[Openvpn-devel] [PATCH] Bugfix: Set broadcast address on interface

[Sebastian Marsching]

From 5e9ec24cefe452c002828104d62d43ae206337e3 Mon Sep 17 00:00:00 2001
From: Sebastian Marsching <sebastian-git-2...@marsching.com>
Date: Fri, 12 Sep 2025 22:34:43 +0200
Subject: [PATCH] Bugfix: Set broadcast address on interface.

This fixes a problem that was introduced in OpenVPN 2.5. Previously,
the ifconfig utility was used for adding the local address to an
interface. This utility automatically sets the correct broadcast address
based on the given unicast address and netmask.

Due to switching to iproute and Netlink, this does not happen
automatically any longer, which means that applications that rely on
broadcasts do not work correctly.

This patch fixes this issue both when using iproute (by telling iproute
to set the broadcast address based on the local address and prefix) and
when using Netlink (by calculating the correct broadcast address and
setting it).

Signed-off-by: Sebastian Marsching <sebastian-git-2...@marsching.com>
---
 src/openvpn/networking_iproute2.c |  2 +-
 src/openvpn/networking_sitnl.c    | 13 ++++++++++++-
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/src/openvpn/networking_iproute2.c 
b/src/openvpn/networking_iproute2.c
index e9be3a45..773571d6 100644
--- a/src/openvpn/networking_iproute2.c
+++ b/src/openvpn/networking_iproute2.c
@@ -150,7 +150,7 @@ net_addr_v4_add(openvpn_net_ctx_t *ctx, const char *iface, 
const in_addr_t *addr

     const char *addr_str = print_in_addr_t(*addr, 0, &ctx->gc);

-    argv_printf(&argv, "%s addr add dev %s %s/%d", iproute_path, iface, 
addr_str, prefixlen);
+    argv_printf(&argv, "%s addr add dev %s %s/%d broadcast +", iproute_path, 
iface, addr_str, prefixlen);
     argv_msg(M_INFO, &argv);
     openvpn_execve_check(&argv, ctx->es, S_FATAL, "Linux ip addr add failed");

diff --git a/src/openvpn/networking_sitnl.c b/src/openvpn/networking_sitnl.c
index 4210e92c..4294a7a5 100644
--- a/src/openvpn/networking_sitnl.c
+++ b/src/openvpn/networking_sitnl.c
@@ -760,7 +760,8 @@ sitnl_addr_set(int cmd, uint32_t flags, int ifindex, 
sa_family_t af_family,
 {
     struct sitnl_addr_req req;
     uint32_t size;
-    int ret = -EINVAL;
+    inet_address_t broadcast;
+    int ret = -EINVAL, i;

     CLEAR(req);

@@ -803,6 +804,16 @@ sitnl_addr_set(int cmd, uint32_t flags, int ifindex, 
sa_family_t af_family,
         SITNL_ADDATTR(&req.n, sizeof(req), IFA_LOCAL, local, size);
     }

+    if (af_family == AF_INET && local && !remote && prefixlen <= 30)
+    {
+        broadcast = *local;
+        for (i = 31; i >= prefixlen; i--)
+        {
+            broadcast.ipv4 |= htonl(1<<(31-i));
+        }
+        SITNL_ADDATTR(&req.n, sizeof(req), IFA_BROADCAST, &broadcast, size);
+    }
+
     ret = sitnl_send(&req.n, 0, 0, NULL, NULL);
     if (ret == -EEXIST)
     {
--
2.50.0

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