden_length = ((uint16_t) (digest[0] ^ cursor[0])) * 256 + (uint16_t) (digest[1] ^ cursor[1]);
+
+       // Keep these for later use
+       working_vector[0] = *cursor;
+       working_vector[1] = *(cursor + 1);
+       cursor += 2;
+
+       if (hidden_length > length - 8)
+       {
+               log(1, 0, s, t, "Hidden length %d too long in AVP of length %d\n", (int) hidden_length, (int) length);
+               return 0;
+       }
+
+       /* Decrypt remainder */
+       done = 2;
+       todo = hidden_length;
+       while (todo)
+       {
+               working_vector[done] = *cursor;
+               *output = digest[done] ^ *cursor;
+               ++output;
+               ++cursor;
+               --todo;
+               ++done;
+               if (done == 16 && todo)
+               {
+                       // Compute new digest
+                       done = 0;
+                       MD5Init(&ctx);
+                       MD5Update(&ctx, config->l2tpsecret, strlen(config->l2tpsecret));
+                       MD5Update(&ctx, &working_vector, 16);
+                       MD5Final(digest, &ctx);
+               }
+       }
+
+       return hidden_length + 6;
+}
+