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HOSTAPD.CONF(5) File Formats Manual HOSTAPD.CONF(5)

hostapd.conf — configuration file for the Host Access Point daemon

hostapd.conf is the configuration file for the hostapd(8) daemon.

The hostapd.conf file is divided into the following main sections:

User-defined variables may be defined and used later, simplifying the configuration file.
Tables provide a mechanism to handle a large number of link layer addresses easily, with increased performance and flexibility.
Global runtime settings for hostapd(8).
Event rules provide a powerful mechanism to trigger certain actions when receiving specified IEEE 802.11 frames.
The concepts and details about the optional IP based roaming in hostapd(8).

The current line can be extended over multiple lines using a backslash (‘\’). Comments can be put anywhere in the file using a hash mark (‘#’), and extend to the end of the current line. Care should be taken when commenting out multi-line text: the comment is effective until the end of the entire block.

Argument names not beginning with a letter, digit, or underscore must be quoted.

Additional configuration files can be included with the include keyword, for example:

include "/etc/hostapd.conf.local"

Macros can be defined that will later be expanded in context. Macro names must start with a letter, digit, or underscore, and may contain any of those characters. Macro names may not be reserved words (for example, set, interface, or hostap). Macros are not expanded inside quotes.

For example:

wlan="ath0"
set iapp handle subtype { ! add notify, radiotap }
set iapp interface $wlan

Tables are named structures which can hold a collection of link layer addresses, masked address ranges, and link layer to IP address assignments. Lookups against tables in hostapd(8) are relatively fast, making a single rule with tables much more efficient, in terms of processor usage and memory consumption, than a large number of rules which differ only in link layer addresses.

Tables are used for hostapd(8) to match specified IEEE 802.11 link layer addresses and address ranges, and the capability to assign link layer to IP addresses and an option netmask is a requirement for advanced IAPP functionality.

Table options may be presented after the table name declaration. The following options are supported:

The table is constant and cannot be later changed from its original definition.

For example:

cisco="00:40:06:ff:ff:ff & ff:ff:ff:00:00:00"

table <black> { $cisco, 00:0d:60:ff:f1:2a }
table <myess> const {
	00:00:24:c3:40:18 -> 10.195.64.24,
	00:00:24:c3:40:19 -> 10.195.64.25,
	00:00:24:c3:40:1a -> 10.195.64.26
}
table <myclient> const {
	00:05:4e:45:d4:b9 -> 172.23.5.1/30
}

The following configuration settings are understood:

interface | {interface0, interface1, ...}
Specify the wireless interface running in Host AP mode. This option could be omitted to use hostapd(8) to log received IAPP messages. Multiple hostap interfaces may be specified as a comma-separated list, surrounded by curly braces.
mode
Specify the Host AP capture mode. The supported modes are:

Capture IEEE 802.11 frames with additional radiotap headers. They will provide optional but useful information like received frame signal levels.
Capture plain IEEE 802.11 frames.
interface | {interface0, interface1, ...}
Enable a channel hopper on the selected wireless interface. Multiple hostap interfaces may be specified as a comma-separated list, surrounded by curly braces.
number
Set the delay in milliseconds for the channel hopper before hopping to the next available channel. The default value is 800 milliseconds.
interface
Specify the mandatory Inter-Access-Point (IAPP) interface. It is important that the IAPP interface is on a trusted network because there is no authentication and an attacker could force disassociation of selected stations on all listening access points.
[address | route] roaming table <table>
Specify a table used for IP Roaming lookups of link layer address to IP address or subnet assignments.
subtype | {subtype0, subtype1, ...}
Specify the IAPP subtypes to use:

[not] add notify
Send and receive ADD.notify messages. This option is enabled by default.
[not] radiotap
Receive radiotap messages. This option is enabled by default.
[not] [address | route] roaming
Enable dynamic roaming of IP addresses or routes. These options are disabled by default.
mode
Specify the IAPP mode. The supported modes are:

[address ipv4addr] [port number] [ttl number]
Use multicast(4) frames. A multicast time-to-live (TTL) of 2 or higher is required to allow multicast forwarding, for example for use with mrouted(8).
[port number]
Use broadcast frames.

The default is multicast using the multicast address 224.0.1.178 and port 3517 with a TTL limited to 1 hop. Some access point vendors still use broadcast with the pre-standard IAPP port 2313.

Event rules provide a powerful way to trigger a certain action when receiving specified IEEE 802.11 frames on the hostap interface. The rules are handled in sequential order, from first to last. Rules are handled without a state: each rule is processed independently from the others and from any previous actions. This behaviour is somewhat different to that of packet filter rules specified in pf.conf(5).

All hostapd(8) event rules are single line statements beginning with the mandatory hostap handle keywords and optional rule options, interface, frame matching, a specified action, a limit, and a minimal rate:

hostap handle [option] [interface] [frame] [action] [limit] [rate]

Some rule statements support the optional keyword not, also represented by the ! operator, for inverse matching.

The optional parts are defined below.

