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LFT(8) System Manager's Manual LFT(8)


lftdisplay the route packets take to a network host/socket; optionally show heuristic network information in transitu


lft [-d dport] [-s sport] [-m min] [-M max] [-a ahead] [-c scatter ms] [-t timeout ms] [-l min ttl] [-q ISN] [-D device] [-H ttl] [-i] [-n] [-E] [-N] [-A] [-T] [-S] [-V] [-v] [<gateway> <...>] host:dport


The Internet is a large and complex aggregation of network hardware, connected together by gateways. Tracking the route one's packets follow (or finding the miscreant gateway that's discarding your packets) can be difficult. (from traceroute(8))
lft sends various TCP SYN and FIN probes (differing from Van Jacobson's UDP-based method) utilizing the IP protocol `time to live' field and attempts to elicit an ICMP TIME_EXCEEDED response from each gateway along the path to some host. lft also listens for various TCP and ICMP messages along the way to assist network managers in ascertaining per-protocol heuristic routing information and can optionally retrieve various information about the networks it traverses.
The only mandatory parameter is the destination host name or IP number. Options toggle the display of more interesting data or change the variables of the trace itself. The (-E) "smart" option tries several combinations in order to improve the chance of a successful trace.
Other options are:
-d dport
Set dport as the destination TCP port of the probes LFT generates. Default is 80. This option is useful to see if packets follow a different route based on protocol destination, a likely scenario when load balancers or proxies are involved. This option may also bypass less sophisticated packet filter configurations.
-s sport
Set sport as the origin TCP port of the probes LFT generates. Default is 53. This option is useful to see if packets follow a different route based on protocol source. This option may also bypass less sophisticated packet filter configurations.
-m min
Set min as the minimum number of probes to send per host. Default is 1 unless "smart" (-E) mode is used.
-M max
Set max as the maximum number of probes to send per host. Default is 5.
-a ahead
Set ahead as the number of hops forward to query before waiting for a response. Default is 5.
-c scatter ms
Set scatter ms as the minimum number of milliseconds to wait between sending probes. Default is 20.
-t timeout ms
Set timeout ms as the maximum number of milliseconds to wait before assuming a probe was lost/discarded. Default is 1000.
-l min ttl
Set min tll as the minimum TTL (time-to-live) on outgoing probes (essentially, the first hop in the line that you want to display). Default is 1.
-q ISN
Set ISN as the ISN (initial sequence number) of the first probe. If unset, one will be automatically generated.
-D device
Set device as the network device or IP address to be used. (e.g., "en1") If unset, one will be automagically selected by default.
-H ttl
Set ttl as the maximum TTL, essentially the maximum route traversal distance in hops. Default is 30.
Disable "stop" on ICMP other than TTL expired.
Print addresses numerically rather than symbolically and numerically. Disables use of the DNS resolver completely.
Enable use of the "smart" engine which tries FIN/SYN/etc tables to improve the chance of a successful trace. The engine also displays other useful information such as stateful inspection firewalls it finds along the way.
Enable lookup and display of network names (e.g., [GNTY-NETBLK-4]). This option queries various registries of network address allocation such as ARIN.
Enable lookup and display of of AS (autonymous system) numbers (e.g., [1]). This option queries various registries of network address allocation such as ARIN.
Enable display of LFT's execution timer. This option places timers on the trace itself and on lookups and name resolution to show where LFT is spending its time, waiting on resolvers, or processing trace packets.
Suppress display of the real-time status bar. This option makes LFT show its completed trace output only, no-frills.
Verbose output. Mostly debug garbage. Likely too verbose for regular use.
Display version information, then exit().
Any hosts listed after these options and before the final host will comprise the loose source route. Since network operators have security concerns with regard to the use of source routing, don't expect the LSRR options to do much for you in most public network environments.


A sample use and output might be:
[edge.lax]$ lft -S 
Hop  LFT trace to vnsc-bak.sys.gtei.net ( 
 1   ln-gateway.centergate.com ( 0.5ms 
 2   isi-acg.ln.net ( 2.3ms 
 3   isi-1-lngw2-atm.ln.net ( 2.5ms 
 4   gigabitethernet5-0.lsanca1-cr3.bbnplanet.net ( 3.0ms 
 5   p6-0.lsanca1-cr6.bbnplanet.net ( 3.4ms 
 6   p6-0.lsanca2-br1.bbnplanet.net ( 3.3ms 
 7   p15-0.snjpca1-br1.bbnplanet.net ( 10.9ms 
 8   so-3-0-0.mtvwca1-br1.bbnplanet.net ( 11.1ms 
 9   p7-0.mtvwca1-dc-dbe1.bbnplanet.net ( 11.0ms 
10   vlan40.mtvwca1-dc1-dfa1-rc1.bbnplanet.net ( 11.1ms 
**   [neglected] no reply packets received from TTLs 11 through 20 
**   [4.2-3 BSD bug] the next gateway may errantly reply with reused TTLs 
21   [target] vnsc-bak.sys.gtei.net ( 11.2ms 

