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GITPROTOCOL-PACK(5) Git Manual GITPROTOCOL-PACK(5)
gitprotocol-pack - How packs are transferred over-the-wire
<over-the-wire-protocol>
Git supports transferring data in packfiles over the ssh://,
git://, http:// and file:// transports. There exist two sets of
protocols, one for pushing data from a client to a server and
another for fetching data from a server to a client. The three
transports (ssh, git, file) use the same protocol to transfer
data. http is documented in gitprotocol-http(5).
The processes invoked in the canonical Git implementation are
upload-pack on the server side and fetch-pack on the client side
for fetching data; then receive-pack on the server and send-pack
on the client for pushing data. The protocol functions to have a
server tell a client what is currently on the server, then for
the two to negotiate the smallest amount of data to send in order
to fully update one or the other.
The descriptions below build on the pkt-line format described in
gitprotocol-common(5). When the grammar indicates PKT-LINE(...),
unless otherwise noted the usual pkt-line LF rules apply: the
sender SHOULD include a LF, but the receiver MUST NOT complain if
it is not present.
An error packet is a special pkt-line that contains an error
string.
error-line = PKT-LINE("ERR" SP explanation-text)
Throughout the protocol, where PKT-LINE(...) is expected, an
error packet MAY be sent. Once this packet is sent by a client or
a server, the data transfer process defined in this protocol is
terminated.
There are three transports over which the packfile protocol is
initiated. The Git transport is a simple, unauthenticated server
that takes the command (almost always upload-pack, though Git
servers can be configured to be globally writable, in which
receive- pack initiation is also allowed) with which the client
wishes to communicate and executes it and connects it to the
requesting process.
In the SSH transport, the client just runs the upload-pack or
receive-pack process on the server over the SSH protocol and then
communicates with that invoked process over the SSH connection.
The file:// transport runs the upload-pack or receive-pack
process locally and communicates with it over a pipe.
The protocol provides a mechanism in which clients can send
additional information in its first message to the server. These
are called "Extra Parameters", and are supported by the Git, SSH,
and HTTP protocols.
Each Extra Parameter takes the form of <key>=<value> or <key>.
Servers that receive any such Extra Parameters MUST ignore all
unrecognized keys. Currently, the only Extra Parameter recognized
is "version" with a value of 1 or 2. See gitprotocol-v2(5) for
more information on protocol version 2.
The Git transport starts off by sending the command and
repository on the wire using the pkt-line format, followed by a
NUL byte and a hostname parameter, terminated by a NUL byte.
0033git-upload-pack /project.git\0host=myserver.com\0
The transport may send Extra Parameters by adding an additional
NUL byte, and then adding one or more NUL-terminated strings:
003egit-upload-pack /project.git\0host=myserver.com\0\0version=1\0
git-proto-request = request-command SP pathname NUL
[ host-parameter NUL ] [ NUL extra-parameters ]
request-command = "git-upload-pack" / "git-receive-pack" /
"git-upload-archive" ; case sensitive
pathname = *( %x01-ff ) ; exclude NUL
host-parameter = "host=" hostname [ ":" port ]
extra-parameters = 1*extra-parameter
extra-parameter = 1*( %x01-ff ) NUL
host-parameter is used for the git-daemon name based virtual
hosting. See --interpolated-path option to git daemon, with the
%H/%CH format characters.
Basically what the Git client is doing to connect to an
upload-pack process on the server side over the Git protocol is
this:
$ echo -e -n \
"003agit-upload-pack /schacon/gitbook.git\0host=example.com\0" |
nc -v example.com 9418
Initiating the upload-pack or receive-pack processes over SSH is
executing the binary on the server via SSH remote execution. It
is basically equivalent to running this:
$ ssh git.example.com "git-upload-pack '/project.git'"
For a server to support Git pushing and pulling for a given user
over SSH, that user needs to be able to execute one or both of
those commands via the SSH shell that they are provided on login.
On some systems, that shell access is limited to only being able
to run those two commands, or even just one of them.
