The term hairpinning comes from the fact that the traffic comes from one source into a router or similar
devices, makes a U-turn and goes back the same way it came.

Hairpinning is only relevant when the firewall is in routed mode since the “turnaround” of traffic
is a routing decision. Also there needs to be another router involved. If the firewall is setup to only
pass traffic between interfaces no hairpinning will be taking place.

Typical hairpinning is done when there is a router inside of the firewall beyond which there is
another network that needs to be reached  to/from the inside network.

 

Another fundament for hairpinning taking place is that not all network equipment has full knowledge
of the network topology. Typically these are computers with only a default route (“default gateway”)
to something but not aware of the fact that the remote network is reachable directly via the other router
without taking the path via the firewall (see picture above).

If the computer had that knowledge it would never involve the firewall. A workaround like this could be
in a windows-host to add “route -p 192.168.2.0 mask 255.255.255.0 192.168.1.2″ from the command
prompt. However this is in most cases not very flexible since it is a manual work at each host.

ver 7.2

Beginning with v7.2 the “same-security permit-intra-interface”-command becomes useful and can
be used for other traffic than vpn-initiated. Now we can do hair- pinning. So, what is needed?
First of all, the “same-security permit -intra-interface”. Also we need to allow inbound  traffic if
we have an access-list applied inbound on the interface.

Let´s have another look at our example:

 

In this example we have one host at 192.168.1.100 using the firewall .1 as default gateway.
The firewall has a route for the 192.168.2.0/24-network via 192.168.1.2 and the route is
directly connected to both networks. Because of this configuration the traffic from
192.168.1.100 is hair-pinned back, to the router.


interface Vlan1
nameif inside
security-level 100
ip address 192.168.1.1 255.255.255.0
!
route inside 192.168.2.0 255.255.255.0 192.168.1.2 1
!
same-security-traffic permit intra-interface
!
no nat-control

All magic lies in the “same-security-traffic”-command. In the example above we have no
access-list applied. If we have we must also open for that traffic:


access-list acl_inside extended permit ip 192.168.2.0 255.255.255.0 192.168.1.0 255.255.255.0
!
access-list acl_inside extended permit ip 192.168.1.0 255.255.255.0 192.168.2.0 255.255.255.0
!
access-list acl_inside extended deny ip any any
!
access-group acl_inside in interface inside

We most likely has a NAT/global configured for the inside network to be able to reach internet.
If we add this to our example we kill our hair-pinning:


nat (inside) 1 0.0.0.0 0.0.0.0
global (outside) 1 interface

Now, when we try to ping from 192.168.1.100 to 192.168.2.200 we get this log output
of the firewall:


%ASA-3-305006: portmap translation creation failed for icmp src inside:192.168.1.100 dst inside:192.168.2.200 (type 8, code 0)
%ASA-3-305006: portmap translation creation failed for icmp src inside:192.168.1.100 dst inside:192.168.2.200 (type 8, code 0)
%ASA-3-305006: portmap translation creation failed for icmp src inside:192.168.1.100 dst inside:192.168.2.200 (type 8, code 0)
%ASA-3-305006: portmap translation creation failed for icmp src inside:192.168.1.100 dst inside:192.168.2.200 (type 8, code 0)
%ASA-3-305006: portmap translation creation failed for icmp src inside:192.168.1.100 dst inside:192.168.2.200 (type 8, code 0)

As soon as we add ANY nat-configuration for an interface we must configure nat for all traffic from
that interface, even hairpinned traffic. We do this with the static-command below. The purpose of this
is to “static” translate traffic from interface “inside” to interface “inside” where the source is “192.168.1.0″
(netmask 255.255.255.0 and translate the source to “192.168.1.0″ (the same address). We also do the same
for the 192.168.2.0-network to ensure that traffic can flow initiated in both directions.


static (inside,inside) 192.168.1.0 192.168.1.0 netmask 255.255.255.0
!
static (inside,inside) 192.168.2.0 192.168.2.0 netmask 255.255.255.0

