Cisco ASA Packet Drop Troubleshooting

As a firewall, the Cisco ASA drops packets. That’s great until it drops packets that you want to permit, and you have no idea what is going on. Fortunately, the ASA supports different tools to show you why and what packets it drops.

In this lesson, we’ll cover the following tools:

• Connection State
• Interface Drops
• Syslog
• ASP Drops
• Packet Capture
• Packet Tracer
• Service Policy
• Conclusion

Here is the topology I use:

We use a simple topology:

• Two “host” devices:
  • These are routers with ip routing disabled, and they use ASA1 as their default gateway.
  • An HTTP server is enabled on both devices.
• ASA1 runs ASAv Version 9.9(2) and has two interfaces:
  • INSIDE: security level 100
  • OUTSIDE: security level 0

In this topology, H1 will be able to initiate a connection to H2. H2 won’t be able to initiate a connection to H1 because we go from a low-security level (0) to a high-security level (100).

hostname ASA1
!
interface GigabitEthernet0/0
 nameif INSIDE
 security-level 100
 ip address 192.168.1.254 255.255.255.0 
!
interface GigabitEthernet0/1
 nameif OUTSIDE
 security-level 0
 ip address 192.168.2.254 255.255.255.0
!
class-map inspection_default
 match default-inspection-traffic
!
policy-map global_policy
 class inspection_default
  inspect ip-options 
  inspect netbios 
  inspect rtsp 
  inspect sunrpc 
  inspect tftp 
  inspect xdmcp 
  inspect dns preset_dns_map 
  inspect ftp 
  inspect h323 h225 
  inspect h323 ras 
  inspect rsh 
  inspect esmtp 
  inspect sqlnet 
  inspect sip  
  inspect skinny  
!
service-policy global_policy global
!
: end

hostname H1
!
no ip routing
!         
interface GigabitEthernet0/1
 ip address 192.168.1.1 255.255.255.0
!
ip default-gateway 192.168.1.254
!
ip http server
no ip http secure-server
!
end

hostname H2
!
no ip routing
!         
interface GigabitEthernet0/1
 ip address 192.168.2.2 255.255.255.0
!
ip default-gateway 192.168.2.254
!
ip http server
no ip http secure-server
!
end

Connection State

The ASA keeps track of all existing connections and their state. Let’s see what this looks like. First, we connect from H1 to H2:

H1#telnet 192.168.2.2 80
Trying 192.168.2.2, 80 ... Open

Now look at the connections with the show conn command:

ASA1# show conn 
1 in use, 1 most used

TCP OUTSIDE  192.168.2.2:80 INSIDE  192.168.1.1:50195, idle 0:00:00, bytes 0, flags U

You can see the flags if you add the detail parameter:

ASA1# show conn detail 
0 in use, 1 most used
Flags: A - awaiting inside ACK to SYN, a - awaiting outside ACK to SYN,
       B - initial SYN from outside, b - TCP state-bypass or nailed,
       C - CTIQBE media, c - cluster centralized,
       D - DNS, d - dump, E - outside back connection, e - semi-distributed,
       F - outside FIN, f - inside FIN,
       G - group, g - MGCP, H - H.323, h - H.225.0, I - inbound data,
       i - incomplete, J - GTP, j - GTP data, K - GTP t3-response
       k - Skinny media, L - LISP triggered flow owner mobility
       l - local director/backup stub flow
       M - SMTP data, m - SIP media, n - GUP
       N - inspected by Snort
       O - outbound data, o - offloaded,
       P - inside back connection,
       Q - Diameter, q - SQL*Net data,
       R - outside acknowledged FIN,
       R - UDP SUNRPC, r - inside acknowledged FIN, S - awaiting inside SYN,
       s - awaiting outside SYN, T - SIP, t - SIP transient, U - up, u - STUN,
       V - VPN orphan, v - M3UA W - WAAS,
       w - secondary domain backup,
       X - inspected by service module,
       x - per session, Y - director stub flow, y - backup stub flow,
       Z - Scansafe redirection, z - forwarding stub flow

Let’s try to connect from H2 to H1:

H2#telnet 192.168.1.1 80
Trying 192.168.1.1, 80 ... 
% Connection timed out; remote host not responding

This connection fails, so it doesn’t show up in the connection overview:

ASA1# show conn
0 in use, 1 most used

At least we know something is going on.

