5.1. Network
The following sections showcase scripts that trace network-related functions and build a profile of network activity.
5.1.1. Network Profiling
This section describes how to profile network activity. nettop.stp provides a glimpse into how much network traffic each process is generating on a machine.
nettop.stp
#! /usr/bin/env stap global ifxmit, ifrecv global ifmerged probe netdev.transmit { ifxmit[pid(), dev_name, execname(), uid()] <<< length } probe netdev.receive { ifrecv[pid(), dev_name, execname(), uid()] <<< length } function print_activity() { printf("%5s %5s %-7s %7s %7s %7s %7s %-15s\n", "PID", "UID", "DEV", "XMIT_PK", "RECV_PK", "XMIT_KB", "RECV_KB", "COMMAND") foreach ([pid, dev, exec, uid] in ifrecv) { ifmerged[pid, dev, exec, uid] += @count(ifrecv[pid,dev,exec,uid]); } foreach ([pid, dev, exec, uid] in ifxmit) { ifmerged[pid, dev, exec, uid] += @count(ifxmit[pid,dev,exec,uid]); } foreach ([pid, dev, exec, uid] in ifmerged-) { n_xmit = @count(ifxmit[pid, dev, exec, uid]) n_recv = @count(ifrecv[pid, dev, exec, uid]) printf("%5d %5d %-7s %7d %7d %7d %7d %-15s\n", pid, uid, dev, n_xmit, n_recv, n_xmit ? @sum(ifxmit[pid, dev, exec, uid])/1024 : 0, n_recv ? @sum(ifrecv[pid, dev, exec, uid])/1024 : 0, exec) } print("\n") delete ifxmit delete ifrecv delete ifmerged } probe timer.ms(5000), end, error { print_activity() }
Note that
function print_activity()
uses the following expressions:
n_xmit ? @sum(ifxmit[pid, dev, exec, uid])/1024 : 0 n_recv ? @sum(ifrecv[pid, dev, exec, uid])/1024 : 0
These expressions are if/else conditionals. The first statement is simply a more concise way of writing the following psuedo code:
if n_recv != 0 then @sum(ifrecv[pid, dev, exec, uid])/1024 else 0
nettop.stp tracks which processes are generating network traffic on the system, and provides the following information about each process:
PID
— the ID of the listed process.UID
— user ID. A user ID of0
refers to the root user.DEV
— which ethernet device the process used to send / receive data (e.g. eth0, eth1)XMIT_PK
— number of packets transmitted by the processRECV_PK
— number of packets received by the processXMIT_KB
— amount of data sent by the process, in kilobytesRECV_KB
— amount of data received by the service, in kilobytes
nettop.stp provides network profile sampling every 5 seconds. You can change this setting by editing
probe timer.ms(5000)
accordingly. Example 5.1, “nettop.stp Sample Output” contains an excerpt of the output from nettop.stp over a 20-second period:
Example 5.1. nettop.stp Sample Output
[...] PID UID DEV XMIT_PK RECV_PK XMIT_KB RECV_KB COMMAND 0 0 eth0 0 5 0 0 swapper 11178 0 eth0 2 0 0 0 synergyc PID UID DEV XMIT_PK RECV_PK XMIT_KB RECV_KB COMMAND 2886 4 eth0 79 0 5 0 cups-polld 11362 0 eth0 0 61 0 5 firefox 0 0 eth0 3 32 0 3 swapper 2886 4 lo 4 4 0 0 cups-polld 11178 0 eth0 3 0 0 0 synergyc PID UID DEV XMIT_PK RECV_PK XMIT_KB RECV_KB COMMAND 0 0 eth0 0 6 0 0 swapper 2886 4 lo 2 2 0 0 cups-polld 11178 0 eth0 3 0 0 0 synergyc 3611 0 eth0 0 1 0 0 Xorg PID UID DEV XMIT_PK RECV_PK XMIT_KB RECV_KB COMMAND 0 0 eth0 3 42 0 2 swapper 11178 0 eth0 43 1 3 0 synergyc 11362 0 eth0 0 7 0 0 firefox 3897 0 eth0 0 1 0 0 multiload-apple [...]
5.1.2. Tracing Functions Called in Network Socket Code
This section describes how to trace functions called from the kernel's
net/socket.c
file. This task helps you identify, in finer detail, how each process interacts with the network at the kernel level.
socket-trace.stp
#! /usr/bin/env stap probe kernel.function("*@net/socket.c").call { printf ("%s -> %s\n", thread_indent(1), probefunc()) } probe kernel.function("*@net/socket.c").return { printf ("%s <- %s\n", thread_indent(-1), probefunc()) }
socket-trace.stp is identical to Example 3.6, “thread_indent.stp”, which was earlier used in SystemTap Functions to illustrate how
thread_indent()
works.
Example 5.2. socket-trace.stp Sample Output
[...] 0 Xorg(3611): -> sock_poll 3 Xorg(3611): <- sock_poll 0 Xorg(3611): -> sock_poll 3 Xorg(3611): <- sock_poll 0 gnome-terminal(11106): -> sock_poll 5 gnome-terminal(11106): <- sock_poll 0 scim-bridge(3883): -> sock_poll 3 scim-bridge(3883): <- sock_poll 0 scim-bridge(3883): -> sys_socketcall 4 scim-bridge(3883): -> sys_recv 8 scim-bridge(3883): -> sys_recvfrom 12 scim-bridge(3883):-> sock_from_file 16 scim-bridge(3883):<- sock_from_file 20 scim-bridge(3883):-> sock_recvmsg 24 scim-bridge(3883):<- sock_recvmsg 28 scim-bridge(3883): <- sys_recvfrom 31 scim-bridge(3883): <- sys_recv 35 scim-bridge(3883): <- sys_socketcall [...]
Example 5.2, “socket-trace.stp Sample Output” contains a 3-second excerpt of the output for socket-trace.stp. For more information about the output of this script as provided by
thread_indent()
, refer to SystemTap Functions Example 3.6, “thread_indent.stp”.
5.1.3. Monitoring Incoming TCP Connections
This section illustrates how to monitor incoming TCP connections. This task is useful in identifying any unauthorized, suspicious, or otherwise unwanted network access requests in real time.
tcp_connections.stp
#! /usr/bin/env stap probe begin { printf("%6s %16s %6s %6s %16s\n", "UID", "CMD", "PID", "PORT", "IP_SOURCE") } probe kernel.function("tcp_accept").return?, kernel.function("inet_csk_accept").return? { sock = $return if (sock != 0) printf("%6d %16s %6d %6d %16s\n", uid(), execname(), pid(), inet_get_local_port(sock), inet_get_ip_source(sock)) }
While tcp_connections.stp is running, it will print out the following information about any incoming TCP connections accepted by the system in real time:
- Current
UID
CMD
- the command accepting the connectionPID
of the command- Port used by the connection
- IP address from which the TCP connection originated
Example 5.3. tcp_connections.stp Sample Output
UID CMD PID PORT IP_SOURCE 0 sshd 3165 22 10.64.0.227 0 sshd 3165 22 10.64.0.227
5.1.4. Monitoring TCP Packets
This section illustrates how to monitor TCP packets received by the system. This is useful in analyzing network traffic generated by applications running on the system.
tcpdumplike.stp
#! /usr/bin/env stap // A TCP dump like example probe begin, timer.s(1) { printf("-----------------------------------------------------------------\n") printf(" Source IP Dest IP SPort DPort U A P R S F \n") printf("-----------------------------------------------------------------\n") } probe tcp.receive { printf(" %15s %15s %5d %5d %d %d %d %d %d %d\n", saddr, daddr, sport, dport, urg, ack, psh, rst, syn, fin) }
While tcpdumplike.stp is running, it will print out the following information about any received TCP packets in real time:
- Source and destination IP address (
saddr
,daddr
, respectively) - Source and destination ports (
sport
,dport
, respectively) - Packet flags
To determine the flags used by the packet, tcpdumplike.stp uses the following functions:
urg
- urgentack
- acknowledgementpsh
- pushrst
- resetsyn
- synchronizefin
- finished
The aforementioned functions return
1
or 0
to specify whether the packet uses the corresponding flag.