The compilation of relevant configuration lines in the firewall to ackomplish this is shown here:


hostname fw
!
interface Vlan1
nameif inside
security-level 100
ip address 192.168.1.1 255.255.255.0
!
interface Vlan222
nameif outside
security-level 0
ip address dhcp setroute
!
boot system disk0:/asa725-k8.bin
!
same-security-traffic permit intra-interface
!
access-list acl_inside extended permit ip 192.168.2.0
255.255.255.0 192.168.1.0 255.255.255.0
!
access-list acl_inside extended permit ip 192.168.1.0
255.255.255.0 192.168.2.0 255.255.255.0
!
access-list acl_inside extended deny ip any any
!
nat-control
!
global (outside) 1 interface
nat (inside) 1 0.0.0.0 0.0.0.0
!
static (inside,inside) 192.168.1.0 192.168.1.0 netmask 255.255.255.0
static (inside,inside) 192.168.2.0 192.168.2.0 netmask 255.255.255.0
!
access-group acl_inside in interface inside
!
route inside 192.168.2.0 255.255.255.0 192.168.1.2 1

Ver 8.3

In this version the nat-concept is totally rewritten with a new command syntax.
More about this can be read about elsewhere, but the technique is the same.

This is the converted version of the configuration snippet above:


hostname fw
!
interface Vlan1
nameif inside
security-level 100
ip address 192.168.1.1 255.255.255.0
!
interface Vlan222
nameif outside
security-level 0
ip address dhcp setroute
boot system disk0:/asa831-k8.bin
!
same-security-traffic permit intra-interface
!
object network obj-192.168.1.0
subnet 192.168.1.0 255.255.255.0
!
object network obj-192.168.2.0
subnet 192.168.2.0 255.255.255.0
!
object network obj_any
subnet 0.0.0.0 0.0.0.0
!
object network obj_any-01
subnet 0.0.0.0 0.0.0.0
!
object network obj-0.0.0.0
host 0.0.0.0
!
access-list acl_inside extended permit ip 192.168.2.0
255.255.255.0 192.168.1.0 255.255.255.0
!
access-list acl_inside extended permit ip 192.168.1.0
255.255.255.0 192.168.2.0 255.255.255.0
!
access-list acl_inside extended deny ip any any
!
object network obj-192.168.1.0
nat (inside,inside) static 192.168.1.0
!
object network obj-192.168.2.0
nat (inside,inside) static 192.168.2.0
!
object network obj_any
nat (inside,outside) dynamic interface
!
object network obj_any-01
nat (inside,outside) dynamic obj-0.0.0.0
!
access-group acl_inside in interface inside
!
route inside 192.168.2.0 255.255.255.0 192.168.1.2 1


Assymetric routing

An issue with the configuration above is that since the firewall is stateful
(which means that it keeps track of TCP-states) and the fact that traffic in one direction
goes via the firewall (from 192.168.1.100 to 192.168.2.200) but the traffic in the other direction
goes direct makes the firewall going bananas. If the traffic initiates from the 1-network all is
fine since the firewall allowes the initial packet and never sees the return traffic. It might build a
quite large list of incomplete sessions that eventually will time out, but it will work:

 

 

However, when the traffic is initiated from the 2-network the first packet that will be seen
by the firewall is the second (SYN-ACK) which is not very appreciated.

 

Beginning with version 8.2 there is a solution for this in the “tcp state bypass”-functionality.
By using this you can with MPF (modular policy framework) specify traffic that should not be
handled stateful.

Here is an example of doing that. Genereally my solution to hairpinning- and/or
assymetric routing-issue is to avoid it. If you have one or more routers in your topology it is
probably a good idea to use that router as the default gateway instead and only direct traffic to
the firewall that should pass it.

The drawback from this is that internet-traffic needs to go via the router instead.
However this seldom causes any trouble since the router is not stateful and has no issues
with doing hairpinning.

Conclusion

Hairpinning of traffic in Cisco Pix/ASA and problems with assymetric routing can be a
pain. There are workarounds but mostly assymetric issues are symptoms of bad network
design rather than configuration issues.

There are more dimensions of hairpinning than just internal traffic turning around to a
WAN-router. Such a common example is U-turning of VPN-traffic, for example traffic
from an VPN-client going via the firewall out to internet or into another vpn-tunnel.
Or spoke-hub-spoke VPN-traffic. This type of traffic seldom gives routing or assymetric issues
but is more a matter of defining proxy ACL:s for vpn-traffic and not doing NAT on that traffic.