This post represents the solution and explanation for quiz-9.
Have a look at the quiz to understand the problem.

The quiz brings up for discussion a well-known problem of BGP peering over a firewall (in this case, a Cisco ASA), problem that is caused by two things: BGP uses TCP Options to perform authentication and the fact that firewalls do not "like" IP nor TCP OPTIONS.

IP OPTIONS are considered a security risk as they have the potential of leaking information about the internet network setup and, by default, firewalls drop all packets with IP OPTIONS.

As regards to the TCP OPTIONS, they can receive different treatment (depending the type of firewall and its configuration):

  • forwarded unchanged, for example: selective ACK, window scale, MSS, etc
  • modified and then forwarded, for example in case of MSS clamping
  • stripped off (actually, replace with NOP TCP Option, that is more like padding), for example: BGP MD5 TCP Option
  • drop them (if they are not known and/or based on firewall configuration)

BGP controls most of the Internet traffic so it is very important to keep it secure. Besides the attacks against the routing protocol itself, BGP shares the same weaknesses as any TCP established application and is vulnerable to:

  • attacks against confidentiality - capture the TCP communication between two devices and learn the information exchanged ("eavesdropping")
  • attacks against integrity - insert forged BGP messages into the BGP peering of two devices ("man in the middle")
  • attacks agains the TCP session:
    • reset the TCP session by falsely terminating the BGP established connection (send a packet with a RST bit set or a packet with SYN bit set, while spoofing all the other fields of the already established connection)
    • turn up a TCP session with an unauthorized party
    • re-routing or looping the connectivity between the BGP peerings (in case of multihop BGP)
  • denial of service (DOS) attacks:
    • send a large number of SYN packets to consume device's memory - SYN flooding
    • saturate the link so that the BGP will time out

There are more methods to protect against these attacks, but today we will only cover BGP Authentication.

RFC 2385 describes a TCP extenstion that enhances BGP security by defining a new TCP OPTION (option-kind 19) that carries an MD5 digest (this means that BGP MD5 is not a function of BGP but an extention of TCP) !

This BGP MD5 will protect against:

  • session hijacking and replay attacks (see above integrity attacks)
  • unauthorized BGP session turn-ups

but it does not protect against:

  • confidentiality - there's no encryption of the packet
  • denial of service - as the router's CPU needs to perform MD5 hashing against all packets sent by attacker

According to the RFC, the MD5 digest is performed against several fields from both the IP and TCP header, as shown in the following diagram:


  • the TCP Pseudo-Header (includes: IP source, IP destination, zero-padded protocol number and TCP segment length)
    • The pseudo-header is called like that because it refers to some fields from Layer 3/IP header
  • the TCP Header excluding options (includes: source port, destination port, sequence numbers, etc)
  • the TCP segment data (if any)
  • the neighbor configured password

Cisco PIX/ASA version 7.x and later has some default protections that impacts the BGP MD5 authentication. By default, Cisco ASA will:
- re-write any TCP OPTION 19 (BGP MD5) with NOP (No Operation) that is used like padding
- randomize the TCP sequence numbers (by offsetting them with a random number)

The BGP MD5 digest needs to match on each side and for this to happen, the information used for hashing needs to be identical, such as: IP source and destination, source and destination port, sequence numbers, TCP segment data, password, etc...

For an authenticated BGP session to get established, it is not enough to allow TCP port 179 via the firewall but also you need to disable the above mentioned protections:

  • disable TCP MD5 option rewriting
  • disable TCP sequence number randomization

This also means, that it's not possible to perform NAT between BGP peers that uses authentication !

In the end, let's see how the configuration looks like on a Cisco ASA to allow the routers to have a successful BGP authenticated session:

access-list ACL_BGP extended permit tcp any any eq bgp
access-list ACL_BGP extended permit tcp any eq bgp any
  tcp-options range 19 19 allow
class-map BGP_MD5
 match access-list ACL_BGP
policy-map global_policy
 class BGP_MD5
 set connection random-sequence-number disable
  set connection advanced-options TCP_MAP_ALLOW_OPTIONS
service-policy global_policy global

Thank you for your comments in the quiz!