Packet analysis using Docker

Why use sFlow for packet analysis? To rephrase the Heineken slogan, sFlow reaches the parts of the network that other technologies cannot reach. Industry standard sFlow is widely supported by switch vendors, embedding wire-speed packet monitoring throughout the network. With sFlow, any link or group of links can be remotely monitored. The alternative approach of physically attaching a probe to a SPAN/Mirror port is becoming much less feasible with increasing network sizes (10's of thousands of switch ports) and link speeds (10, 100, and 400 Gigabits). Using sFlow for packet capture doesn't replace traditional packet analysis, instead sFlow extends the capabilities of existing packet capture tools into the high speed switched network.

This article describes the sflow/tcpdump  and sflow/tshark Docker images, which provide a convenient way to analyze packets captured using sFlow.

Run the following command to analyze packets using tcpdump:

$ docker run -p 6343:6343/udp -p 8008:8008 sflow/tcpdump

19:06:42.000000 ARP, Reply 10.0.0.254 is-at c0:ea:e4:89:b0:98 (oui Unknown), length 64
19:06:42.000000 IP 10.0.0.236.548 > 10.0.0.70.61719: Flags [P.], seq 3380015689:3380015713, ack 515038158, win 41992, options [nop,nop,TS val 1720029042 ecr 904769627], length 24
19:06:42.000000 IP 10.0.0.236.548 > 10.0.0.70.61719: Flags [P.], seq 149816:149832, ack 510628, win 41992, options [nop,nop,TS val 1720029087 ecr 904770068], length 16
19:06:42.000000 IP 10.0.0.236.548 > 10.0.0.70.61719: Flags [P.], seq 149816:149832, ack 510628, win 41992, options [nop,nop,TS val 1720029087 ecr 904770068], length 16

The normal tcpdump options can be used. For example, to select DNS packets:

$ docker run -p 6343:6343/udp -p 8008:8008 sflow/tcpdump -vv port 53
reading from file -, link-type EN10MB (Ethernet)
19:08:49.000000 IP (tos 0x0, ttl 64, id 22316, offset 0, flags [none], proto UDP (17), length 65)
    10.0.0.70.43801 > dns.google.53: [udp sum ok] 35941+ A? clients2.google.com. (37)
19:09:00.000000 IP (tos 0x0, ttl 255, id 16813, offset 0, flags [none], proto UDP (17), length 66)
    10.0.0.64.50675 > 10.0.0.1.53: [udp sum ok] 57874+ AAAA? p49-imap.mail.me.com. (38)

The following command selects TCP SYN packets:

$ docker run -p 6343:6343/udp sflow/tcpdump 'tcp[tcpflags] == tcp-syn'
reading from file -, link-type EN10MB (Ethernet)
19:10:37.000000 IP 10.0.0.30.46786 > 10.0.0.162.1179: Flags [S], seq 2993962362, win 29200, options [mss 1460,sackOK,TS val 20531427 ecr 0,nop,wscale 9], length 0

Capture 10 packets to a file and then exit:

$ docker run -v $PWD:/pcap -p 6343:6343/udp sflow/tcpdump -w /pcap/packets.pcap -c 10
reading from file -, link-type EN10MB (Ethernet)

A tcpdump Tutorial with Examples — 50 Ways to Isolate Traffic provides an overview of the capabilities of tcpdump with useful examples.

Run the following command to analyze packets using tshark - a terminal based version of Wireshark:

$ docker run -p 6343:6343/udp -p 8008:8008 sflow/tshark
Capturing on '-'
    1   0.000000   10.0.0.236 → 10.0.0.70    AFP 1518 [Reply without query?]
    2   0.000000   10.0.0.236 → 10.0.0.70    AFP 1518 [Reply without query?]
    3   0.000000   10.0.0.114 → 10.0.0.72    SSH 1518 Server: Encrypted packet (len=1448)

Packets can be filtered using Display Filters. For example, the following command selects DNS traffic:

$ docker run -p 6343:6343/udp -p 8008:8008 sflow/tshark -Y 'dns'
Capturing on '-'
  328  22.000000      8.8.8.8 → 10.0.0.70    DNS 136 Standard query response 0xfce4 AAAA img.youtube.com CNAME ytimg.l.google.com AAAA
  472  36.000000    10.0.0.52 → 10.0.0.1     DNS 79 Standard query 0x173e AAAA www.nytimes.com

Print ip source, destination, protocol and packet lengths:

$ docker run -p 6343:6343/udp -p 8008:8008 sflow/tshark -T fields -e ip.src -e ip.dst -e ip.proto -e ip.len
Capturing on '-'
10.0.0.70 10.0.0.236 6 1500
10.0.0.236 10.0.0.70 6 52
10.0.0.70 10.0.0.236 6 1500
10.0.0.236 10.0.0.70 6 52
10.0.0.70 10.0.0.236 6 1500

Capture 100 packets and print summary of the protocols:

$ docker run -p 6343:6343/udp -p 8008:8008 sflow/tshark -q -z io,phs -c 100
Capturing on '-'
100 packets captured

===================================================================
Protocol Hierarchy Statistics
Filter: 

eth                                      frames:100 bytes:85721
  ip                                     frames:99 bytes:85657
    tcp                                  frames:97 bytes:85119
      dsi                                frames:61 bytes:82122
        _ws.short                        frames:54 bytes:77180
        afp                              frames:6 bytes:4856
          _ws.short                      frames:5 bytes:4766
      _ws.short                          frames:15 bytes:1050
      http                               frames:1 bytes:499
        _ws.short                        frames:1 bytes:499
      iscsi                              frames:1 bytes:118
        iscsi.flags                      frames:1 bytes:118
          scsi                           frames:1 bytes:118
            _ws.short                    frames:1 bytes:118
    ipv6                                 frames:2 bytes:538
      tcp                                frames:2 bytes:538
        tls                              frames:2 bytes:538
          _ws.short                      frames:2 bytes:538
  arp                                    frames:1 bytes:64
    _ws.short                            frames:1 bytes:64
===================================================================

