aprsdigi man page

aprsdigi — APRS(™) digipeater


aprsdigi options


Aprsdigi is a specialized Amateur Packet Radio (AX.25) UI-frame digipeater for the Automatic Position Reporting Systems, APRS(tm). It uses the Linux kernel AX.25 network stack as well as the SOCK_PACKET facility to listen for packets on one or more radio interfaces (ports) and repeat those packets -- with several possible modifications -- on the same or other interfaces. Aprsdigi can also use the Internet to tunnel connections among other APRS digipeaters and nodes using IPv4 or IPv6 UDP unicast or multicast.

Aprsdigi implements conventional packet radio AX.25 digipeating, in which a packet is digipeated if the next hop (non-repeated) digipeater ("via") callsign matches the AX.25 port's callsign and sub-station ID (SSID) or an alias callsign and SSID.

There are a number of extensions to conventional digipeating that have been proposed for use in the APRS community. Some of these features have been adopted by Terminal Node Controller (TNC) manufacturers, notably Paccomm and Kantronics. Aprsdigi implements most if not all of the commercialy adopted and proposed features. See the APRS 1.0 Protocol Specification at www.tapr.org for protocol documentation. Aprsdigi attempts to minimally comply with the protocol specification as well as support experimental APRS features. Specific features implemented include:

Single-interface conventional UI-frame digipeating.
Cross-interface digipeating (also known as bridging, routing or gatewaying) and one-to-many fanout.
Substitution of a digipeated alias with the interface's callsign (typically used to substitute RELAY, WIDE or TRACE aliases).
WIDEn-n flooding algorithim.
TRACEn-n route recording.
Mic-Encoder(tm) support, including SSID-based digipeating, decompression of packets into the conventional APRS MIM format. (The Mic-Encoder compression is also used by other products such as the Kenwood TH-D7A and D700, and TAPR PIC-Encoder).
TheNet X1J4 node beacon text translation (removal of the “TheNet X1J4 (alias)” prefix from the btext).

General Options

-v --verbose
Produce verbose debugging output.
-T --testing
Test mode: listen to my packets too. This mode is useful for off-air experimentation and configuration testing. Do not use it on-air.
-D --kill_dupes
Suppress Duplicate packets. Remembers duplicate packets for the number of seconds given by the -k option and will not repeat them more than once. This reduces conjestion caused when several digipeaters that share a common flooding alias (e.g. WIDE) have overlapping footprints, causing geometric duplication of packets addressed via “WIDE,WIDE” for example.
-L --kill_loops
Suppress Looping packets. Similar in function to duplicate packet suppression, but looks back through the list of already digipeated callsigns in the packet's digipeat list and kills any packets that list a callsign belonging to this aprsdigi. Note that only real callsigns are compared. Generic flooding aliases are not. Therefore, loop detection is only useful when callsign substitution is used.
-V --version
Print program version and exit.
-n|s|e|w --north|south|east|west
Set North|South|East|West SSID directional path.
-d --digipath
Set SSID omnidirectional next-hops when operating in a non flooding network (e.g. when WIDEn-n is not an option).
-f --flood
Set flooding alias. Use “-f WIDE” to enable WIDEn-n flooding. Use -f multiple times to define several flooding aliases.
-F --trace
Set flooding trace callsign. Use “-F TRACE” to enable TRACE and TRACEn-n flooding. Use -F multiple times to define several trace aliases.
-k --keep secs
Remember old packets for this long for duplicate packet detection. Default is 28 seconds.
-l --logfile file
Log digipeated packets to this file.

Per-Interface Options

Put these options before each -p --interface to set new values as needed. The values you set are remembered for subsequent -p's so options you want to set for all interfaces need only be specified once, before the first -p. But you have to remember to unset an option if you don't want it to apply to subsequent interfaces.

