DOKK / manpages / debian 10 / lft / lft.8.en
LFT(8) System Manager's Manual LFT(8)

lft — display the route packets take to a network host/socket using one of several layer-4 protocols and methods; optionally show heuristic network information in transitu

lft [-d dport] [-s sport] [-m retry min] [-M retry max] [-a ahead] [-c scatter ms] [-t timeout ms] [-l min ttl] [-H max ttl] [-L length] [-q ISN] [-D device] [-f device] [-G icons path] [-ACEINPRSTUVbeghinpruvxyz] [<gateway> <...>] target: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 was developed to automate a solution to the above, taking into account that modern networks contain many configurations of load balancers, proxies, and stateful firewalls.

lft implements numerous network tracing methods and strategies. Generally, lft sends various types of layer-4 probes utilizing the IP protocol `time to live' field and attempts to elicit an ICMP `time exceeded in transit' response from each gateway along the path to some host. RFC 1393 Traceroute Using an IP Option is also available as one of several tracing methods.

lft additionally listens for various messages along the way to assist network managers in ascertaining per-protocol heuristic routing information. lft can optionally retrieve various information about the networks it traverses using a variety of sources such as registries, routing arbiters, etc.

The only mandatory parameter is the target host name or IP number. Options toggle the display of more interesting data or change the variables of the trace itself. The (-E/-e) adaptive option tries several combinations of TCP states (changing flags inside the probes it sends) in order to improve the chances of a successful trace and expose stateful packet filters.

Other options are:

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.
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.
Automatically select a pseudo-random source port. This option may be useful if your local packet filter or proxy doesn't allow you to use source ports outside of the dymanic range allocation.
min
Set min as the minimum number of re-attempts to send per host. Default is 1 unless adaptive (-E) mode is used.
max
Set max as the maximum number of re-attempts to send per host. Default is 3.
ahead
Set ahead as the number of hops forward to query before waiting for a response. Default is 5.
scatter ms
Set scatter ms as the minimum number of milliseconds to wait between sending probes. Default is 20.
timeout ms
Set timeout ms as the maximum number of milliseconds to wait before assuming a probe was lost/discarded. Default is 1000.
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.
ISN
Set ISN as the ISN (initial sequence number) of the first probe. If unset, one will be automatically generated using a pseudo-random, time-seeded algorithm.
length
Set length as the size of probe packets in bytes. This includes layer-3 and layer-4 headers, but does not include layer-2 headers. For example, setting the length to 1500 would create a 1500-byte probe packet which would result in a 1514-byte frame on an Ethernet network.
device
Set device as the network device or address to receive traffic. (e.g., "en1" or "1.2.3.4") If unset, lft will attempt to determine and acquire the appropriate interface based on routing.
device
Set device as the network device or address to transmit traffic. (e.g., "en1" or "1.2.3.4") If unset, lft will attempt to determine and acquire the appropriate interface based on routing. This serves to operate lft in a passive mode where you may transmit from a (potentially) spoofed IP address on one interface, yet receive on another. This allows you to trace from a different IP address whose traffic you can see in order to intercept replies.
ttl
Set ttl as the maximum TTL, essentially the maximum route traversal distance in hops. Default is 30.
icons path
Set icons path as the path to GraphViz icons in connection with GraphViz output.
Set the ToS (Type of Serice) bit on outgoing IP datagrams. The ToS will be set to the differentiated services request minimize-delay.
Disable "stop" on ICMP other than TTL expired.
Print addresses numerically rather than symbolically and numerically. Disables use of the DNS resolver completely.
Print addresses symbolically rather than symbolically and numerically. If the DNS resolver fails to resolve an address, the address is printed numerically.
Enable use of the adaptive engine which tries several combinations of TCP states (changing flags inside the probes it sends) in order to improve the chances of a successful trace. The engine also displays other useful information such as stateful inspection firewalls or broken IP stacks encountered along the way.
Enable use of TCP packets with the FIN flag set. This strategy fools unsophisticated packet filters that don't maintain a proper state table. Such devices will forward the packet to its destination rather than filter it, assuming a handshake has already taken place and the probes are part of an existing and valid TCP stream.
Enable use of UDP-based probes instead of TCP-based probes. This strategy is similar to the traditional traceroute method, but many of LFT's other options (such as source and destination port selection) are still available. By default, LFT's UDP probes have a small payload (unlike LFT's TCP probes that carry no payload).
Enable lookup and display of network or AS names (e.g., [GNTY-NETBLK-4]). This option queries Prefix WhoIs, RIPE NCC, or the RADB (as requested). In the case of Prefix WhoIs or RADB, the network name is displayed. In the case of RIPE NCC, the AS name is displayed.
Enable RFC 1393 tracing method using ICMP and an IP option. While this strategy has been formalized in an RFC, few network equipment vendors support it.
Enable use of ICMP-based probes instead of TCP-based probes. This strategy is sometimes the fastest, however firewalls commonly filter ICMP at network borders. ICMP probes are echo request (ping) packets.
Enable TCP basic tracing method. Unlike the default method, the basic method generates TCP probes without relying upon sequence numbers being conveyed correctly. This makes LFT more comptabile with networks employing NAT, but is slower than the default method. TCP basic may also be used with adaptive mode (-E).
Enable lookup and display of of AS (autonomous system) numbers (e.g., [1]). This option queries one of several whois servers (see options 'C' and 'r') in order to ascertain the origin ASN of the IP address in question. By default, LFT uses the pWhoIs service whose ASN data tends to be more accurate and more timely than using the RADB as it is derived from the Internet's global routing table. See www.pwhois.org
Force use of the RIPE NCC RIS whois service to lookup ASNs. This is an alternative source of timely ASN-related information built using the Internet's global routing table. See www.ripe.net/projects/ris
Force use of the Cymru whois service to lookup ASNs. This is an alternative source of timely ASN-related information built using the Internet's global routing table. See www.cymru.com
Force use of the RADB whois service to lookup ASNs. This tends to be quick, but incomplete and usually inaccurate with regard to the 'actual' Internet routing table. See www.radb.net
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. Use with -V (verbose) to display additional detail.
Display all times in UTC/GMT0. This option also enables the -T option automatically.
Suppress display of the real-time status bar. This option makes LFT show its completed trace output only, no-frills.
Enable XML output and suppress all other output to stdout.
Enable GraphViz output and suppress all other output to stdout.
Enable network seam testing in connection with GraphViz output.
Display verbose output. Use more V's for more info.
Display version information, then exit(1).