In an ssh:// format URI, it’s absolute in the URI, so the / after
the host name (or port number) is sent as an argument, which is
then read by the remote git-upload-pack exactly as is, so it’s
effectively an absolute path in the remote filesystem.
git clone ssh://user@example.com/project.git
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v
ssh user@example.com "git-upload-pack '/project.git'"
In a "user@host:path" format URI, it’s relative to the user’s
home directory, because the Git client will run:
git clone user@example.com:project.git
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v
ssh user@example.com "git-upload-pack 'project.git'"
The exception is if a ~ is used, in which case we execute it
without the leading /.
ssh://user@example.com/~alice/project.git,
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v
ssh user@example.com "git-upload-pack '~alice/project.git'"
Depending on the value of the protocol.version configuration
variable, Git may attempt to send Extra Parameters as a
colon-separated string in the GIT_PROTOCOL environment variable.
This is done only if the ssh.variant configuration variable
indicates that the ssh command supports passing environment
variables as an argument.
A few things to remember here:
• The "command name" is spelled with dash (e.g.
git-upload-pack), but this can be overridden by the client;
• The repository path is always quoted with single quotes.
When one Git repository wants to get data that a second
repository has, the first can fetch from the second. This
operation determines what data the server has that the client
does not then streams that data down to the client in packfile
format.
When the client initially connects the server will immediately
respond with a version number (if "version=1" is sent as an Extra
Parameter), and a listing of each reference it has (all branches
and tags) along with the object name that each reference
currently points to.
$ echo -e -n "0045git-upload-pack /schacon/gitbook.git\0host=example.com\0\0version=1\0" |
nc -v example.com 9418
000eversion 1
00887217a7c7e582c46cec22a130adf4b9d7d950fba0 HEAD\0multi_ack thin-pack
side-band side-band-64k ofs-delta shallow no-progress include-tag
00441d3fcd5ced445d1abc402225c0b8a1299641f497 refs/heads/integration
003f7217a7c7e582c46cec22a130adf4b9d7d950fba0 refs/heads/master
003cb88d2441cac0977faf98efc80305012112238d9d refs/tags/v0.9
003c525128480b96c89e6418b1e40909bf6c5b2d580f refs/tags/v1.0
003fe92df48743b7bc7d26bcaabfddde0a1e20cae47c refs/tags/v1.0^{}
0000
The returned response is a pkt-line stream describing each ref
and its current value. The stream MUST be sorted by name
according to the C locale ordering.
If HEAD is a valid ref, HEAD MUST appear as the first advertised
ref. If HEAD is not a valid ref, HEAD MUST NOT appear in the
advertisement list at all, but other refs may still appear.
The stream MUST include capability declarations behind a NUL on
the first ref. The peeled value of a ref (that is "ref^{}") MUST
be immediately after the ref itself, if presented. A conforming
server MUST peel the ref if it’s an annotated tag.
advertised-refs = *1("version 1")
(no-refs / list-of-refs)
*shallow
flush-pkt
no-refs = PKT-LINE(zero-id SP "capabilities^{}"
NUL capability-list)
list-of-refs = first-ref *other-ref
first-ref = PKT-LINE(obj-id SP refname
NUL capability-list)
other-ref = PKT-LINE(other-tip / other-peeled)
other-tip = obj-id SP refname
other-peeled = obj-id SP refname "^{}"
shallow = PKT-LINE("shallow" SP obj-id)
capability-list = capability *(SP capability)
capability = 1*(LC_ALPHA / DIGIT / "-" / "_")
LC_ALPHA = %x61-7A
Server and client MUST use lowercase for obj-id, both MUST treat
obj-id as case-insensitive.
See protocol-capabilities.txt for a list of allowed server
capabilities and descriptions.
After reference and capabilities discovery, the client can decide
to terminate the connection by sending a flush-pkt, telling the
server it can now gracefully terminate, and disconnect, when it
does not need any pack data. This can happen with the ls-remote
command, and also can happen when the client already is up to
date.
