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:
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Configure interfaces and assign an IP address to interfaces.
[edit] user@host# set interfaces ge-0/0/1 unit 0 family inet address 192.0.2.0/24 user@host# set interfaces ge-0/0/3 unit 0 family inet address 198.51.100.0/24
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Configure security zones trustZone and
untrustZone and assign interfaces to them.
[edit] user@host# set security zones security-zone trustZone host-inbound-traffic system-services all user@host# set security zones security-zone trustZone host-inbound-traffic protocols all user@host# set security zones security-zone trustZone interfaces ge-0/0/1.0 user@host# set security zones security-zone untrustZone host-inbound-traffic system-services all user@host# set security zones security-zone untrustZone host-inbound-traffic protocols all user@host# set security zones security-zone untrustZone interfaces ge-0/0/3.0
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Configure security policies from untrustZone to
trustZone.
[edit] user@host# set security policies from-zone untrustZone to-zone trustZone policy policy1 match source-address any user@host# set security policies from-zone untrustZone to-zone trustZone policy policy1 match destination-address any user@host# set security policies from-zone untrustZone to-zone trustZone policy policy1 match application any user@host# set security policies from-zone untrustZone to-zone trustZone policy policy1 then permit user@host# set security policies default-policy deny-all
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Configure security screens and attach them to
untrustZone.
[edit] user@host# set security screen ids-option untrustScreen icmp fragment user@host# set security zones security-zone untrustZone screen untrustScreen user@host# set security screen ids-option untrustScreen alarm-without-drop
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Configure syslog.
[edit] user@host# set system syslog file syslog any any user@host# set system syslog file syslog archive size 10000000 user@host# set system syslog file syslog structured-data user@host# set security policies from-zone untrustZone to-zone trustZone policy policy1 then log session-init user@host# set security policies from-zone untrustZone to-zone trustZone policy policy1 then log session-close
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Commit the configuration.
[edit] user@host# commit