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LDAP request cause SYN-ACK Attack on Firewall


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we have a Load Balancing Virtual Server for our LDAP (secure) as SSL_BRIDGE running.

Since this week (we increased the number of Mobile-Phone users) this VS is causing SYN-ACK Attacks on our Firewall. So connection is dropped with message "First packet isn't SYN"


MonServiceBinding_xx.xxx.xxx.xx:636_(FMO_ldaps-16)(svc_adcontroller.internal.domain.local_LDAPS): DOWN; Last response: Failure - Probe timed out.


Someone any idea howto prevent this? Some setting we can change? 


Config from VS:

Listen Priority-

Listen Policy Expression NONE

Redirection Mode IP

Range 1

IPset -


AppFlow Logging ENABLED

Retain Connections on Cluster NO


Persistence  SSLSESSION

Time-out (mins) 2

Backup Persistence NONE

Backup Time-out (mins) 2

IPv4 Netmask

IPv6 Mask Length 128


Health Threshold 0

Client Idle Time-out 180

Minimum Autoscale Members 0

Maximum Autoscale Members 0

ICMP Virtual Server Response PASSIVE

Priority Queuing

Sure Connect

Down State Flush ENABLED

Layer 2 Parameters OFF

Trofs Persistence ENABLED

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What monitor are your services using, SSL_BRIDGE uses tcp-default by default.  The tcp monitor sends syns, waiting for a syn-ack but with no actual tcp request. Some devices will see too many of those and treat as an attack and not a probe.  Change to ping, or a different monitor type that won't trip up your firewalls.


But I also agree, be sure you don't actually want the netscaler to do packet inspection itself so end-to-end ssl might be better than ssl_bridge depending on context.

The monitor type might be the current problem though.

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Sure, Rhonda is right. I didn't think about it. Things would get better immediately if you would use TCP-secure, as this monitor has to complete the TCP handshake and establish a SSL connection, prior to sending the reset. Or even better, the built in LDAP monitor. Don't forget to check the secure checkbox :)

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Thx for your Help to both of you,  i will take a look into this (changing from ssl_bridge to ssl_tcp)  later on. 


but for now, it is fixed :-)

Our Firewall-Team found the error on their side. Some wired/wrong Routing-Table.



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