Today's network infrastructure typically will not have fragmented packets destined for the router's control or management plane. In most cases, it is safe to apply packet filters which will prevent fragmented packets from arriving on the router. Usually, fragmented packets received by a router indicate a problem with the network or a DDOS attack against the router. In either case, fragmented packets should be dropped to protect the router's control and management plane.
Below is a sample firewall filter to demonstrate this recommendation:
[edit firewall family inet filter fragment]
user@junos# show
term first-frag {
from {
first-fragment;
}
then {
discard;
}
term next-frag {
from {
is-fragment;
}
then {
discard;
}
Caution: Some routing protocols, such as BGP and OSPF, may rely upon fragmented traffic being received by the RE. As with any control plane firewall filter, perform careful testing in your environment to insure that dropping all fragmented traffic will not have a negative impact. If necessary, add explicit exceptions for fragmented BGP and/or OSPF traffic to the sample firewall filter above.
Note that some platforms -- most notably the EX Series -- do not support the
'first-fragment' filter criterion. In these cases, simply discarding all fragments via
'is-fragment' will be sufficient. Additionally, the EX-8200 does not support either criteria, in which case the only option is to upgrade.
The trigger for this issue is specific to fragmented ICMP packets. While the recommendation of the Juniper Networks SIRT and a security best common practice is to discard all fragments destined to the router's control plane, customers who wish to more specifically define this sample filter may add an additional criterion,
'protocol icmp', to each term. This will cause the filter to only discard fragmented ICMP packets.
Information for how Juniper Networks uses CVSS can be found at KB 16446 "Common Vulnerability Scoring System (CVSS) and Juniper's Security Advisories."