The rule option will modify the behaviour of handling the statement. There are two possible options, quick and skip. If either the keyword quick or the keyword skip is specified, no further event rules will be handled for this frame after processing this rule successfully. The keyword skip additionally skips any further IAPP processing of the frame, which is normally done after handling the event rules.

The rule interface specifies the hostap interface the rule is matched on. The available interface list is specified by the global set hostap interface configuration setting.

on [not] interface

If not given, the event rule is matched on all available hostap interfaces.

The frame description specifies a mechanism to match IEEE 802.11 frames.

Match all frames.
[type] [dir] [from] [to] [bssid] [radiotap]
Apply rules to frames matching the given parameters. The parameters are explained below.

The type parameter specifies the frame type to match on. The frame type may be specified in the following ways:

type any
Match all frame types.
[not] data
Match data frames. Presence of the not keyword negates the match and will match all non-data frames.
[not] management [[not] subtype]
Match management frames. The subtype argument may be specified to optionally match management frames of the given subtype. The subtype match may be negated by specifying the not keyword. See the Management Frame Subtypes section below for available subtypes specifications.

The dir parameter specifies the direction the frame is being sent. The direction may be specified in the following ways:

dir any
Match all directions.
framedir
Match frames with the given direction framedir. See the Frame Directions section below for available direction specifications.

The radiotap rules allow parsing and matching of the extra information reported by the radiotap header. Support for the specified radiotap headers is optional and the specific parameters depend on the radiotap elements reported by the wireless interface. Support for the radiotap data link type can be verified with the tcpdump(8) command. These rules require hostap mode radiotap in the global configuration.

[operator] percentage %
Match the signal quality of the received frame.
[operator] value (GHz | )
Match the transmit rate of the received frame.
[operator] rate Mb
Match the frequency of the received frame, in Mbps.

The radiotap rules support the following operators. If omitted, the specified value will be checked if it is equal or not.

=	(equal)
!=	(not equal)
<	(less than)
<=	(less than or equal)
>	(greater than)
>=	(greater than or equal)

The from, to, and bssid parameters specify the IEEE 802.11 address fields to match on. They can be specified in the following ways:

(from | | ) any
Allow all addresses for the specified address field.
(from | | ) [not] <table>
Allow allow addresses from the given table (see Tables above) for the specified address field.
(from | | ) [not] lladdr
Allow the given address lladdr for the specified address field.

An optional action is triggered if a received IEEE 802.11 frame matches the frame description. The following actions are supported:

type [dir] from to bssid
Send an arbitrary constructed frame to the wireless network. The arguments are as follows.

The type describes the IEEE 802.11 frame type to send, specified in the frame control header. The following frames types are supported at present:

type data
Send a data frame. This is normally used to encapsulate ordinary IEEE 802.3 frames into IEEE 802.11 wireless frames.
type management subtype
Send a management frame with the specified subtype. Management frames are used to control states and to find access points and IBSS nodes in IEEE 802.11 networks. See the Management Frame Subtypes section below for available subtypes specifications.

The dir describes the direction the IEEE 802.11 frame will be sent. It has the following syntax:

dir framedir

See the Frame Directions section below for available direction specifications.

The from, to, and bssid arguments specify the link layer address fields used in IEEE 802.11 frames. All address fields are mandatory in the frame action. The optional fourth address field used by wireless distribution systems (WDS) is currently not supported. Each argument is specified by a keyword of the same name (from, to, or bssid) followed by one of the following address specifications:

lladdr
Specify the link layer addresses used in the IEEE 802.11 frame address field. The link layer address ‘ff:ff:ff:ff:ff:ff’ is the IEEE 802.11 broadcast address.
refaddr
Fill in a link layer address from the previously matched IEEE 802.11 frame. &from will use the source link layer address; &to the destination link layer address; and &bssid the BSSID link layer address of the previously matched frame.
Use a random link layer address in the specified IEEE 802.11 frame address field. Multicast and broadcast link layer addresses will be skipped.
iapp-type
Send a hostapd(8) specific IAPP frame with a raw IEEE 802.11 packet dump of the received frame to the wired network. The only supported iapp-type is radiotap.
[verbose]
Write informational messages to the local system log (see syslogd(8)) or standard error. If the Rule Rate has been specified, log will print the actual rate.
| lladdr
Add or remove the specified node from the internal kernel node table.
Resend the received IEEE 802.11 frame.

It is possible to limit handling of specific rules with the limit keyword:

limit number sec | usec

In some cases it is absolutely necessary to use limited matching to protect hostapd(8) against excessive flooding with IEEE 802.11 frames. For example, beacon frames will be normally received every 100 ms.

It is possible to tell hostapd(8) to trigger the action only after a specific rate of matched frames.

rate number / number sec

This will help to detect excessive flooding of IEEE 802.11 frames. For example, de-auth flooding is a denial of service (DoS) attack against IEEE 802.11 wireless networks.