The (-S) option was used to suppress the real-time status bar for clean output. LFT's "**" notifiers in between hops 10 and 21 represent additional useful information: the first is a "[neglected]" indicator that lets us know that none of the probes sent with the TTLs indicated elicited responses. This could be for a variety of reasons, but the cause of this specific occurrence is described in the next informative message which indicates that this is likely the result of a bug in the 4.[23] BSD network code (and its derivatives): BSD 4.x (x < 3) sends an unreachable message using whatever TTL remains in the original datagram. Since, for gateways, the remaining TTL is zero, the ICMP "time exceeded" is guaranteed to not make it back to us. LFT does its best to identify this condition rather than print lots and lots of hops that don't exist (trying to reach a high enough TTL).
Now, using the smart engine option:

[edge.lax]$ lft -E -S 
Hop  LFT trace to vnsc-pri.sys.gtei.net ( 
 1   ln-gateway.centergate.com ( 0.5/0.5ms 
 2   isi-acg.ln.net ( 2.1/2.3ms 
 3   isi-1-lngw2-atm.ln.net ( 2.6/7.1ms 
 4   gigabitethernet5-0.lsanca1-cr3.bbnplanet.net ( 6.1/3.9ms 
**   [firewall] the next gateway may statefully inspect packets 
 5   p0-0-0.lsanca1-csr1.bbnplanet.net ( 155.4/3.7ms 
 6   [target] vnsc-pri.sys.gtei.net ( 22.6/3.7/*/*/*/*/*ms 

In the scenario above, the smart engine was able to identify a stateful, packet-inspecting firewall in the path. Another example with more options:

[edge.lax]$ lft -S -A -T -m 2 -d 80 -s 53 www.yahoo.com 
Hop  LFT trace to w9.scd.yahoo.com ( 
 1   [AS226] ln-gateway.centergate.com (  1 ms 
 2   [AS226] isi-acg.ln.net (  2 ms 
 3   [AS226] isi-1-lngw2-atm.ln.net (  3 ms 
 4   [AS1] gigether5-0.lsanca1-cr3.bbnplanet.net (  3 ms 
 5   [AS1] p6-0.lsanca1-cr6.bbnplanet.net (  5 ms 
 6   [AS1] p6-0.lsanca2-br1.bbnplanet.net (  3 ms 
 7   [AS1] p1-0.lsanca2-cr2.bbnplanet.net (  3 ms 
 8   [AS16852] pos4-0.core1.LosAngeles1.Level3.net (  3 ms 
 9   [AS3356] so-4-0-0.mp1.LosAngeles1.Level3.net (  3 ms 
10   [AS3356] so-3-0-0.mp2.SanJose1.Level3.net (  11 ms 
11   [AS3356] gige10-0.ipcolo4.SanJose1.Level3.net (  11 ms 
12   [AS3356] cust-int.level3.net (  52 ms 
13   [AS10310] vl17.bas2.scd.yahoo.com (  53 ms 
14   [AS10310] w9.scd.yahoo.com ( [target]  54 ms 

LFT's trace took 5.23 seconds. Resolution required 3.58 seconds.
And why not request netblock lookups?

[edge.lax]$ lft -S -N www.microsoft.com 
Hop  LFT trace to www.us.microsoft.com ( 
 1   [LOS-NETTOS-BLK4] ln-gateway.centergate.com (  2 ms 
 2   [LOS-NETTOS] isi-acg.ln.net (  3 ms 
 3   [LOS-NETTOS] isi-1-lngw2-pos.ln.net (  5 ms 
 4   [GNTY-4-0] gigether5-0.lsanca1-cr3.bbnplanet.net (  4 ms 
 5   [GNTY-4-0] p6-0.lsanca1-cr6.bbnplanet.net (  3 ms 
 6   [GNTY-4-0] p6-0.lsanca2-br1.bbnplanet.net (  3 ms 
 7   [GNTY-4-0] p15-0.snjpca1-br1.bbnplanet.net (  10 ms 
 8   [GNTY-4-0] p9-0.snjpca1-br2.bbnplanet.net (  11 ms 
 9   [GNTY-4-0] so-1-0-0.sttlwa2-br1.bbnplanet.net (  27 ms 
10   [GNTY-4-0] so-0-0-0.sttlwa1-hcr1.bbnplanet.net (  28 ms 
11   [GNTY-4-0] so-7-0-0.sttlwa1-hcr2.bbnplanet.net (  28 ms 
12   [GNTY-4-0] p1-0.sttlwa1-cr2.bbnplanet.net (  29 ms 
13   [GNTY-4-0] p2-0.msseattle.bbnplanet.net (  32 ms 
16   [MICROSOFT-GLOBAL-NET] [prohibited]  35 ms 

Running LFT as a non-root user

As distributed by Debian, lft can only be run by the root user. To allow regular users to run lft, the sysadmin needs to read the file /usr/share/doc/lft/HOWTO-UsersLFT.txt and follow the instructions in that file.


Nils McCarthy, Victor Oppleman, Ugen Antsilevitch, and other helpers around the world. The source is currently maintained and administered by MainNerve, Inc.


To report bugs, send e-mail to <lft-bugs@mainnerve.com>


traceroute(8), netstat(1), whois(1)


The lft command first appeared in 1998 as 'fft'. Renamed as a result of confusion with fast fourier transforms, lft stands for 'layer four trace.'
August 17, 2002 LFT