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@setfilename guile-netlink.info

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@settitle guile-netlink

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@copying

Copyright @copyright{} 2020 Julien Lepiller

@quotation

Permission is granted to copy, distribute and/or modify this document

under the terms of the GNU Free Documentation License, Version 1.3 or

any later version published by the Free Software Foundation; with no

Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.  A

copy of the license is included in the section entitled ``GNU Free

Documentation License''.

@end quotation

@end copying

@titlepage

@end titlepage

@contents

@node Top

@top guile-netlink

This document describes guile-netlink version @value{VERSION}, a guile

implementation of the netlink protocol.

@menu

* Introduction::           What is netlink?

* API Reference::          Description of the library interface.

@detailmenu

--- The Detailed Node Listing ---

API Reference

* Common API::             Common functions and data types for defining netlink

                           protocols.

* Netlink API::            Common structures and data types for every protocols.

* Rtnetlink API::          The ROUTE_NETLINK protocol.

@end detailmenu

@end menu

@node Introduction

@chapter Introduction

Netlink is an inter-process communication protocol that can be used for

communication between processes, or with the kernel.  It is implemented by

Linux.

Many protocols exist on top of Netlink.  The most famous are used to configure

network-related functions in the kernel, such as firewall, route table or

IP addresses of interfaces.

This library implements the low-level bits of the code by providing data

structures that are close to their C counterpart, and basic procedures to

initiate communication.

@node API Reference

@chapter API Reference

@node Common API

@section Common API

Guile-netlink implements a common API for expressing other protocols.  This

section describes how to use this API to augment guile-netlink with additional

protocols.

@node Data Types

@subsection Data Types

Guile-netlink defines data types that are used in the various Netlink protocols.

We need to be able to serialize and deserialize data that guile-netlink

understands, but we also want to let users of guile-netlink extend this process

easily.  This need has lead to the creating of the following data structure,

defined in @code{(netlink data}).

@deffn {Datatype} nl-data

@table @asis

@item @code{data}

The data that is held by this record.

@item @code{size-proc}

A procedure that takes a data (of the same type as the data recorded in the

@code{data} field) and returns the size of its serialization.

@item @code{serialization-proc}

A procedure that takes a data (of the same type as the data recorded in the

@code{data} field), the position at which to start serializing, and a

bytevector in which to serialize.  This procedure should modify the bytevector

and its return value is ignored.

@end table

@end deffn

The module also defines the following function, that takes a @code{nl-data}

structure and provides its serialization in a bytevector:

@deffn {Scheme Procedure} serialize @var{data} @var{pos} @var{bv}

Takes a @code{nl-data} structure as @var{data}, a position @var{pos} in

the bytevector @var{bv}, and returns an unspecified value.

This function updates the bytevector and adds the serialization of @var{data}

into @var{bv} at @var{pos}.

@end deffn

By providing a @code{nl-data} structure, we defer the knowledge of how to

serialize the data to the structure itself, instead of the @code{serialize}

function.  This allows for more flexibility and extensibility, as the user

of the procedure can pass any kind of data, even if it is not yet supported by

guile-netlink.

Similarly, we need to be able to deserialize netlink answers into a data

structure.  To do so, we also defer the knowledge of the datastructure to

deserialize to, to a decoder structure that is passed to the deserialization

procedure.  @code{(netlink data)} also defines the following procedures to

deserialize data:

@deffn {Scheme Procedure} deserialize @var{type} @var{decoder} @var{bv} @var{pos}

Takes a bytevector @var{bv} and starts deserializing the data starting at

position @var{pos}.  To do so, it uses the @var{type} variable as the lookup

key in the @var{decoder}.  @var{type} is a symbol that represents the type of

data to deserialize to.

The decoder is a structure that associates each known type to its deserializer

(a function that takes a decoder, a bytevector and a position and returns some

data) and an alist that associates a type (an integer, as returned by the

protocol in use) to the proper decoder of that type.

@end deffn

@deffn {Scheme Procedure} get-current-deserialize @var{decoder} @var{current-type}

Takes a decoder and a type, and returns the deserialization procedure associated

with the type (a symbol) in @var{decoder}.

@end deffn

@deffn {Scheme Procedure} get-next-deserialize @var{decoder} @var{current-type} @

    @var{target-type}

Takes a decoder, a type (a symbol that represents the type of data being

deserialized) and another type (an integer as returned by the protocol), and

returns the deserialization procedure needed to continue decoding the data

associated with the currently being deserialized data.

For example, when decoding an answer in the netlink protocol, we first deserialize

the header into a @code{message} structure.  That header contains a type field

that contains an integer constant representing the type of data of the body.

Similarly, when deserializing a routing attribute in the rtnetlink protocol,

we first find a header of the attribute that defines an integer constant

corresponding to the type of attribute in the body.

By knowing the context in which the type is declared, this procedure can return

the correct deserializing procedure.  For instance, when deserializing a

@code{message}, type @code{16} means @code{RTM_NEWLINK} in the rtnetlink

protocol, whereas it means @code{IFLA_OPERSTATE} when deserializing a

@code{route-attribute}.