The subtype describes the IEEE 802.11 frame subtype, specified in the frame control header. The choice of subtypes depends on the used frame type. hostapd(8) currently only supports management frame subtypes. Most frame subtypes require an additional subtype-specific header in the frame body, but currently only the deauth and disassoc reason codes are supported:

A beacon frame. Wireless access points and devices running in master or hostap mode continuously send beacon frames to indicate their presence, traffic load, and capabilities.
[reason]
A deauthentication frame with an optional reason code. Deauthenticated stations will lose any IEEE 802.11 operational state.
[reason]
A disassociation frame with an optional reason code.
An association request frame.
An association response frame.
An announcement traffic indication message (ATIM frame).
[open request | response]
An authentication frame.
A probe request frame. Probe requests are used to probe for access points and IBSS nodes.
A probe response frame.
A re-association request frame.
A re-association response frame.

The reason defines a descriptive reason for the actual or of a station:

Disassociated due to inactivity.
Disassociated because the sending station is leaving or has left the wireless network.
Disassociated because the access point has reached its limit of associated stations.
Previous authentication no longer valid.
Deauthenticated because the sending station is leaving or has left the wireless network.
IEEE 802.11i extension.
IEEE 802.11i extension.
Frame received from unauthenticated station.
Frame received from an associated but unauthenticated station.
Frame received from unassociated station.
IEEE 802.11i extension.
IEEE 802.11i extension.
Unspecified reason.

The direction a frame is being transmitted (framedir) can be specified in the following ways:

dir no ds
No distribution system direction is used for management frames.
dir to ds
A frame sent from a station to the distribution system, the access point.
dir from ds
A frame from the distribution system, the access point, to a station.
dir ds to ds
A frame direction used by wireless distribution systems (WDS) for wireless access point to access point communication.

# Log probe requests locally
hostap handle type management subtype probe request \
    with log

# Detect flooding of management frames except beacons.
# This will detect some possible denial of service attacks
# against the IEEE 802.11 protocol.
hostap handle skip type management subtype ! beacon \
    with log \
    rate 100 / 10 sec

# Log rogue access points via IAPP, limited to every second,
# and skip further IAPP processing.
hostap handle skip type management subtype beacon bssid !<myess> \
    with iapp type radiotap limit 1 sec

# Send deauthentication frames to stations associated to rogue APs
hostap handle type data bssid !<myess> with frame type management \
    subtype deauth reason auth expire \
    from &bssid to &from bssid &bssid

# Send authentication requests from random station addresses to
# rogue access points. This is a common way to test the quality of
# various hostap implementations.
hostap handle skip type management subtype beacon bssid <pentest> \
    with frame type management subtype auth \
    from random to &bssid bssid &bssid

# Re-inject a received IEEE 802.11 frame on the interface ath0
hostap handle on ath0 type management subtype auth with resend

# Remove a blacklisted node from the kernel node tree
hostap handle type management subtype auth from <blacklist> \
    with node delete &from

# Log rogue access points with a strong signal quality on
# channel 3 (2.422GHz) transmitting frames with 1Mbps.
hostap handle type management subtype beacon bssid !<myess> \
    signal >= 50% txrate 1Mb freq 2.422GHz \
    with log

In a traditional wireless network, multiple access points are members of a single layer 3 broadcast domain. The traffic is bridged between physical collision domains, as with the bridge(4) interface in OpenBSD. This may cause problems in large wireless networks with a heavy load of broadcast traffic, like broadcasted ARP, DHCP or ICMP requests.

hostapd(8) implements IP based roaming to build wireless networks without the requirement of a single broadcast domain. This works as follows:

  1. Every access point running hostapd(8) is a router to an individual internal broadcast domain, using the bridge(4) interface.
  2. An increased multicast TTL is used for IAPP communication between access points in multiple network segments. Multicast routing is required in the network infrastructure, like an OpenBSD router running mrouted(8).
  3. The configuration file hostapd.conf is used to assign IP subnets to link layer addresses. If a station with the specified link layer address successfully associates to the access point, hostapd(8) will configure the specified IP address and subnet on the wireless interface.
  4. The IAPP ADD.notify message is used to notify other access points running hostapd(8) to remove the station and any assigned IP addresses or subnets from the wireless interface.
  5. A dynamic routing daemon like ospfd(8) or bgpd(8) running on the access point will be used to announce the new IP route to the internal network and routers.

For example:

# Assign IP addresses to layer 2 addresses
table <clients> {
	00:02:6f:42:d0:01 -> 172.23.5.1/30,
	00:05:4e:45:d3:b8 -> 172.23.5.4/30,
	00:04:2e:12:03:e0 -> 172.23.5.8/30
}

# Global options
set hostap interface ath0
set hostap mode radiotap
set iapp interface sis0
set iapp address roaming table <clients>
set iapp handle subtype address roaming
set iapp mode multicast ttl 2

/etc/hostapd.conf
Default location of the configuration file.
/etc/examples/hostapd.conf
Example configuration file.

hostapd(8)

The hostapd(8) program was written by Reyk Floeter <reyk@openbsd.org>.

IP Roaming requires statically assigned IP addresses of stations and does not support DHCP at present.

April 23, 2020 OpenBSD-7.3