UDP dont fragment bit

Robert Watson rwatson at FreeBSD.org
Tue Sep 20 05:45:43 PDT 2005

On Sun, 18 Sep 2005, Milscvaer wrote:

> FreeBSD ought to add per-socket socket options to allow a programmer to 
> turn on and off the don't fragment bit for UDP and the UDP checksum, on 
> a per socket basis.
> FreeBSD needs to implement this feature. We requested this same feature 
> years ago and I cannot believe such a simple, basic feature is still not 
> implemented, or not documented in the setsockopt manpage anyhow. This 
> needs to be implemented. We need it. Trust me.

For whatever reason, it turns out that you and only you have requested 
this feature.  For typicaly network debugging applications, a raw socket 
is used, which allows the direct frobbing the the IP DF bit.  For example, 
in traceroute(8).

Could you tell us a bit more about the application and proposed use? 
Presumably forcing the DF bit isn't much use unless you can also retrieve 
useful reporting on the ICMP errors associated with needing the fragment?

Robert N M Watson

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