Otherwise, it enters the negotiation phase, where the client and
server determine what the minimal packfile necessary for
transport is, by telling the server what objects it wants, its
shallow objects (if any), and the maximum commit depth it wants
(if any). The client will also send a list of the capabilities it
wants to be in effect, out of what the server said it could do
with the first want line.
upload-request = want-list
*shallow-line
*1depth-request
[filter-request]
flush-pkt
want-list = first-want
*additional-want
shallow-line = PKT-LINE("shallow" SP obj-id)
depth-request = PKT-LINE("deepen" SP depth) /
PKT-LINE("deepen-since" SP timestamp) /
PKT-LINE("deepen-not" SP ref)
first-want = PKT-LINE("want" SP obj-id SP capability-list)
additional-want = PKT-LINE("want" SP obj-id)
depth = 1*DIGIT
filter-request = PKT-LINE("filter" SP filter-spec)
Clients MUST send all the obj-ids it wants from the reference
discovery phase as want lines. Clients MUST send at least one
want command in the request body. Clients MUST NOT mention an
obj-id in a want command which did not appear in the response
obtained through ref discovery.
The client MUST write all obj-ids which it only has shallow
copies of (meaning that it does not have the parents of a commit)
as shallow lines so that the server is aware of the limitations
of the client’s history.
The client now sends the maximum commit history depth it wants
for this transaction, which is the number of commits it wants
from the tip of the history, if any, as a deepen line. A depth of
0 is the same as not making a depth request. The client does not
want to receive any commits beyond this depth, nor does it want
objects needed only to complete those commits. Commits whose
parents are not received as a result are defined as shallow and
marked as such in the server. This information is sent back to
the client in the next step.
The client can optionally request that pack-objects omit various
objects from the packfile using one of several filtering
techniques. These are intended for use with partial clone and
partial fetch operations. An object that does not meet a
filter-spec value is omitted unless explicitly requested in a
want line. See rev-list for possible filter-spec values.
Once all the want’s and 'shallow’s (and optional 'deepen) are
transferred, clients MUST send a flush-pkt, to tell the server
side that it is done sending the list.
Otherwise, if the client sent a positive depth request, the
server will determine which commits will and will not be shallow
and send this information to the client. If the client did not
request a positive depth, this step is skipped.
shallow-update = *shallow-line
*unshallow-line
flush-pkt
shallow-line = PKT-LINE("shallow" SP obj-id)
unshallow-line = PKT-LINE("unshallow" SP obj-id)
If the client has requested a positive depth, the server will
compute the set of commits which are no deeper than the desired
depth. The set of commits starts at the client’s wants.
The server writes shallow lines for each commit whose parents
will not be sent as a result. The server writes an unshallow line
for each commit which the client has indicated is shallow, but is
no longer shallow at the currently requested depth (that is, its
parents will now be sent). The server MUST NOT mark as unshallow
anything which the client has not indicated was shallow.
Now the client will send a list of the obj-ids it has using have
lines, so the server can make a packfile that only contains the
objects that the client needs. In multi_ack mode, the canonical
implementation will send up to 32 of these at a time, then will
send a flush-pkt. The canonical implementation will skip ahead
and send the next 32 immediately, so that there is always a block
of 32 "in-flight on the wire" at a time.
upload-haves = have-list
compute-end
have-list = *have-line
have-line = PKT-LINE("have" SP obj-id)
compute-end = flush-pkt / PKT-LINE("done")
If the server reads have lines, it then will respond by ACKing
any of the obj-ids the client said it had that the server also
has. The server will ACK obj-ids differently depending on which
ack mode is chosen by the client.
In multi_ack mode:
• the server will respond with ACK obj-id continue for any
common commits.
• once the server has found an acceptable common base commit
and is ready to make a packfile, it will blindly ACK all have
obj-ids back to the client.
• the server will then send a NAK and then wait for another
response from the client - either a done or another list of
have lines.
In multi_ack_detailed mode:
• the server will differentiate the ACKs where it is signaling
that it is ready to send data with ACK obj-id ready lines,
and signals the identified common commits with ACK obj-id
common lines.
Without either multi_ack or multi_ack_detailed:
• upload-pack sends "ACK obj-id" on the first common object it
finds. After that it says nothing until the client gives it a
"done".