Interface drops

The ASA keeps track of drops on the interface. Here’s where you find this:

ASA1# show interface GigabitEthernet 0/1 | include packets dropped
        10 packets dropped

We see the ASA drops packets on the interface, but we have no idea what. You can use clear interface to reset this counter.

Syslog

Syslog is one of the best tools to figure out what is going on with your packet drops. The ASA has over 2000 unique syslog messages. You can send syslog messages to different locations. For debugging, the console works fine. Let’s see how we can use this.

First, you need to enable logging up to the debug level:

ASA1(config)# logging enable
ASA1(config)# logging buffered debugging

Optionally, you can increase the buffer size so you can store more syslog messages. For this example, I’m disabling the timestamps so that the messages are easier to look at:

ASA1(config)# logging buffer-size 1000000
ASA1(config)# no logging timestamp

Let’s try to connect from H2 to H1 on TCP port 80:

H2#telnet 192.168.1.1 80
Trying 192.168.1.1, 80 ... 
% Connection timed out; remote host not responding

Now check out your log with the show logging command:

ASA1(config)# show logging
Syslog logging: enabled
    Facility: 20
    Timestamp logging: disabled
    Hide Username logging: enabled
    Standby logging: disabled
    Debug-trace logging: disabled
    Console logging: disabled
    Monitor logging: disabled
    Buffer logging: level debugging, 11 messages logged
    Trap logging: disabled
    Permit-hostdown logging: disabled
    History logging: disabled
    Device ID: disabled
    Mail logging: disabled
    ASDM logging: disabled
%ASA-5-111008: User 'enable_15' executed the 'logging buffer-size 1000000' command.
%ASA-5-111010: User 'enable_15', running 'CLI' from IP 0.0.0.0, executed 'logging buffer-size 1000000'
%ASA-5-111008: User 'enable_15' executed the 'no logging timestamp' command.
%ASA-5-111010: User 'enable_15', running 'CLI' from IP 0.0.0.0, executed 'no logging timestamp'
%ASA-7-111009: User 'enable_15' executed cmd: show logging
%ASA-2-106001: Inbound TCP connection denied from 192.168.2.2/13279 to 192.168.1.1/80 flags SYN  on interface OUTSIDE
%ASA-2-106001: Inbound TCP connection denied from 192.168.2.2/13279 to 192.168.1.1/80 flags SYN  on interface OUTSIDE
%ASA-2-106001: Inbound TCP connection denied from 192.168.2.2/13279 to 192.168.1.1/80 flags SYN  on interface OUTSIDE
%ASA-2-106001: Inbound TCP connection denied from 192.168.2.2/13279 to 192.168.1.1/80 flags SYN  on interface OUTSIDE

Above, we see the syslog messages and the code. If you want to know what each message means, you have to check the Cisco ASA Series Syslog Messages. In this case, we see message code 106001. Here is the explanation for this message, taken directly from Cisco.com:

Explanation An attempt was made to connect to an inside address is denied by the security policy that is defined for the specified traffic type. The IP address displayed is the real IP address instead of the IP address that appears through NAT. Possible tcp_flags values correspond to the flags in the TCP header that were present when the connection was denied. For example, a TCP packet arrived for which no connection state exists in the ASA, and it was dropped. The tcp_flags in this packet are FIN and ACK.

