Commit e4fc408e authored by Daniel Borkmann's avatar Daniel Borkmann Committed by David S. Miller
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packet: nlmon: virtual netlink monitoring device for packet sockets

Currently, there is no good possibility to debug netlink traffic that
is being exchanged between kernel and user space. Therefore, this patch
implements a netlink virtual device, so that netlink messages will be
made visible to PF_PACKET sockets. Once there was an approach with a
similar idea [1], but it got forgotten somehow.

I think it makes most sense to accept the "overhead" of an extra netlink
net device over implementing the same functionality from PF_PACKET
sockets once again into netlink sockets. We have BPF filters that can
already be easily applied which even have netlink extensions, we have
RX_RING zero-copy between kernel- and user space that can be reused,
and much more features. So instead of re-implementing all of this, we
simply pass the skb to a given PF_PACKET socket for further analysis.

Another nice benefit that comes from that is that no code needs to be
changed in user space packet analyzers (maybe adding a dissector, but
not more), thus out of the box, we can already capture pcap files of
netlink traffic to debug/troubleshoot netlink problems.

Also thanks goes to Thomas Graf, Flavio Leitner, Jesper Dangaard Brouer.


Signed-off-by: default avatarDaniel Borkmann <>
Signed-off-by: default avatarDavid S. Miller <>
parent bcbde0d4
......@@ -240,6 +240,16 @@ config VIRTIO_NET
This is the virtual network driver for virtio. It can be used with
lguest or QEMU based VMMs (like KVM or Xen). Say Y or M.
config NLMON
tristate "Virtual netlink monitoring device"
This option enables a monitoring net device for netlink skbs. The
purpose of this is to analyze netlink messages with packet sockets.
Thus applications like tcpdump will be able to see local netlink
messages if they tap into the netlink device, record pcaps for further
diagnostics, etc. This is mostly intended for developers or support
to debug netlink issues. If unsure, say N.
endif # NET_CORE
......@@ -22,6 +22,7 @@ obj-$(CONFIG_TUN) += tun.o
obj-$(CONFIG_VETH) += veth.o
obj-$(CONFIG_VIRTIO_NET) += virtio_net.o
obj-$(CONFIG_VXLAN) += vxlan.o
obj-$(CONFIG_NLMON) += nlmon.o
# Networking Drivers
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/netdevice.h>
#include <linux/netlink.h>
#include <net/net_namespace.h>
#include <linux/if_arp.h>
struct pcpu_lstats {
u64 packets;
u64 bytes;
struct u64_stats_sync syncp;
static netdev_tx_t nlmon_xmit(struct sk_buff *skb, struct net_device *dev)
int len = skb->len;
struct pcpu_lstats *stats = this_cpu_ptr(dev->lstats);
stats->bytes += len;
return NETDEV_TX_OK;
static int nlmon_is_valid_mtu(int new_mtu)
return new_mtu >= sizeof(struct nlmsghdr) && new_mtu <= INT_MAX;
static int nlmon_change_mtu(struct net_device *dev, int new_mtu)
if (!nlmon_is_valid_mtu(new_mtu))
return -EINVAL;
dev->mtu = new_mtu;
return 0;
static int nlmon_dev_init(struct net_device *dev)
dev->lstats = alloc_percpu(struct pcpu_lstats);
return dev->lstats == NULL ? -ENOMEM : 0;
static void nlmon_dev_uninit(struct net_device *dev)
static struct netlink_tap nlmon_tap;
static int nlmon_open(struct net_device *dev)
return netlink_add_tap(&nlmon_tap);
static int nlmon_close(struct net_device *dev)
return netlink_remove_tap(&nlmon_tap);
static struct rtnl_link_stats64 *
nlmon_get_stats64(struct net_device *dev, struct rtnl_link_stats64 *stats)
int i;
u64 bytes = 0, packets = 0;
for_each_possible_cpu(i) {
const struct pcpu_lstats *nl_stats;
u64 tbytes, tpackets;
unsigned int start;
nl_stats = per_cpu_ptr(dev->lstats, i);
do {
start = u64_stats_fetch_begin_bh(&nl_stats->syncp);
tbytes = nl_stats->bytes;
tpackets = nl_stats->packets;
} while (u64_stats_fetch_retry_bh(&nl_stats->syncp, start));
packets += tpackets;
bytes += tbytes;
stats->rx_packets = packets;
stats->tx_packets = 0;
stats->rx_bytes = bytes;
stats->tx_bytes = 0;
return stats;
static u32 always_on(struct net_device *dev)
return 1;
static const struct ethtool_ops nlmon_ethtool_ops = {
.get_link = always_on,
static const struct net_device_ops nlmon_ops = {
.ndo_init = nlmon_dev_init,
.ndo_uninit = nlmon_dev_uninit,
.ndo_open = nlmon_open,
.ndo_stop = nlmon_close,
.ndo_start_xmit = nlmon_xmit,
.ndo_get_stats64 = nlmon_get_stats64,
.ndo_change_mtu = nlmon_change_mtu,
static struct netlink_tap nlmon_tap __read_mostly = {
.module = THIS_MODULE,
static void nlmon_setup(struct net_device *dev)
dev->type = ARPHRD_NETLINK;
dev->tx_queue_len = 0;
dev->netdev_ops = &nlmon_ops;
dev->ethtool_ops = &nlmon_ethtool_ops;
dev->destructor = free_netdev;
dev->flags = IFF_NOARP;
/* That's rather a softlimit here, which, of course,
* can be altered. Not a real MTU, but what is to be
* expected in most cases.
dev->mtu = NLMSG_GOODSIZE;
static __init int nlmon_register(void)
int err;
struct net_device *nldev;
nldev = = alloc_netdev(0, "netlink", nlmon_setup);
if (unlikely(nldev == NULL))
return -ENOMEM;
err = register_netdev(nldev);
if (unlikely(err))
return err;
static __exit void nlmon_unregister(void)
struct net_device *nldev =;
MODULE_AUTHOR("Daniel Borkmann <>");
MODULE_AUTHOR("Mathieu Geli <>");
MODULE_DESCRIPTION("Netlink monitoring device");
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