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The IP_MTU_DISCOVER description from "man 7 ip" is too short. As far as I understand IP_PMTUDISC_DONT/IP_PMTUDISC_DO values enable/disable packet fragmentation on a route.

What are IP_PMTUDISC_WANT and IP_PMTUDISC_PROBE? What do they need for? How they affect IP traffic? Can they be used with UDP?

Thanks.

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1 Answer 1

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MTU discovery doesn't necessarily mean that it's going to enable or disable packet fragmentation - it means that it's going to use path MTU discovery to adjust MTU sizes for a path to avoid fragmentation.

Don't mess with these settings unless you have a very good reason to do so, and you fully understand the changes that you're making; MTU discovery is a very good thing.

IP_PMTUDISC_WANT lets the setting be specifically defined on each route; in practice, it's essentially the same as IP_PMTUDISC_DO.

IP_PMTUDISC_PROBE is just for if you want to be able to send packets larger than the observed path MTU; this is probably not useful to you.

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Thank you for the answer. I have IP_PMTUDISC_WANT set by default on my Ubuntu. And in my network I have some routers which block ICMP packets. When I send UDP packets larger than MTU in some segment, the receiver never receive these packets. And now I understand why. It looks that I need to set IP_PMTUDISC_DONT for my UDP traffic. –  Roman Aug 9 '11 at 8:23

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