When there is much traffic going on, you’ll need to filter these messages. You can either use include to filter the message:

ASA1(config)# show logging | include 106001
%ASA-2-106001: Inbound TCP connection denied from 192.168.2.2/13279 to 192.168.1.1/80 flags SYN  on interface OUTSIDE
%ASA-2-106001: Inbound TCP connection denied from 192.168.2.2/13279 to 192.168.1.1/80 flags SYN  on interface OUTSIDE
%ASA-2-106001: Inbound TCP connection denied from 192.168.2.2/13279 to 192.168.1.1/80 flags SYN  on interface OUTSIDE
%ASA-2-106001: Inbound TCP connection denied from 192.168.2.2/13279 to 192.168.1.1/80 flags SYN  on interface OUTSIDE

Or get rid of what you don’t need with exclude:

ASA1(config)# show logging | exclude 111008|111009|111010|302010
Syslog logging: enabled
    Facility: 20
    Timestamp logging: disabled
    Hide Username logging: enabled
    Standby logging: disabled
    Debug-trace logging: disabled
    Console logging: disabled
    Monitor logging: disabled
    Buffer logging: level debugging, 19 messages logged
    Trap logging: disabled
    Permit-hostdown logging: disabled
    History logging: disabled
    Device ID: disabled
    Mail logging: disabled
    ASDM logging: disabled
%ASA-2-106001: Inbound TCP connection denied from 192.168.2.2/13279 to 192.168.1.1/80 flags SYN  on interface OUTSIDE
%ASA-2-106001: Inbound TCP connection denied from 192.168.2.2/13279 to 192.168.1.1/80 flags SYN  on interface OUTSIDE
%ASA-2-106001: Inbound TCP connection denied from 192.168.2.2/13279 to 192.168.1.1/80 flags SYN  on interface OUTSIDE
%ASA-2-106001: Inbound TCP connection denied from 192.168.2.2/13279 to 192.168.1.1/80 flags SYN  on interface OUTSIDE

These syslog messages are handy to figure out what is going on.

ASP drops

Another useful tool is to check the Accelerated Security Path (ASP) drops with the show asp drop command. This command gives an overview of packets that the ASA drops with a reason. Let’s have a look:

ASA1# show asp drop

Frame drop:
  Flow is denied by configured rule (acl-drop)                                 3

Last clearing: 12:12:46 UTC Apr 24 2020 by enable_15

Flow drop:

Last clearing: 12:12:46 UTC Apr 24 2020 by enable_15

Above, we see 3 hits because of “acl-drop”. You can reset this counter with the clear asp drop command if needed. There is a huge list of reasons, which you can find on the Cisco “show asp drop” command reference page.

Packet Capture

We can also capture packets to take a closer look. There are two options:

• Capture ASP dropped packets
• Capture any packets you want.

Let’s check both options.

ASP Drops Capture

The show asp drop command tells us why something is dropped with a counter, but that’s it. It doesn’t tell us exactly what is dropped. Let’s capture some packets so we can see them. We do this with the capture command:

ASA1(config)# capture ASP_DROPS type asp-drop acl-drop

The command above supports some extra parameters. For example, you could capture only specific protocol numbers (AH, ESP, GRE, etc.) or add an access-list. If you have a lot of traffic, you probably want a specific capture, but in my case, this is fine.

Let’s generate some more traffic from H2 to H1:

H2#telnet 192.168.1.1 80
Trying 192.168.1.1, 80 ... 
% Connection timed out; remote host not responding

Here are the packets we captured:

ASA1(config)# show capture ASP_DROPS

4 packets captured

   1: 12:16:18.018919       192.168.2.2.30833 > 192.168.1.1.80: S 178607441:178607441(0) win 4128  Drop-reason: (acl-drop) Flow is denied by configured rule

   2: 12:16:20.021666       192.168.2.2.30833 > 192.168.1.1.80: S 178607441:178607441(0) win 4128  Drop-reason: (acl-drop) Flow is denied by configured rule

   3: 12:16:24.028456       192.168.2.2.30833 > 192.168.1.1.80: S 178607441:178607441(0) win 4128  Drop-reason: (acl-drop) Flow is denied by configured rule

   4: 12:16:32.034986       192.168.2.2.30833 > 192.168.1.1.80: S 178607441:178607441(0) win 4128  Drop-reason: (acl-drop) Flow is denied by configured rule

4 packets shown

The above output is interesting. This output tells us which packets get dropped with the reason. When you are finished, don’t forget to get rid of the capture: