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Configuring Serval DNA

Serval Project, March 2016

Introduction

The Serval DNA daemon, also called servald, is designed to operate in a wide variety of environments (GNU/Linux, Android, Solaris, iOS) with different kinds of network links (Wi-Fi, Ethernet, packet radio, dummy), different levels of resources (CPU, memory, persistent storage) and for different purposes (development, testing, deployment).

In order to adapt to all of these requirements, servald can be configured to adjust its behaviour in the following areas:

Configuration options

The Serval DNA configuration is expressed as an unordered set of textual LABEL=VALUE pairs called options. For example:

debug.verbose=true
interfaces.0.file=/var/tmp/serval/dummy
interfaces.0.socket_type=file
log.file.directory_path=/var/tmp/serval-logs
log.file.duration=20m
log.file.rotate=10
rhizome.direct.peer.0.host=129.96.12.91
server.respawn_on_crash=true

An option LABEL is a sequence of one or more US-ASCII alphanumeric words separated by period characters, eg, log.file.directory_path. If a LABEL is not recognised, it is unsupported.

An option VALUE is a string of US-ASCII characters, excluding newline (character 10), which is parsed according to the option's type:

  • decimal integer, eg: 10, 0, -1000000
  • boolean, eg: true, false, on, off, 1, 0, yes, no
  • internet address (in_addr), eg: 192.168.1.1
  • time interval, eg: 12h, 1w3d, 2h15m30s
  • absolute path, eg: /var/lib/serval
  • relative path, eg: ../lib/hostlist
  • Serval ID (SID), eg: EEBF3AC19E7EE58722A0F6D4A4D5894A72F5C71030C3399FE75808DCF6C6254B
  • scaled decimal integer with optional suffix k (10^3), K (2^10), m (10^6), M (2^20), g (10^9) or G (2^30), eg: 1M = 1,048,576

If a VALUE does not parse correctly, it is invalid.

Configuration persistence

servald stores its configuration option settings in a file called serval.conf in its instance directory. It reads this file upon every invocation. This means that each instance's option settings persist until changed or until its serval.conf file is altered or removed.

The format of the file is as shown in the example above: each option is represented by a single line in the file. Each line may have one of the forms:

[ WHITE ] LABEL "=" VALUE "\n"
[ WHITE ] "#" COMMENT "\n"
[ WHITE ] "\n"

where WHITE is any sequence of zero or more space or tab characters (as classified by isspace(3)), COMMENT is any sequence of characters except newline, and LABEL and VALUE are as defined above.

Blank lines are ignored, as are lines beginning with the comment character #. VALUE is parsed very strictly: all spaces are significant. A leading or trailing space in VALUE can cause a numerical option to be invalid.

The order of the lines in the file has no significance. Changing the order of lines will produce exactly the same result.

Invalid configuration

Although serval.conf is usually written and read only by servald, in fact it is an external file which may be modified, so servald has no control over its contents. The semantics of the configuration loading anticipate the possibility of encountering a defective file:

  • If serval.conf is syntactically malformed, then servald will log a warning, skip the malformed line and continue parsing;

  • If the same LABEL appears more than once, then servald will log a warning and ignore the second and all subsequent lines with the duplicate LABEL;

  • If an unsupported LABEL is encountered (which could be a mis-spelling of a supported option), then servald will log a warning and ignore the line;

  • If a configuration option has an invalid VALUE, then servald will log a warning and leave the option with its built-in default value;

  • If a logical relation, such as mutually incompatible or mandatory options, is violated, then servald will log a warning and ignore the illogical options or use default values.

In all the above cases, most servald commands will reject the defective file by logging an error and exiting with error status (255). In this case, logging is done using options salvaged from the defective file (see the config dump command, described below).

Some “permissive” commands, such as help, stop, and the various config commands described below, will not fail on a defective configuration file. Instead they will log a warning and carry on using options salvaged from the defective file. This means that servald may always be used to inspect and correct the configuration, and to stop a running daemon, despite a defective configuration file.