Capture 100 packets and print a summary of the IP traffic by address:

$ docker run -p 6343:6343/udp -p 8008:8008 sflow/tshark -q -z endpoints,ip -c 100
Capturing on '-'
100 packets captured

================================================================================
IPv4 Endpoints
Filter:
                       |  Packets  | |  Bytes  | | Tx Packets | | Tx Bytes | | Rx Packets | | Rx Bytes |
10.0.0.70                     95         81713         44           25507          51           56206   
10.0.0.236                    91         80820         50           55956          41           24864   
10.0.0.30                      6          2369          2            1508           4             861   
10.0.0.16                      1           587          1             587           0               0   
10.0.0.28                      1           587          0               0           1             587   
10.0.0.160                     1          1258          0               0           1            1258   
10.0.0.172                     1           218          1             218           0               0   
================================================================================

The following command prints packet decodes as JSON:

$ docker run -p 6343:6343/udp -p 8008:8008 sflow/tshark -T json
Capturing on '-'
[
  {
"_index": "packets-2019-09-06",
"_type": "pcap_file",
"_score": null,
"_source": {
"layers": {
"frame": {
"frame.interface_id": "0",
"frame.interface_id_tree": {
"frame.interface_name": "-"
          },
"frame.encap_type": "1",
"frame.time": "Sep  6, 2019 19:41:12.000000000 UTC",
"frame.offset_shift": "0.000000000",
"frame.time_epoch": "1567798872.000000000",
"frame.time_delta": "0.000000000",
"frame.time_delta_displayed": "0.000000000",
"frame.time_relative": "0.000000000",
"frame.number": "1",
"frame.len": "64",
"frame.cap_len": "60",
"frame.marked": "0",
"frame.ignored": "0",
"frame.protocols": "eth:ethertype:arp"
        },
"eth": {
"eth.dst": "70:10:6f:d8:13:30",
"eth.dst_tree": {
"eth.dst_resolved": "HewlettP_d8:13:30",
"eth.addr": "70:10:6f:d8:13:30",
"eth.addr_resolved": "HewlettP_d8:13:30",
"eth.lg": "0",
"eth.ig": "0"
          },
"eth.src": "98:4b:e1:03:4a:61",
"eth.src_tree": {
"eth.src_resolved": "HewlettP_03:4a:61",
"eth.addr": "98:4b:e1:03:4a:61",
"eth.addr_resolved": "HewlettP_03:4a:61",
"eth.lg": "0",
"eth.ig": "0"
          },
"eth.type": "0x00000806",
"eth.padding": "00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00"
        },
"arp": {
"arp.hw.type": "1",
"arp.proto.type": "0x00000800",
"arp.hw.size": "6",
"arp.proto.size": "4",
"arp.opcode": "1",
"arp.src.hw_mac": "98:4b:e1:03:4a:61",
"arp.src.proto_ipv4": "10.0.0.30",
"arp.dst.hw_mac": "00:00:00:00:00:00",
"arp.dst.proto_ipv4": "10.0.0.232"
        },
"_ws.short": "[Packet size limited during capture: Ethertype truncated]"
      }
    }
  },

The tshark -T ek option formats the JSON output as a single line per packet making the output easy to parse in scripts. For example, the following emerging.py script downloads the Emerging Threats compromised IP address database, parses the JSON records, checks to see if source and destination addresses can be found in the database, and prints out information on any matches:

#!/usr/bin/env python

from sys import stdin
from json import loads
from requests import get

blacklist = set()
r = get('https://rules.emergingthreats.net/blockrules/compromised-ips.txt')
for line in r.iter_lines():
  blacklist.add(line)

for line in stdin:
  msg = loads(line)
  try:
    time = msg['timestamp']
    layers = msg['layers']
    ip = layers["ip"]
    src = ip["ip_ip_src"]
    dst = ip["ip_ip_dst"]
    if src in blacklist or dst in blacklist:
      print "%s %s %s" % (time,src,dst)
  except KeyError:
    pass

The following command runs the script:

$ docker run -p 6343:6343/udp -p 8008:8008 sflow/tshark -T ek | ./tshark.py

See the TShark man page for more options.

Forwarding using sFlow-RT describes how to set up and tear down sFlow streams using the sFlow-RT analytics engine. This is a simple way to direct a stream of sFlow to a desktop running sflowtool. For example, suppose sflowtool is running on host 10.0.0.30 and sFlow-RT is running on host 10.0.0.1, the following command would start a session:

curl -H "Content-Type:application/json" -X PUT --data '{"address":"10.0.0.30"}' \
http://10.0.0.1:8008/forwarding/tcpdump/json

and the following command would end the session:

curl -X DELETE http://10.0.0.1:8008/forwarding/tcpdump/json

Note: The sflow/sflow-rt Docker image is a convenient way to run sFlow-RT:

docker run -p 8008:8008 -p 6343:6343/udp sflow/sflow-rt

Finally, Triggered remote packet capture using filtered ERSPAN, shows how the broad visibility provided by sFlow can be combined with hardware filtering to trigger full packet capture of selected traffic.