[PATCH networking v2] fix #6255: SNAT: fix incorrect IP collection for sdn config

[Lukas Sichert <l.sichert@proxmox.com>]

On a host with a SimpleZone or EVPN and SNAT configured, when changing
the ip address of the bridge that handles the SNAT, the changes are not
reflected in /etc/network/interfaces.d/sdn. This happens because the ip
address is resolved via 'ip route get 8.8.8.8'. By the time this command
is executed, no ifreload has been executed yet, so ip route still
contains the old ip address.

Determine the IP version (IPv4 or IPv6) of the address used with 'ip
route'. Then parse the incoming /etc/network/interfaces file and look up
the interface currently used by 'ip route'. If the interface is still
present and has a gateway and ip address with the correct version, but
the ip address has changed, update the ip address in the SDN
configuration accordingly and log the change to the journal.

Signed-off-by: Lukas Sichert <l.sichert@proxmox.com>
---

Notes:
    changes from v1 to v2: (thanks @Gabriel, @Stefan)
    -add handling for IPV6 addresses
    -remove warnings for missing ip/gateway, as they might trigger
      in some legitimate scenarios
    -add canonical formatting for IPv4 and IPV6
    -execute make tidy
    -small restructuring of if clauses for earlier exit
    -fix typos
    
    concerning the ignoring of live routing tables:
    
    The function only determines the currently used interface (via 'ip
    route') and its ip address and returns these two. The gateway configured
    in /etc/network/interfaces is only used to verify that the interface is
    still a valid candidate after an 'ifreload -a'. As it cannot switch
    interfaces relative what to 'ip route' reports it should not be able
    influence gateway selection.
    
    If I am missing a more obscure way or an edge case where gateway
    selection could still be affected, please let me know.
    
    Also I have thought about adding "... does not match with its actual ip
    address reported by ip route ...", but after this would be displayed
    'ifreload -a' gets executed and 'ip route' changes.

 src/PVE/Network/SDN/Zones/EvpnPlugin.pm   |  5 ++--
 src/PVE/Network/SDN/Zones/Plugin.pm       | 30 +++++++++++++++++++++--
 src/PVE/Network/SDN/Zones/SimplePlugin.pm |  5 ++--
 3 files changed, 34 insertions(+), 6 deletions(-)

diff --git a/src/PVE/Network/SDN/Zones/EvpnPlugin.pm b/src/PVE/Network/SDN/Zones/EvpnPlugin.pm
index 8e7ddfd..91b8a7e 100644
--- a/src/PVE/Network/SDN/Zones/EvpnPlugin.pm
+++ b/src/PVE/Network/SDN/Zones/EvpnPlugin.pm
@@ -252,8 +252,9 @@ sub generate_sdn_config {
         if ($subnet->{snat}) {
 
             #find outgoing interface
-            my ($outip, $outiface) =
-                PVE::Network::SDN::Zones::Plugin::get_local_route_ip($checkrouteip);
+            my ($outip, $outiface) = PVE::Network::SDN::Zones::Plugin::get_local_route_ip(
+                $checkrouteip, $interfaces_config,
+            );
             if ($outip && $outiface && $is_evpn_gateway) {
                 #use snat, faster than masquerade
                 push @iface_config,
diff --git a/src/PVE/Network/SDN/Zones/Plugin.pm b/src/PVE/Network/SDN/Zones/Plugin.pm
index 826ebdf..1d18acc 100644
--- a/src/PVE/Network/SDN/Zones/Plugin.pm
+++ b/src/PVE/Network/SDN/Zones/Plugin.pm
@@ -8,6 +8,7 @@ use PVE::IPRoute2;
 use PVE::JSONSchema;
 use PVE::Cluster;
 use PVE::Network;
+use PVE::SafeSyslog;
 
 use PVE::JSONSchema qw(get_standard_option);
 use base qw(PVE::SectionConfig);
@@ -279,7 +280,7 @@ sub del_bridge_fdb {
 #helper
 
 sub get_local_route_ip {
-    my ($targetip) = @_;
+    my ($targetip, $interfaces_config) = @_;
 
     my $ip = undef;
     my $interface = undef;
@@ -296,6 +297,31 @@ sub get_local_route_ip {
 
         },
     );
+    my $ipversion = Net::IP::ip_is_ipv6($targetip) ? 6 : 4;
+    my $interface_in_config = $interfaces_config->{ifaces}->{$interface};
+    my $ip_address_in_config = undef;
+    my $gateway_in_config = undef;
+
+    if ($ipversion == 6) {
+        $ip_address_in_config = PVE::Network::canonical_ip($interface_in_config->{address6});
+        $gateway_in_config = PVE::Network::canonical_ip($interface_in_config->{gateway6});
+    } else {
+        $ip_address_in_config = PVE::Network::canonical_ip($interface_in_config->{address});
+        $gateway_in_config = PVE::Network::canonical_ip($interface_in_config->{gateway});
+    }
+
+    # if the device currently used for routing still has a valid description in /network/interfaces/, use it
+    if ($interface_in_config && $gateway_in_config) {
+        if ($ip_address_in_config eq $ip) {
+            return ($ip_address_in_config, $interface);
+        } else {
+            syslog(
+                "warning",
+                "ip address $ip_address_in_config of interface $interface in /etc/network/interfaces does not match with its ip address reported by ip route: $ip, switching to $ip_address_in_config for SNAT",
+            );
+            return ($ip_address_in_config, $interface);
+        }
+    }
     return ($ip, $interface);
 }
 
@@ -323,7 +349,7 @@ sub find_local_ip_interface_peers {
 
     #if peer is remote, find source with ip route
     foreach my $address (@{$peers}) {
-        my ($ip, $interface) = get_local_route_ip($address);
+        my ($ip, $interface) = get_local_route_ip($address, $network_config);
         return ($ip, $interface);
     }
 }
diff --git a/src/PVE/Network/SDN/Zones/SimplePlugin.pm b/src/PVE/Network/SDN/Zones/SimplePlugin.pm
index f5cd18e..ac772e7 100644
--- a/src/PVE/Network/SDN/Zones/SimplePlugin.pm
+++ b/src/PVE/Network/SDN/Zones/SimplePlugin.pm
@@ -112,8 +112,9 @@ sub generate_sdn_config {
         push @iface_config, "up ip route add $cidr dev $vnetid" if $mask == 32 && $ipversion == 4;
         if ($subnet->{snat}) {
             #find outgoing interface
-            my ($outip, $outiface) =
-                PVE::Network::SDN::Zones::Plugin::get_local_route_ip($checkrouteip);
+            my ($outip, $outiface) = PVE::Network::SDN::Zones::Plugin::get_local_route_ip(
+                $checkrouteip, $interfaces_config,
+            );
             if ($outip && $outiface) {
                 #use snat, faster than masquerade
                 push @iface_config,
-- 
2.47.3