yafscii - YAF Flow Printer


    yafscii [--in INPUT_SPECIFIER] [--out OUTPUT_SPECIFIER]
            [--nextdir PROCESSED_INPUT_DIRECTORY]
            [--faildir FAILED_INPUT_DIRECTORY]
            [--poll POLLING_DELAY] [--lock]
            [--log LOG_SPECIFIER] [--loglevel LOG_LEVEL]
            [--verbose] [--version] [--daemon] [--foreground]
            [--tabular] [--mac] [--print-header]


yafscii takes IPFIX flow data files generated by yaf(1) and prints them in an ASCII format loosely analogous to that produced by tcpdump(1), with one flow per line. The text output format is detailed in the "OUTPUT" section below. yafscii is generally intended to be used to print single files for verification or debugging purposes, or to operate in a pipe with yaf, but it can be used as a daemon as well.

yafscii ignores yaf stats records and most of the data in yaf's subrecords. The ipfix2json(1) and ipfixDump(1) applications from libfixbuf are able to print all records generated by yaf (or another IPFIX tool) and include support for printing subrecords.


Input Options

The input specifier determines where yafscii will read its input from. If the input specifier is not given, yaf defaults to reading from standard input.


INPUT_SPECIFIER is an input specifier. This is a filename, a directory name, a file glob pattern (in which case it should be escaped or quoted to prevent the shell from expanding the glob pattern), or the string - to read from standard input.

Output Options

The output specifier determines where yaf will send its output. If reading standard input, output defaults to standard output. If reading from files on disk, output defaults to one file per input file, named as the input file in the same directory as the input file with a .txt extension.


OUTPUT_SPECIFIER is an output specifier. This should be a filename or a directory name, or the string - to write to standard output.


Use tabular output mode, which is designed for easy parsability over human readability. See the Tabular Output section below for details.


Used with --tabular mode to print source and destination MAC Addresses.

Used with --tabular mode to print column headers.

Daemon Options

These options are used to run yafscii in daemon mode for batch processing of pcap dumpfiles.


Run yafscii in daemon mode. Instead of processing its input then exiting, yafscii will continually look for new input matching its input specifier. This will cause yaf to fork into the background and exit.


Instead of forking in --daemon mode, stay in the foreground. Useful for debugging.


Use lockfiles for concurrent file access protection. Highly recommended in --daemon mode, especially if two daemons are interacting through a given directory.


POLLING_DELAY is the polling delay in seconds; how long yaf will wait for new input when none is available. The default is 60 seconds.


When reading from files, if this option is present, input files will be moved to PROCESSED_INPUT_DIRECTORY after they are successfully processed. The special string delete will cause successfully processed input to be removed instead. This option is required in daemon mode.


When reading from files, if this option is present, input files will be moved to FAILED_INPUT_DIRECTORY if processing failed. The special string delete will cause failed input to be removed instead. This option is required in daemon mode.

Logging Options

These options are used to specify how log messages are routed. yaf can log to standard error, regular files, or the UNIX syslog facility.


Specifies destination for log messages. LOG_SPECIFIER can be a syslog(3) facility name, the special value stderr for standard error, or the absolute path to a file for file logging. Standard error logging is only available in --daemon mode if --foreground is present. The default log specifier is stderr if available, user otherwise.

--loglevel LOG_LEVEL

Specify minimum level for logged messages. In increasing levels of verbosity, the supported log levels are quiet, error, critical, warning, message, info, and debug. The default logging level is warning.


Equivalent to --loglevel debug.


If present, print version and copyright information to standard error and exit.


Human-Readable Output

yafscii's default output format, like that of tcpdump, is designed to be easily human-readable, at the possible expense of ease of automated parsing. Each flow is represented by a single output line representing the flow itself, followed by zero or more indented lines containing flow payload in hexdump format. This section details each flow format, where each field specifier is as follows:

start-time, end-time

Flow start or end time in ISO 8601 format, with milliseconds (YYYY-MM-DD hh:mm:ss.ssss). Start time is printed with a date; end time is not. End time is only present if the flow has a non-zero duration.


Flow duration in fractional seconds. Only present if the flow has a non-zero duration.


IP protocol identifier in decimal format.

sip, dip

Source or destination IPv4 address in dotted-quad format or IPv6 address in RFC 2373 format.

sp, dp

Source or destination transport port in decimal format.

type, code

ICMP type or code in decimal format.

isn, risn

Forward or reverse initial TCP sequence number in hexadecimal format.

iflags, riflags, uflags, ruflags

Foward or reverse first-packet TCP flags; forward or reverse nth-packet TCP flags union; where each flags bit is represented by the first character in the flag's name: FIN, SYN, RST, PSH, ACK, URG, ECE, CWR. The character 0 means no flags are set (and will appear in the nth-packet field for single-packet TCP flows).

tag, rtag

Forward or reverse first-packet 802.1q VLAN tag in hexadecimal format.

srcMacAddress, destMacAddress

Source or Destination MAC Address. These are only printed if --mac is used.

pkt, rpkt

Forward or reverse packet count in decimal format.

oct, roct

Forward or reverse octet count in decimal format.


