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    Configuring ICMP Fragment Screen

    This topic describes how to configure detection of an ICMP fragment attack.

    If an ICMP packet is large, then it must be fragmented. When the ICMP fragment protection screen option is enabled, the Junos OS blocks any ICMP packet that has many fragment flags set or that has an offset value indicated in the offset field.

    To enable detection of an ICMP fragment IDS attack:

    1. Configure interfaces and assign an IP address to interfaces.
      user@host# set interfaces ge-0/0/1 unit 0 family inet address 192.0.2.0/24user@host# set interfaces ge-0/0/3 unit 0 family inet address 198.51.100.0/24
    2. Configure security zones trustZone and untrustZone and assign interfaces to them.
      user@host# set security zones security-zone trustZone host-inbound-traffic system-services alluser@host# set security zones security-zone trustZone host-inbound-traffic protocols alluser@host# set security zones security-zone trustZone interfaces ge-0/0/1.0user@host# set security zones security-zone untrustZone host-inbound-traffic system-services alluser@host# set security zones security-zone untrustZone host-inbound-traffic protocols alluser@host# set security zones security-zone untrustZone interfaces ge-0/0/3.0
    3. Configure security policies from untrustZone to trustZone.
      user@host# set security policies from-zone untrustZone to-zone trustZone policy policy1 match source-address anyuser@host# set security policies from-zone untrustZone to-zone trustZone policy policy1 match destination-address anyuser@host# set security policies from-zone untrustZone to-zone trustZone policy policy1 match application anyuser@host# set security policies from-zone untrustZone to-zone trustZone policy policy1 then permituser@host# set security policies default-policy deny-all
    4. Configure security screens and attach them to untrustZone.
      user@host# set security screen ids-option untrustScreen icmp fragmentuser@host# set security zones security-zone untrustZone screen untrustScreenuser@host# set security screen ids-option untrustScreen alarm-without-drop
    5. Configure syslog.
      user@host# set system syslog file syslog any anyuser@host# set system syslog file syslog archive size 10000000user@host# set system syslog file syslog explicit-priorityuser@host# set system syslog file syslog structured-datauser@host# set security policies from-zone untrustZone to-zone trustZone policy policy1 then log session-inituser@host# set security policies from-zone untrustZone to-zone trustZone policy policy1 then log session-close
    6. Commit the configuration.
      user@host# commit
     

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    Modified: 2016-08-01