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Differential D45854

if_gif(4): Support the IFF_LINK0 flag to change the MTU handling for IPv6
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Authored by hrs on Jul 3 2024, 8:35 AM.
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network
manpages
Summary

The gif(4) interface always used IPV6_MINMTU when the outer protocol was
IPv6 so that packets would not be dropped between the tunnel endpoints.
The interface MTU was ignored because path MTU discovery was too expensive.

This change adds IFF_LINK0 flag support to revert it to normal behavior,
and documents the backgrounds. While using IPV6_MINMTU is still
the safest option for IPv6, ignoring the configured interface MTU is
confuses the users, and using a larger MTU is sometimes useful.

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rG FreeBSD src repository
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Buildable 58471
Build 55359: arc lint + arc unit

Event Timeline

hrs created this revision.Jul 3 2024, 8:35 AM
Herald added subscribers: glebius, melifaro, ae, imp. · View Herald TranscriptJul 3 2024, 8:35 AM
hrs requested review of this revision.Jul 3 2024, 8:35 AM
Harbormaster completed remote builds in B58471: Diff 140501.Jul 3 2024, 8:35 AM
hrs added reviewers: network, manpages.Jul 3 2024, 8:36 AM
ae added a comment.Jul 3 2024, 10:35 AM

Maybe it would be better implement such feature via named flag, like ignore_source is implemented? Also if_gre(4) has the same problem.

emaste added a subscriber: emaste.Aug 14 2024, 5:48 PM
zlei added a subscriber: zlei.Aug 15 2024, 6:14 AM
kazubu_jtime.net added a subscriber: kazubu_jtime.net.Aug 25 2024, 5:53 AM
bcr accepted this revision as: manpages.Sep 5 2024, 7:13 AM
bcr added a subscriber: bcr.

OK for the man page part of the change.

meta added a subscriber: meta.Mar 17 2025, 3:36 PM

+1 to implement this using a named flag.

Herald added a subscriber: ziaee. · View Herald TranscriptMar 17 2025, 3:36 PM
meta mentioned this in D51297: if_gif(4): Support the NOCLAMP flag to change the MTU handling for IPv6.Jul 14 2025, 8:25 AM
meta added a comment.Jul 14 2025, 8:33 AM

Raised D51297 for the named flag version.

meta mentioned this in rG93c2d7d5265c: if_gif(4): Support the NOCLAMP flag to change MTU handling for IPv6.Jul 21 2025, 1:48 PM
hrs mentioned this in D51866: if_gif: Use the MTU on the gif interface for the outgoing packets.Aug 11 2025, 11:14 PM
hrs abandoned this revision.Aug 12 2025, 12:55 AM

D51866 is created because another approach seems reasonable.

meta mentioned this in D52594: if_gif: Use the MTU on the gif interface for the outgoing packets.Sep 17 2025, 3:46 PM

Revision Contents
Changeset List

PathSize
share/
man/
man4/
152 lines
sys/
netinet6/
13 lines

Diff 140501

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share/man/man4/gif.4

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sys/netinet6/in6_gif.c

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