OSPFv3 authentication

Most IPv4 routing protocols support some form of neighbor authentication, provided by either a plaintext password or MD5 HMAC. However, OSPFv3 (OSPF for IPv6) doesn't include any authentication capabilities of its own; instead, it relies entirely on IPsec to secure communications between neighbors. This is beneficial in simplifying the OSPFv3 protocol and standardizing its authentication mechanism.

Continuing from our OSPFv3 configuration lab, we can identify several points in the network where adding authentication would be prudent. In the real world, of course, it's considered best practice to implement authentication for all adjacencies, regardless of the apparent need.

First, because end users may be present in the 2001:db8:0:12::/64 subnet connecting R1 and R2, adjacency authentication in this subnet is critical. Second, since a diligent network engineer never trusts his service provider, we'll also implement authentication across the frame relay network. The IPsec Authentication Header (AH) protocol will be used to preserve packet integrity. Unfortunately, IOS doesn't currently support IPsec payload encryption via Encapsulating Security Payload (ESP). ESP can be configured with null encryption, but this has the same affect as configuring AH. (For reference, the OSPFv3 IPsec configuration guide for IOS can be found here.)

Enabling Authentication on the Interface

Currently, IOS requires IPsec for OSPFv3 be configured separate from the normal configuration of an IPsec policy; fortunately the entire configuration is accomplished with one line. As with legacy authentication, IPsec authentication can be configured either per-interface or per-area. We'll configure per-interface authentication between R1 and R2.

The first parameter to specify is the Security Policy Index (SPI). The SPI functions similarly to key numbers in a key chain, but is communicated via AH and must match between both ends of the adjacency. The SPI number is arbitrary, but must be between 256 and 4,294,967,295 (the 32-bit ceiling). We'll use the SPI of 256 for our lab.

Next we must decide on the type of authentication and the key string to use. AH provides authentication via either MD5 or SHA1, with a string length of 128 or 160 bits, respectively. Since SHA1 is the stronger algorithm, we'll choose it, and specify a random bit string as our key. Note that the key lengths must be exact: 40 hex digits (or 32 for MD5). For reasons outlined in section 7 of RFC 4552, the same keying is used for the SA in each direction between two OSPFv3 neighbors; this is in contrast to normal IPsec implementations, in which each unidirectional SA is built from its own unique keying via IKE.

R1(config-if)# ipv6 ospf authentication ipsec spi 256 sha1
 24e692732d80fac4f6dc2b9abfb73678ef660bab

If you're feeling especially lazy, you can generate an SHA1 hash from random data with dd and sha1sum on a Linux or UNIX box:

LinuxBox$ dd if=/dev/urandom count=1024 | sha1sum
954644a966d69bc14f6148b0be865b803f3bc9c9  -
1024+0 records in
1024+0 records out
524288 bytes (524 kB) copied, 0.0940048 s, 5.6 MB/s

After configuring authentication on FastEthernet0/1 at both routers, you may notice the OSPFv3 adjacency drop and reform. The OSPFv3 interface display verifies that SHA1 authentication is in use:

R1# show ipv6 ospf interface f0/1
FastEthernet0/1 is up, line protocol is up 
  Link Local Address FE80::1, Interface ID 5
  Area 1, Process ID 1, Instance ID 0, Router ID 1.1.1.1
  Network Type BROADCAST, Cost: 10
  SHA1 authentication SPI 256, secure socket UP (errors: 0)
  Transmit Delay is 1 sec, State BDR, Priority 1 
  Designated Router (ID) 2.2.2.2, local address FE80::2
  Backup Designated router (ID) 1.1.1.1, local address FE80::1
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
  Hello due in 00:00:03
  Index 1/1/1, flood queue length 0
  Next 0x0(0)/0x0(0)/0x0(0)
  Last flood scan length is 6, maximum is 6
  Last flood scan time is 4 msec, maximum is 4 msec
  Neighbor Count is 1, Adjacent neighbor count is 1 
  Adjacent with neighbor 2.2.2.2  (Designated Router)
  Suppress hello for 0 neighbor(s)

Incidentally, we can also verify the state of the IPsec AH security associations in use:

R1# show crypto ipsec sa

interface: FastEthernet0/1
 Crypto map tag: (none), local addr FE80::1

IPsecv6 policy name: OSPFv3-1-256
   IPsecv6-created ACL name: FastEthernet0/1-ipsecv6-ACL
   ...
 inbound ah sas:
  spi: 0x100(256)
    transform: ah-sha-hmac ,
    in use settings ={Transport, }
    conn id: 1, flow_id: SW:1, crypto map: (none)
    no sa timing
    replay detection support: N
    Status: ACTIVE
...
 outbound ah sas:
  spi: 0x100(256)
    transform: ah-sha-hmac ,
    in use settings ={Transport, }
    conn id: 2, flow_id: SW:2, crypto map: (none)
    no sa timing
    replay detection support: N
    Status: ACTIVE

To see what the AH-secured OSPF packets look like on the wire, check out this packet capture. Look for the AH extension in the IPv6 header.

Enabling Authentication for the Area

Configuring authentication for an area follows the same logic as for an interface. First we enter OSFPv3 router configuration, then specify the authentication parameters for the area:

R1(config)# ipv6 router ospf 1
R1(config-rtr)# area 0 authentication ipsec spi 500 sha1
 954644a966d69bc14f6148b0be865b803f3bc9c9

Again, the SPI is an arbitrary value, and the key string must be the appropriate length for whichever hashing method (MD5 or SHA1) is chosen. The above configuration enables IPsec authentication for all of R1's interfaces in area 0 (which in our case is just Serial1/0). We can use the show ipv6 ospf interface and show crypto ipsec sa commands as we did in the first section to verify OSPFv3 authentication is in use. The show crypto ipsec policy also provides a convenient overview of the authentication policies in use:

R1# show crypto ipsec policy
Crypto IPsec client security policy data

Policy name:      OSPFv3-1-256
Policy refcount:  1
Inbound  AH SPI:  256 (0x100)
Outbound AH SPI:  256 (0x100)
Inbound  AH Key:  24E692732D80FAC4F6DC2B9ABFB73678EF660BAB
Outbound AH Key:  24E692732D80FAC4F6DC2B9ABFB73678EF660BAB
Transform set:    ah-sha-hmac

Crypto IPsec client security policy data

Policy name:      OSPFv3-1-500
Policy refcount:  1
Inbound  AH SPI:  500 (0x1F4)
Outbound AH SPI:  500 (0x1F4)
Inbound  AH Key:  954644A966D69BC14F6148B0BE865B803F3BC9C9
Outbound AH Key:  954644A966D69BC14F6148B0BE865B803F3BC9C9
Transform set:    ah-sha-hmac