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dhcp: ignore the Router option when there are Classless Static Routes

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RFC 3442 says:

   If the DHCP server returns both a Classless Static Routes option and
   a Router option, the DHCP client MUST ignore the Router option.

Currently the internal client is ignoring the Router option only if
the Classless Static Routes option doesn't include a default route,
which is different from what is recommended in the RFC. Fix the behavior.

Fixes: 6adade6f21 ('dhcp: add nettools dhcp4 client')

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/issues/834
(cherry picked from commit 7651ef0386)
This commit is contained in:
Beniamino Galvani 2026-02-18 15:43:52 +01:00 • committed by Jan Vaclav
parent 5ea7d65fd1
commit a2479fc012
2 changed files with 9 additions and 12 deletions
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2
NEWS
View file

@ -7,6 +7,8 @@ Overview of changes since NetworkManager-1.56
* Allow persisting the managed state across reboots from nmcli and the D-Bus API.
* Allow changing the device's administrative state in the kernel at the same
time as a change to the managed state from nmcli and the D-Bus API.
* The DHCPv4 internal client now ignores option 3 (Router) if the lease
contains option 121 (Classless Static Route), as recommended by RFC 3442.
=============================================
NetworkManager-1.56

19
src/core/dhcp/nm-dhcp-nettools.c
View file

@ -418,7 +418,6 @@ lease_parse_routes(NDhcp4ClientLease *lease,
in_addr_t gateway;
uint8_t plen;
guint32 m;
gboolean has_router_from_classless = FALSE;
gboolean has_classless = FALSE;
guint32 default_route_metric_offset = 0;
const guint8 *l_data;
@ -434,7 +433,7 @@ lease_parse_routes(NDhcp4ClientLease *lease,
* We will however also parse one of the options into the "l3cd" for configuring routing.
* Thereby we prefer 121 over 249 over 33.
*
* Preferring 121 over 33 is defined by RFC 3443.
* Preferring 121 over 33 is defined by RFC 3442.
* Preferring 121 over 249 over 33 is made up as it makes sense (the MS docs are not very clear).
*/
for (i = 0; i < 2; i++) {
@ -460,8 +459,7 @@ lease_parse_routes(NDhcp4ClientLease *lease,
if (plen == 0) {
/* if there are multiple default routes, we add them with differing
* metrics. */
m = default_route_metric_offset++;
has_router_from_classless = TRUE;
m = default_route_metric_offset++;
} else
m = 0;
@ -495,7 +493,7 @@ lease_parse_routes(NDhcp4ClientLease *lease,
nm_str_buf_append_printf(sbuf, "%s/%d %s", dest_str, (int) plen, gateway_str);
if (has_classless) {
/* RFC 3443: if the DHCP server returns both a Classless Static Routes
/* RFC 3442: if the DHCP server returns both a Classless Static Routes
* option and a Static Routes option, the DHCP client MUST ignore the
* Static Routes option. */
continue;
@ -539,13 +537,10 @@ lease_parse_routes(NDhcp4ClientLease *lease,
continue;
}
if (has_router_from_classless) {
/* If the DHCP server returns both a Classless Static Routes option and a
* Router option, the DHCP client MUST ignore the Router option [RFC 3442].
*
* Be more lenient and ignore the Router option only if Classless Static
* Routes contain a default gateway (as other DHCP backends do).
*/
if (has_classless) {
/* RFC 3442: if the DHCP server returns both a Classless Static Routes
* option and a Router option, the DHCP client MUST ignore the Router
* option. */
continue;
}