Service set
Service set refers to all devices in a WLAN according to the 802.11 standard of the Institute of Electrical and Electronics Engineers . Some related terms appear in this structure.
Basic service set
A Basic Service Set (BSS) is created by synchronizing the basic parameters of several devices, which one of the devices starts by sending a proposal in the START format and others can accept in the JOIN format . By JOIN only possible are essentially data transmission rates confirmed.
Which of the two types of BSS is created is determined by whether connections are made to other networks such as other BSS:
- An Independent Basic Service Set (IBSS) is a BSS without connection to other networks, a closed network. An IBSS can be an ad hoc network .
- An Extended Service Set (ESS) is a BSS with a connection to other networks, including other standards such as Ethernet . An ESS is an infrastructure network .
The starting device continuously sends a clock signal , and in an IBSS each device is equally responsible for this task, so that such a BSS can continue without the starting device.
Basic Service Set Identification
The Basic Service Set Identification (BSSID) uniquely identifies each BSS. It either corresponds to the MAC address of the wireless access point or is generated as a random number .
Service Set Identifier
A service set identifier (SSID) is a freely selectable name of a service set that makes it accessible. Since this identifier often has to be entered manually into devices by a user, it is often a character string that is easy for humans to read, and it is therefore generally referred to as the (radio) network name of the WLAN.
A wireless access point can carry several SSIDs at the same time, which became more important with the introduction of authorization for WLANs. Such multiple SSIDs enable a wireless access point to offer multiple virtual local area networks .
An SSID can be up to 32 bytes long and accordingly contain up to 32 octet characters; ASCII characters are normally used for this. Since the IEEE 802.11-2012 standard, it can be signaled that these are encoded as UTF-8. If a device sends a data frame with an empty SSID, wireless access points send any SSID they support in response. The device can use these responses to generate a list that enables the user to select a service set.
SSID, ESSID and BSSID
By assigning different network IDs, you can also operate different wireless LANs at the same location. The SSID (Service Set Identifier) is set on the base station. ESSID (Extended Service Set Identifier) and BSSID (Basic Service Set Identifier) can be used in an SSID. An ad hoc network without access points is known as a BSSID. If a wireless infrastructure network is operated that has an access point, it is referred to as ESSID or simply SSID. In order to connect several base stations to a network (on OSI layer 2), set the same SSID on them; it is then referred to as ESSID ( Extended SSID ).
Clients (i.e. the radio subscribers) either specify the ESSID explicitly or try to join any network with the ESSID "*". In the latter case, it depends on the configuration of the base station whether the client is allowed to log into the network.
Hidden networks
As an apparent security measure, the broadcast beacon of the SSID can be switched off on some base stations. Before joining it is then necessary to enter it in addition to any password that may be used. However, this setting means that the WLAN clients regularly have to actively search for all saved network names of such "hidden" networks, even if the base station is not in the vicinity. This can easily be exploited to attack these end devices by simulating the presence of the access point by the attacker . Furthermore, the effect of security by covering up is rather small; This method can only keep inexperienced, casual intruders away from the wireless network.
The SSID can be determined quickly in an actively used WLAN. As soon as an attacker eavesdrops on a client using a WLAN sniffer when joining the network or just searching for a stored network, he also learns the previously suppressed SSID. In order not to have to wait for this process, the attacker can use forged data packets to throw clients out of the network. Since they usually log in again automatically, they can find out the suppressed SSID in no time at all. The passive network sniffer Kismet, for example, then has no problems recording the SSID.
WLAN-based location
In order to be able to offer WLAN-based localization , location mapping services record the SSIDs and locations of access points. Some of these services promise not to include WLANs in their databases or to delete them from their databases if their network name ends with the character string "_nomap".
Individual evidence
- ↑ a b c IEEE Std 802.11-2007 (PDF; 14.54 MB) Accessed May 1, 2012.
- ↑ The Wi-Fi Protected Access 2 (WPA2) / Wireless Provisioning Services Information Element (WPS IE) Update ... . Microsoft . Retrieved October 6, 2012.
- ↑ Henrique Devide: Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications Sponsored by the LAN / MAN Standards Committee . S. 478 ( academia.edu [accessed October 15, 2018]).
- ↑ Voice over WLAN roaming . Cisco Systems . Retrieved October 6, 2012.
- ↑ Why Non-Broadcast Networks are not a Security Feature. Microsoft Technet, accessed February 17, 2016 .
Web links
- Vivek Ramachandran, WLAN Security Megaprimer Part 6: Pwning hidden SSIDs . Video demonstration of active and passive exposure of hidden SSIDs.