Fun with IPsec stateful failover

A Cisco ASA does not always determine the egress interface of a packet based on the routing table. Instead, it’s possible that a NAT fun with IPsec stateful failover is overriding the routing table. If you specify an optional interface, then the ASA uses the NAT configuration to determine the egress interface.

The only exception is for identity NAT, which always uses a route lookup, regardless of the NAT configuration. For identity NAT, the default behavior is to use the NAT configuration, but you have the option to always use a route lookup instead. If you do not specify a specific interface, then the ASA uses a route lookup to determine the egress interface. My scenario is a routed firewall, not transparent. 1, the ASA would always do a route lookup to determine the egress interface.

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2 and higher, the ASA will not do a route lookup on identity NATs by default. Playing with this a bit on an ASA running 8. NAT statement unless both the source and destination interfaces were defined. I am more convinced than ever that it’s a mistake to think of the ASA as a router. The device simply does not follow the packet forwarding logic of Cisco IOS. Writer, podcaster, and speaker covering enterprise IT.

I was never able to find a good resource for packet processing thru ASA. Such things are being taught on JUNOS for security platform very well. I try to think of a firewall as a device that’s built for the purpose of breaking RFCs. Since much of a firewall’s activities are not RFC compliant.

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And I’ve always thought that learning what a firewall wants to do and not do by default is half the battle. We have always suspected that NAT processing happens before route processing in ASA and can override routing selection, but could not find the confirmation in Cisco docs. So it always was a bit of reverse engineering challenge between man and the machine. I wonder if Juniper’s Netscreen does the same. I just have one thing to add.

When any of the interfaces is ANY, than route-lookup is performed by default and that is why you cannot add it on the end of your NAT statement. It has helped confirm the behavior we were seeing with identity NAT in 8. I’ve been trying to lab this up and use packet tracer to see a scenario where this would happen. ASDM makes it look like the match. Could someone outline a scenario for me where this would happen? Below is what I ran into with three interfaces. NAT’s, which is what caused the problem in this example.

When a connection from the OUTSIDE was meant to be built to egress on the DMZ interface, it ended up matching the first rule, and the ASA built the connection incorrectly using the INSIDE interface as egress, even though the routing table had the correct information. Thank you for this, very helpful for me today. Neither I nor TAC was able to get NAT to override the default routing in 9. ASA Services Module implicitly perform a destination route lookup for Twice NAT entries with an identity destination translation even when the route-lookup keyword is not used. The bug is fixed in 9. 5, which can turn out to be a painful upgrade if your firewall was operating as intended without the route-lookup keyword.

A Cisco ASA does not always determine the egress interface of a packet based on the routing table. Instead, it’s possible that a NAT rule is overriding the routing table. If you specify an optional interface, then the ASA uses the NAT configuration to determine the egress interface. The only exception is for identity NAT, which always uses a route lookup, regardless of the NAT configuration. For identity NAT, the default behavior is to use the NAT configuration, but you have the option to always use a route lookup instead.

If you do not specify a specific interface, then the ASA uses a route lookup to determine the egress interface. My scenario is a routed firewall, not transparent. 1, the ASA would always do a route lookup to determine the egress interface. 2 and higher, the ASA will not do a route lookup on identity NATs by default.

Playing with this a bit on an ASA running 8. NAT statement unless both the source and destination interfaces were defined. I am more convinced than ever that it’s a mistake to think of the ASA as a router. The device simply does not follow the packet forwarding logic of Cisco IOS.

Writer, podcaster, and speaker covering enterprise IT. I was never able to find a good resource for packet processing thru ASA. Such things are being taught on JUNOS for security platform very well. I try to think of a firewall as a device that’s built for the purpose of breaking RFCs. Since much of a firewall’s activities are not RFC compliant. And I’ve always thought that learning what a firewall wants to do and not do by default is half the battle.

Fun with IPsec stateful failover

We have always suspected that NAT processing happens before route processing in ASA and can override routing selection, but could not find the confirmation in Cisco docs. So it always was a bit of reverse engineering challenge between man and the machine. I wonder if Juniper’s Netscreen does the same. I just have one thing to add. When any of the interfaces is ANY, than route-lookup is performed by default and that is why you cannot add it on the end of your NAT statement.

