TRACEROUTE(8)                                         Traceroute For Linux                                         TRACEROUTE(8)



NAME
       traceroute - print the route packets trace to network host

SYNOPSIS
       traceroute [-46dFITUnreAV] [-f first_ttl] [-g gate,...]
               [-i device] [-m max_ttl] [-p port] [-s src_addr]
               [-q nqueries] [-N squeries] [-t tos]
               [-l flow_label] [-w waittime] [-z sendwait]
               [-UL] [-P proto] [--sport=port] [-M method] [-O mod_options]
               [--mtu] [--back]
               host [packet_len]
       traceroute6  [options]

DESCRIPTION
       traceroute  tracks the route packets taken from an IP network on their way to a given host. It utilizes the IP protocol's
       time to live (TTL) field and attempts to elicit an ICMP TIME_EXCEEDED response from each gateway along the  path  to  the
       host.

       traceroute6 is equivalent to traceroute -6

       The only required parameter is the name or IP address of the destination host .  The optional packet_len`gth is the total
       size of the probing packet (default 60 bytes for IPv4 and 80 for IPv6). The specified size can be ignored in some  situa-
       tions or increased up to a minimal value.

       This  program attempts to trace the route an IP packet would follow to some internet host by launching probe packets with
       a small ttl (time to live) then listening for an ICMP "time exceeded" reply from a gateway.  We start our probes  with  a
       ttl  of one and increase by one until we get an ICMP "port unreachable" (or TCP reset), which means we got to the "host",
       or hit a max (which defaults to 30 hops). Three probes (by default) are sent at each ttl setting and a  line  is  printed
       showing  the  ttl,  address  of  the gateway and round trip time of each probe. The address can be followed by additional
       information when requested. If the probe answers come from different gateways, the address of each responding system will
       be printed.  If there is no response within a 5.0 seconds (default), an "*" (asterisk) is printed for that probe.

       After the trip time, some additional annotation can be printed: !H, !N, or !P (host, network or protocol unreachable), !S
       (source route failed), !F (fragmentation needed), !X (communication administratively  prohibited),  !V  (host  precedence
       violation),  !C  (precedence cutoff in effect), or !<num> (ICMP unreachable code <num>).  If almost all the probes result
       in some kind of unreachable, traceroute will give up and exit.

       We don't want the destination host to process the UDP probe packets, so the destination port is set to an unlikely  value
       (you  can  change it with the -p flag). There is no such a problem for ICMP or TCP tracerouting (for TCP we use half-open
       technique, which prevents our probes to be seen by applications on the destination host).

       In the modern network environment the traditional traceroute methods can not be always applicable, because of  widespread
       use  of  firewalls.  Such firewalls filter the "unlikely" UDP ports, or even ICMP echoes.  To solve this, some additional
       tracerouting methods are implemented (including tcp), see LIST OF AVAILABLE METHODS below. Such methods try to  use  par-
       ticular  protocol  and  source/destination port, in order to bypass firewalls (to be seen by firewalls just as a start of
       allowed type of a network session).

OPTIONS
       --help Print help info and exit.

       -4, -6 Explicitly force IPv4 or IPv6 tracerouting. By default, the program will try to resolve the name given, and choose
              the  appropriate protocol automatically. If resolving a host name returns both IPv4 and IPv6 addresses, traceroute
              will use IPv4.

       -I     Use ICMP ECHO for probes

       -T     Use TCP SYN for probes

       -d     Enable socket level debugging (when the Linux kernel supports it)

       -F     Do not fragment probe packets. (For IPv4 it also sets DF bit, which tells intermediate  routers  not  to  fragment
              remotely as well).

              Varying  the size of the probing packet by the packet_len command line parameter, you can manually obtain informa-
              tion about the MTU of individual network hops. The --mtu option (see below) tries to do this automatically.

              Note, that non-fragmented features (like -F or --mtu) work properly since the Linux kernel  2.6.22  only.   Before
              that  version,  IPv6  was always fragmented, IPv4 could use the once the discovered final mtu only (from the route
              cache), which can be less than the actual mtu of a device.

       -f first_ttl
              Specifies with what TTL to start. Defaults to 1.

       -g gateway
              Tells traceroute to add an IP source routing option to the outgoing packet that tells the  network  to  route  the
              packet  through  the  specified gateway (most routers have disabled source routing for security reasons).  In gen-
              eral, several gateway's is allowed (comma separated). For IPv6, the form of num,addr,addr...   is  allowed,  where
              num is a route header type (default is type 2). Note the type 0 route header is now deprecated (rfc5095).

