babeld − ad-hoc network routing daemon
babeld option... [ −− ] interface...
Babel is a loop-avoiding distance-vector routing protocol roughly based on DSDV and AODV, but with provisions for link cost estimation and redistribution of routes from other routing protocols.
While it is optimised for wireless mesh networks, Babel will also work efficiently on wired networks.
Display babeld’s version and quit.
Specify the link-local multicast address to be used by the protocol. The default is ff02:0:0:0:0:0:1:6.
Specify the UDP port number to be used by the protocol. The default is 6696.
Set the name of the file used for preserving long-term information between invocations of the babeld daemon. If this file is deleted, the daemon will run in passive mode for 3 minutes when it is next started (see -P below), and other hosts might initially ignore it. The default is /var/lib/babel-state.
Specify the interval in seconds at which scheduled hello packets are sent on wireless interfaces. The default is 4 seconds.
Specify the interval in seconds at which scheduled hello packets are sent on wired interfaces. The default is 4 seconds.
−z kind [,factor]
Enable diversity-sensitive routing. The value kind defines the diversity algorithm used, and can be one of 0 (no diversity), 1 (per-interface diversity with no memory), 2 (per-channel diversity with no memory), or 3 (per-channel diversity with memory). The value factor specifies by how much the cost of non-interfering routes is multiplied, in units of 1/256; the default is 128 (i.e. division by 2).
Specify the half-time in seconds of the exponential decay used for smoothing metrics for performing route selection; the value 0 disables smoothing. The default is 4s.
Specify the priority value used when installing routes into the kernel. The default is 0.
Allow duplicating external routes when their kernel priority is at least priority. Do not use this option unless you know what you are doing, as it can cause persistent route flapping.
Use IFF_RUNNING (carrier sense) when determining interface availability.
Don’t optimise wired links, assume all interfaces are wireless unless explicitly overridden in the configuration file.
Do not perform split-horizon processing on wired interfaces. Split-horizon is not performed on wireless interfaces.
Use a random router-id. The default is to use persistent router-ids derived from the MAC address of the first interface, which is easier to debug and more reliably prevents routing loops but may sometimes cause a node to be unreachable for 120 seconds just after boot.
Do not flush unfeasible (useless) routes. This is useful in order to announce more information to a front-end (see −g).
Debug level. A value of 1 requests a routing table dump at every iteration through the daemon’s main loop. A value of 2 additionally requests tracing every message sent or received. A value of 3 additionally dumps all interactions with the OS kernel. The default is 0.
−g port, −g path
Set up a local configuration server on port port or at path in read-only mode. The protocol is described in the section Local Configuration Protocol below.
−G port, −G path
Set up a local configuration server on port port or at path in read-write mode. This allows any local user to change babeld’s configuration, and may therefore be a security issue.
Use the given kernel routing table for routes inserted by babeld.
Export routes from the given kernel routing table. This can be specified multiple times in order to export routes from more than one table.
Specify the name of the configuration file. This flag can be repeated multiple times. The default is /etc/babeld.conf.
Specify a configuration statement directly on the command line.
Daemonise at startup.
Specify a file to log random ’’how do you do?’’ messages to. This defaults to standard error if not daemonising, and to /var/log/babeld.log otherwise.
Specify a file to write our process id to, use no pidfile if set to the empty string. The default is /var/run/babeld.pid.
The list of interfaces on which the protocol should operate.
The configuration file is a sequence of lines each of which specifies a global option, an interface specification or a filtering rule. Comments are introduced by an octothorp ’’#’’ and terminate at the end of the line.
This specifies the link-local multicast address to be used by the protocol, and is equivalent to the command-line option −m.
This specifies the UDP port number to be used by the protocol, and is equivalent to the command-line option −p.
This specifies the priority value used when installing routes into the kernel, and is equivalent to the command-line option −k.
Reflect route metrics as kernel priorities. The priority effectively used is kernel-priority + metric.
This allows duplicating external routes when their kernel priority is at least priority. Do not use this option unless you know what you are doing, as it can cause persistent route flapping.
This specifies whether to use a random router-id, and is equivalent to the command-line option −r.
This specifies whether to use native source-specific IPv6 forwarding rather than multiple routing tables. The default is chosen automatically depending on the kernel version.
This specifies the debugging level, and is equivalent to the command-line option −d.
