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Old 3rd January 2019
apfelgluck apfelgluck is offline
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Join Date: Sep 2016
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Default [pf] MTU question


I have questions about this line I see on this site.

match all scrub (no-df max-mss 1440)
What is the use of this rule if we allow with pf the discovery of the MTU by the hosts with a rule like :

pass inet proto icmp all icmp-type { echoreq, unreach } from $LAN to any
With the above rule, the hosts are supposed to discover the MTU by themselve and to adapt to it.
Correct ?

So why forcing the PTU at an arbitrary value ?
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Old 3rd January 2019
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jggimi jggimi is offline
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Join Date: May 2008
Location: USA
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As I perceive it, these are two different provisioning recommendations.
  • Path MTU Discovery permits an endpoint to find an optimum MTU for a route.
  • The scrub instruction directs PF to conduct incoming packet normalization.
If you don't get better clarity from someone here, try asking Peter directly. He's open to receiving clarifying questions about his tutorial.
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Old 3rd January 2019
TronDD TronDD is offline
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I'm not sure exactly where you're looking but in the "scrub" section, the rule is
match *in* all scrub...
You're limiting the MTU of incoming packages while your second rule is allowing outgoing connections to find the maximum MTU size.
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