Example 5.4. tcpdumplike.stp Sample Output
----------------------------------------------------------------- Source IP Dest IP SPort DPort U A P R S F ----------------------------------------------------------------- 209.85.229.147 10.0.2.15 80 20373 0 1 1 0 0 0 92.122.126.240 10.0.2.15 80 53214 0 1 0 0 1 0 92.122.126.240 10.0.2.15 80 53214 0 1 0 0 0 0 209.85.229.118 10.0.2.15 80 63433 0 1 0 0 1 0 209.85.229.118 10.0.2.15 80 63433 0 1 0 0 0 0 209.85.229.147 10.0.2.15 80 21141 0 1 1 0 0 0 209.85.229.147 10.0.2.15 80 21141 0 1 1 0 0 0 209.85.229.147 10.0.2.15 80 21141 0 1 1 0 0 0 209.85.229.147 10.0.2.15 80 21141 0 1 1 0 0 0 209.85.229.147 10.0.2.15 80 21141 0 1 1 0 0 0 209.85.229.118 10.0.2.15 80 63433 0 1 1 0 0 0 [...]
5.1.5. Monitoring Network Packets Drops in Kernel
The network stack in Linux can discard packets for various reasons. Some Linux kernels include a tracepoint,
kernel.trace("kfree_skb")
, which easily tracks where packets are discarded. dropwatch.stp uses kernel.trace("kfree_skb")
to trace packet discards; the script summarizes which locations discard packets every five-second interval.
dropwatch.stp
#! /usr/bin/env stap ############################################################ # Dropwatch.stp # Author: Neil Horman <nhorman@redhat.com> # An example script to mimic the behavior of the dropwatch utility # http://fedorahosted.org/dropwatch ############################################################ # Array to hold the list of drop points we find global locations # Note when we turn the monitor on and off probe begin { printf("Monitoring for dropped packets\n") } probe end { printf("Stopping dropped packet monitor\n") } # increment a drop counter for every location we drop at probe kernel.trace("kfree_skb") { locations[$location] <<< 1 } # Every 5 seconds report our drop locations probe timer.sec(5) { printf("\n") foreach (l in locations-) { printf("%d packets dropped at %s\n", @count(locations[l]), symname(l)) } delete locations }
The
kernel.trace("kfree_skb")
traces which places in the kernel drop network packets. The kernel.trace("kfree_skb")
has two arguments: a pointer to the buffer being freed ($skb
) and the location in kernel code the buffer is being freed ($location
). The dropwatch.stp script provides the function containing $location
where possible. The information to map $location
back to the function is not in the instrumentation by default. On Systemtap 1.4 the --all-modules
option will include the required mapping information and the following command can be used to run the script:
stap --all-modules dropwatch.stp
On older versions of Systemtap you can use the following command to emulate the
--all-modules
option:
stap -dkernel \ `cat /proc/modules | awk 'BEGIN { ORS = " " } {print "-d"$1}'` \ dropwatch.stp
Running the dropwatch.stp script 15 seconds would result in output similar in Example 5.5, “dropwatch.stp Sample Output”. The output lists the number of misses for each tracepoint location with either the function name or the address.
Example 5.5. dropwatch.stp Sample Output
Monitoring for dropped packets 1762 packets dropped at unix_stream_recvmsg 4 packets dropped at tun_do_read 2 packets dropped at nf_hook_slow 467 packets dropped at unix_stream_recvmsg 20 packets dropped at nf_hook_slow 6 packets dropped at tun_do_read 446 packets dropped at unix_stream_recvmsg 4 packets dropped at tun_do_read 4 packets dropped at nf_hook_slow Stopping dropped packet monitor
When the script is being compiled on one machine and run on another the
--all-modules
and /proc/modules
directory are not available; the symname
function will just print out the raw address. To make the raw address of packet drops more meaningful, refer to the /boot/System.map-`uname -r`
file. This file lists the starting addresses for each function, allowing you to map the addresses in the output of Example 5.5, “dropwatch.stp Sample Output” to a specific function name. Given the following snippet of the /boot/System.map-`uname -r`
file, the address 0xffffffff8149a8ed maps to the function unix_stream_recvmsg
:
[...] ffffffff8149a420 t unix_dgram_poll ffffffff8149a5e0 t unix_stream_recvmsg ffffffff8149ad00 t unix_find_other [...]