This specifies the TCP port on which babeld will listen for connections from a configuration client in read-only mode, and is equivalent to the command-line option −g.
This specifies the TCP port on which babeld will listen for connections from a configuration client in read-write mode, and is equivalent to the command-line option −G. This allows any local user to change babeld’s configuration, and may therefore be a security issue.
This specifies the filesystem path to a socket on which babeld will listen for connections from a configuration client in read-only mode, and is equivalent to the command-line option −g.
This specifies the filesystem path to a socket on which babeld will listen for connections from a configuration client in read-write mode, and is equivalent to the command-line option −G. Any user with write access to that socket will be able to change babeld’s configuration.
This specifies the kernel routing table to use for routes inserted by babeld, and is equivalent to the command-line option −t.
This specifies a kernel routing table from which routes are redistributed by babeld, and can be specified multiple times with a cumulative effect. This is equivalent to the command-line option −T.
This specifies whether to use carrier sense for determining interface availability, and is equivalent to the command-line option −l.
This specifies the diversity algorithm to use; true is equivalent to kind 3. The default is false (do not use any diversity algorithm).
This specifies by how much the cost of non-interfering routes should be multiplied, in units of 1/256. The default is 128 (division by 2).
This specifies the half-life in seconds of the exponential decay used for smoothing metrics for performing route selection, and is equivalent to the command-line option −M.
This specifies whether to daemonize at startup, and is equivalent to the command-line option −D.
If this flag is set, no kernel (sysctl) setup is performed on startup. This can be useful when running in environments where system permissions prevent setting kernel parameters, for instance inside a Linux container.
Specify the router-id explicitly, as a modified EUI-64 or a MAC-48 address. If two nodes have the same router-id, bad things will happen. Don’t use this option unless you know what you are doing.
This specifies the name of the file used for preserving long-term information between invocations of the babeld daemon, and is equivalent to the command-line option −S.
This specifies the name of the file used to log random messages to, and is equivalent to the command-line option −L.
This specifies the name of the file to which babeld writes out its process id, and is equivalent to the command-line option −I.
This specifies the index of the first routing table to use for source-specific routes. The default is 10.
This specifies smallest (highest) rule priority used with source-specific routes. The default is 100.
An interface is configured by a line with the following format:
interface name [parameter...]
where name is the name of the interface (something like eth0). The default value of an interface parameter can be specified changed by a line of the form
parameter can be one of:
The default is auto unless the −w command-line flag was specified.
This specifies whether link quality estimation should be performed on this interface. The default is to perform link quality estimation on wireless interfaces only.
This specifies whether to perform split-horizon processing on this interface. The default is to perform split-horizon processing on on wired interfaces, unless the −s flag was set.
This defines the cost of receiving frames on the given interface under ideal conditions (no packet loss); how this relates to the actual cost used for computing metrics of routes going through this interface depends on whether link quality estimation is being done. The default is 256 if the interface is wireless, and 96 otherwise.
Sets the channel for this interface. The value channel can be either an integer, or one of the strings interfering or noninterfering. The default is to autodetect the channel number for wireless interfaces, and noninterfering for other interfaces.
This specifies whether the network is "far away", in the sense that networks behind it don’t interfere with networks in front of it. By default, networks are not far away.
This defines the interval between hello packets sent on this interface. The default is specified with the −h and −H command-line flags.
This defines the interval between full routing table dumps sent on this interface; since Babel uses triggered updates and doesn’t count to infinity, this can be set to a fairly large value, unless significant packet loss is expected. The default is four times the hello interval.
Enable sending timestamps with each Hello and IHU message in order to compute RTT values. The default is true for tunnel interfaces, and false otherwise.
This specifies the decay factor for the exponential moving average of RTT samples, in units of 1/256. Must be between 1 and 256, inclusive. Higher values discard old samples faster. The default is 42.
This specifies the minimum RTT, in milliseconds, starting from which we increase the cost to a neighbour. The additional cost is linear in (rtt - rtt−min). The default is 10 ms.
This specifies the maximum RTT, in milliseconds, above which we don’t increase the cost to a neighbour. The default is 120 ms.
This specifies the maximum cost added to a neighbour because of RTT, i.e. when the RTT is higher or equal than rtt−max. The default is 96 if the interface is of type tunnel, and 0 otherwise.
