Fixes broken cryptographic authentication in OSPF

Cryptographic authentication in OSPF is defective by
design - there might be several packets independently
sent to the network (for example HELLO, LSUPD and LSACK)
where they might be reordered and that causes crypt.
sequence number error.

That can be workarounded by not incresing sequence number
too often. Now we update it only when last packet was sent
before at least one second. This can constitute a risk of
replay attacks, but RFC supposes something similar (like time
in seconds used as CSN).

1 files changed, 22 insertions, 8 deletions

diff --git a/proto/ospf/packet.c b/proto/ospf/packet.c
index c6b233f8..23785fe8 100644
--- a/proto/ospf/packet.c
+++ b/proto/ospf/packet.c
@@ -75,13 +75,25 @@ ospf_pkt_finalize(struct ospf_iface *ifa, struct ospf_packet *pkt)
 
       pkt->checksum = 0;
 
+      /* Perhaps use random value to prevent replay attacks after
+	 reboot when system does not have independent RTC? */
       if (!ifa->csn)
-        ifa->csn = (u32) time(NULL);
+	{
+	  ifa->csn = (u32) now;
+	  ifa->csn_use = now;
+	}
+
+      /* We must have sufficient delay between sending a packet and increasing 
+	 CSN to prevent reordering of packets (in a network) with different CSNs */
+      if ((now - ifa->csn_use) > 1)
+	ifa->csn++;
+
+      ifa->csn_use = now;
 
       pkt->u.md5.keyid = passwd->id;
       pkt->u.md5.len = OSPF_AUTH_CRYPT_SIZE;
       pkt->u.md5.zero = 0;
-      pkt->u.md5.csn = htonl(ifa->csn++);
+      pkt->u.md5.csn = htonl(ifa->csn);
       tail = ((void *)pkt) + ntohs(pkt->length);
       MD5Init(&ctxt);
       MD5Update(&ctxt, (char *) pkt, ntohs(pkt->length));
@@ -184,12 +196,14 @@ ospf_pkt_checkauth(struct ospf_neighbor *n, struct ospf_iface *ifa, struct ospf_
 
       if (n)
       {
-        if(ntohs(pkt->u.md5.csn) < n->csn)
-        {
-          OSPF_TRACE(D_PACKETS, "OSPF_auth: lower sequence number");
-          return 0;
-        }
-        n->csn = ntohs(pkt->u.md5.csn);
+	u32 rcv_csn = ntohl(pkt->u.md5.csn);
+	if(rcv_csn < n->csn)
+	{
+	  OSPF_TRACE(D_PACKETS, "OSPF_auth: lower sequence number (rcv %d, old %d)", rcv_csn, n->csn);
+	  return 0;
+	}
+
+	n->csn = rcv_csn;
       }
 
       MD5Init(&ctxt);