It has helped confirm the behavior we were seeing with identity NAT in 8. I’ve been trying to lab this up and use packet tracer to see a scenario where this would happen. ASDM makes it look like the match. Could someone outline a scenario for me where this would happen?

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Below is what I ran into with three interfaces. NAT’s, which is what caused the problem in this example. When a connection from the OUTSIDE was meant to be built to egress on the DMZ interface, it ended up matching the first rule, and the ASA built the connection incorrectly using the INSIDE interface as egress, even though the routing table had the correct information. Thank you for this, very helpful for me today. Neither I nor TAC was able to get NAT to override the default routing in 9.

ASA Services Module implicitly perform a destination route lookup for Twice NAT entries with an identity destination translation even when the route-lookup keyword is not used. The bug is fixed in 9. 5, which can turn out to be a painful upgrade if your firewall was operating as intended without the route-lookup keyword. A Cisco ASA does not always determine the egress interface of a packet based on the routing table. Instead, it’s possible that a NAT rule is overriding the routing table. If you specify an optional interface, then the ASA uses the NAT configuration to determine the egress interface. The only exception is for identity NAT, which always uses a route lookup, regardless of the NAT configuration.

For identity NAT, the default behavior is to use the NAT configuration, but you have the option to always use a route lookup instead. If you do not specify a specific interface, then the ASA uses a route lookup to determine the egress interface. My scenario is a routed firewall, not transparent. 1, the ASA would always do a route lookup to determine the egress interface. 2 and higher, the ASA will not do a route lookup on identity NATs by default.

Playing with this a bit on an ASA running 8. NAT statement unless both the source and destination interfaces were defined. I am more convinced than ever that it’s a mistake to think of the ASA as a router. The device simply does not follow the packet forwarding logic of Cisco IOS. Writer, podcaster, and speaker covering enterprise IT. I was never able to find a good resource for packet processing thru ASA. Such things are being taught on JUNOS for security platform very well.

Ledger

I try to think of a firewall as a device that’s built for the purpose of breaking RFCs. Since much of a firewall’s activities are not RFC compliant. And I’ve always thought that learning what a firewall wants to do and not do by default is half the battle. We have always suspected that NAT processing happens before route processing in ASA and can override routing selection, but could not find the confirmation in Cisco docs. So it always was a bit of reverse engineering challenge between man and the machine. I wonder if Juniper’s Netscreen does the same. I just have one thing to add.

When any of the interfaces is ANY, than route-lookup is performed by default and that is why you cannot add it on the end of your NAT statement. It has helped confirm the behavior we were seeing with identity NAT in 8. I’ve been trying to lab this up and use packet tracer to see a scenario where this would happen. ASDM makes it look like the match. Could someone outline a scenario for me where this would happen? Below is what I ran into with three interfaces.

NAT’s, which is what caused the problem in this example. When a connection from the OUTSIDE was meant to be built to egress on the DMZ interface, it ended up matching the first rule, and the ASA built the connection incorrectly using the INSIDE interface as egress, even though the routing table had the correct information. Thank you for this, very helpful for me today. Neither I nor TAC was able to get NAT to override the default routing in 9. ASA Services Module implicitly perform a destination route lookup for Twice NAT entries with an identity destination translation even when the route-lookup keyword is not used.

The bug is fixed in 9. 5, which can turn out to be a painful upgrade if your firewall was operating as intended without the route-lookup keyword. A Cisco ASA does not always determine the egress interface of a packet based on the routing table. Instead, it’s possible that a NAT rule is overriding the routing table. If you specify an optional interface, then the ASA uses the NAT configuration to determine the egress interface.

The only exception is for identity NAT, which always uses a route lookup, regardless of the NAT configuration. For identity NAT, the default behavior is to use the NAT configuration, but you have the option to always use a route lookup instead. If you do not specify a specific interface, then the ASA uses a route lookup to determine the egress interface. My scenario is a routed firewall, not transparent. 1, the ASA would always do a route lookup to determine the egress interface. 2 and higher, the ASA will not do a route lookup on identity NATs by default.

