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rjordan's avatar
rjordan
Icon for Nimbostratus rankNimbostratus
Oct 07, 2011

Disabling Reject Unmatched Packets vs Dropping with a Packet Filter

We have several virtual servers that require external auditing/scanning for various compliance certifications. Some of our compliance vendors assume that the open/filtered state from dropped UDP packets on our routers are indeed open because they get a closed state from rejected packets from the LBs. It is a pretty bad assumption, but we have to deal with it anyway.

 

 

I know I can enable the packet filter to explicitly permit the necessary ports and then set Unhandled Packet Action to DISCARD. I'm just worried that I might miss something in my rules and break a bunch of stuff.

 

 

The other alternative seems to be setting TM.RejectUnmatched to False. My concern is that I don't know all the scenarios that RSTs are used for. I guess if a client somehow gets out of sync and sends a SYN that it no longer has a connection for, the LB would not send a RST. It would just allow the client to time out? Is anyone else using this option? It is much easier to implement but I don't know if I should be worried any other side effects.

 

config
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  • nitass's avatar
    nitass
    Icon for Employee rankEmployee
    Oct 08, 2011
    not sure if i understand correctly. u don't want bigip to send reset or unreachable if port is not opening (listening), do u?

    if so, is wildcard virtual server with discard action helpful? 

    [root@iris:Active] config  b virtual list
virtual bar {
   snat automap
   destination 172.28.17.33:http
   ip protocol tcp
   rules myrule
   profiles {
      http {}
      tcp {}
   }
}

[root@iris:Active] config  b db|grep -i match
TM.ContinueMatching = false
TM.RejectUnmatched = true

[root@iris:Active] config  tcpdump -nni 0.0 host 172.28.16.50
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on 0.0, link-type EN10MB (Ethernet), capture size 108 bytes
18:02:39.077784 IP 172.28.16.50.44906 > 172.28.17.33.53:  30724+ A? www.google.com. (32)
18:02:39.077824 IP 172.28.17.33 > 172.28.16.50: ICMP 172.28.17.33 udp port 53 unreachable, length 36

[root@iris:Active] config  b virtual wildcard destination any:any mask 0.0.0.0 rule discard_rule
[root@iris:Active] config  b virtual wildcard list
virtual wildcard {
   destination any:any
   mask 0.0.0.0
   rules discard_rule
}
[root@iris:Active] config  b rule discard_rule list
rule discard_rule {
   when CLIENT_ACCEPTED {
        discard
}
}

[root@iris:Active] config  tcpdump -nni 0.0 host 172.28.16.50
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on 0.0, link-type EN10MB (Ethernet), capture size 108 bytes
18:04:12.533123 IP 172.28.16.50.58033 > 172.28.17.33.53:  1454+ A? www.google.com. (32)
18:04:17.533315 IP 172.28.16.50.58033 > 172.28.17.33.53:  1454+ A? www.google.com. (32)
18:04:22.533661 IP 172.28.16.50.58033 > 172.28.17.33.53:  1454+ A? www.google.com. (32)

3 packets captured
3 packets received by filter
0 packets dropped by kernel