GENERIC

-?

Display short form help text about command line options and exit.

-V

Print version and exit.

-d

Tells a verbose story of options and patterns chosen, files opened, and so on. Multiple -d options can be given to increase verbosity and frequency of trace messages.

RUNTIME

-y

Enable Linux seccomp-bpf sandbox if available (compile option).

-b

Run in background as daemon and drop privileges, using set*uid(), set*gid() functions, unless options -N is given or only reading from files.

-o user=...

Specify the user to drop privileges to (default nobody).

-o group=...

Specify the group to drop privileges to (default nobody).

-N

Do not attempt to drop privileges, this is implicit if only reading offline pcap files.

-S

Print stats counters on standard error when closed the packet dump file (see -w).

-B datetime

Start collecting at a specific time. datetime should be specified as "YYYY-MM-DD HH:MM:SS". The program will sleep(3) until the start time, or it will skip all packets related to an earlier time if used with an offline pcap(3) file, and then begin capturing/processing packets.

-E datetime

Stop collecting at a specific time. datetime should be specified as "YYYY-MM-DD HH:MM:SS". dnscap will exit when it sees a packet (live or offline pcap(3) file) with timestamp greater or equal to it.

INPUT

-r file

Select an offline pcap(3) file produced by this utility or by tcpdump(1) (or simiar tools) as the input packet source. Can be given as "-" to indicate standard input.

-i if

Select an interface to be monitored.

On BSD systems, the default is the first interface that was configured at system boot time.

On Linux systems, the default might be to monitor all interfaces but most commonly it will also capture on the first interface. This depends on the libpcap version.

If you want to make sure you're capturing on all interfaces then use the special "any" or "all" (depends on system).

More than one interface may be selected which will cause output to be interleaved from all selected interfaces.

-p

Asks that the interface not be put into promiscuous mode. Note that even without this option, the interface could be in promiscuous mode for some other reason.

-M

Enable monitor mode on interfaces.

-D

Enable immediate mode on interfaces.

Option -p, -M and -D are libpcap specific options, see pcap(3) for more information on their meaning.

-o pcap_buffer_size=num

Set the pcap(3) buffer size to num bytes when capturing packets. This can be used to increase the buffer so that packets are not missed/dropped while processing or rotating packet dumps.

-o use_layers=yes

Enable pcap-thread layers, this will let pcap-thread parse the network layers and call back with UDP, TCP or ICMP traffic.

This options is required for IP defragmentation (see -o defrag_ipv4=yes and -o defrag_ipv6=yes), TCP reassembly (see -o reassemble_tcp=yes) and parsing ongoing TCP sessions (see -o parse_ongoing_tcp=yes).

OUTPUT

-o dump_format=format

Specify the output format to use. Default is pcap.

-g

Produce diagnostic output to standard error, showing the presentation form of DNS messages which passed through all of the filters. If -w is also used, then every message will be dumped in both binary and presentation form.

-w base

Dump the captured packets to successive binary files in pcap(3) format with DLT_RAW datalink type. Each file will have a name like "%s.%s.%06u" where the first %s is base, second %s is the time as hours, minutes and seconds (%H%M%S), and %06u is the microseconds. The argument "-" may be given to send the binary output to standard output.

By default, dnscap will close its packet dump file only when interrupted. You can change that behavior with options -t, -c, and -C.

-W suffix

The provided suffix is added to the dump file name, e. g.: ".pcap". If the suffix ends with ".gz" then files will be automatically gzip compressed. If gzip compression is requested but not supported (i.e. because of lack of system support) an error will be generated.

-1

Flush the output after every packet. Mostly this is useful when the packet dump is standard output, and has been piped to tcpdump(1).

-t lim

Set a time interval, specified in seconds. When writing to a file, the packet dump file will be closed and reopened (creating a new dump file) when time() % lim is zero. Note that the first file will usually be shorter than lim seconds. If the packet dump file is standard output or if -g is used, then dnscap will exit after the first interval.

-c lim

Set a size limit, measured in packets. When writing to a file, the packet dump file will be closed when lim number of packets has been written. If option -k is not used (see below) or the packet dump file is standard output, or if -g is used, then dnscap will exit after reaching the limit.

