From 3e22e330048e38b708b9cfffbfd31aaf5791a7e9 Mon Sep 17 00:00:00 2001
From: Ryan Wilson <ryan.wilson@white.space>
Date: Thu, 2 Jul 2026 12:47:00 +0100
Subject: [PATCH] fix(network-vmnet): install explicit subnet route, not just
 interface-scoped default
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vmnet_network_create() only results in an interface-scoped default
route for the container subnet, never an explicit non-scoped route.
Routing to containers is therefore entirely dependent on which global
default route currently wins on the host. Any other tool that installs
its own default route (a VPN client, a Tailscale exit node, etc.) can
shadow it and silently break all container connectivity, even though
this subnet's own route should normally win under standard
longest-prefix-match rules.

Install an explicit route ourselves via route(8), matching the existing
Process-based pattern this codebase already uses for pfctl in
PacketFilter.swift. Best-effort: failures are logged, not thrown, since
this is a resilience improvement rather than a hard requirement for the
network to otherwise start successfully. It also doesn't fully
eliminate exposure to tools that actively remove routes they don't own
(see tailscale/tailscale#18653) — only to the simpler and more common
case of a competing default route being added without deleting others.

Fixes apple/container#1881.
---
 .../Server/ReservedVmnetNetwork.swift         | 56 +++++++++++++++++++
 1 file changed, 56 insertions(+)

diff --git a/Sources/Services/NetworkVmnet/Server/ReservedVmnetNetwork.swift b/Sources/Services/NetworkVmnet/Server/ReservedVmnetNetwork.swift
index 5b0fee6ad..dc4551660 100644
--- a/Sources/Services/NetworkVmnet/Server/ReservedVmnetNetwork.swift
+++ b/Sources/Services/NetworkVmnet/Server/ReservedVmnetNetwork.swift
@@ -170,6 +170,18 @@ public final class ReservedVmnetNetwork: ContainerNetworkServer.Network {
         let runningSubnet = try CIDRv4(lower: lower, upper: upper)
         let runningGateway = IPv4Address(runningSubnet.lower.value + 1)
 
+        // `vmnet_network_create` only installs an interface-scoped default route
+        // for this subnet, not an explicit non-scoped one — so routing to
+        // containers is entirely dependent on which *global* default route
+        // currently wins on the host. Any other tool that installs its own
+        // default route (a VPN client, a Tailscale exit node, etc.) can shadow
+        // it and break container connectivity even though this subnet's own
+        // route would normally win under standard longest-prefix-match rules.
+        // Install an explicit route ourselves so container traffic doesn't
+        // depend on default-route ordering at all. Best-effort: failure here
+        // shouldn't prevent the network from otherwise starting successfully.
+        Self.installExplicitSubnetRoute(subnet: lower, mask: IPv4Address(maskValue), gateway: runningGateway, log: log)
+
         var prefixAddr = in6_addr()
         var prefixLength = UInt8(0)
         vmnet_network_get_ipv6_prefix(network, &prefixAddr, &prefixLength)
@@ -199,4 +211,48 @@ public final class ReservedVmnetNetwork: ContainerNetworkServer.Network {
             ipv6Subnet: runningV6Subnet,
         )
     }
+
+    /// Installs an explicit, non-scoped route for `subnet` via `gateway`, so
+    /// routing to this network's containers doesn't depend on which default
+    /// route currently wins on the host (see the call site for why). This is
+    /// a defense-in-depth measure, not a hard requirement — some other tool
+    /// can still remove or shadow the route later (see apple/container#1881
+    /// and tailscale/tailscale#18653 for one such case), so failures here are
+    /// logged rather than thrown; they shouldn't prevent the network from
+    /// otherwise starting successfully.
+    private static func installExplicitSubnetRoute(subnet: IPv4Address, mask: IPv4Address, gateway: IPv4Address, log: Logger) {
+        let process = Foundation.Process()
+        process.executableURL = URL(fileURLWithPath: "/sbin/route")
+        process.arguments = [
+            "add", "-net", subnet.description,
+            "-netmask", mask.description,
+            "-interface", gateway.description,
+        ]
+        let stderr = Pipe()
+        process.standardOutput = Pipe()
+        process.standardError = stderr
+
+        do {
+            try process.run()
+        } catch {
+            log.warning(
+                "failed to launch route(8) to install explicit subnet route",
+                metadata: ["subnet": "\(subnet)", "mask": "\(mask)", "gateway": "\(gateway)", "error": "\(error)"]
+            )
+            return
+        }
+        process.waitUntilExit()
+
+        guard process.terminationStatus == 0 else {
+            let message = String(data: stderr.fileHandleForReading.readDataToEndOfFile(), encoding: .utf8) ?? ""
+            log.warning(
+                "route(8) failed to install explicit subnet route",
+                metadata: [
+                    "subnet": "\(subnet)", "mask": "\(mask)", "gateway": "\(gateway)",
+                    "status": "\(process.terminationStatus)", "message": "\(message)",
+                ]
+            )
+            return
+        }
+    }
 }