File use-32-prefix-udp-backend.patch of Package flannel

From ed425bdd6fefacb0f06b35fa8f4caedf042dc84d Mon Sep 17 00:00:00 2001
From: "Cel A. Skeggs" <cela@mit.edu>
Date: Thu, 1 Feb 2018 17:49:12 -0500
Subject: [PATCH] backend/udp: Use a /32 prefix for the flannel0 interface

This avoids the kernel's creation of broadcast routes, which prevent
communication from the host with the zeroth subnet to containers on any
other hosts.
---
 backend/udp/udp_network.go | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/backend/udp/udp_network.go b/backend/udp/udp_network.go
index 1f9752f8..242ebf49 100644
--- a/backend/udp/udp_network.go
+++ b/backend/udp/udp_network.go
@@ -153,9 +153,15 @@ func configureIface(ifname string, ipn ip.IP4Net, mtu int) error {
 		return fmt.Errorf("failed to lookup interface %v", ifname)
 	}
 
-	err = netlink.AddrAdd(iface, &netlink.Addr{IPNet: ipn.ToIPNet(), Label: ""})
+	// Ensure that the device has a /32 address so that no broadcast routes are created.
+	// This IP is just used as a source address for host to workload traffic (so
+	// the return path for the traffic has an address on the flannel network to use as the destination)
+	ipnLocal := ipn
+	ipnLocal.PrefixLen = 32
+
+	err = netlink.AddrAdd(iface, &netlink.Addr{IPNet: ipnLocal.ToIPNet(), Label: ""})
 	if err != nil {
-		return fmt.Errorf("failed to add IP address %v to %v: %v", ipn.String(), ifname, err)
+		return fmt.Errorf("failed to add IP address %v to %v: %v", ipnLocal.String(), ifname, err)
 	}
 
 	err = netlink.LinkSetMTU(iface, mtu)