Scroll to navigation

bwctld.limits(5) File Formats Manual bwctld.limits(5)


bwctld.limits - Bandwidth Control daemon policy configuration file


The bwctld.limits file is used to define the policy configuration for the bwctld program. It allows the system administrator to allocate the resources in a variety of ways.

There are two parts to the policy configuration:

Who is making the request? This can be very specific to an individual user or it can be more general in that the connection is coming from some particular network.
Now that the connection has been generally identified, what will bwctld allow it to do?

The authentication is done by assigning a userclass to each new connection as it comes in. Each userclass has a set of limits associated with it. The userclasses are hierarchical, so a connection must pass the limit restrictions of the given userclass as well as all parent classes.

Within the bwcltd.limits file, assign lines are used to assign a userclass to a given connection. limit lines are used to define a userclass and set the limits associated with that userclass. The file is read sequentially, and it is not permitted to use a classname before it is defined using a limit line.

The format of this file is:

  • Comment lines are any line where the first non-whitespace character is '#'. These lines are counted to return line numbers in error messages but are otherwise ignored by bwctld.
  • Lines may be continued using the semi-standard '\' character followed immediately by a newline. This is the only valid place for the '\' character. If it is found elsewhere a syntax error is reported.
  • Blank lines are treated as comment lines.
  • All other lines must conform to the syntax of a limit line or an assign line.


This directive is used to define the userclass hierarchy. It defines the classname as well as the limits associated with that class. A classname may only be defined once. The format of the limit directive is:

limit classname with limtype=value[,limtype=value]*

classname defines the name of the class with the given limits. Whitespace is used as a separator but is otherwise ignored. classname may be used as a directory name component within bwctld, so take care not to use characters that would be invalid. (i.e. '*' or '/' would be particularly bad.)

limtype and value indicate the particular type of limit and value to apply to this userclass. The available settings for limtype are:

limtype valid values default
allow_open_mode on/off on
allow_tcp on/off on
allow_udp on/off off
bandwidth integer (b/s) 0 (unlimited)
duration integer (seconds) 0 (unlimited)
event_horizon integer (seconds) 0 (unlimited)
max_time_error integer (seconds) 0 (unlimited)
parent previously defined classname null
pending integer 0 (unlimited)
This limit is only useful if the class is assigned to a netmask. It is used to limit specific IP/netmask identities to only encrypted or authenticated mode transactions or to allow open mode.
Allow TCP Iperf tests for userclass.
Allow UDP Iperf tests for userclass.
Maximum amount of bandwidth to allow userclass to use in a UDP Iperf test. 0 indicates unlimited by policy, but remember this is checked all the way to the root of the hierarchy. (If you want an unlimited userclass, your root must be unlimited, and the whole path down to the given userclass.)
Maximum duration of a single Iperf test for this userclass.
Maximum window into the future to look when trying to schedule a test for this userclass.
Maximum amount of time error to allow for tests in this class. The time error is the sum of the errors reported by NTP on the two involved systems. The larger the time error, the larger the duration of the reservation because the time error is used to ensure tests don't overlap. There is a limit on this to defend against DOS attacks where a client could report large errors to ensure other clients can not allocate test reservations.
The first limit line cannot have a parent since none have been defined yet. As such, the first line defines the root of your class hierarchy. All remaining limit lines MUST assign a parent. (It is hierarchical, after all.)
Maximum number of pending reservations for this userclass.
The assign directive is used to assign a userclass to a given connection. Basically, it authenticates the connection. The format of the assign directive is:

assign authtype [args] classname

authtype identifies the type of authentication being used. Whitespace is used as a separator but is otherwise ignored. classname must have been previously defined with the limit directive earlier in the file.

The available settings for authtype are:

Used if no other assignment matches. It takes no args.
Assign a specific subnet to a given userclass. subnet must be specified using VLSM notation (IP/nbits). The only arg is the subnet. For example:
would match only the loopback IPv4 address.
would match only the loopback IPv6 address.
would match all hosts on the 192.168.1.XXX network.

There must be no set bits in the non-masked portion of the address part of the subnet specification. i.e., would be an invalid subnet due to the bit set in the fourth octet.

Assign a specific user to a given userclass. The user must be defined in the bwctld.keys file.


bwctld determines if it should allow a connection from the client based upon the authentication mode of the request and the source IP address of the connection. If the client connection is in authenticated or encrypted mode, the daemon does not do any filtering based upon the source address of the connection. (See the -A option to bwctl and the authmode option in bwctld.conf.) In these modes, bwctld simply uses the identity of the connection to determine the userclass limits. If the connection is made in open mode, then bwctld first uses the source address to determine if bwctld should allow an open mode connection from that subnet at all. (This is the purpose of the allow_open_mode limtype described above.) If open mode is allowed from this subnet, then the userclass is determined by the closest subnet match defined by the assign net lines in the bwctld.limits file.


An initial limit line might look like:

limit root with \
bandwidth=900m, \
duration=0, \
allow_udp=on, \
allow_tcp=on, \

This would create a userclass named root. Because no parent is specified, this must be the first userclass defined in the file. This userclass has very liberal limits (UDP enabled with 900m limit). However, open mode authentication is not enabled for this userclass, so the connections that get these limits must successfully authenticate using an AES key.

If an administrator also wants to create a userclass that is used to deny all requests, they might add:

limit jail with \
parent=root, \
allow_udp=off, \
allow_tcp=off, \

This would create a userclass named jail. Because UDP and TCP tests have both been denied, no tests will be allowed. Also, allow_open_mode is off, so initial connections that are not in authenticated or encrypted mode would be dropped immediately anyway. (It would not make much sense to assign a user identity to this userclass. If you don't want connections from a particular user, the best thing to do is to remove that user from the bwctld.keys file.

If the administrator wanted to allow a limited amount of open tests, they could define a userclass like:

limit open with \
parent=root, \
allow_open_mode=on, \
allow_udp=off, \
allow_tcp=on, \
duration=30, \
event_horizon=300, \

This could be used to allow TCP throughput tests by random connections. It limits those tests to 30 seconds in duration, and only allows them to be scheduled within the next 5 minutes (event_horizon=300). Additionally, it only allows this userclass to have 5 currently pending reservations. This ensures that this userclass can only schedule 50% of the next 5 minutes. The advantage of this kind of setup is that the administrator can define other userclasses with a larger event_horizon allowing then to have priority over this class. (Suggestions for other methods of priority scheduling should be sent to

Now, these three userclasses might be assigned to specific connections in the following ways:

# default open
assign default open

# badguys subnet
assign net jail

# network admins
assign user joe root
assign user jim root
assign user bob root

This set of assign lines specifically denies access from any open mode connection from the badguys subnet. It specifically allows access to authenticated or encrypted mode transactions that can authenticate as the identities joe jim or bob (even from the badguys subnet). All other connections would match the assign default rule and get the limits associated with the open userclass.


bwctl(1), bwctld(8), bwctld.limits(5), bwctld.keys(5), and the web site.

For details on Iperf3, see the web site.

For details on Iperf, see the web site.

For details on Nuttcp, see the web site.

For details on Owamp, see the web site.


This material is based in part on work supported by the National Science Foundation (NSF) under Grant No. ANI-0314723. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the NSF.