• upload-pack sends "NAK" on a flush-pkt if no common object
has been found yet. If one has been found, and thus an ACK
was already sent, it’s silent on the flush-pkt.
After the client has gotten enough ACK responses that it can
determine that the server has enough information to send an
efficient packfile (in the canonical implementation, this is
determined when it has received enough ACKs that it can color
everything left in the --date-order queue as common with the
server, or the --date-order queue is empty), or the client
determines that it wants to give up (in the canonical
implementation, this is determined when the client sends 256 have
lines without getting any of them ACKed by the server - meaning
there is nothing in common and the server should just send all of
its objects), then the client will send a done command. The done
command signals to the server that the client is ready to receive
its packfile data.
However, the 256 limit only turns on in the canonical client
implementation if we have received at least one "ACK %s continue"
during a prior round. This helps to ensure that at least one
common ancestor is found before we give up entirely.
Once the done line is read from the client, the server will
either send a final ACK obj-id or it will send a NAK. obj-id is
the object name of the last commit determined to be common. The
server only sends ACK after done if there is at least one common
base and multi_ack or multi_ack_detailed is enabled. The server
always sends NAK after done if there is no common base found.
Instead of ACK or NAK, the server may send an error message (for
example, if it does not recognize an object in a want line
received from the client).
Then the server will start sending its packfile data.
server-response = *ack_multi ack / nak
ack_multi = PKT-LINE("ACK" SP obj-id ack_status)
ack_status = "continue" / "common" / "ready"
ack = PKT-LINE("ACK" SP obj-id)
nak = PKT-LINE("NAK")
A simple clone may look like this (with no have lines):
C: 0054want 74730d410fcb6603ace96f1dc55ea6196122532d multi_ack \
side-band-64k ofs-delta\n
C: 0032want 7d1665144a3a975c05f1f43902ddaf084e784dbe\n
C: 0032want 5a3f6be755bbb7deae50065988cbfa1ffa9ab68a\n
C: 0032want 7e47fe2bd8d01d481f44d7af0531bd93d3b21c01\n
C: 0032want 74730d410fcb6603ace96f1dc55ea6196122532d\n
C: 0000
C: 0009done\n
S: 0008NAK\n
S: [PACKFILE]
An incremental update (fetch) response might look like this:
C: 0054want 74730d410fcb6603ace96f1dc55ea6196122532d multi_ack \
side-band-64k ofs-delta\n
C: 0032want 7d1665144a3a975c05f1f43902ddaf084e784dbe\n
C: 0032want 5a3f6be755bbb7deae50065988cbfa1ffa9ab68a\n
C: 0000
C: 0032have 7e47fe2bd8d01d481f44d7af0531bd93d3b21c01\n
C: [30 more have lines]
C: 0032have 74730d410fcb6603ace96f1dc55ea6196122532d\n
C: 0000
S: 003aACK 7e47fe2bd8d01d481f44d7af0531bd93d3b21c01 continue\n
S: 003aACK 74730d410fcb6603ace96f1dc55ea6196122532d continue\n
S: 0008NAK\n
C: 0009done\n
S: 0031ACK 74730d410fcb6603ace96f1dc55ea6196122532d\n
S: [PACKFILE]
Now that the client and server have finished negotiation about
what the minimal amount of data that needs to be sent to the
client is, the server will construct and send the required data
in packfile format.
See gitformat-pack(5) for what the packfile itself actually looks
like.
If side-band or side-band-64k capabilities have been specified by
the client, the server will send the packfile data multiplexed.
Each packet starting with the packet-line length of the amount of
data that follows, followed by a single byte specifying the
sideband the following data is coming in on.
In side-band mode, it will send up to 999 data bytes plus 1
control code, for a total of up to 1000 bytes in a pkt-line. In
side-band-64k mode it will send up to 65519 data bytes plus 1
control code, for a total of up to 65520 bytes in a pkt-line.
The sideband byte will be a 1, 2 or a 3. Sideband 1 will contain
packfile data, sideband 2 will be used for progress information
that the client will generally print to stderr and sideband 3 is
used for error information.
