Reduce the interface MTU by 2 when MPPE has been successfully negotiated.

This is necessary because MPPE will combine the protocol id with the
payload received on the tun interface, encrypt it, then prepend its
own protocol id, effectively increasing the payload by two bytes.

1 files changed, 31 insertions, 1 deletions

diff --git a/usr.sbin/ppp/ccp.c b/usr.sbin/ppp/ccp.c
index e8f0a41..b3a80f8 100644
--- a/usr.sbin/ppp/ccp.c
+++ b/usr.sbin/ppp/ccp.c
@@ -299,6 +299,19 @@ ccp_Required(struct ccp *ccp)
   return 0;
 }
 
+/*
+ * Report whether it's possible to increase a packet's size after
+ * compression (and by how much).
+ */
+int
+ccp_MTUOverhead(struct ccp *ccp)
+{
+  if (ccp->fsm.state == ST_OPENED)
+    return algorithm[ccp->out.algorithm]->o.MTUOverhead;
+
+  return 0;
+}
+
 static void
 CcpInitRestartCounter(struct fsm *fp, int what)
 {
@@ -474,7 +487,24 @@ CcpLayerUp(struct fsm *fp)
   /* We're now up */
   struct ccp *ccp = fsm2ccp(fp);
   struct ccp_opt **o;
-  int f;
+  int f, fail;
+
+  for (f = fail = 0; f < NALGORITHMS; f++)
+    if (IsEnabled(ccp->cfg.neg[algorithm[f]->Neg]) &&
+        (*algorithm[f]->Required)(&ccp->fsm) &&
+        (ccp->in.algorithm != f || ccp->out.algorithm != f)) {
+      /* Blow it all away - we haven't negotiated a required algorithm */
+      log_Printf(LogWARN, "%s: Failed to negotiate (required) %s\n",
+                 fp->link->name, protoname(algorithm[f]->id));
+      fail = 1;
+    }
+
+  if (fail) {
+    ccp->his_proto = ccp->my_proto = -1;
+    fsm_Close(fp);
+    fsm_Close(&fp->link->lcp.fsm);
+    return 0;
+  }
 
   log_Printf(LogCCP, "%s: LayerUp.\n", fp->link->name);