       -i interface
              Specifies  the  interface  through  which  traceroute  should  send packets. By default, the interface is selected
              according to the routing table.

       -m max_ttl
              Specifies the maximum number of hops (max time-to-live value) traceroute will probe. The default is 30.

       -N squeries
              Specifies the number of probe packets sent out simultaneously.  Sending several probes concurrently can  speed  up
              traceroute considerably. The default value is 16.
              Note that some routers and hosts can use ICMP rate throttling. In such a situation specifying too large number can
              lead to loss of some responses.

       -n     Do not try to map IP addresses to host names when displaying them.

       -p port
              For UDP tracing, specifies the destination port base traceroute will use (the  destination  port  number  will  be
              incremented by each probe).
              For ICMP tracing, specifies the initial ICMP sequence value (incremented by each probe too).
              For TCP specifies just the (constant) destination port to connect.

       -t tos For  IPv4,  set  the  Type  of  Service  (TOS)  and Precedence value. Useful values are 16 (low delay) and 8 (high
              throughput). Note that in order to use some TOS precedence values, you have to be super user.
              For IPv6, set the Traffic Control value.

       -w waittime
              Set the time (in seconds) to wait for a response to a probe (default 5.0 sec).

       -q nqueries
              Sets the number of probe packets per hop. The default is 3.

       -r     Bypass the normal routing tables and send directly to a host on an attached network.  If the  host  is  not  on  a
              directly-attached  network,  an error is returned.  This option can be used to ping a local host through an inter-
              face that has no route through it.

       -s source_addr
              Chooses an alternative source address. Note that you must select  the  address  of  one  of  the  interfaces.   By
              default, the address of the outgoing interface is used.

       -z sendwait
              Minimal  time  interval  between  probes (default 0).  If the value is more than 10, then it specifies a number in
              milliseconds, else it is a number of seconds (float point values allowed too).  Useful when some routers use rate-
              limit for ICMP messages.

       -e     Show  ICMP  extensions  (rfc4884).  The  general  form  is  CLASS/TYPE:  followed by a hexadecimal dump.  The MPLS
              (rfc4950) is shown parsed, in a form: MPLS:L=label,E=exp_use,S=stack_bottom,T=TTL (more objects separated by / ).

       -A     Perform AS path lookups in routing registries and print results directly after the corresponding addresses.

       -V     Print the version and exit.

       There is a couple of additional options, intended for an advanced usage (another trace methods etc.):

       --sport=port
              Chooses the source port to use. Implies -N 1.  Normally source ports (if applicable) are chosen by the system.

       --fwmark=mark
              Set the firewall mark for outgoing packets (since the Linux kernel 2.6.25).

       -M method
              Use specified method for traceroute operations. Default traditional udp method has name default, icmp (-I) and tcp
              (-T) have names icmp and tcp respectively.
              Method-specific options can be passed by -O .  Most methods have their simple shortcuts, (-I means -M icmp, etc).

       -O option
              Specifies  some  method-specific  option.  Several  options are separated by comma (or use several -O on cmdline).
              Each method may have its own specific options, or many not have them at all.  To print information about available
              options, use -O help.

       -U     Use  UDP  to particular destination port for tracerouting (instead of increasing the port per each probe). Default
              port is 53 (dns).

       -UL    Use UDPLITE for tracerouting (default port is 53).

       -P protocol
              Use raw packet of specified protocol for tracerouting. Default protocol is 253 (rfc3692).

       --mtu  Discover MTU along the path being traced. Implies -F -N 1.  New mtu is printed once in a  form  of  F=NUM  at  the
              first  probe  of a hop which requires such mtu to be reached. (Actually, the correspond "frag needed" icmp message
              normally is sent by the previous hop).

              Note, that some routers might cache once the seen information on a fragmentation. Thus you can receive  the  final
              mtu from a closer hop.  Try to specify an unusual tos by -t , this can help for one attempt (then it can be cached
              there as well).
              See -F option for more info.

       --back Print the number of backward hops when it seems different with the forward direction. This number  is  guessed  in
              assumption  that  remote  hops  send reply packets with initial ttl set to either 64, or 128 or 255 (which seems a
              common practice). It is printed as a negate value in a form of '-NUM' .

LIST OF AVAILABLE METHODS
       In general, a particular traceroute method may have to be chosen by -M name, but most of the methods  have  their  simple
       cmdline switches (you can see them after the method name, if present).

   default
       The traditional, ancient method of tracerouting. Used by default.

