September 17, 2016
It is good practice to configure an individual MD5 password for each BGP peer, but this is not enough. Why?
- Resource consumption attacks against TCP connections protected with MD5 as the router must verify the MD5 signature of packets it receives
- Many routers are based on Linux as there base operating system and there is a weakness which allows an attacker to insert arbitrary data into TCP connection. For more details click on this link.
The classical BGP TTL security is based on using a low TTL, generally 1, for single-hop BGP connections. The measure is effective in preventing a BGP connection from being established from a peer more than one hop away. Why? routers decrement the TTL when routing the packets and won’t route packets with a TTL of 1. So if your BGP session is a multi hop connection over one router a TTL of 2 makes sure the packet travels only over one router and not more. That sounds fine but has still has a drawback, as packets with a TTL of 1 are trivial to spoof, so rogue packets will still reach the router – leading to the problems described above.
GTSM (Global TTL Security Mechanism; RFC 5082, which obsolated RFC 3682) suggests the opposite approach. Instead of using a TTL value of 1, it suggests a value of 255 and discarding any packets received with have a TTL lower than 255 minus the hop count for this BGP session. Doing that an attacker is not able to perform the attack as the TTL gets decremented by every router (something the attacker can’t prevent). So setting the TTL on the router to 255 and having no multi hop BGP session allows to drop all packets on the receiving router which are lower than 255. This way only an attack in the same subnet is possible.
After the theory here the actual doing:
For Cisco routers is quite easy, just add
ttl-security hops 1 (for a BGP session in the local subnet) to the peering config
neighbor x.x.x.x command. For Mikrotik routers it is a little more complicated. For IPv4 connections configure following (should be default anyway):
/routing bgp peer set 0 ttl=255
For dropping incoming packets just use the firewall on the Mikrotik with a command like this:
/ip firewall filter add action=drop chain=input log=yes log-prefix="RFC 5082 block" src-address=xxx.xxx.xxx.xxx ttl=less-than:255
For IPv6 it is a little more complicated as setting the TTL for bgp peer configuration does have no effect (I’ve reported the bug already) but there is a simple workaround possible. Just use the mangle function of the firewall to set the correct hop count:
/ipv6 firewall mangle add action=change-hop-limit chain=output dst-address=xxx:xxx:xxx::xxx/128 new-hop-limit=set:255
Now you need only to filter the IPv6 packets:
/ipv6 firewall filter add action=drop chain=input log=yes log-prefix="RFC 5082 block" src-address=xxx:xxx:xxx::xxx/128 hop-limit=less-than:248
This configuration is quite easy and minimal invasive but should help a lot against attacks on your BGP routers.