1 files changed, 121 insertions, 0 deletions
diff --git a/shared/n-dhcp4/src/util/socket.c b/shared/n-dhcp4/src/util/socket.c
new file mode 100644
index 00000000..c25f76fd
--- /dev/null
+++ b/shared/n-dhcp4/src/util/socket.c
@@ -0,0 +1,121 @@
+/*
+ * Socket Utilities
+ */
+
+#include <assert.h>
+#include <c-stdaux.h>
+#include <errno.h>
+#include <net/if.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <sys/socket.h>
+#include "socket.h"
+
+/**
+ * socket_SIOCGIFNAME() - resolve an ifindex to an ifname
+ * @socket:                     socket to operate on
+ * @ifindex:                    index of network interface to resolve
+ * @ifname:                     buffer to store resolved name
+ *
+ * This uses the SIOCGIFNAME ioctl to resolve an ifindex to an ifname. The
+ * buffer provided in @ifnamep must be at least IFNAMSIZ bytes in size. The
+ * maximum ifname length is IFNAMSIZ-1, and this function always
+ * zero-terminates the result.
+ *
+ * This function is similar to if_indextoname(3) provided by glibc, but it
+ * allows to specify the target socket explicitly. This allows the caller to
+ * control the target network-namespace, rather than relying on the network
+ * namespace of the running process.
+ *
+ * Return: 0 on success, negative kernel error code on failure.
+ */
+int socket_SIOCGIFNAME(int socket, int ifindex, char (*ifnamep)[IFNAMSIZ]) {
+        struct ifreq req = { .ifr_ifindex = ifindex };
+        int r;
+
+        r = ioctl(socket, SIOCGIFNAME, &req);
+        if (r < 0)
+                return -errno;
+
+        /*
+         * The linux kernel guarantees that an interface name is always
+         * zero-terminated, and it always fully fits into IFNAMSIZ bytes,
+         * including the zero-terminator.
+         */
+        memcpy(ifnamep, req.ifr_name, IFNAMSIZ);
+        return 0;
+}
+
+/**
+ * socket_bind_if() - bind socket to a network interface
+ * @socket:                     socket to operate on
+ * @ifindex:                    index of network interface to bind to, or 0
+ *
+ * This binds the socket given via @socket to the network interface specified
+ * via @ifindex. It uses the underlying SO_BINDTODEVICE ioctl of the linux
+ * kernel. However, if available, if prefers the newer SO_BINDTOIFINDEX ioctl,
+ * which avoids resolving the interface name temporarily, and thus does not
+ * suffer from a race-condition.
+ *
+ * Return: 0 on success, negative error code on failure.
+ */
+int socket_bind_if(int socket, int ifindex) {
+        char ifname[IFNAMSIZ] = {};
+        int r;
+
+        c_assert(ifindex >= 0);
+
+        /*
+         * We first try the newer SO_BINDTOIFINDEX. If it is not available on
+         * the running kernel, we fall back to SO_BINDTODEVICE. This, however,
+         * requires us to first resolve the ifindex to an ifname. Note that
+         * this is racy, since the device name might theoretically change
+         * asynchronously.
+         *
+         * Using 0 as ifindex will remove the device-binding. For
+         * SO_BINDTOIFINDEX we simply pass-through the 0 to the kernel, which
+         * recognizes this correctly. For SO_BINDTODEVICE we pass the empty
+         * string, which the kernel recognizes as a request to remove the
+         * binding.
+         *
+         * The commit introducing SO_BINDTOIFINDEX first appeared in linux-5.1:
+         *
+         *     commit f5dd3d0c9638a9d9a02b5964c4ad636f06cf7e2c
+         *     Author: David Herrmann <dh.herrmann@gmail.com>
+         *     Date:   Tue Jan 15 14:42:14 2019 +0100
+         *
+         *         net: introduce SO_BINDTOIFINDEX sockopt
+         *
+         * In older kernels, setsockopt(2) is guaranteed to return ENOPROTOOPT
+         * for this ioctl.
+         */
+
+#ifdef SO_BINDTOIFINDEX
+        r = setsockopt(socket,
+                       SOL_SOCKET,
+                       SO_BINDTOIFINDEX,
+                       &ifindex,
+                       sizeof(ifindex));
+        if (r >= 0)
+                return 0;
+        else if (errno != ENOPROTOOPT)
+                return -errno;
+#endif /* SO_BINDTOIFINDEX */
+
+        if (ifindex > 0) {
+                r = socket_SIOCGIFNAME(socket, ifindex, &ifname);
+                if (r)
+                        return r;
+        }
+
+        r = setsockopt(socket,
+                       SOL_SOCKET,
+                       SO_BINDTODEVICE,
+                       ifname,
+                       strlen(ifname));
+        if (r < 0)
+                return -errno;
+
+        return 0;
+}