       Probe  packets  are udp datagrams with so-called "unlikely" destination ports.  The "unlikely" port of the first probe is
       33434, then for each next probe it is incremented by one. Since the ports are expected to be unused, the destination host
       normally  returns  "icmp unreach port" as a final response.  (Nobody knows what happens when some application listens for
       such ports, though).

       This method is allowed for unprivileged users.

   icmp       -I
       Most usual method for now, which uses icmp echo packets for probes.
       If you can ping(8) the destination host, icmp tracerouting is applicable as well.

       This method may be allowed for unprivileged users since the kernel 3.0 (IPv4 only), which supports  new  dgram  icmp  (or
       "ping") sockets. To allow such sockets, sysadmin should provide net/ipv4/ping_group_range sysctl range to match any group
       of the user.
       Options:

       raw    Use only raw sockets (the traditional way).
              This way is tried first by default (for compatibility reasons), then new dgram icmp sockets as fallback.

       dgram  Use only dgram icmp sockets.

   tcp        -T
       Well-known modern method, intended to bypass firewalls.
       Uses the constant destination port (default is 80, http).

       If some filters are present in the network path, then most probably any "unlikely" udp ports (as for default  method)  or
       even  icmp echoes (as for icmp) are filtered, and whole tracerouting will just stop at such a firewall.  To bypass a net-
       work filter, we have to use only allowed protocol/port combinations. If we trace for some,  say,  mailserver,  then  more
       likely -T -p 25 can reach it, even when -I can not.

       This  method  uses  well-known "half-open technique", which prevents applications on the destination host from seeing our
       probes at all.  Normally, a tcp syn is sent. For non-listened ports we receive tcp reset, and all  is  done.  For  active
       listening  ports  we  receive tcp syn+ack, but answer by tcp reset (instead of expected tcp ack), this way the remote tcp
       session is dropped even without the application ever taking notice.

       There is a couple of options for tcp method:

       syn,ack,fin,rst,psh,urg,ece,cwr
              Sets specified tcp flags for probe packet, in any combination.

       flags=num
              Sets the flags field in the tcp header exactly to num.

       ecn    Send syn packet with tcp flags ECE and CWR (for Explicit Congestion Notification, rfc3168).

       sack,timestamps,window_scaling
              Use the corresponding tcp header option in the outgoing probe packet.

       sysctl Use current sysctl (/proc/sys/net/*) setting for the tcp header options above and ecn.  Always set by default,  if
              nothing else specified.

       mss=num
              Use value of num for maxseg tcp header option (when syn).

       info   Print  tcp flags of final tcp replies when the target host is reached.  Allows to determine whether an application
              listens the port and other useful things.

       Default options is syn,sysctl.

   tcpconn
       An initial implementation of tcp method, simple using connect(2) call, which does full tcp session  opening.  Not  recom-
       mended for normal use, because a destination application is always affected (and can be confused).

   udp        -U
       Use udp datagram with constant destination port (default 53, dns).
       Intended to bypass firewall as well.

       Note,  that unlike in tcp method, the correspond application on the destination host always receive our probes (with ran-
       dom data), and most can easily be confused by them. Most cases it will not respond to our  packets  though,  so  we  will
       never see the final hop in the trace. (Fortunately, it seems that at least dns servers replies with something angry).

       This method is allowed for unprivileged users.

   udplite    -UL
       Use udplite datagram for probes (with constant destination port, default 53).

       This method is allowed for unprivileged users.
       Options:

       coverage=num
              Set udplite send coverage to num.

   raw        -P proto
       Send raw packet of protocol proto.
       No protocol-specific headers are used, just IP header only.
       Implies -N 1.
       Options:

       protocol=proto
              Use IP protocol proto (default 253).

NOTES
       To  speed up work, normally several probes are sent simultaneously.  On the other hand, it creates a "storm of packages",
       especially in the reply direction. Routers can throttle the rate of icmp responses, and some of replies can be  lost.  To
       avoid  this, decrease the number of simultaneous probes, or even set it to 1 (like in initial traceroute implementation),
       i.e.  -N 1

       The final (target) host can drop some of the simultaneous probes, and might even answer only the latest ones. It can lead
       to  extra  "looks like expired" hops near the final hop. We use a smart algorithm to auto-detect such a situation, but if
       it cannot help in your case, just use -N 1 too.

       For even greater stability you can slow down the program's work by -z option, for example  use  -z  0.5  for  half-second
       pause between probes.

       If  some  hops  report nothing for every method, the last chance to obtain something is to use ping -R command (IPv4, and
       for nearest 8 hops only).

SEE ALSO
       ping(8), ping6(8), tcpdump(8), netstat(8)



Traceroute                                               11 October 2006                                           TRACEROUTE(8)