-C lim

Set a size limit, measured in bytes. When writing to a file, the packet dump file will be closed when lim number of bytes (or larger then) has been written. If option -k is not used or the packet dump file is standard output, or if -g is used, then dnscap will exit after reaching the limit.

When using the above options -t, -c, and -C together, the order of applying them are 1) time interval, 2) number of packets and 3) number of bytes.

-k cmd

After each dump file specified by -w is closed, this command will be executed in a non-blocking subprocess with the file name as its one argument. This can be used to submit the finished file to other processing systems.

If this option is used together with -c or -C and the output is a packet dump file, then it will be reopened (creating a new dump file) before continuing.

NETWORK

-U str

Append "and str" to the BPF/pcap filter.

-o bpf_hosts_apply_all=yes

This changes the BPF generation so that any host restriction will come after ICMP, fragments, ports or DNS section to allow it to apply for ICMP and fragments also. The default behavior is to only apply hosts to the ports or DNS section.

-6

Used to suppress the use of packet filter patterns that cause problems when processing IPv6 packets. As of version 2.0.0 this option is deprecated and filters have been reworked to only match IPv4 packets, IPv6 filtering are processed at a higher level.

-f

Selects fragments (which could include unrelated flows since fragments do not contain port numbers), and includes fragments in the binary output. Necessary if you intend to do IP Reassembly. Note that all fragments will be collected, not just those using the DNS port number, since fragments don't have port numbers. Beware this option if you also handle a lot of NFS traffic.

-T

Selects TCP packets. SYN, FIN, and RST packets are collected if they pass the layer 2, port, and host filters (although hosts need not be in the correct direction); they are not tested against filter options that require a DNS header such as -m, -s, or -e. All DNS messages in the stream is captured if it passes all filter options.

Each TCP packet with payload will be tagged as DNS, unless -o reassemble_tcp=yes is used, with the support of having the DNS length arrive before the message in an own packet. Ongoing TCP connections can be inspected by using -o parse_ongoing_tcp=yes. TCP packets are processed as they arrive so missing, unaligned data or DNS message split over multiple packets will produce parsing errors. Using extended option -o allow_reset_tcpstate=yes may allow dnscap to recover from these scenarios.

-I

Select ICMP and ICMPv6 packets.

-l vlan

Captures only 802.1Q encapsulated packets, and selects specific vlans to be monitored. Can be specified more than once to select multiple vlans. VLAN id 4095 can be used to specify all vlans.

-L vlan

Captures 802.1Q encapsulated packets matching the specified vlans AND packets without VLAN tags. Can be specified more than one to select multiple vlans. VLAN id 4095 can be used to specify all vlans.

-u port

Capture only packets on this UDP port, and treat as DNS traffic. The default port is 53. Note that there is no way to select multiple UDP ports, as would be necessary to capture both DNS (port 53) and mDNS (port 5353) traffic.

-o defrag_ipv4=yes

-o defrag_ipv6=yes

Enable IPv4/IPv6 defragmentation in pcap-thread, requires -o use_layers=yes.

When enabled, the following options are also available:

-o parse_ongoing_tcp=yes

dnscap will normally not look at TCP unless it sees the start of it. This enables state tracking when a new TCP stream is found but no SYN/ACK has been seen. Each TCP packet with payload will be tagged as DNS.

-o allow_reset_tcpstate=yes

Allow the TCP state to be reseted, this is used in diagnostic output and plugins when parsing the DNS in a TCP packet fails to try and recover from missing or unaligned data.

-o reassemble_tcp=yes

Enable reassembly of TCP packets, this will not parse each packet as an own DNS message but will store TCP segments until they can be reassembled. It will expect the DNS message length to come first and then wait for the full length of data to arrive until passing to outputs and plugins.

Since the number of saved segments are limited and fixed, if the TCP steam becomes corrupt then processing may stop. Recovering from this can be done by enabling which will reset state and free all saved segments to try and start over.

-o reassemble_tcp_faultreset=num

This controls the number of faults (num) that can happen before the state is reseted (as described above), faults are if the segments buffer are full or if the sequence is outside the TCP window. The default is zero which means it will reset the state as soon as the segment buffer is full.