The difference between the first packet seen in the forward direction and the first packet seen in the reverse direction, which is an estimate of the round-trip time, in milliseconds in decimal format.


If not present, the flow ended normally (i.e., by TCP RST or FIN). Otherwise, the end-reason is one of the following strings:


Flow was expired by idle timeout. No packets were received for IDLE_TIMEOUT seconds (see yaf(1)) and the flow was presumed closed.


Flow was expired by active timeout. The flow's duration was longer than ACTIVE_TIMEOUT seconds (see yaf(1)) and the flow was flushed from the flow table.


Flow was still active in the flow table at the end of the dumpfile or at yaf(1) shutdown time; it was flushed as the flow table was cleared.


Flow was prematurely flushed as idle because more than FLOW_TABLE_MAX flows (see yaf(1)) were active in the flow table.


yaf forced a write of the flow, but the flow remained open. This is only seen if yaf operated with the --udp-uniflow flag, which exports each UDP packet as a flow record, but allows the flow to remain open until it closes naturally by idle and active timeouts (see yaf(1)).


The application label, if yaf(1) was built with application labeling enabled and the application labeler was able to identify the payload in the flow.


The Shannon-Fano Entropy for the forward then the reverse flow payload if the payload existed and yaf(1) was built with entropy enabled.

Each flow line format is as follows:

Unidirectional IP flow

start-time [- end-time (duration sec)] ip proto sip => dip [vlan tag] (pkt/oct ->) [end-reason]

Unidirectional UDP flow

start-time [- end-time (duration sec)] udp sip:sp => dip:dp [vlan tag] (pkt/oct ->) [end-reason]

Unidirectional TCP flow

start-time [- end-time (duration sec)] tcp sip:sp => dip:dp isn iflags/uflags [vlan tag] (pkt/oct ->) [end-reason]

Unidirectional ICMP flow

start-time [- end-time (duration sec)] icmp [type:code] sip => dip [vlan tag] (pkt/oct ->) [end-reason]

Bidirectional IP flow

start-time [- end-time (duration sec)] ip proto sip => dip [vlan tag:rtag] (pkt/oct <-> rpkt/roct) rtt rtt ms [end-reason]

Bidirectional UDP flow

start-time [- end-time (duration sec)] udp sip:sp => dip:dp [vlan tag:rtag] (pkt/oct <-> rpkt/roct) rtt rtt ms [end-reason]

Bidirectional TCP flow

start-time [- end-time (duration sec)] tcp sip:sp => dip:dp isn:risn iflags/uflags:riflags/ruflags [vlan tag:rtag] (pkt/oct <-> rpkt/roct) rtt rtt ms [end-reason]

If present, the payload follows each flow line. Forward direction payload lines are prefixed with the string ->, and reverse direction payload lines are prefixed with the string <-. Payload is only taken from the first packet for non-TCP flows (see yaf(1)).

Tabular Output

In --tabular mode, yafscii prints its output as a table, without a header, with one flow per line and no payload information. Each column is separated by a pipe character. Columns have constant width and are filled with leading zeroes or spaces as appropriate. Every column appears in each row whether it is present in the flow data or not; non-present columns are represented with a 0. All columns are formatted as they are in the human-readable output, except end-time which appears with a data and rtt which is expressed in fractional seconds instead of decimal milliseconds. For ICMP flows, ICMP type and code appear in the dp field, which has the value 256(type) + code. srcMacAddress and destMacAddress will only print if --mac is used. The order of columns is as follows:

start-time| end-time| duration| rtt| proto| sip| sp| dip| dp| srcMacAddress| destMacAddress| iflags| uflags| riflags| ruflags| isn| risn| tag| rtag| pkt| oct| rpkt| roct| applabel| entropy| rentropy| end-reason


yafscii responds to SIGINT or SIGTERM by terminating input processing and exiting.


Known issues are listed in the README file in the YAF tools source distribution. Bug reports and feature requests may be sent directly to the Network Situational Awareness team at <>.


Brian Trammell, Chris Inacio, Michael Duggan, and the CERT Network Situational Awareness Group Engineering Team, <>.


yaf(1), ipfix2json(1), ipfixDump(1)