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Disable bad bugfix in 9.405-5 "Fix [NUTM-2840]: [AWS] UTM ignores MTU sent by DHCP server"

Do not do this if you don't feel comfortable messing up your UTM. 

I'm pretty shure this voids the warranty.  But my UTM is pretty useless using a MTU of 576 from my ISP.

The 9.405-5 upgrade introduces a mandatory, non disable, usage of the MTU provided with DHCP, if one is provided.

A lot of us have ISP's that provide bad MTU values. Like my own ISP giving a MTU of 576 (Confirmed with wireshark).

This is what you need to do to disable the usage of MTU from DHCP. Beware, you will be touching the system, and also.. it will not update MTU based on any DHCP.

(I'm not telling you how to get into the UTM, if you don't know... you have no business being there... better wait for the fix.)

In the 

/var/chroot-dhcpc/etc

There is a file named: default.conf

cat default.conf

interface "[<INTERFACE>]" {
timeout 20;
retry 60;
script "/usr/sbin/dhcp_updown.plx";
request subnet-mask, broadcast-address, time-offset,
routers, domain-name, domain-name-servers, host-name,
domain-search, nis-domain, nis-servers,
ntp-servers, interface-mtu;
[<HOSTNAME>]
}

"interface-mtu" : If you remove that (not the following ;!!!), and take your interface down/up, your MTU is possible to edit by hand again in the GUI.

AND ... it will use the number you give it, not the dumb MTU value one of your ISP's let be in their equipment because they did not bother to change it.

Finally I have a UTM back up and working, and I can get back to business.



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  • Nice catch! Thanks for the workaround :-)

    ----

    Best regards Martin ;-)

    Sophos UTM Certified Engineer 9.5
    Sophos  XG  Certified Engineer 17.1
    Homelab: 1 x SG210 XG v18 - 3xAPX530 - 1 x SG210 v9.7 - 1 x UTM 220 v9.7 - 1 x SG135 v9.7 (All Fullguard Plus licenses)

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  • Nice catch! Thanks for the workaround :-)

    ----

    Best regards Martin ;-)

    Sophos UTM Certified Engineer 9.5
    Sophos  XG  Certified Engineer 17.1
    Homelab: 1 x SG210 XG v18 - 3xAPX530 - 1 x SG210 v9.7 - 1 x UTM 220 v9.7 - 1 x SG135 v9.7 (All Fullguard Plus licenses)

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