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n-dhcp4: discard NAKs from other servers in SELECTING

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I got a report of a scenario where multiple servers reply to a REQUEST
in SELECTING, and all servers send NAKs except the one which sent the
offer, which replies with a ACK. In that scenario, n-dhcp4 is not able
to obtain a lease because it restarts from INIT as soon as the first
NAK is received. For comparison, dhclient can get a lease because it
ignores all NAKs in SELECTING.

Arguably, the network is misconfigured there, but it would be great if
n-dhcp4 could still work in such scenario.

According to RFC 2131, ACK and NAK messages from server must contain a
server-id option. The RFC doesn't explicitly say that the client
should check the option, but I think it's a reasonable thing to do, at
least for NAKs.

This patch stores the server-id of the REQUEST in SELECTING, and
compares it with the server-id from NAKs, to discard other servers'
replies.

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1144
This commit is contained in:
Beniamino Galvani 2022-03-10 12:07:49 +01:00
parent 4cc0f41e45
commit 118561e284
2 changed files with 20 additions and 0 deletions
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19
src/n-dhcp4/src/n-dhcp4-c-connection.c
View file

@ -705,6 +705,7 @@ int n_dhcp4_c_connection_select_new(NDhcp4CConnection *connection,
message->userdata.start_time = offer->userdata.start_time;
message->userdata.base_time = offer->userdata.base_time;
message->userdata.client_addr = client.s_addr;
message->userdata.server_id = server.s_addr;
n_dhcp4_incoming_get_xid(offer, &xid);
n_dhcp4_outgoing_set_xid(message, xid);
@ -1224,6 +1225,24 @@ int n_dhcp4_c_connection_dispatch_io(NDhcp4CConnection *connection,
serv_addr, sizeof(serv_addr)));
}
if (type == N_DHCP4_MESSAGE_NAK &&
connection->request->userdata.server_id != INADDR_ANY) {
struct in_addr server;
r = n_dhcp4_incoming_query_server_identifier(message, &server);
if (r)
return N_DHCP4_E_AGAIN;
if (connection->request->userdata.server_id != server.s_addr) {
n_dhcp4_log(connection->log_queue,
LOG_DEBUG,
"discarded NAK with wrong server-id %s",
inet_ntop(AF_INET, &server,
serv_addr, sizeof(serv_addr)));
return N_DHCP4_E_AGAIN;
}
}
switch (type) {
case N_DHCP4_MESSAGE_OFFER:
case N_DHCP4_MESSAGE_ACK:

1
src/n-dhcp4/src/n-dhcp4-private.h
View file

@ -202,6 +202,7 @@ struct NDhcp4Outgoing {
uint8_t type;
uint8_t message_type;
uint32_t client_addr;
uint32_t server_id;
uint64_t start_time;
uint64_t base_time;
uint64_t send_time;