Configuration reloading

A running daemon re-loads its configuration whenever the serval.conf file is changed. It does this by periodically checking the file's size and modification time, and if they have changed, parses the file and updates its own internal copy of the configuration settings.

As described above, the servald start command will not start a daemon process if the serval.conf file is defective. However, the file may become defective after the daemon has started.

In this case, the daemon will not terminate, nor load the defective file, but will log an error and continue execution with its internal configuration unchanged. The daemon's internal configuration will no longer be consistent with the contents of the file. If the daemon is stopped or killed, it cannot be re-started while serval.conf remains defective.

Despite detecting a defective configuration file, the daemon continues to check for changes to the file, and attempts to re-load whenever it changes. As soon as the defective serval.conf is fixed, the running daemon will load it successfully and continue execution with the new configuration. The internal configuration and the file contents will once again be consistent.

Daemon instances

It order to support more than one Serval DNA daemon running on the same host, each daemon can be configured to use its own instance directory, as follows:

  • A daemon's instance directory can be set at run time by setting the SERVALINSTANCE_PATH environment variable prior to starting the daemon. This overrides the daemon's standard file system layout. Once a daemon is running, the only way to change its instance directory is to stop it and start another daemon with a different value for the environment variable.

  • If no instance directory is set at run time, then if servald was built with the ./configure INSTANCE_PATH=DIR option, then the servald executable will use the instance directory in DIR by default, not the standard file system layout.

  • On an Android system, if none of the above are used, then servald will use the instance directory /data/data/org.servalproject/var/serval-node, not the standard file system layout.

  • If none of the above apply, then the daemon uses the standard file system layout. Only one daemon can run with the standard layout on a host, since the single, common PID file will prevent more than one being started.

The main use for multiple instances on a single host is for testing, and so the instance directory is used extensively in the automated tests. Environments other than Android are unlikely to use an instance path, so the standard file system layout is most likely to be used in most other deployments.

File system layout

Standard file system layout

By default, servald locates its files such as configuration, logs, Rhizome storage, etc. in accordance with the Filesystem Heirarchy Standard 2.3:

  • the servald executable is installed in /usr/local/bin/servald
  • the configuration is stored in /etc/serval/serval.conf
  • the keyring is stored in /etc/serval/serval.keyring
  • the PID file is /var/run/serval/servald.pid
  • the daemon creates its process information files under /var/run/serval/proc/
  • the daemon creates and rotates log files files under /var/log/serval/
  • the daemon creates and updates a symbolic link to the latest log file in /var/log/servald.log
  • the Rhizome store is in /var/cache/serval/ (unless overridden by configuration):
    • the SQLite database is /var/cache/serval/rhizome.db
    • large payloads are stored under /var/cache/serval/blob/
  • the SQLite temporary directory is /tmp/serval/sqlite3tmp/
  • dummy interface files are created under /tmp/serval/ (unless overridden by configuration)

The servald start command will create all sub-directories within the standard FHS paths as needed, and any failure will cause the daemon not to start.

The following build-time configuration options are available to alter the paths described above, which can help adapt servald to systems which use a different software installation convention (eg, all packages are installed under /opt/packagename) or which have a volatile /var directory (eg, on OpenWRT, /var is a symlink to /tmp):

  • If servald is built with the ./configure --prefix=DIR option, then the executable is installed in DIR/bin instead of /usr/local/bin

  • If servald is built with the ./configure --sysconfdir=DIR option, then it will use DIR in place of /etc

  • If servald is built with the ./configure --localstatedir=DIR option, then it will use DIR in place of /var

  • If servald is built with the ./configure SERVAL_ETC_PATH=DIR option, then it will use DIR in place of /etc/serval

  • If servald is built with the ./configure SERVAL_RUN_PATH=DIR option, then it will use DIR in place of /var/run/serval

  • If servald is built with the ./configure SYSTEM_LOG_PATH=DIR option, then it will use DIR in place of /var/log

  • If servald is built with the ./configure SERVAL_LOG_PATH=DIR option, then it will use DIR in place of /var/log/serval

  • If servald is built with the ./configure RHIZOME_STORE_PATH=DIR option, then it will use DIR in place of /var/cache/serval for the Rhizome store

  • If servald is built with the ./configure SERVAL_TMP_PATH=DIR option, then it will use DIR in place of /tmp/serval

The servald config paths command will display all the paths in use, based on the built-in defaults as overridden by configuration settings and run-time environment variables available to the command. This command will work even if configuration is defective, so is a useful diagnostic tool.

