Solution: Cisco Firewall Causing 500 Firewall Error on SMTP Email Send

Solution: Cisco Firewall Causing 500 Firewall Error on SMTP Email Send

I recently made some changes to our Cisco UC540 Firewall/Router/PBX unit  and a problem I had long thought we’d seen the back of reared it’s ugly head, random emails bouncing with an error similar to the following:

#500 Firewall Error ##

As has been documented in numerous places, this is caused by Cisco firewalls being over-zealous with their outbound ESMTP packet inspection, it can be disabled easily enough (instructions further down). What frustrated me is that I know I saved the configuration last time I did this and performed my usual backup but here the problem was again.
After a little research, it turns out that changes to the UC540 via CCA can – without warning or notification – re-enable the packet filter and so re-awaken the demon problem. Unfortunately it doesn’t seem there is anyway around this at the moment.

How to Disable SMTP Packet Inspection

These instructions work on our Cisco UC540 box but may be slightly different for yours, for anyone looking for a reference for the Cisco Command Line Interface, I highly recommend keeping a paper copy of the CCNA Portable Command Guide around:

Telnet in to your unit

Before you do anything else (including entering Config t mode), type:

Show run int FastEthernet0/0

Current configuration : 309 bytes
interface FastEthernet0/0
description $FW_OUTSIDE$
ip address
ip access-group 104 in
ip nat outside
ip inspect SDM_MEDIUM out
ip virtual-reassembly in
duplex auto
speed auto
service-policy input sdmappfwp2p_SDM_MEDIUM
service-policy output sdmappfwp2p_SDM_MEDIUM

Note down the portion in bold (SDM_MEDIUM)

Enter the configuration mode by entering the following:

Config T

Enter the following command replacing SDM_MEDIUM with whatever your script returned:

no ip inspect name SDM_MEDIUM esmtp

If done correctly you will get no confirmation and no error which in Cisco-land is a good thing, programming a confirmation message is obviously beyond Cisco’s developers.

If you’ve got a Cisco unit that you’re now responsible for I’d also recommend getting the CCNA Command Quick Reference – good to have in times of need!

Bob McKay

About Bob McKay

Bob is a Founder of Seguro Ltd, a full time father and husband, part-time tinkerer-with-wires, coder, Muay Thai practitioner, builder and cook. Big fan of equality, tolerance and co-existence.

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