NAME¶
lft —
display the route packets take to
a network host/socket; optionally show heuristic network information in
transitu
SYNOPSIS¶
| 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 |
DESCRIPTION¶
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.
- -i
- Disable "stop" on ICMP other than TTL
expired.
- -n
- Print addresses numerically rather than symbolically and
numerically. Disables use of the DNS resolver completely.
- -E
- 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.
- -N
- Enable lookup and display of network names (e.g.,
[GNTY-NETBLK-4]). This option queries various registries of network
address allocation such as ARIN.
- -A
- 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.
- -T
- 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.
- -S
- Suppress display of the real-time status bar. This option
makes LFT show its completed trace output only, no-frills.
- -V
- Verbose output. Mostly debug garbage. Likely too verbose
for regular use.
- -v
- 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.
EXAMPLES¶
A sample use and output might be:
[edge.lax]$ lft -S 4.2.2.2
Hop LFT trace to vnsc-bak.sys.gtei.net (4.2.2.2):80/tcp
1 ln-gateway.centergate.com (206.117.161.1) 0.5ms
2 isi-acg.ln.net (130.152.136.1) 2.3ms
3 isi-1-lngw2-atm.ln.net (130.152.180.21) 2.5ms
4 gigabitethernet5-0.lsanca1-cr3.bbnplanet.net (4.24.4.249) 3.0ms
5 p6-0.lsanca1-cr6.bbnplanet.net (4.24.4.2) 3.4ms
6 p6-0.lsanca2-br1.bbnplanet.net (4.24.5.49) 3.3ms
7 p15-0.snjpca1-br1.bbnplanet.net (4.24.5.58) 10.9ms
8 so-3-0-0.mtvwca1-br1.bbnplanet.net (4.24.7.33) 11.1ms
9 p7-0.mtvwca1-dc-dbe1.bbnplanet.net (4.24.9.166) 11.0ms
10 vlan40.mtvwca1-dc1-dfa1-rc1.bbnplanet.net (128.11.193.67) 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 (4.2.2.2) 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 4.2.2.1
Hop LFT trace to vnsc-pri.sys.gtei.net (4.2.2.1):80/tcp
1 ln-gateway.centergate.com (206.117.161.1) 0.5/0.5ms
2 isi-acg.ln.net (130.152.136.1) 2.1/2.3ms
3 isi-1-lngw2-atm.ln.net (130.152.180.21) 2.6/7.1ms
4 gigabitethernet5-0.lsanca1-cr3.bbnplanet.net (4.24.4.249) 6.1/3.9ms
** [firewall] the next gateway may statefully inspect packets
5 p0-0-0.lsanca1-csr1.bbnplanet.net (4.24.4.10) 155.4/3.7ms
6 [target] vnsc-pri.sys.gtei.net (4.2.2.1) 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 (66.218.71.88):80/tcp
1 [AS226] ln-gateway.centergate.com (206.117.161.1) 1 ms
2 [AS226] isi-acg.ln.net (130.152.136.1) 2 ms
3 [AS226] isi-1-lngw2-atm.ln.net (130.152.180.21) 3 ms
4 [AS1] gigether5-0.lsanca1-cr3.bbnplanet.net (4.24.4.249) 3 ms
5 [AS1] p6-0.lsanca1-cr6.bbnplanet.net (4.24.4.2) 5 ms
6 [AS1] p6-0.lsanca2-br1.bbnplanet.net (4.24.5.49) 3 ms
7 [AS1] p1-0.lsanca2-cr2.bbnplanet.net (4.25.112.1) 3 ms
8 [AS16852] pos4-0.core1.LosAngeles1.Level3.net (209.0.227.57) 3 ms
9 [AS3356] so-4-0-0.mp1.LosAngeles1.Level3.net (209.247.10.193) 3 ms
10 [AS3356] so-3-0-0.mp2.SanJose1.Level3.net (64.159.1.130) 11 ms
11 [AS3356] gige10-0.ipcolo4.SanJose1.Level3.net (64.159.2.42) 11 ms
12 [AS3356] cust-int.level3.net (64.152.81.62) 52 ms
13 [AS10310] vl17.bas2.scd.yahoo.com (66.218.64.150) 53 ms
14 [AS10310] w9.scd.yahoo.com (66.218.71.88) [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 (207.46.197.113):80/tcp
1 [LOS-NETTOS-BLK4] ln-gateway.centergate.com (206.117.161.1) 2 ms
2 [LOS-NETTOS] isi-acg.ln.net (130.152.136.1) 3 ms
3 [LOS-NETTOS] isi-1-lngw2-pos.ln.net (130.152.80.30) 5 ms
4 [GNTY-4-0] gigether5-0.lsanca1-cr3.bbnplanet.net (4.24.4.249) 4 ms
5 [GNTY-4-0] p6-0.lsanca1-cr6.bbnplanet.net (4.24.4.2) 3 ms
6 [GNTY-4-0] p6-0.lsanca2-br1.bbnplanet.net (4.24.5.49) 3 ms
7 [GNTY-4-0] p15-0.snjpca1-br1.bbnplanet.net (4.24.5.58) 10 ms
8 [GNTY-4-0] p9-0.snjpca1-br2.bbnplanet.net (4.24.9.130) 11 ms
9 [GNTY-4-0] so-1-0-0.sttlwa2-br1.bbnplanet.net (4.0.3.229) 27 ms
10 [GNTY-4-0] so-0-0-0.sttlwa1-hcr1.bbnplanet.net (4.24.11.202) 28 ms
11 [GNTY-4-0] so-7-0-0.sttlwa1-hcr2.bbnplanet.net (4.24.10.234) 28 ms
12 [GNTY-4-0] p1-0.sttlwa1-cr2.bbnplanet.net (4.24.10.241) 29 ms
13 [GNTY-4-0] p2-0.msseattle.bbnplanet.net (4.25.89.6) 32 ms
14 [MICROSOFT-GLOBAL-NET] 207.46.154.9 32 ms
15 [MICROSOFT-GLOBAL-NET] 207.46.155.17 33 ms
16 [MICROSOFT-GLOBAL-NET] 207.46.129.51 [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.
AUTHOR¶
Nils McCarthy, Victor Oppleman, Ugen Antsilevitch, and other helpers around the
world. The source is currently maintained and administered by MainNerve, Inc.
REPORTING BUGS¶
To report bugs, send e-mail to <lft-bugs@mainnerve.com>
SEE ALSO¶
traceroute(8),
netstat(1),
whois(1)
HISTORY¶
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.'
