Elements / Tunnels

CheckGREHeader

checks GRE header

SYNOPSIS§

CheckGREHeader([keywords OFFSET, CHECKSUM, KEY, SEQUENCE, VERBOSE, DETAILS])

Ports: 1 input, 1-2 outputs
Processing: agnostic

DESCRIPTION§

Input packets should have GRE headers starting OFFSET bytes in. Default OFFSET is zero. Checks that the packet's length is reasonable, and that the GRE version, checksum, key, and sequence fields are valid.

CheckGREHeader emits valid packets on output 0. Invalid packets are pushed out on output 1, unless output 1 was unused; if so, drops invalid packets.

CheckGREHeader prints a message to the console the first time it encounters an incorrect GRE packet (but see VERBOSE below).

Keyword arguments are:

• CHECKSUM — Boolean. If true, then check each header's checksum for presence and validity. Default is false.
• KEY — Unsigned. If not 0, check each header's key field for presence and validity (equal to this value). Default is false.
• SEQUENCE — Boolean. If true, then check each header's sequence number for presence and validity. Default is false.
• OFFSET — Unsigned integer. Byte position at which the IP header begins. Default is 0.
• VERBOSE — Boolean. If it is true, then a message will be printed for every erroneous packet, rather than just the first. False by default.
• DETAILS — Boolean. If it is true, then CheckGREHeader will maintain detailed counts of how many packets were dropped for each possible reason, accessible through the drop_details handler. False by default.

ELEMENT HANDLERS§

• drops (read-only) — Returns the number of incorrect packets CheckGREHeader has seen.
• drop_details (read-only) — Returns a text file showing how many erroneous packets CheckGREHeader has seen, subdivided by error. Only available if the DETAILS keyword argument was true.

SEE ALSO§

GREEncap, StripGREHeader

Generated by click-elem2man from ../elements/tunnel/checkgreheader.hh:27 on 2026/11/02.