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device: do not set MTU twice in stage3

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The pair of _commit_mtu() calls in activate_stage3_ip_config() are very
heavily commented, but it is still not clear why would there be two of
them.

Remove one, and try to clarify the situation in an updated comment.

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/2231
This commit is contained in:
Lubomir Rintel 2025-06-30 15:58:39 +02:00 • committed by Íñigo Huguet
parent 9e0551aefd
commit c2ba552b5b
1 changed files with 14 additions and 12 deletions
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26
src/core/devices/nm-device.c
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@ -13454,13 +13454,21 @@ activate_stage3_ip_config(NMDevice *self)
nm_device_get_ip_iface(self));
}
/* We currently will attach ports in the state change NM_DEVICE_STATE_IP_CONFIG above.
* Note that kernel changes the MTU of bond ports, so we want to commit the MTU
* afterwards!
/*
* Let's make sure MTU matches what is configured. The reason it's done at this
* precise location is twofold:
*
* This might reset the MTU to something different from the bond controller and
* it might not be a working configuration. But it's what the user asked for, so
* let's do it! */
* (1) Attaching ports above might affect the MTU.
*
* We currently will attach ports in the state change NM_DEVICE_STATE_IP_CONFIG
* above. This might reset the MTU to something different from the bond controller
* and it might not be a working configuration. But it's what the user asked for.
*
* (2) When MTU is under 1280 IPv6 can not work.
*
* Kernel will not expose sysctls, create or accept addresses that are needed for IPv6
* configuration when the MTU is too small (under 1280).
*/
_commit_mtu(self);
if (!nm_device_managed_type_is_external(self)
@ -13469,12 +13477,6 @@ activate_stage3_ip_config(NMDevice *self)
&& !NM_IN_STRSET(priv->ipv6_method,
NM_SETTING_IP6_CONFIG_METHOD_DISABLED,
NM_SETTING_IP6_CONFIG_METHOD_IGNORE)) {
/* Ensure the MTU makes sense. If it was below 1280 the kernel would not
* expose any ipv6 sysctls or allow presence of any addresses on the interface,
* including LL, which * would make it impossible to autoconfigure MTU to a
* correct value. */
_commit_mtu(self);
/* Any method past this point requires an IPv6LL address. Use NM-controlled
* IPv6LL if this is not an assumed connection, since assumed connections
* will already have IPv6 set up.