IPS against Skype for Business (on prem at customer site)

Hi everyone,

we are having issues with the customers skype for business (still on prem) because of IPS.

After a while the voice stops and our users at the office (it is working from home or data plan) cannot voip anymore.

The IPS log shows the IP(s) of the customer and UDP flood.

As i do not know which customers use what IP, i cannot (and i don't wanna) configure all IPs (ranges are not working)  in IPS exceptions. Is there an easy way (without disabling UDP flooding) to accomplish good voice quality for everyone?

Or can i whitelist "german or some european countries" for UDP flooding?

Log entry where 213.95.x is the customers skype server

2021:11:17-14:10:56 fw3str-1 ulogd[35793]: id="2105" severity="info" sys="SecureNet" sub="ips" name="UDP flood detected" action="UDP flood" fwrule="60013" initf="eth5" srcmac="3c:13:cc:53:df:80" dstmac="00:1a:8c:f0:4e:65" srcip="213.95.x" dstip="62.96.x" proto="17" length="650" tos="0x00" prec="0x00" ttl="122" srcport="53916" dstport="20388"



  • Hallo Stephan,

    Usually, Amodin is spot on, but only his second suggestion can work.  To narrow down what you might need there, show us what result you get from the following commands:

    • zgrep '"UDP flood"' /var/log/ips/2021/11/*|grep -oP 'srcip=".*?"'|sort -n|uniq -c|sort -n
    • zgrep '"UDP flood"' /var/log/ips/2021/11/*|grep -oP 'dstip=".*?"'|sort -n|uniq -c|sort -n
    • zgrep '"UDP flood"' /var/log/ips/2021/11/*|grep -oP 'dstport=".*?"'|sort -n|uniq -c|sort -n

    Cheers - Bob

    Sophos UTM Community Moderator
    Sophos Certified Architect - UTM
    Sophos Certified Engineer - XG
    Gold Solution Partner since 2005
    MediaSoft, Inc. USA
  • Thanks - but this is only a solution afterwards. My colleagues are already angry.
    Only solution seems to be that i have to turn off "UDP flooding" then?

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