@end deffn

guile-netlink provides the following default decoder for the rtnetlink

protocol in @code{(netlink deserialize)}:

@deffn {Scheme Variable} %default-route-decoder

Contains the default decoder for the NETLINK_ROUTE protocol.

@end deffn

For convenience, guile-netlink defines the following structures that can be used

to create a custom decoder.

@deffn {Scheme Variable} %default-message-decoder

Contains the default association list for the common message types of netlink,

associating each of them to a deserialization procedure.

@end deffn

@deffn {Scheme Procedure} default-route-attr-decoder @var{deserialize-addr}

Creates the default association list for a route protocol, given the specified

address deserializer.  This is useful because the @code{IFA_ADDRESS},

@code{IFA_BROADCAST}, etc, contain a different type of address depending on

the message type or its header.  This is defined an @code{(netlink route attrs)}

and used by the following variables:

@end deffn

@deffn {Scheme Variable} %default-route-link-attr-decoder

Contains the default association list for the known types of routing attributes

for link messages.  This list is defined in @code{(netlink route attrs)}.

@end deffn

@node Constants

@subsection Constants

Guile-netlink defines constants used by the Netlink protocols in the

@code{(netlink constant)} module.  The constants are the ones present in the

kernel and are too numerous to list here.  Please see the source for the

complete list.

The module also defines the following macro:

@deffn {Scheme Macro} define-enum @var{integer->symbol} @var{name-spec} ...

This macros defines an enumeration.  @var{integer->symbol} is the name of

a procedure that is publicly defined, that takes an integer and returns the

associated symbol in the enumeration.

The macro also publicly defines variables whose names are in @var{name-spec}

to an integer.

A @var{name-spec} is either a single name, and the associated value is 0 for

the first @var{name-spec}, or one more than the previous @var{name-spec}.

It can also be a pair of a name and an integer, in which case the associated

value is that integer.  For instance:

@example

(define-enum get-foo FOO0 FOO1 (FOO10 10) FOO11 FOO12)

(get-foo 9) -> #<unspecified>

(get-foo 0) -> FOO0

FOO11 -> 11

@end example

@end deffn

@node Netlink Connections

@subsection Netlink Connections

The @code{(netlink connection)} module defines the following procedures, used

to connect and communicate with another process or the kernel using a netlink

socket.

@deffn {Scheme Procedure} get-addr @var{family} @var{pid} @var{groups}

Return a bytevector that represents a netlink address.  @var{family}

should be @code{AF_NETLINK}, @var{pid} is the PID of the process with which

to communicate or 0 for the kernel. @var{groups} is an integer representing

the set of broadcast groups to which the connection subscribes.

@end deffn

@deffn {Scheme Procedure} connect @var{proto} @var{addr}

Creates a netlink socket for @var{proto} and binds it to @var{addr}.

@var{proto} is the integer representing the protocol.  For instance, rtnetlink

can be selected by usin @code{NETLINK_ROUTE} (defined in

@code{(netlink constant)}).

@var{addr} is a bytevector, as returned by @code{get-addr}.

@end deffn

@deffn {Scheme Procedure} connect-route [#:groups @code{0}]

This procedure is a wrapper for @code{connect} that creates a socket for the

rtnetlink protocol, binds it to the kernel and returns it.  By passing the

optional @var{groups} keyword, you can select broadcast groups to subscribe to.

@end deffn

@deffn {Scheme Procedure} close-socket @var{socket}

Closes a netlink socket.  The socket cannot be used afterwards.

@end deffn

@deffn {Scheme Procedure} send-msg @var{msg} @var{sock} [#:@var{addr}]

Send @var{msg} (it must be of type message, @xref{Netlink Headers}) to

@var{addr} using @var{sock}.  If not passed, @var{addr} is the address of

the kernel.

@end deffn

@deffn {Scheme Procedure} receive-msg @var{sock} [#:@var{addr}]

Receives a message from @var{sock} from @var{addr}.  This procedure is blocking.

If not passed, @var{addr} defaults to the address of the kernel.  This

procedure returns the message as a bytevector, that you can deserialize with

@code{deserialize} (@xref{Data Types})

@end deffn

@deffn {Scheme Procedure} receive-and-decode-msg @var{sock} @var{decoder} @

    [#:@var{addr}]

Receives one or more messages from @var{sock} from @var{addr}.  this procedure

is blocking.  If not passed, @var{addr} defaults to the address of the kernel.

This procedure returns a list of messages that were decoded using @var{decoder}.

When the answer has the @code{NLM_F_MULTI} flag, this procedure decodes the next

message, until it receives a @code{NLMSG_DONE} message.  It returns the list

of every netlink messages it received, including the @code{NLMSG_DONE}.

@end deffn

@node Netlink API

@section Netlink API

This section introduces the data structures used for all the netlink protocols.

First, we introduce the structure of a netlink message, then we present the

standard types of netlink messages, that can be used with every protocol.