If no side-band capability was specified, the server will stream
the entire packfile without multiplexing.
Pushing data to a server will invoke the receive-pack process on
the server, which will allow the client to tell it which
references it should update and then send all the data the server
will need for those new references to be complete. Once all the
data is received and validated, the server will then update its
references to what the client specified.
The protocol itself contains no authentication mechanisms. That
is to be handled by the transport, such as SSH, before the
receive-pack process is invoked. If receive-pack is configured
over the Git transport, those repositories will be writable by
anyone who can access that port (9418) as that transport is
unauthenticated.
The reference discovery phase is done nearly the same way as it
is in the fetching protocol. Each reference obj-id and name on
the server is sent in packet-line format to the client, followed
by a flush-pkt. The only real difference is that the capability
listing is different - the only possible values are
report-status, report-status-v2, delete-refs, ofs-delta, atomic
and push-options.
Once the client knows what references the server is at, it can
send a list of reference update requests. For each reference on
the server that it wants to update, it sends a line listing the
obj-id currently on the server, the obj-id the client would like
to update it to and the name of the reference.
This list is followed by a flush-pkt.
update-requests = *shallow ( command-list | push-cert )
shallow = PKT-LINE("shallow" SP obj-id)
command-list = PKT-LINE(command NUL capability-list)
*PKT-LINE(command)
flush-pkt
command = create / delete / update
create = zero-id SP new-id SP name
delete = old-id SP zero-id SP name
update = old-id SP new-id SP name
old-id = obj-id
new-id = obj-id
push-cert = PKT-LINE("push-cert" NUL capability-list LF)
PKT-LINE("certificate version 0.1" LF)
PKT-LINE("pusher" SP ident LF)
PKT-LINE("pushee" SP url LF)
PKT-LINE("nonce" SP nonce LF)
*PKT-LINE("push-option" SP push-option LF)
PKT-LINE(LF)
*PKT-LINE(command LF)
*PKT-LINE(gpg-signature-lines LF)
PKT-LINE("push-cert-end" LF)
push-option = 1*( VCHAR | SP )
If the server has advertised the push-options capability and the
client has specified push-options as part of the capability list
above, the client then sends its push options followed by a
flush-pkt.
push-options = *PKT-LINE(push-option) flush-pkt
For backwards compatibility with older Git servers, if the client
sends a push cert and push options, it MUST send its push options
both embedded within the push cert and after the push cert. (Note
that the push options within the cert are prefixed, but the push
options after the cert are not.) Both these lists MUST be the
same, modulo the prefix.
After that the packfile that should contain all the objects that
the server will need to complete the new references will be sent.
packfile = "PACK" 28*(OCTET)
If the receiving end does not support delete-refs, the sending
end MUST NOT ask for delete command.
If the receiving end does not support push-cert, the sending end
MUST NOT send a push-cert command. When a push-cert command is
sent, command-list MUST NOT be sent; the commands recorded in the
push certificate is used instead.
The packfile MUST NOT be sent if the only command used is delete.
A packfile MUST be sent if either create or update command is
used, even if the server already has all the necessary objects.
In this case the client MUST send an empty packfile. The only
time this is likely to happen is if the client is creating a new
branch or a tag that points to an existing obj-id.
The server will receive the packfile, unpack it, then validate
each reference that is being updated that it hasn’t changed while
the request was being processed (the obj-id is still the same as
the old-id), and it will run any update hooks to make sure that
the update is acceptable. If all of that is fine, the server will
then update the references.
A push certificate begins with a set of header lines. After the
header and an empty line, the protocol commands follow, one per
line. Note that the trailing LF in push-cert PKT-LINEs is not
optional; it must be present.
Currently, the following header fields are defined:
pusher ident
Identify the GPG key in "Human Readable Name
<email@address[1]>" format.
pushee url
The repository URL (anonymized, if the URL contains
authentication material) the user who ran git push intended
to push into.
nonce nonce
The nonce string the receiving repository asked the pushing
user to include in the certificate, to prevent replay
attacks.
