1tcpconnect(8) System Manager's Manual tcpconnect(8)
2
6 tcpconnect - Trace TCP active connections (connect()). Uses Linux
7 eBPF/bcc.
8
10 tcpconnect [-h] [-c] [-t] [-p PID] [-P PORT] [-4 | -6] [-L] [-u UID]
11 [-U] [--cgroupmap MAPPATH] [--mntnsmap MAPPATH] [-d]
12
14 This tool traces active TCP connections (eg, via a connect() syscall;
15 accept() are passive connections). This can be useful for general trou‐
16 bleshooting to see what connections are initiated by the local server.
17 All connection attempts are traced, even if they ultimately fail.
19 This works by tracing the kernel tcp_v4_connect() and tcp_v6_connect()
21 functions using dynamic tracing, and will need updating to match any
22 changes to these functions.
23 When provided with the -d or --dns option, this tool will also corre‐
25 late connect calls with the most recent DNS query that matches the IP
26 connected. This feature works by tracing the kernel udp_recvmsg()
27 function to collect DNS responses.
28 Since this uses BPF, only the root user can use this tool.
30
32 CONFIG_BPF and bcc.
33 If using the -d or --dns option, you must have the dnslib and ca‐
35 chetools python packages installed. You can install them with pip3 or
36 with apt on Ubuntu 18.04+ using the python3-dnslib and python3-ca‐
37 chetools packages.
38
40 -h Print usage message.
41 -t Include a timestamp column.
43 -c Count connects per src ip and dest ip/port.
45 -p PID Trace this process ID only (filtered in-kernel).
47 -P PORT
49 Comma-separated list of destination ports to trace (filtered in-
50 kernel).
51 -4 Trace IPv4 family only.
53 -6 Trace IPv6 family only.
55 -L Include a LPORT column.
57 -U Include a UID column.
59 -u UID Trace this UID only (filtered in-kernel).
61 --cgroupmap MAPPATH
63 Trace cgroups in this BPF map only (filtered in-kernel).
64 --mntnsmap MAPPATH
66 Trace mount namespaces in this BPF map only (filtered in-ker‐
67 nel).
68 -d Shows the most recent DNS query for the IP address in the con‐
70 nect call. This is likely related to the TCP connection details
71 in the other columns, but is not guaranteed. This feature works
72 by tracing the udp_recvmsg kernel function and tracking DNS re‐
73 sponses received by the server. It only supports UDP DNS pack‐
74 ets up to 512 bytes in length. The python code keeps a cache of
75 10k DNS responses in memory for up 24 hours.
76 If the time difference in milliseconds between when the system
78 received a DNS response and when a connect syscall was traced
79 using an IP in that DNS response is greater than 100ms, this
80 tool will report this delta after the query. These deltas
81 should be relatively short for most applications. A long delay
82 between the response and connect could be either anomalous ac‐
83 tivity or indicate a misattribution between the DNS name re‐
84 quested and the IP that the connect syscall is using.
85 The -d option may not be used with the count feature (option -c)
87
89 Trace all active TCP connections:
90 # tcpconnect
91 Trace all TCP connects, and include timestamps:
93 # tcpconnect -t
94 Trace all TCP connects, and include most recent matching DNS query for
96 each connected IP
97 # tcpconnect -d
98 Trace PID 181 only:
100 # tcpconnect -p 181
101 Trace ports 80 and 81 only:
103 # tcpconnect -P 80,81
104 Trace IPv4 family only:
106 # tcpconnect -4
107 Trace IPv6 family only:
109 # tcpconnect -6
110 Trace all TCP connects, and include LPORT:
112 # tcpconnect -L
113 Trace all TCP connects, and include UID:
115 # tcpconnect -U
116 Trace UID 1000 only:
118 # tcpconnect -u 1000
119 Count connects per src ip and dest ip/port:
121 # tcpconnect -c
122 Trace a set of cgroups only (see special_filtering.md from bcc sources
124 for more details):
125 # tcpconnect --cgroupmap /sys/fs/bpf/test01
126 Trace a set of mount namespaces only (see special_filtering.md from bcc
128 sources for more details):
129 # tcpconnect --mntnsmap /sys/fs/bpf/mnt_ns_set
130
132 TIME(s)
133 Time of the call, in seconds.
134 UID User ID
136 PID Process ID
138 COMM Process name
140 IP IP address family (4 or 6)
142 SADDR Source IP address.
144 LPORT Source port
146 DADDR Destination IP address.
148 DPORT Destination port
150 CONNECTS
152 Accumulated active connections since start.
153 QUERY Shows the most recent DNS query for the IP address in the con‐
155 nect call. This is likely related to the TCP connection details
156 in the other columns, but is not guaranteed.
157
159 This traces the kernel tcp_v[46]_connect functions and prints output
160 for each event. As the rate of this is generally expected to be low (<
161 1000/s), the overhead is also expected to be negligible. If you have an
162 application that is calling a high rate of connect()s, such as a proxy
163 server, then test and understand this overhead before use.
164 If you are using the -d option to track DNS requests, this tool will
166 trace the udp_recvmsg function and generate an event for any packets
167 from UDP port 53. This event contains up to 512 bytes of the UDP
168 packet payload. Typical applications do not extensively use UDP, so
169 the performance overhead of tracing udp_recvmsg is expected to be neg‐
170 ligible, However, if you have an application that receives many UDP
171 packets, then you should test and understand the overhead of tracing
172 every received UDP message. Furthermore, performance overhead of run‐
173 ning this tool on a DNS server is expected to be higher than average
174 because all DNS response packets will be copied to userspace.
175
177 This is from bcc.
178 https://github.com/iovisor/bcc
180 Also look in the bcc distribution for a companion _examples.txt file
182 containing example usage, output, and commentary for this tool.
183
185 Linux
186
188 Unstable - in development.
189
191 Brendan Gregg
192
194 tcptracer(8), tcpaccept(8), funccount(8), tcpdump(8)
195USER COMMANDS 2020-02-20 tcpconnect(8)