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As far as security zones are concerned, we apply policies to it to control the transit traffic. And we also apply host-inbound-traffic to a zone or interfaces. If we have both of the above configurations, why there is junos-host zone to prevent the traffic destined to the device itself as "host-inbound-traffic" also does the same thing. Please help to correct me if im wrong somewhere or am missing something.

This whole concept of junos-host zone and security zones along with policies is confusing.

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Junos-host zone can be used to add an additional check for traffic destined to SRX. If you don't configure any security policy to-zone junos-host, the traffic/packet will be validated based on host-inbound-traffic configured under security zones. If you configure security policy to-zone junos-host, that policy check will be done additionaly to host-inbound-traffic/services specified under zones.

For example, if you allow SSH/Telnet/OSPF under interface ge-0/0/0.0, but configure a security policy to-zone junos-host allowing SSH, then Telnet/OSPF wont work. Only SSH will work.

More to read: Link-1 Link-2

Juniper SRX traffic flow

Inbound packet will pass in this order:

  1. Input interface filter, if set
  2. Zone host-inbound-traffic
  3. Zone-to-Zone policy

If traffic passed on 1 step, it can be still denied on 2 or 3. Assume:

  1. No interface filter applied
  2. host-inbound-traffic set to system-services ssh
  3. Policy from zone zone1 to-zone junos-host allows only ICMP ping

As result, not ICMP ping, not SSH will be allowed. ICMP ping will be dropped on step 2; SSH will be dropped on step 3.

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    So correct me if I am wrong. If security policy to-zone junos-host is configured, its applicable to the whole device(all the interfaces included). So even if, host-inbound-traffic is configured for an interface or a zone(say Red-zone, allow only ssh) but the junos-host is configured to allow only ping, then it means the interfaces part of that Red-zone cannot allow ssh as the junos-host is configured to allow only ping to those interfaces.
    – RRHS
    Commented Nov 21, 2018 at 9:07
  • junos-host it's device itself. So yes, if you will configure policy from some zone, lets say RED to junos-host, and you will allow only ICMP ping, but your RED zone host-inbound-services will have ICMP ping and SSH - only ICMP ping will be allowed. You can think of it like consecutive levels of security. Let me check some docs, I will edit my answer with traffic flow sequence...
    – Andrey Prokhorov
    Commented Nov 21, 2018 at 9:42
  • Understood. Its all about levels and junos-host isnt given any priority over the host-inbound-traffic. And yes, please attach any docs if you have, it would be very helpful.
    – RRHS
    Commented Nov 21, 2018 at 11:31
  • @RRHS Check theese: Security Policies Feature Guide for Security Devices, KB24227
    – Andrey Prokhorov
    Commented Nov 21, 2018 at 12:20
  • Just one more query. Are configuring security zones a necessity? And i have also read that there has to be a minimum of 2 security zones to be configured. But some of the devices which i have used have only 1(security-zone HOST) and VSRX has none. Could you please shed some light on it.
    – RRHS
    Commented Nov 22, 2018 at 13:59

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