-o reassemble_tcp_bfbparsedns=yes

Enable an additional layer (experimental) of reassembly that uses LDNS to parse the payload before accepting it. If the DNS is invalid it will move 2 bytes within the payload and treat it as a new payload, taking the DNS length again and restart the process.

DNS

-m [qun]

Capture only messages of designated types; query, update, and notify). Multiple types can be given at the same time, for example -m qn will select query and notify messages. Multiple -m can not be used to specify multiple types. Default is query.

-e [nytfsxir]

Among responses, consider nonzero DNS TC or DNS RCODE to indicate an error, and select only responses which do not have (n), or which have (y), these conditions. The default is to only select non-errors among responses. If both non-error and error responses are to be selected, specify both the n and y options here.

To be more specific, use one or more condition-specific options, as follows:

-h ir

Hide initiator or responder of each captured transaction. Hiding an initiator means wiping out the address and port number. Hiding a responder means to wipe out the address only. This wiping occurs on the copy of the packet sent to the pcap(3) dump output, and both the IP and UDP checksums will be recomputed in that case.

-s ir

Select messages which are initiations and/or responses. This is done by checking the DNS header flag QR and source/destination port against the DNS port (see -u). Default is both.

-a host

Capture only transactions having these initiators. Can be specified more than once to select multiple initiators. If a host name is used, then all of that host's addresses whether IPv4 or IPv6 are added to the recognition pattern.

-z host

Capture only transactions having these responders. Can be specified more than once to select multiple responders. If a host name is used, then all of that host's addresses whether IPv4 or IPv6 are added to the recognition pattern.

-A host

Capture only transactions NOT having these initiators.

-Z host

Capture only transactions NOT having these responders.

-Y host

Drop responses having these responders. Similar to -Z in spirit. However, -Y applies only to responses and does not cause any additions to the BPF filter string.

-x pat

If one or more -x options are provided, then DNS messages will only be selected if the printable representation of the QNAME or any RR matches at least one of the provided pat patterns.

-X pat

If one or more -X options are provided, then DNS messages matching these patterns will not be selected.

If both options are used then the message must first be matched by -x and then not matched by all -X regex. See regex(3) and re_format(7) for more information about extended regular expression syntax.

-q num|str

Only select DNS messages where QTYPE matches the specified type. Can not be used together with -Q.

-Q num|str

Only select DNS messages where QTYPE does not matches the specified type. Can not be used together with -q.

PLUGINS

-P /path/to/plugin.so ...

Load and use the specified plugin, full path to plugin must be supplied. Any options given after this are sent to the plugin.

Once a double dash, "--", is encountered after -P, processing of the command line options will go back to dnscap.

Using this you can chain and use multiple plugins at once:

  -P /path/to/plugin_one.so -a opt -- -P /path/to/plugin_two.so -b opt

    

To show the plugins option help, run it with -?:

  -P /path/to/plugin_one.so -?

    

Plugins are loaded, executed and given the packets to process in the order given on command line.

These bundled plugins are installed in /usr/local/lib/dnscap:

CBOR

-o cbor_chunk_size=bytes

Specify the number of bytes of CBOR to construct before flushing the output, must be a non zero positive number.

CBOR DNS STREAM (CDS)

-o cds_cbor_size=bytes

Number of bytes of memory to use before flushing to file.

-o cds_message_size=bytes

Number of bytes of memory to use for each DNS packet.

-o cds_max_rlabels=num

Number of labels (num) to keep in the reverse label index.

-o cds_min_rlabel_size=num

The minimum size of a label (num) to be able to use the reverse label index.

-o cds_use_rdata_index=yes

Use the resource data index, default is no.

-o cds_rdata_index_min_size=num

The minimum size of the data (num) to be able to use the resource data index.

-o cds_use_rdata_rindex=yes

Use the resource data reverse index, default is no.

-o cds_rdata_rindex_size=num

Number of resource data (num) to keep in the resource data reverse index.

-o cds_rdata_rindex_min_size=num

The minimum size of the data (num) to be able to use the resource data reverse index.