The GPG signature lines are a detached signature for the contents
recorded in the push certificate before the signature block
begins. The detached signature is used to certify that the
commands were given by the pusher, who must be the signer.
After receiving the pack data from the sender, the receiver sends
a report if report-status or report-status-v2 capability is in
effect. It is a short listing of what happened in that update. It
will first list the status of the packfile unpacking as either
unpack ok or unpack [error]. Then it will list the status for
each of the references that it tried to update. Each line is
either ok [refname] if the update was successful, or ng [refname]
[error] if the update was not.
report-status = unpack-status
1*(command-status)
flush-pkt
unpack-status = PKT-LINE("unpack" SP unpack-result)
unpack-result = "ok" / error-msg
command-status = command-ok / command-fail
command-ok = PKT-LINE("ok" SP refname)
command-fail = PKT-LINE("ng" SP refname SP error-msg)
error-msg = 1*(OCTET) ; where not "ok"
The report-status-v2 capability extends the protocol by adding
new option lines in order to support reporting of reference
rewritten by the proc-receive hook. The proc-receive hook may
handle a command for a pseudo-reference which may create or
update one or more references, and each reference may have
different name, different new-oid, and different old-oid.
report-status-v2 = unpack-status
1*(command-status-v2)
flush-pkt
unpack-status = PKT-LINE("unpack" SP unpack-result)
unpack-result = "ok" / error-msg
command-status-v2 = command-ok-v2 / command-fail
command-ok-v2 = command-ok
*option-line
command-ok = PKT-LINE("ok" SP refname)
command-fail = PKT-LINE("ng" SP refname SP error-msg)
error-msg = 1*(OCTET) ; where not "ok"
option-line = *1(option-refname)
*1(option-old-oid)
*1(option-new-oid)
*1(option-forced-update)
option-refname = PKT-LINE("option" SP "refname" SP refname)
option-old-oid = PKT-LINE("option" SP "old-oid" SP obj-id)
option-new-oid = PKT-LINE("option" SP "new-oid" SP obj-id)
option-force = PKT-LINE("option" SP "forced-update")
Updates can be unsuccessful for a number of reasons. The
reference can have changed since the reference discovery phase
was originally sent, meaning someone pushed in the meantime. The
reference being pushed could be a non-fast-forward reference and
the update hooks or configuration could be set to not allow that,
etc. Also, some references can be updated while others can be
rejected.
An example client/server communication might look like this:
S: 006274730d410fcb6603ace96f1dc55ea6196122532d refs/heads/local\0report-status delete-refs ofs-delta\n
S: 003e7d1665144a3a975c05f1f43902ddaf084e784dbe refs/heads/debug\n
S: 003f74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/master\n
S: 003d74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/team\n
S: 0000
C: 00677d1665144a3a975c05f1f43902ddaf084e784dbe 74730d410fcb6603ace96f1dc55ea6196122532d refs/heads/debug\n
C: 006874730d410fcb6603ace96f1dc55ea6196122532d 5a3f6be755bbb7deae50065988cbfa1ffa9ab68a refs/heads/master\n
C: 0000
C: [PACKDATA]
S: 000eunpack ok\n
S: 0018ok refs/heads/debug\n
S: 002ang refs/heads/master non-fast-forward\n
Part of the git(1) suite
1. email@address
mailto:email@address
This page is part of the git (Git distributed version control
system) project. Information about the project can be found at
⟨http://git-scm.com/⟩. If you have a bug report for this manual
page, see ⟨http://git-scm.com/community⟩. This page was obtained
from the project's upstream Git repository
⟨https://github.com/git/git.git⟩ on 2023-12-22. (At that time,
the date of the most recent commit that was found in the
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(which is not part of the original manual page), send a mail to
man-pages@man7.org
Git 2.43.0.174.g055bb6 2023-12-20 GITPROTOCOL-PACK(5)
Pages that refer to this page: git(1), gitprotocol-capabilities(5), gitprotocol-http(5), gitprotocol-v2(5)
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HTML rendering created 2023-12-22 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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