-C (-c) --[no]subst_mycall
Do (not) perform callsign substitution. When enabled, aliases are replaced with the interface's callsign when repeated.
-M (-m) --[no]mice_xlate
Do (not) perform Mic-E to MIM translation. When enabled, compressed Mic-E reports are expanded into one MIM-style position report packet and optionally a second telemetry packet if telemetry was supplied in the Mic-E packet.
-X (-x) --[no]x1j4_xlate
Do (not) perform X1J4 translation. When enabled, the leading “TheNet X1J4 (alias)” text is removed when digipeated. This allows non-compliant APRS implementations to detect an APRS position report in an X1J4 beacon.
-i --idinterval secs
Seconds between ID transmissions. Set to 0 to disable IDs on this interface. Default is 570 (9 minutes 30 seconds). IDs are only sent if the interface transmitted anything since the last ID. ID packets are addressed to the “ID” callsign, have no digipeat path, and list the callsign and aliases for the interface the ID is being transmitted on.
-t --tag text
Text to append to received packets. Use -t - to reset to empty. Use this, for example, when gatewaying Mic-E packets from a voice repeater to the APRS net frequency to indicate where the report originated.
-3 --3rdparty
Enable 3rd party tunneling. Packets tunneled to a 3rd party interface are sent with the unused digipeaters removed from the digipeater list. Packets tunneled from a 3rd party interface have the Source Path Header prepended to the packet payload prefixed by the "}" character.
-0 --no3rdparty
Enable transparent tunneling. No special tricks are done when sending to or receiving from a tunneled interface. If the interface does not natively support AX.25 addresses (from-call, to-call, and digipeater list), then the address header is prepended to the payload in "cooked" format. Likewise, a cooked prepended header is stripped from a cooked interface and put back in the AX.25 address when going from a non-AX.25 to AX.25 interface.
-o r --norx
Disable receiving on the following interface(s).
-o R --rx
Enable receiving on the following interface(s).
-o t --notx
Disable transmitting on the following interface(s).
-o T --tx
Enable transmitting on the following interface(s).
-o s --notxsame
Disable retransmitting a received packet on the same interface.
-o S --txsame
Enable retransmitting a received packet on the same interface.
-o d --duplicate intf
Duplicate received packets without modification to the given interface (port).
-p --interface ax25:port:alias1,alias2,...
AX25 interface name (port) and optional list of aliases. The primary callsign is obtained from the interface's configuration. (See ifconfig(8)).
-p --interface udp:host/port/ttl:alias1,alias2,...
IP host name or address and list of aliases. IP addresses may be IPv4 unicast or multicast or IPv6 unicast. The primary callsign is obtained from the first alias.
-p --interface unix:filename:alias1,alias2,...
Unix file and list of aliases. Useful for debugging by setting up a simulated APRS network on one machine. You may want to make your FIFOs explicitly transmit- or receive-only to avoid confusion. The primary callsign is obtained from the first alias.
-B|b --[no]bud
addr Is similar to a TNC-2's BUDLIST. Use -B --bud to accept or -b --nobud to ignore packets from a sender or group of senders. Budlists are attached to each interface and can be reset with --bud -
You can set up a global budlist once, or per-interface budlists. The format of addr varies based on the interface type:

--bud ax25:callsign-ssid matches only a given digipeater callsign and SSID. For example, -B ax25:n0clu-14.

--bud ax25:callsign matches all SSIDs for the given callsign. For example -B ax25:n0clu.

--bud ip:hostname matches one Internet host name (IPv6 or IPv4). For example -B ip:n0clu.ampr.net

--bud ip:address/maskbits matches all IP addresses that have the given prefix. For example --bud ip: matches the entire class-A network. --bud ip: matches the entire class-B network. --bud ip:fe80::201:3ff:fe9a:38c6 matches a single IPv6 host. --bud ip:2002:905::/32 matches the 32-bit IPv6 prefix.

Runtime Controls

aprsdigi responds to the following signals:

Print cumulative statistics. For each port, the following counters are displayed: packets received and how many of those where ignored, duplicates, loops, mic-E formatted; packets transmitted and how many of those where conventional digipeats, flooding digipeats (WIDEn-n), SSID-based digipeats, and IDs. If a log file was specified with the -l --logfile option, then the statistics are written to that file. Otherwise they are written to stderr.
Prints the statistics and then resets all counters to zero.

All other normal termination signals cause final statistics to print before aprsdigi exits.

