PXAR Format

Background

We are currently building a go library for the PXAR Archive format, and have been gathering information about the format by dissecting the archives and gathering information from the proxmox repository and other tools (mainly @tizbac s proxmoxbackupclient_go)

Official Overview (V1 format)

Base Entry Types

Name	Identifier	Description
PXAR_ENTRY	0xd5956474e588acef	Stores metadata information about the entry
PXAR_ENTRY_V1	0x11da850a1c1cceff	Older version of PXAR_ENTRY
PXAR_FILENAME	0x16701121063917b3	Stores the filename of the following item
PXAR_PAYLOAD	0x28147a1b0b7c1a25	The entry payload, e.g. symlink target or file contents
PXAR_SYMLINK	0x27f971e7dbf5dc5f	Stores symlink target and metadata
PXAR_HARDLINK	0x51269c8422bd7275	Stores hardlink
PXAR_GOODBYE	0x2fec4fa642d5731d	PXAR_GOODBYE contains hashes of all files in a folder
PXAR_GOODBYE_TAIL_MARKER	0xef5eed5b753e1555	End of goodbye record
PXAR_DEVICE	0x9fc9e906586d5ce9	Stores information about device records
PXAR_XATTR	0x0dab0229b57dcd03	Stores extended attributes for a file/folder
PXAR_ACL_USER	0x2ce8540a457d55b8	User ACL Entries
PXAR_ACL_GROUP	0x136e3eceb04c03ab	Group ACL Entries
PXAR_ACL_GROUP_OBJ	0x10868031e9582876	ACL_GROUP_OBJ
PXAR_ACL_DEFAULT	0xbbbb13415a6896f5	various default ACL fields
PXAR_ACL_DEFAULT_USER	0xc89357b40532cd1f	User ACL Entries
PXAR_ACL_DEFAULT_GROUP	0xf90a8a5816038ffe	Group ACL Entries
PXAR_FCAPS	0x2da9dd9db5f7fb67	File capability information, Linux disk format
PXAR_QUOTA_PROJID	0xe07540e82f7d1cbb	Quota information

Order

File

• PXAR_ENTRY
• PXAR_FILENAME
• PXAR_XATTR
• PXAR_ACL
• PXAR_FCAPS
• PXAR_QUOTA_PROJID
• PXAR_PAYLOAD

Folder

• PXAR_ENTRY
• PXAR_FILENAME
  • Files, links, nested folders
• PXAR_GOODBYE

Common

• All records include a descriptor header (table below)
• All numbers are little-endian

Field	Type	Description	Example
Type	uint64	The identifier from the table above	0xd5956474e588acef
Length	uint64	The length of the following record + 16 (length of self)	56

PXAR_ENTRY

Length: Fixed, linux stat struct length as uint64

Field	Type	Description	Example
Mode	uint64	File or folder mode	0o777
Flags	uint64	Flags on the object	0
Uid	uint32	User ID	1000
Gid	uint32	Group ID	1000
MtimeSecs	uint64	ModifiedTime Seconds	0
MtimeNanos	uint32	ModifiedTime Nanoseconds	0
MtimePadding	uint32	ModifiedTime Padding	0

PXAR_FILENAME

Length: Variable, max length 4096 + 1 (null-termination)

Field	Type	Description	Example
Filename	[]byte	The decoded filename	test.txt
Null Termination	byte	Terminates the filename with a null byte	0x00

PXAR_PAYLOAD

Length: Variable, max length MAX(uint64) - size of headers

Field	Type	Description	Example
Content	[]byte	The decoded filename	hello world

PXAR_SYMLINK, PXAR_HARDLINK

Length: Variable, max length 4096

Field	Type	Description	Example
Content	[]byte	The symlink target	/etc/target

PXAR_GOODBYE, PXAR_GOODBYE_TAIL

Length: Variable, max length MAX(uint64) - size of headers. 24*len(PXAR_GOODBYE)

Field	Type	Description	Example
Hash	uint64	The filename hash for a given file. The last record in an archive (PXAR_GOODBYE_TAIL) always has the hash value 0xef5eed5b753e1555	0xabcdef1234567890
Offset	uint64	The offset where the referenced item starts	10
Len	uint64	The length of the referenced item	100