Instance directory

If servald is started with an instance directory, then all configuration, state, and temporary files are stored in or beneath that directory, denoted IDIR:

  • the configuration is stored in IDIR/serval.conf
  • the keyring is stored in IDIR/serval.keyring
  • the PID file is IDIR/servald.pid
  • the daemon creates its process information files under IDIR/proc/
  • the daemon creates and rotates log files files under IDIR/log/
  • the daemon creates and updates a symbolic link to the latest log file in IDIR/servald.log
  • the Rhizome store is in IDIR/ (unless overridden by configuration):
    • the SQLite database is IDIR/rhizome.db
    • large payloads are stored under IDIR/blob/
  • the SQLite temporary directory is IDIR/sqlite3tmp/
  • dummy interface files are created under IDIR/ (unless overridden by configuration)

The servald start command will create its instance directory (and all enclosing parent directories) if it does not already exist. Failure will cause the daemon not to start.

About the examples

The examples in this document are Bourne shell commands, using standard quoting and variable expansion. Commands issued by the user are prefixed with the shell prompt $ to distinguish them from the output of the command. Single and double quotes around arguments are part of the shell syntax, so are not seen by the command. Lines ending in backslash \ continue the command on the next line.

Configuration commands

config set

To set a configuration option:

$ servald config set name.of.option 'value'
$

config del

To unset (remove) a configuration option, returning it to its default value:

$ servald config del name.of.option
$

config set del

Several set and del commands can be chained together in a single command:

$ servald config set debug.verbose 1 \
                 del debug.dnahelper \
                 set log.file.path /tmp/log.txt
$

config get

To examine a single option's current value as defined in the serval.conf file (invalid, unsupported and illogical options may be examined, but not duplicate options):

$ servald config get name.of.option
name.of.option=value
$

To examine all option settings defined in the serval.conf file, including invalid, unsupported and illogical options, but not duplicate options:

$ servald config get
interfaces=+eth0,+wifi0
name.of.option=value
name.of.other_option=value2
$

config schema

To list the names and types of all supported configuration options (the “configuration schema”):

$ servald config schema
debug.broadcasts=(boolean)
debug.dnahelper=(boolean)
debug.dnaresponses=(boolean)
...
server.chdir=(absolute_path)
server.interface_path=(str_nonempty)
server.respawn_on_crash=(boolean)
$

The configuration schema, with its default values, is defined in the conf_schema.h source header file.

config dump

To examine all current valid configuration option settings, as produced by parsing serval.conf and omitting invalid, unsupported, duplicate and illogical options (ie, the configuration used by permissive commands and for logging):

$ servald config dump --full
debug.broadcasts=false
debug.dnahelper=false
debug.dnaresponses=false
...
rhizome.rhizome_mdp_block_size=512
server.chdir=/
server.respawn_on_crash=true
$

Omitting the --full argument omits all options which have their default value, leaving only the minimal settings that need be present in serval.conf to produce the current configuration:

$ servald config dump
debug.rhizome=true
interfaces.0.file=/var/serval-node/dummy
interfaces.0.socket_type=file
rhizome.direct.peer.0.host=129.128.127.126
server.respawn_on_crash=true
$

Logging configuration

servald logging is controlled by the following config options:

log.console.level=debug|info|hint|warn|error|none
log.console.dump_config=BOOLEAN
log.console.show_pid=BOOLEAN
log.console.show_time=BOOLEAN

log.android.level=debug|info|hint|warn|error|none
log.android.dump_config=BOOLEAN
log.android.show_pid=BOOLEAN
log.android.show_time=BOOLEAN

log.file.level=debug|info|hint|warn|error|none
log.file.dump_config=BOOLEAN
log.file.show_pid=BOOLEAN
log.file.show_time=BOOLEAN
log.file.path=PATH
log.file.directory_path=PATH
log.file.duration=INTERVAL
log.file.rotate=UINT

There are three log output destinations, each of which can be configured independently of the others:

  • The console log destination is the standard error of the servald process, which is available in all command invocations of servald, but not in the background daemon process (the daemon closes all its standard IO streams when started in background mode).