Ssid-Based Routing

SSID-based routing uses a non-zero sub-station ID in the destination callsign, an empty digipeater path to indicate that the APRS digipeater should repeat the packet after filling in an appropriate digipeater path. For example, a packet sent to “T1QS4W-3” would be repeated with a modifed destination of “APRS VIA WIDE3-3” (in a network that supports WIDEn-n flooding). A packet sent to “APRS-11” would be repeated to the West unproto path, as defined with the --west option. A table of SSID values and their paths follows:

SSID unproto path
---- ------------
0    none 
1    WIDE1-1
2    WIDE2-2
3    WIDE3-3
4    WIDE4-4
5    WIDE5-5
6    WIDE6-6
7    WIDE7-7
10   EAST  UNPROTO path
11   WEST  UNPROTO path
14   EAST  UNPROTO path + WIDE
15   WEST  UNPROTO path + WIDE

SSID digipeating was first introduced with the Mic-Encoder but works with any destination callsign with a non-zero SSID. The theory behind destination SSID digipeating is described in more detail in the APRSdos README, MIC-E.TXT. Basically, the idea is to minimize packet lengths and to have the manager of the WIDE APRS digipeater determine the most appropriate directional digipeat paths, removing the burden from the mobile user.

Aprsdigi also fits into a non WIDEn-n network by using the same algorithm for selection of subset of digipeaters from a list supplied with the --digipath option as the MIC-E. That is, SSIDs of 1, 2 or 3 select that number of digipeaters from the first three digipeaters in the --digipath list. SSIDs of 4, 5, 6, or 7, start at the fourth digipeater in the list.

Flooding Aliases

APRS flooding (WIDEn-n) digipeating works by repeating any received packet whose next hop digipeater has a flooding alias (specified with the --flood option), and the SSID is 1 or greater. The SSID is decremented by one, and the packet is repeated. Furthermore, to prevent broadcast storms, recently transmitted packets are remembered for a period of time specified by the --keep option and are not repeated if they are heard within that time period.

Unlike conventional digipeating, in which the digipeater callsign/alias is flagged as “repeated”, the flooding mode does not do this. Once the SSID decrements to zero, then a flooding alias is treated just like any other alias, and does get marked as repeated upon transmission.


“Flooding” Trace aliases (TRACEn-n; --trace option) are treated like flooding aliases with the addition that, besides decrementing the SSID, the current interface's callsign is inserted in front of the trace alias, providing a record-route function. “Plain” trace aliases (TRACE; also --trace option) are simply substituted in the conventional ( --subst_mycall ) manner.

Multi Port Operation

In single port operation, there is only one interface specified with --interface. All packets are received and some are retransmitted on the same interface, depending on whether they match the criteria for retransmission after translation of the digpeater path from one of the APRS-specific formats:

Mic-E TO-call SSID-based route.
WIDEn-n/TRACEn-n flooding.

or a conventional next-hop (non-repeated) digipeater matching the callsign or one of the aliases for the interface.

The decision to transmit is made by matching the next hop callsign/alias with the table of callsigns and aliases you supply to --interface.

In multi-port operation, this same technique simply extends to several interfaces. Besides each interface's unique callsign, you can give the same alias to several interfaces. This results in a one-to-many fanout which might be useful for dual frequency operation such as a general use APRS net frequency and an event-specific frequency.

By using different flags for Mic-E expansions, etc. you can tailor these fanouts differently on each of these interfaces, perhaps keeping Mic-E packets compressed on one frequency while decompressing them on another.

Duplicating Packets

The --dupe intf option will duplicate a packet received on one interface to the interface name given. If you want to duplicate to several other interface, repeat --dupe intf for each interface. The packet is duplicated verbatim as received. No callsign substitution, flooding or other processing or checking such as whether the packet still has any non-repeated digipeaters in the list is checked. This feature is meant to provide a means to simply repeat received packets verbatim, on an RF interface, for example, out an interface that might be an Ethernet, that has APRS client applications running on it (or aprsd listening on a UDP interface). Digipeating without the normal processing can be dangerous since the digipeater list is never used up. Because of this, packets received on a given interface will never be blindly duplicated back to the same interface, regardless of the option setting.


Note that TRACEn-n vs. plain TRACE do different things: TRACEn-n *inserts* calls into the digipath while decrementing ssid, e.g.:


Killing Looping Packets

Kill looping packets (--kill_loops option):


Normally n2ygk-7 would respond to this, but, by finding one of mycall earlier in the path, I know to ignore it.


