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ndisc: preserve router preferences

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If we add multiple default routes with the same metric and different
preferences, kernel merges them into a single ECMP route, with overall
preference equal to the preference of the first route
added. Therefore, the preference of individual routes is not
respected.

To avoid that, add routes with different metrics if they have
different preferences, so that they are not merged together.

We could configure only the route(s) with highest preference ignoring
the others, and the effect would be the same. However, it is better to
add all routes so that users can easily see from "ip route" that there
are multiple routers available.

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/issues/1468
https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1983

Fixes: 032b4e4371 ('core: use router preference for IPv6 routes')
This commit is contained in:
Beniamino Galvani 2024-07-08 10:46:08 +02:00
parent 815505fc43
commit c437625a76
1 changed files with 17 additions and 0 deletions
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17
src/core/ndisc/nm-ndisc.c
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@ -170,6 +170,9 @@ nm_ndisc_data_to_l3cd(NMDedupMultiIndex *multi_idx,
}
if (rdata->gateways_n > 0) {
guint metric_offset = 0;
NMIcmpv6RouterPref prev_pref = NM_ICMPV6_ROUTER_PREF_INVALID;
NMPlatformIP6Route r = {
.rt_source = NM_IP_CONFIG_SOURCE_NDISC,
.ifindex = ifindex,
@ -180,6 +183,20 @@ nm_ndisc_data_to_l3cd(NMDedupMultiIndex *multi_idx,
};
for (i = 0; i < rdata->gateways_n; i++) {
/* If we add multiple default routes with the same metric and
* different preferences, kernel merges them into a single ECMP
* route, with overall preference equal to the preference of the
* first route added. Therefore, the preference of individual routes
* is not respected.
* To avoid that, add routes with different metrics if they have
* different preferences, so that they are not merged together. Here
* the gateways are already ordered by increasing preference. */
if (i != 0 && rdata->gateways[i].preference != prev_pref) {
metric_offset++;
}
prev_pref = rdata->gateways[i].preference;
r.metric = metric_offset;
r.gateway = rdata->gateways[i].address;
r.rt_pref = rdata->gateways[i].preference;
nm_assert((NMIcmpv6RouterPref) r.rt_pref == rdata->gateways[i].preference);