Use host to match traffic to or from an IP address, net for a subnet, and port for a transport port. Add src or dst to limit direction, and combine conditions with and, or, and parentheses. Quote the complete filter expression so your shell passes it to tcpdump as one argument.
How to write a basic tcpdump IP filter
In tcpdump, the expression after the command selects which packets are dumped. These examples use documentation-only addresses and ports; replace them with the address, network, or port you need.
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# Traffic to or from one host
tcpdump -n -- 'host 192.0.2.10'
# Traffic to or from a subnet
tcpdump -n -- 'net 192.0.2.0/24'The -n option avoids translating addresses and ports into names in the output. The Linux tcpdump manual recommends putting options before -- and passing the expression as a single quoted string. See the Linux tcpdump(8) manual.
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How to filter by direction
Without a direction qualifier, host, net, and port match either the source or destination. Use src or dst when only one side matters. The OpenBSD tcpdump(8) manual documents src or dst as the default direction.
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| Filter | What it matches |
|---|---|
host 192.0.2.10 |
Packets with that host as source or destination |
src host 192.0.2.10 |
Packets sourced by that host |
dst host 192.0.2.10 |
Packets destined for that host |
src net 192.0.2.0/24 |
Packets sourced by an address in that subnet |
How to filter by port and protocol
A port filter without a protocol qualifier matches TCP or UDP traffic. Add tcp or udp if you need to restrict the transport protocol; a number alone does not imply TCP. This behavior is described in the OpenBSD tcpdump(8) manual.
# TCP or UDP traffic involving port 443
tcpdump -n -- 'port 443'
# TCP traffic involving port 443
tcpdump -n -- 'tcp port 443'
# UDP traffic involving port 53
tcpdump -n -- 'udp port 53'To match both a destination host and destination port, join the two conditions with and:
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tcpdump -n -- 'dst host 192.0.2.10 and dst port 443'How to combine conditions without ambiguity
Use and, or, and not to build a filter. Put parentheses around alternatives when they are part of a larger condition so the intended grouping is explicit. Quote the whole expression, including parentheses.
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tcpdump -n -- 'src host 192.0.2.10 and (dst port 443 or dst port 8443)'The parentheses make clear that the source-host condition applies alongside either destination-port option. The filter grammar and these Boolean operators are documented in the OpenBSD tcpdump(8) manual.
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How to filter an IPv6 subnet
Specify a prefix length when you want an IPv6 network match, for example:
tcpdump -n -- 'ip6 net 2001:db8:1234::/48'The versioned OpenBSD manual explains that an IPv6 network number without a netmask length is treated as a host match, not as a subnet. Check the installed tcpdump or libpcap documentation when adapting syntax to a particular platform: OpenBSD 7.5 tcpdump(8) manual.
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What a capture filter does—and does not—guarantee
A filter selects packets available to tcpdump at the capture point; it cannot make traffic visible if the chosen interface or capture location does not see it. Supply an expression when you want to narrow output: the OpenBSD manual notes, “If no expression is given, all packets on the net will be dumped.” Less common primitives and link-layer direction qualifiers can depend on the platform and packet-capture library. In particular, ra, ta, addr1, addr2, addr3, and addr4 are specialized for IEEE 802.11 wireless LAN link layers, not substitutes for IP source and destination.
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