A filtering rule is defined by a single line with the following format:
filter selector... action
Filter specifies the filter to which this entry will be added, and can be one of in (applied to routes learned from Babel neighbours), out (applied to routes announced to Babel neighbours), redistribute (applied to routes redistributed from the kernel), or install (applied to routes installed into the kernel).
selector specifies the conditions under which the
given statement matches. It can be one of
This entry only applies to routes in the given prefix.
This entry only applies to routes with a prefix length equal to plen.
This entry only applies to routes with a prefix length less or equal to plen.
This entry only applies to routes with a prefix length greater or equal to plen.
This entry only applies to routes with a source prefix in the given prefix.
This entry only applies to routes with a source prefix length equal to plen.
This entry only applies to routes with a source prefix length less or equal to plen.
This entry only applies to routes with a source prefix length greater or equal to plen.
This entry only applies to routes learned from a neighbour with link-local address address.
This entry only applies to routes originated by a router with router-id id.
This entry only applies to kernel routes with kernel protocol number p. If neither proto nor local is specified, this entry applies to all non-local kernel routes with a protocol different from "boot".
This entry only applies to local addresses.
For an input filter, this specifies the interface over which the route is learned. For an output filter, this specifies the interface over which this route is advertised. For a redistribute statement, this specifies the interface over which the route forwards packets.
Action specifies the action to be taken when this entry matches. It can have one of the following values:
Allow this route, without changing its metric (or setting its metric to 0 in case of a redistribute filter).
Ignore this route.
For an input or output filter, allow this route after increasing its metric by value. For a redistribute filter, redistribute this route with metric value.
For a redistribute filter, set the source prefix of this route to prefix.
In an install filter, specify the kernel routing table to use. For source-specific routes, this only works reliably for IPv6, and only when ipv6-subtrees is true.
If action is not specified, it defaults to allow.
By default, babeld redistributes all local addresses, and no other routes. In order to make sure that only the routes you specify are redistributed, you should include the line
redistribute local deny
as the last line in your configuration file.
If babeld is invoked with the flag −g, it accepts TCP connections from local clients on the given port and address ::1 (the IPv6 localhost address), or on the given UNIX-domain socket path if the argument starts with ’/’. When a client connects, babeld replies with BABEL followed with the supported version of the local configuration protocol (currently 1.0). This is followed with a number of informational lines (version etc.), terminated by ok. The client can then send requests, one per line. To each request, babeld replies with one or more lines of data terminated by one of ok, no, or bad.
The following requests are currently defined:
any configuration file directive, including interface;
monitor and unmonitor;
You can participate in a Babel network by simply running
# babeld wlan0
where wlan0 is the name of your wireless interface.
In order to gateway between multiple interfaces, just list them all on the command line:
# babeld wlan0 eth0 sit1
On an access point, you’ll probably want to redistribute some external routes into Babel:
# babeld \
−C ’redistribute metric 256’ \
or, if you want to constrain the routes that you redistribute,
# babeld \
−C ’redistribute proto 11 ip ::/0 le 64 metric 256’ \
−C ’redistribute proto 11 ip 0.0.0.0/0 le 24 metric 256’ \
If your want to redistribute kernel routes as source-specific to the network, with the 2001:DB8:0:1::/64 prefix:
redistribute src-prefix 2001:DB8:0:1::/64
For more information about source-specific routing, please see
Matthieu Boutier and Juliusz Chroboczek. Source-sensitive routing. In Proc. IFIP Networking 2015. 2015.
available online at
The default location of the configuration file.
The default location of the file storing long-term state.
The default location of the pid file.
The default location of the log file.
Dump Babel’s routing tables to standard output or to the log file.
Check interfaces and kernel routes right now, then reopen the log file.
Babel is a completely insecure protocol: any attacker able to inject IP packets with a link-local source address can disrupt the protocol’s operation. This is no different from unsecured neighbour discovery or ARP.
Usage of the −G flag allows any user logged on the local host to change babeld’s configuration.
Since Babel uses link-local IPv6 packets only, there is no need to update firewalls to allow forwarding of Babel protocol packets. If local filtering is being done, UDP datagrams to the port used by the protocol should be allowed. As Babel uses unicast packets in some cases, it is not enough to just allow packets destined to Babel’s multicast address.