  • The android log destination is available in servald executables built for the Android platform, and sends to the Android Log buffer that is accessible via the adb logcat command. On non-Android platforms, the log.android configuration options are supported but have no effect.

  • The file log destination is a log file created and appended directly by the servald process using the O_APPEND option of open(2) and a single write(2) system call per log line (so concurrent servald processes will not corrupt each others' log lines). If the file does not exist, servald will create it and all its enclosing directories as needed.

All log destinations support the following configuration options:

  • log.DESTINATION.level Log messages below this level are not sent to the destination. The lowest level is debug, and the highest is error. Setting this option to none suppresses all log messages.

  • log.DESTINATION.dump_config If true, then the current configuration is written to the destination (in servald config dump format), prior to other messages.

  • log.DESTINATION.show_pid If true, then every line written to this destination is prefixed with the Process ID of the process that produced it.

  • log.DESTINATION.show_time If true, then every line written to this destination is prefixed with the system time in millisecond resolution (if available) in the format HH:MM:SS.mmm.

In addition, the file destination has these extra configuration options:

  • log.file.directory_path If set, log files are created in this directory, which is created if it does not exist. Relative paths are interpreted relative to the log sub-directory of the instance directory. The default setting (empty string) causes log files to be created within the log sub-directory of the instance directory.

  • log.file.path If set, all log messages are appended directly to the file at the given path. If the path is not absolute, it is interpreted relative the log.file.directory_path option. If log.file.path is not set, then log files have names of the form serval-YYYYMMDDHHMMSS.log, using the date/time of creation of the file.

  • log.file.duration If non zero, then a new log file is created every new interval. Interval boundaries are measured from the Unix epoch, so if the interval is an integral divisor of one day then a new file will always start at midnight. The interval can be given as a plain number of seconds, but a convenient scaled notation is supported: [Nw][Nd][Nh][Nm][N[s]], eg, 2h40m20s means two hours plus 40 minutes plis 20 seconds.

  • log.file.rotate If non zero, then old log files are deleted so that no more than this many files exist at one time.

Every log message is written to all destinations according to their configuration.

Network interfaces

The servald daemon periodically scans its operating system's network interfaces and uses its interfaces configuration option to select which to ignore and which to use.

For example, the following configuration will use any interface whose name starts with eth (eg, eth0, eth1) on port 7333 and any interface whose name starts with wifi or wlan but is not wifi0 or wlan0 as a Wi-Fi on the default port number:

$ servald config set interfaces.0.match 'eth*' \
                 set interfaces.0.type ethernet \
                 set interfaces.0.port 7333 \
                 set interfaces.1.match 'wifi0,wlan0' \
                 set interfaces.1.exclude true \
                 set interfaces.2.match 'wifi*,wlan*' \
                 set interfaces.2.type wifi

The following configuration is equivalent to the above example, but uses the “legacy”, single-option syntax (see below):

$ servald config set interfaces \
    '+eth=ethernet:7333,-wifi0,-wlan0,+wifi=wifi,+wlan=wifi'

The following two equivalent configurations will use all available interfaces, treating all as Wi-Fi (the default type) with a 400 µs inter-packet delay (the default packet interval for Wi-Fi):

$ servald config set interfaces.0.match '*'
$ servald config set interfaces '+'

