Additional block types for PCAP Next Generation (pcapng) Capture File Format

3.1. systemd Journal Export Block

The systemd Journal Export Block is a lightweight container for systemd Journal Export Format entry data.¶

One of the primary components of the systemd System and Service Manager is the "Journal", a message logging system that uses arrays of key-value pairs. Journal entries are stored in a database-like file on disk but can be serialized to easily parseable "Journal Export Format" data or to a JSON object. The block described here is limited to Journal Export Format data only.¶

A systemd Journal Export Block contains a single systemd Journal Export Format entry. Each entry MUST contain a __REALTIME_TIMESTAMP= field. If a timestamp for the block is required it can be derived from this field. Each entry MUST be zero-padded to 32 bits. Although the primary use of this block is intended for importing data from systemd, it could potentially be used to include arbitrary key-value data in a capture file.¶

Figure 1 shows the format of the Journal Export Block.¶

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 0 |                    Block Type = 0x00000009                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 4 |                      Block Total Length                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 8 /                                                               /
   /                         Journal Entry                         /
   /              variable length, padded to 32 bits               /
   /                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Block Total Length                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The systemd Journal Export Block has the following fields:¶

• Block Type: The block type of the Journal Export Block is 9.¶
• Block Total Length: total size of this block, as described in [I-D.tuexen-opsawg-pcapng], section "Section Blocks".¶
• Journal Entry: A journal entry as described in the Journal Export Format documentation. Entries consist of a series of field names followed by text or binary field data. Common field names can be found in the systemd.journal-fields documentation. The __REALTIME_TIMESTAMP= field MUST be present and valid as described above. Entries are not guaranteed to be a multiple of four octets and must be zero-padded. This allows the length of the entry to be determined by finding the last non-zero octet in the Journal Entry data. An entry may contain an entry separator (trailing newline) as described in the Journal Export Format specification¶

3.2. Alternative Packet Blocks (experimental)

Can some other packet blocks (besides the ones described in the previous paragraphs) be useful?¶

3.3. Compression Block (experimental)

The Compression Block is optional. A file can contain an arbitrary number of these blocks. A Compression Block, as the name says, is used to store compressed data. Its format is shown in Figure 2.¶

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 0 |                        Block Type = ?                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 4 |                      Block Total Length                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 8 |  Compr. Type  |                                               /
   +-+-+-+-+-+-+-+-+                                               /
   /                                                               /
   /                       Compressed Data                         /
   /                                                               /
   /  variable length, octet-aligned and padded to end on a 32-bit /
   /                         boundary                              /
   /                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Block Total Length                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The fields have the following meaning:¶

• Block Type: The block type of the Compression Block is not yet assigned.¶
• Block Total Length: total size of this block, as described in [I-D.tuexen-opsawg-pcapng], section "Section Blocks".¶
• Compression Type (8 bits): an unsigned value that specifies the compression algorithm. Possible values for this field are 0 (uncompressed), 1 (Lempel-Ziv), 2 (Gzip), other?? Probably some kind of dumb and fast compression algorithm could be effective with some types of traffic (for example web), but which?¶
• Compressed Data: data of this block. Once decompressed, it is made of other blocks.¶

3.4. Encryption Block (experimental)

The Encryption Block is optional. A file can contain an arbitrary number of these blocks. An Encryption Block is used to store encrypted data. Its format is shown in Figure 3.¶

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 0 |                        Block Type = ?                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 4 |                      Block Total Length                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 8 |   Encr. Type  |                                               /
   +-+-+-+-+-+-+-+-+                                               /
   /                                                               /
   /                       Encrypted Data                          /
   /                                                               /
   /                 variable length, octet-aligned                /
   /                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Block Total Length                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The fields have the following meaning:¶

• Block Type: The block type of the Encryption Block is not yet assigned.¶
• Block Total Length: total size of this block, as described in [I-D.tuexen-opsawg-pcapng], section "Section Blocks".¶
• Encryption Type (8 bits): an unsigned value that specifies the encryption algorithm. Possible values for this field are ??? (TODO) NOTE: this block should probably contain other fields, depending on the encryption algorithm. To be defined precisely.¶
• Encrypted Data: data of this block. Once decrypted, it originates other blocks.¶

3.5. Fixed Length Block (experimental)

The Fixed Length Block is optional. A file can contain an arbitrary number of these blocks. A Fixed Length Block can be used to optimize the access to the file. Its format is shown in Figure 4. A Fixed Length Block stores records with constant size. It contains a set of Blocks (normally Enhanced Packet Blocks or Simple Packet Blocks), of which it specifies the size. Knowing this size a priori helps to scan the file and to load some portions of it without truncating a block, and is particularly useful with cell-based networks like ATM.¶

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 0 |                        Block Type = ?                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 4 |                      Block Total Length                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 8 |          Cell Size            |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               /
   /                                                               /
   /                        Fixed Size Data                        /
   /                                                               /
   /                 variable length, octet-aligned                /
   /                                                               /
   /                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Block Total Length                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The fields have the following meaning:¶

• Block Type: The block type of the Fixed Length Block is not yet assigned.¶
• Block Total Length: total size of this block, as described in [I-D.tuexen-opsawg-pcapng], section "Section Blocks".¶
• Cell size (16 bits): an unsigned value that indicates the size of the blocks contained in the data field.¶
• Fixed Size Data: data of this block.¶

3.6. Directory Block (experimental)

If present, this block contains the following information:¶

• number of indexed packets (N)¶
• table with position and length of any indexed packet (N entries)¶

A directory block MUST be followed by at least N packets, otherwise it MUST be considered invalid. It can be used to efficiently load portions of the file to memory and to support operations on memory mapped files. This block can be added by tools like network analyzers as a consequence of file processing.¶

3.7. Traffic Statistics and Monitoring Blocks (experimental)

One or more blocks could be defined to contain network statistics or traffic monitoring information. They could be use to store data collected from RMON or Netflow probes, or from other network monitoring tools.¶

3.8. Event/Security Block (experimental)

This block could be used to store events. Events could contain generic information (for example network load over 50%, server down...) or security alerts. An event could be:¶

• skipped, if the application doesn't know how to do with it¶
• processed independently by the packets. In other words, the applications skips the packets and processes only the alerts¶
• processed in relation to packets: for example, a security tool could load only the packets of the file that are near a security alert; a monitoring tool could skip the packets captured while the server was down.¶