@node Netlink Headers

@subsection Netlink Headers

The @code{(netlink message)} module defines the message structure that contains

a netlink message.  It is composed of a header and a body, and is the data

structure to pass to @code{send-msg} (@xref{Netlink Connections}).

This module defines the following data structure:

@deffn {Datatype} message

@table @asis

@item @code{type}

The type of data in the body of the message.  For instance, @code{RTM_GETLINK}.

@item @code{flags}

The set of flags that are set in the header.  For instance,

@code{(logior NLM_F_REQUEST NLM_F_DUMP)}.

@item @code{seq}

The sequence number of the message.  If this message is an answer to a request,

it must keep the same sequence number.  Otherwise, you must generate a new and

unique sequence number, to track the answers.

@item @code{pid}

The PID of the receiving process, or 0 for the kernel.

@item @code{data}

The actual body, as an @code{nl-data} structure.

@end table

@end deffn

@node Standard Message Types

@subsection Standard Message Types

The @code{(netlink standard)} module defines the set of standard message types

and their data type.

@deffn {Datatype} error-message

@table @asis

@item @code{err}

The error code, as a negative number.

@item @code{hdr}

The message on which this error applies.

@end table

@deffn {Scheme Variable} no-data

This variable defines the absence of data.  This is useful when a structure

is expecting a body part, but the protocol specifically defines that it should

not take any data in some cases.  For instance, a @code{NLMSG_NOOP} message

takes no data, so the @code{data} field of the message will contain this

@code{no-data} value.

@end deffn

@end deffn

@node Rtnetlink API

@section Rtnetlink API

@cindex rtnetlink

@cindex ROUTE_NETLINK

This section describes the support for rtnetlink in guile-netlink.  Rtnetlink

is the protocol responsible for everything related to network routing.  It

allows you to manage links, addresses, routing tables, neighbor chaces,

routing rules, queueing disciplines, traffic classes, traffic filters and

more.

@node Routing Attributes

@subsection Routing Attributes

The @code{(netlink route attrs)} module defines the following data types:

@deffn {Datatype} route-attr

This defines a header structure for the attribute, as well as its body.

@table @asis

@item @code{type}

This is the type of the attribute, for instance @code{IFLA_ADDRESS}.

@item @code{data}

This is the body of the attribute, ie.@: its value.

@end table

@end deffn

The module also defines additional data types that are not represented as

a record, but by a simple type.  For each of the following types, there is

a @code{make-*-route-attr} procedure to produce a @code{nl-data} value

for this type.  There is also @code{deserialize-route-attr-data-*} procedure

to deserialize a value of this type.

@table @asis

@item @code{u8}

A one-byte unsigned integer

@item @code{u16}

A two-bytes unsigned integer

@item @code{u32}

A four-bytes unsigned integer

@item @code{s32}

A four-bytes signed integer

@item @code{string}

A string

@item @code{ethernet}

An ethernet address.  Its value is a string that represents that address,

for instnace @code{"01:23:45:67:89:ab"}

@item @code{ipv4}

An IPv4 address.  Its value is a string that represents that address,

for instnace @code{"192.0.2.152"}

@item @code{ipv6}

An IPv6 address.  Its value is a string that represents that address,

for instnace @code{"2001:db8::0123:4567:89ab:cdef"}

@item @code{bv}

A bytevector.  This is used by default when the type is not supported.

@end table

@node Link Messages

@subsection Link Messages

The @code{(netlink route link)} package defines the following data type:

@deffn {Datatype} link-message

This datatype represents a link message with its routing attributes.  This type

of message is expected when using the @var{RTM_*LINK} message types.

@table @asis

@item @code{family}

The network family, defined as @code{AF_UNSPEC} in the rtnetlink documentation,

although it holds different values in practice.

@item @code{type}

The device type.

@item @code{index}

The index of the device.  This is used to select a specific device by its index,

or 0 to not filter by device index.

@item @code{flags}

The device flags.  See @code{man 7 netdevices} for a list.

@item @code{attrs}

A list of attributes.  This field must contain a list of @code{nl-data}

structures, not a structure by itself.

@end table

@end deffn

@node Address Messages

@subsection Address Messages

The @code{(netlink route addr)} package defines the following data type:

@deffn {Datatype} addr-message

This datatype represents an address message with its routing attributes.  This

type of message is expected when using the @var{RTM_*ADDR} message types.

@table @asis

@item @code{family}

The network family, either @code{AF_INET} for IPv4 addresses, or @code{AF_INET6}

for IPv6 addresses.

@item @code{prefix-len}

The prefix length, i.e.@: the length of the prefix mask, in bits, if defined

for the address family.

@item @code{flags}

Address flags.  This can be a flag word of @code{IFA_F_SECONDARY} for secondary

address (old alias interface), @code{IFA_F_PERMANENT} for a permanent

address set by the user and other undocumented flags.

@item @code{scope}

The address scope.

@item @code{index}

The index of the device this address is for.

@item @code{attrs}

A list of attributes.  This field must contain a list of @code{nl-data}

structures, not a structure by itself.

@end table

@end deffn

@bye