Network interface rules

As shown in the first example above, the interfaces config option contains a numbered list of rules that are applied to all detected system interfaces in order of ascending number. The general form of an interface rule is:

interfaces.UINT.match=PATTERN[, PATTERN ...]
interfaces.UINT.file=PATH
interfaces.UINT.exclude=BOOLEAN
interfaces.UINT.socket_type=SOCKTYPE
interfaces.UINT.port=PORT
interfaces.UINT.encapsulation=ENCAPSULATION
interfaces.UINT.default_route=BOOLEAN
interfaces.UINT.prefer_unicast=BOOLEAN
interfaces.UINT.send_broadcasts=BOOLEAN
interfaces.UINT.type=IFTYPE
interfaces.UINT.mdp_tick_ms=UINT_NONZERO
interfaces.UINT.packet_interval=UINT_NONZERO

where:

  • PATTERN is a shell wildcard pattern
  • BOOLEAN is true, false, 1, 0, yes, no, on or off
  • SOCKTYPE is dgram, stream or file
  • ENCAPSULATION is overlay or single
  • IFTYPE is wifi, ethernet, catear or other
  • PORT is an unsigned decimal integer in the range 1 to 65535
  • UINT is any unsigned decimal integer (with no + or - prefix)
  • UINT_NONZERO is an unsigned decimal integer ≥ 1
  • PATH is an absolute or relative file path
  • IN_ADDR is an Internet address as accepted by inet_aton(3), ie, N.N.N.N where N is an integer in the range 0 to 255.

Exactly one of the match and file options must be set on an interface. If neither or both are set, it is an error; the whole interface rule is omitted from the configuration and serval.conf is treated as defective (see above).

match

If a rule specifies a match option, then each PATTERN is applied to the names of the real system interfaces using the fnmatch(3) standard library function. If any PATTERN matches, then the rule's exclude option is checked: if true, then the interface is not activated, otherwise a socket on that system interface is opened and the interface's socket_type is implicitly set to dgram. (It is invalid to explicitly set socket_type to other than dgram for a match interface.)

exclude

If a rule sets its a exclude option to true, then the interface will never be activated. This can be useful for writing negative match rules to exclude specific interface names before a final match-all rule.

file

If a rule specifies a file path, then an interface is created only if the given file exists. The rule's exclude option is ignored. The interface's socket_type determines how the file is written and read.

socket_type

An interface's socket_type determines how the its socket file descriptor is written and read: descriptor is written and read:

  • file (the default) creates a dummy interface for closed communication with other servald daemons on the same host. If the dummy file does not exist, a warning is logged and the interface is not activated.

  • stream reads and writes the file as though it were a character special device. If the file does not exist, an error is logged and the interface is not activated.

  • dgram is not valid for a file interface.

type

The type option affects the default settings of the packet_interval and mdp_tick_ms options, for convenience. In future it may also change the way the interface behaves, for example, an ethernet interface may automatically assume that broadcast packets will be filtered out, so will start using MDP unicast protocols immediately rather than waiting to detect that broadcast packets are not acknowledged.

packet_interval

The packet_interval option controls the maximum rate at which packets are tramsmitted on the interface. It sets the average interval, in microseconds, between individual packets. If the interval is less than the time it takes to transmit a packet, then packets will be sent at maximum speed with no intervening delay. Otherwise, delays are inserted between packets as needed to keep to the average.

mdp_tick_ms

The mdp_tick_ms option controls the time interval, in milliseconds, between MDB broadcast announcements on the interface. If set to zero, it disables MDP announcements altogether on the interface (called “tickless” mode). If not set, then the value of the mdp.iftype.IFTYPE.tick_ms option is used. If that is not set, then servald uses a built-in interval that depends on the type.

encapsulation

The encapsulation option controls how MDP messages are written to the interface's socket:

  • overlay (the default) aggregates as many messages as possible into overlay packets that fit into each UDP packet, in order to take full advantage of the bandwidth on conventional Wi-Fi interfaces which have a fixed packet size (the IEEE 802.11 MTU) over the air;

  • single sends each MDP message on its own to the socket using SLIP encoding, and is suited to data links with a variable packet size on the air (eg, a serial connection to a packet radio modem).