Any hosts listed after these options and before the final host/target will comprise the loose source route. Since network operators have security concerns regarding the use of source routing, don't expect the LSRR options to do anything for you in most public networks.

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 adaptive 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 adaptive 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   [226] ln-gateway.centergate.com (206.117.161.1)  1 ms
 2   [226] isi-acg.ln.net (130.152.136.1)  2 ms
 3   [226] isi-1-lngw2-atm.ln.net (130.152.180.21)  3 ms
 4   [1] gigether5-0.lsanca1-cr3.bbnplanet.net (4.24.4.249)  3 ms
 5   [1] p6-0.lsanca1-cr6.bbnplanet.net (4.24.4.2)  5 ms
 6   [1] p6-0.lsanca2-br1.bbnplanet.net (4.24.5.49)  3 ms
 7   [1] p1-0.lsanca2-cr2.bbnplanet.net (4.25.112.1)  3 ms
 8   [16852] pos4-0.core1.LosAngeles1.Level3.net (209.0.227.57)  3 ms
 9   [3356] so-4-0-0.mp1.LosAngeles1.Level3.net (209.247.10.193)  3 ms
10   [3356] so-3-0-0.mp2.SanJose1.Level3.net (64.159.1.130)  11 ms
11   [3356] gige10-0.ipcolo4.SanJose1.Level3.net (64.159.2.42)  11 ms
12   [3356] cust-int.level3.net (64.152.81.62)  52 ms
13   [10310] vl17.bas2.scd.yahoo.com (66.218.64.150)  53 ms
14   [10310] w9.scd.yahoo.com (66.218.71.88) [target]  54 ms

LFT's trace took 5.23 seconds.  Resolution required 3.58 seconds.

Note the -Ar above displays ASNs using the RADB as a whois source. A better option may have been to use the -A alone or perhaps -AC.

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

If traces don't appear to go anywhere, there are a number of things to try. If you are receiving an error related to permissions, be sure the lft executable is set-uid root so it may execute with root-level permissions required to utilize raw sockets on most operating systems.

If you do not receive permissions-related errors, but traces still don't go anywhere, first activate verbose output by adding -VV to your command line options. Then, reading the verbose output, if you see trace probes going out, but no replies being detected (as indicated by "RCVD" tags), you may: Use the TCP basic (-b) method if you wish to use TCP probes and you fear NAT may be causing your trace to fail. Alternatively, select a different trace method and protocol such as UDP (-u) or ICMP (-p).

If you are attempting to use RFC 1393 (-P) and your trace is failing, this is likely because network equipment somewhere in the path does not conform to RFC 1393. Your only option is to select an alternative tracing method or protocol.

If you are attempting to utilize adaptive mode (-E/-e) and traces fail, first try enabling NAT compatibility using TCP basic (-b). If traces still fail, the most likely reason is a close-proximity stateful firewall in your network, which prevents this feature from working.

Victor Oppleman, Eugene Antsilevitch, Sergey Kondryukov and other helpers around the world.

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

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

The lft command first appeared in 1998 as 'fft'. Renamed as a result of confusion with fast fourier transforms, lft stands for 'layer four traceroute.' Thanks also to Nils McCarthy for writing 'FFT', LFT's predecessor.

August 17, 2002 LFT