ICMP Echo-request (Ping)

Shorewall 'Ping' management has evolved over time with the latest change coming in Shorewall version 1.3.14. In that version, a new option (OLD_PING_HANDLING) was added to /etc/shorewall/shorewall.conf. The value of that option determines the overall handling of ICMP echo requests (pings).

Shorewall Versions >= 1.3.14 with OLD_PING_HANDLING=No in /etc/shorewall/shorewall.conf

In 1.3.14, Ping handling was put under control of the rules and policies just like any other connection request. In order to accept ping requests from zone z1 to zone z2 where the policy for z1 to z2 is not ACCEPT, you need a rule in /etc/shoreall/rules of the form:
ACCEPT    z1    z2    icmp    8
Example:

To permit ping from the local zone to the firewall:
ACCEPT    loc    fw    icmp    8
If you would like to accept 'ping' by default even when the relevant policy is DROP or REJECT, create /etc/shorewall/icmpdef if it doesn't already exist and in that file place the following command:
run_iptables -A icmpdef -p icmp --icmp-type 8 -j ACCEPT
With that rule in place, if you want to ignore 'ping' from z1 to z2 then you need a rule of the form:
DROP    z1    z2    icmp    8
Example:

To drop ping from the internet, you would need this rule in /etc/shorewall/rules:
DROP    net    fw    icmp    8

Shorewall Versions < 1.3.14 or with OLD_PING_HANDLING=Yes in /etc/shorewall/shorewall.conf

There are several aspects to the old Shorewall Ping management:
  1. The noping and filterping interface options in /etc/shorewall/interfaces.
  2. The FORWARDPING option in /etc/shorewall/shorewall.conf.
  3. Explicit rules in /etc/shorewall/rules.
There are two cases to consider:
  1. Ping requests addressed to the firewall itself; and
  2. Ping requests being forwarded to another system. Included here are all cases of packet forwarding including NAT, DNAT rule, Proxy ARP and simple routing.
These cases will be covered separately.

Ping Requests Addressed to the Firewall Itself

For ping requests addressed to the firewall, the sequence is as follows:
  1. If neither noping nor filterping are specified for the interface that receives the ping request then the request will be responded to with an ICMP echo-reply.
  2. If noping is specified for the interface that receives the ping request then the request is ignored.
  3. If filterping is specified for the interface then the request is passed to the rules/policy evaluation.

Ping Requests Forwarded by the Firewall

These requests are always passed to rules/policy evaluation.

Rules Evaluation

Ping requests are ICMP type 8. So the general rule format is:

    Target    Source    Destination    icmp    8

Example 1. Accept pings from the net to the dmz (pings are responded to with an ICMP echo-reply):

    ACCEPT    net    dmz    icmp    8

Example 2. Drop pings from the net to the firewall

    DROP    net    fw    icmp    8

Policy Evaluation

If no applicable rule is found, then the policy for the source to the destination is applied.
  1. If the relevant policy is ACCEPT then the request is responded to with an ICMP echo-reply.
  2. If FORWARDPING is set to Yes in /etc/shorewall/shorewall.conf then the request is responded to with an ICMP echo-reply.
  3. Otherwise, the relevant REJECT or DROP policy is used and the request is either rejected or simply ignored.

Updated 1/21/2003 - Tom Eastep

Copyright © 2001, 2002, 2003 Thomas M. Eastep.