default_route

The default_route option, if true, causes all MDP messages with an unresolved recipient address (SID) to be sent to this interface instead of just dropped. This will allow the node to use Serval Infrastructure to route its packets. Many interfaces may have the default_route set to true, but only the first one will be used as the default route.

prefer_unicast

The prefer_unicast option, if true, causes the interface to send to unicast IP addresses instead of the broadcast IP address if both have been observed to reach the destination.

send_broadcasts

Normally, any MDP message with an unresolvable recipient gets broadcast on all active interfaces. The send_broadcasts option, if false, prevents the interface from sending any broadcast packets whenever a recipient address (SID) cannot be resolved to an interface.

Network interface “legacy” syntax

Instead of using the multi-option schema described above, the interfaces configuration option can be set using a less capable “legacy” format, for compatibility with older config files. The “legacy” interfaces syntax is a single text string consisting of a comma-separated list of rule stanzas, each stanza having one of the following forms:

+
-
+PREFIX=IFTYPE
+PREFIX=IFTYPE:PORT
+PREFIX=IFTYPE:PORT:IGNORED
-PREFIX
+>PATH

The rule + matches all interfaces.

The rule - excludes all interfaces.

Rules beginning with +PREFIX match any interface whose name starts with PREFIX; so for example a rule starting with +foo is equivalent to a match option with a single PATTERN of foo*

The rule -PREFIX excludes interfaces whose name starts with PREFIX.

The rule +>PATH specifies a dummy interface (see below) with no address or netmask or broadcast filter.

Interface rules are numbered in the order they appear, and are applied in that order. For example, an interfaces option of +,-eth0 will not reject the eth0 interface because the leading + will match it first, but -eth0,+ will reject eth0 and accept all others.

The “legacy” format is only provided for backward compatibility and will eventually be deprecated and removed. The “legacy” interfaces configuration is incompatible with the modern form; an instance that uses one cannot use the other.

Dummy interface

Sometimes it is helpful to run an isolated group of connected servald instances on a single machine for testing purposes. To make this possible, servald supports a dummy network interface.

A dummy interface is simply a regular file to which all instances append their network packets. The file grows without limit. Each instance advances its own read pointer through the file, packet by packet, skipping packets which are not addressed to it. A single dummy file simulates a lossless mesh network with total connectivity, ie, all nodes that read and write the file are neighbours.

To use a dummy interface, first create an empty file, eg, /tmp/dummy, and for each servald instance, include the dummy file in its interfaces list, eg:

$ servald config set interfaces.0.file '/tmp/dummy'

NOTE: Because dummynets are files, not sockets, the poll(2) system call does not work on them. As a result the servald daemon main loop has slightly different behaviour and timing characteristics when a dummynet is in use.

If a dummy interface's file PATH is not absolute (ie, does not start with /) then it is interpreted relative to the instance directory.

The following config options adorn a dummy interface with properties that real interfaces normally obtain directly from the operating system:

interfaces.UINT.dummy_address=IN_ADDR
interfaces.UINT.dummy_netmask=IN_ADDR
interfaces.UINT.drop_unicasts=BOOLEAN
interfaces.UINT.drop_broadcasts=BOOLEAN

The dummy_address option sets the interface's unicast (receive) IP address.

The dummy_netmask option sets the interface's unicast (receive) IP network mask, which together with dummy_address determines the interface's IP broadcast address.

The drop_unicasts option, if true, will drop overlay frames addressed to the interface's unicast IP address, so that only broadcast packets will be read.

The drop_broadcasts option, if true, will drop overlay frames addressed to the interface's broadcast IP address, so that only unicast packets will be read. This can simulate the effects of the Fi-Fi drivers on some Android devices that filter out broadcast packets (to prevent the device from waking up unless there is traffic explicitly sent to it).

Copyright 2013 Serval Project Inc.
Copyright 2016 Flinders University
CC-BY-4.0 Available under the Creative Commons Attribution 4.0 International licence.