Track a packet as it goes through the kernel (linux)

Question

I have two machines which are set up to use Ip-Security and machine A (lets call them A and B) has a socket which is bound to a particular UDP port on the local machine and it p

Answer 1

Yes, as Dan said, SystemTap is useful. But my favorite is ftrace.

For reference:

Path of UDP packet in linux kernel

So for tracing the network traffic in general, put the following in a bash shell and run it as root:

mkdir /debug
mount -t debugfs nodev /debug
mount -t debugfs nodev /sys/kernel/debug
echo '*' >/debug/tracing/set_ftrace_filter
echo function_graph >/debug/tracing/current_tracer
echo 1 >/debug/tracing/tracing_on
sleep 20
echo 0 >/debug/tracing/tracing_on
cat /debug/tracing/trace > /tmp/tracing.out$$

And so on the receiving ingress path:

 5)               |              tcp_recvmsg() {
 5)               |                lock_sock_nested() {
 5)   0.042 us    |                  _cond_resched();
 5)               |                  _raw_spin_lock_bh() {
 5)   0.040 us    |                    local_bh_disable();
 5)   0.414 us    |                  }
 5)   0.040 us    |                  _raw_spin_unlock();
 5)   0.040 us    |                  local_bh_enable();
 5)   1.814 us    |                }
 5)               |                skb_copy_datagram_iovec() {
 5)   0.042 us    |                  _cond_resched();
 5)   0.588 us    |                }
 5)   0.042 us    |                tcp_rcv_space_adjust();
 5)               |                __kfree_skb() {
 5)               |                  skb_release_all() {
 5)               |                    skb_release_head_state() {
 5)   0.044 us    |                      sock_rfree();
 5)   0.670 us    |                    }
 5)               |                    skb_release_data() {
 5)               |                      put_page() {
 5)   0.049 us    |                        put_compound_page();
 5)   0.449 us    |                      }

and this:

Netlink processing:

 6)               |          rtnetlink_rcv() {
 6)               |            mutex_lock() {
 6)   0.090 us    |              _cond_resched();
 6)   1.455 us    |            }
 6)               |            netlink_rcv_skb() {
 6)               |              rtnetlink_rcv_msg() {
 6)   0.150 us    |                mutex_unlock();
 6)               |                __netlink_dump_start() {
 6)               |                  netlink_lookup() {
 6)   0.091 us    |                    _raw_read_lock();
 6)   0.100 us    |                    netlink_compare();
 6)   1.791 us    |                  }
 6)               |                  mutex_lock() {
 6)   0.095 us    |                    _cond_resched();
 6)   0.913 us    |                  }
 6)   0.100 us    |                  try_module_get();
 6)   0.090 us    |                  mutex_unlock();

and this is the ingress also:

 3)               |                                                tcp_v4_rcv() {
 3)               |                                                  sk_filter() {
 3)               |                                                    security_sock_rcv_skb() {
 3)   0.076 us    |                                                      cap_socket_sock_rcv_skb();
 3)   0.867 us    |                                                    }
 3)   1.630 us    |                                                  }
 3)   0.076 us    |                                                  _raw_spin_lock();
 3)   0.477 us    |                                                  tcp_prequeue();
 3)               |                                                  tcp_v4_do_rcv() {
 3)   0.088 us    |                                                    tcp_md5_do_lookup();
 3)   0.109 us    |                                                    tcp_parse_md5sig_option();
 3)   0.072 us    |                                                    ipv4_dst_check();
 3)               |                                                    tcp_rcv_established() {
 3)   0.076 us    |                                                      tcp_parse_aligned_timestamp.part.34();
 3)               |                                                      tcp_queue_rcv() {
 3)               |                                                        tcp_try_coalesce.part.41() {
 3)   0.835 us    |                                                          skb_try_coalesce();
 3)   1.722 us    |                                                        }
 3)   2.637 us    |                                                      }

And this is the egress (starting from the syscall "sendmsg()"):

 5)               |  SyS_sendmsg() {
 5)               |    __sys_sendmsg() {
 5)               |      sockfd_lookup_light() {
 5)   0.080 us    |        fget_light();
 5)   0.502 us    |      }
 5)               |      ___sys_sendmsg() {
 5)   0.117 us    |        copy_msghdr_from_user();
 5)   0.101 us    |        verify_iovec();
 5)               |        sock_sendmsg() {
 5)               |          security_socket_sendmsg() {
 5)               |            apparmor_socket_sendmsg() {
 5)   0.092 us    |              aa_revalidate_sk();
 5)   0.580 us    |            }
 5)   1.044 us    |          }
 5)               |          unix_stream_sendmsg() {
 5)   0.113 us    |            wait_for_unix_gc();
 5)               |            security_socket_getpeersec_dgram() {
 5)   0.044 us    |              apparmor_socket_getpeersec_dgram();
 5)   0.479 us    |            }
 5)               |            sock_alloc_send_pskb() {
 5)               |              __alloc_skb() {
 5)               |                kmem_cache_alloc_node() {
 5)   0.042 us    |                  _cond_resched();
 5)   0.648 us    |                }
 5)               |                __kmalloc_reserve.isra.27() {
 5)               |                  __kmalloc_node_track_caller() {
 5)   0.074 us    |                    kmalloc_slab();
 5)   0.040 us    |                    _cond_resched();
 5)   0.504 us    |                    __slab_alloc();
 5)   1.878 us    |                  }
 5)   2.276 us    |                }
 5)   0.175 us    |                ksize();
 5)   4.217 us    |              }

Hope you enjoy....