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Playing with this a bit on an ASA running 8. NAT statement unless both the source and destination interfaces were defined. I am more convinced than ever that it’s a mistake to think of the ASA as a router. The device simply does not follow the packet forwarding logic of Cisco IOS. Writer, podcaster, and speaker covering enterprise IT.

I was never able to find a good resource for packet processing thru ASA. Such things are being taught on JUNOS for security platform very well. I try to think of a firewall as a device that’s built for the purpose of breaking RFCs. Since much of a firewall’s activities are not RFC compliant.

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And I’ve always thought that learning what a firewall wants to do and not do by default is half the battle. We have always suspected that NAT processing happens before route processing in ASA and can override routing selection, but could not find the confirmation in Cisco docs. So it always was a bit of reverse engineering challenge between man and the machine. I wonder if Juniper’s Netscreen does the same. I just have one thing to add. When any of the interfaces is ANY, than route-lookup is performed by default and that is why you cannot add it on the end of your NAT statement.

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It has helped confirm the behavior we were seeing with identity NAT in 8. I’ve been trying to lab this up and use packet tracer to see a scenario where this would happen. ASDM makes it look like the match. Could someone outline a scenario for me where this would happen?

Below is what I ran into with three interfaces. NAT’s, which is what caused the problem in this example. When a connection from the OUTSIDE was meant to be built to egress on the DMZ interface, it ended up matching the first rule, and the ASA built the connection incorrectly using the INSIDE interface as egress, even though the routing table had the correct information. Thank you for this, very helpful for me today. Neither I nor TAC was able to get NAT to override the default routing in 9. ASA Services Module implicitly perform a destination route lookup for Twice NAT entries with an identity destination translation even when the route-lookup keyword is not used.

The bug is fixed in 9. 5, which can turn out to be a painful upgrade if your firewall was operating as intended without the route-lookup keyword. A Cisco ASA does not always determine the egress interface of a packet based on the routing table. Instead, it’s possible that a NAT rule is overriding the routing table. If you specify an optional interface, then the ASA uses the NAT configuration to determine the egress interface.

The only exception is for identity NAT, which always uses a route lookup, regardless of the NAT configuration. For identity NAT, the default behavior is to use the NAT configuration, but you have the option to always use a route lookup instead. If you do not specify a specific interface, then the ASA uses a route lookup to determine the egress interface. My scenario is a routed firewall, not transparent.

1, the ASA would always do a route lookup to determine the egress interface. 2 and higher, the ASA will not do a route lookup on identity NATs by default. Playing with this a bit on an ASA running 8. NAT statement unless both the source and destination interfaces were defined. I am more convinced than ever that it’s a mistake to think of the ASA as a router. The device simply does not follow the packet forwarding logic of Cisco IOS. Writer, podcaster, and speaker covering enterprise IT.

I was never able to find a good resource for packet processing thru ASA. Such things are being taught on JUNOS for security platform very well. I try to think of a firewall as a device that’s built for the purpose of breaking RFCs. Since much of a firewall’s activities are not RFC compliant. And I’ve always thought that learning what a firewall wants to do and not do by default is half the battle. We have always suspected that NAT processing happens before route processing in ASA and can override routing selection, but could not find the confirmation in Cisco docs. So it always was a bit of reverse engineering challenge between man and the machine.

I wonder if Juniper’s Netscreen does the same. I just have one thing to add. When any of the interfaces is ANY, than route-lookup is performed by default and that is why you cannot add it on the end of your NAT statement. It has helped confirm the behavior we were seeing with identity NAT in 8. I’ve been trying to lab this up and use packet tracer to see a scenario where this would happen. ASDM makes it look like the match. Could someone outline a scenario for me where this would happen?

Below is what I ran into with three interfaces. NAT’s, which is what caused the problem in this example. When a connection from the OUTSIDE was meant to be built to egress on the DMZ interface, it ended up matching the first rule, and the ASA built the connection incorrectly using the INSIDE interface as egress, even though the routing table had the correct information. Thank you for this, very helpful for me today. Neither I nor TAC was able to get NAT to override the default routing in 9. ASA Services Module implicitly perform a destination route lookup for Twice NAT entries with an identity destination translation even when the route-lookup keyword is not used.