Following is a sample invocation of aprsdigi running on two ports. This is a contrived example that tries to show all the features. Comments to the right describe each feature.

aprsdigi \
   --verbose \                                 # verbose
   --north "N2YGK-2 WB2ZII WA2YSM-14" \        # North digi path
   --south "N2YGK-3 WB2ZII WA2JNF-4" \         # South ...
   --east "N2YGK-3 WB2ZII KD1LY" \             # East ...
   --west "N2YGK-2 WB2ZII N2MH-15" \           # West ...
   --flood "WIDE" \                            # WIDEn-n flooding
   --trace "TRACE" \                           # TRACEn-n tracing
   --kill_dupes \                              # kill dupes
   --kill_loops \                              # kill loops
   --mice_xlate \                              # do Mic-E translation
   --subst_mycall \                            # do callsign substituton
   --tag " via 147.06 (WB2ZII/R)" \            # add this tag to rec'd pkts
   --nobud "ax25:NOCALL" \                     # ignore pkts from NOCALL
   --dupe udp: \			# dupe everything heard 
   --int ax25:sm0:RELAY,WIDE,TRACE \           # ax25 soundmodem intf
   --nomice_xlate \                            # turn off Mic-E translation
   --x1j4_xlate \                              # do X1J4 translation
   --nosubst_mycall \                          # turn off callsign subst.
   --tag - \                                   # clear the tag
   --int ax25:ax0:RELAY,WIDE,FOO,TRACE \       # ax25 ax0 intf.
   --bud - \                                   # clear the budlist
   --bud ip: \                 # allow only from this IP host
   --int udp:,RELAY,WIDE,TRACE \ # multicast 
   --int udp:	# to this mcast group

opening UDP socket on
UDP address info: family 2 type 2 proto 17 next 0x0
Linux APRS(tm) digipeater
Copyright (c) 1996,1997,1999,2001,2002,2003 Alan Crosswell, n2ygk@weca.org
Version: aprsdigi aprsdigi-2.4.3
This is free software covered under the GNU Public License.
There is no warranty.  See the file COPYING for details.

# configuration:
 budlist 1 deny NOCALL/48
 budlist 2 permit
interface ax25:sm0
 callsign N2YGK-2
 alias RELAY
 alias WIDE
 alias TRACE
 option SUBST_MYCALL on
 option MICE_XLATE on
 option X1J4_XLATE off
 option I_TX on
 option I_RX on
 option I_TXSAME on
 option idinterval 570 #(09:30)
 option tag  via 147.06 (WB2ZII/R)
 budlist 1
interface ax25:ax0
 callsign N2YGK-3
 alias RELAY
 alias WIDE
 alias FOO
 option SUBST_MYCALL off
 option MICE_XLATE off
 option X1J4_XLATE on
 option I_TX on
 option I_RX on
 option I_TXSAME on
 option idinterval 570 #(09:30)
 option tag #(none)
 budlist 2
interface udp:
 callsign N2YGK-4
 alias RELAY
 alias WIDE
 alias FOO
 option SUBST_MYCALL off
 option MICE_XLATE off
 option X1J4_XLATE on
 option I_TX on
 option I_RX on
 option I_TXSAME off
 option idinterval 570 #(09:30)
 option tag #(none)
 budlist 2
# end of configuration

My callsigns and aliases (routing table):
Callsign  Interfaces...
N2YGK-2   sm0 
RELAY     sm0       ax0
WIDEn-n   sm0       ax0
TRACEn-n  sm0 
N2YGK-3   ax0 
FOO       ax0
SSID-based directional routing:

N:        N2YGK-2   WB2ZII    WA2YSM-14 
S:        N2YGK-3   WB2ZII    WA2JNF-4  
E:        N2YGK-3   WB2ZII    KD1LY     
W:        N2YGK-2   WB2ZII    N2MH-15   
keep dupes for: 28 seconds
log file: (none)
kill dupes: ON loops: ON  testing: OFF


Aprsdigi should not be confused with a Wes Johnson's DOS program of the same name. This code has most recently been tested with the Linux 2.4.20 kernel under Red Hat Fedora Core 1. The command line syntax is ugly and will eventually be replaced by a configuration file. Short options are deprecated and will dissappear in a future release. Please use long options.



See Also

call(1), listen(1), beacon(1), ax25(4), kissattach(8), ifconfig(8), aprsmon(1), http://www.tapr.org


Alan Crosswell, n2ygk@weca.org

APRS and the Mic-Encoder are Trademarks of APRS Engineering LLC.


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