V-217051

Discussion

GTSM is designed to protect a router's IP-based control plane from DoS attacks. Many attacks focused on CPU load and line-card overload can be prevented by implementing GTSM on all Exterior Border Gateway Protocol speaking routers. GTSM is based on the fact that the vast majority of control plane peering is established between adjacent routers; that is, the Exterior Border Gateway Protocol peers are either between connecting interfaces or between loopback interfaces. Since TTL spoofing is considered nearly impossible, a mechanism based on an expected TTL value provides a simple and reasonably robust defense from infrastructure attacks based on forged control plane traffic.

Check Content

Verify that a filter has been configured to only allow BGP packets with a TTL of 255 as shown in the example below.

firewall {
        …
        …
        …
    filter GTSM_FILTER {
        term TTL_SECURITY {
            from {
                protocol tcp;
                ttl-except 255;
                port bgp;
            }
            then {
                syslog;
                discard;
            }
        }
        term ELSE_ACCEPT {
            then accept;
        }
    }
}

Verify that the filter is applied to all interfaces connecting to eBGP peers.

interfaces {
…
…
…
  ge-0/0/0  {
        unit 0 {
            family inet {
                filter {
                    input-list [INBOUND_FILTER GTSM_FILTER];
                }
                address x.x.x.x/30;
            }
         }
    }
}

Configure the router to send all BGP packets with a TTL of 255 as shown in the example below.

If the router is not configured to use GTSM for all Exterior Border Gateway Protocol peering sessions, this is a finding.