traceroute – print the route packets take to network host
traceroute 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.
traceroute [ -dFInrvx ] [ -f first_ttl ] [ -g gateway ]
[ -i iface ] [ -m max_ttl ] [ -p port ]
[ -q nqueries ] [ -s src_addr ] [ -t tos ]
[ -w waittime ] [ -z pausemsecs ]
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. Traceroute utilizes 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.
The only mandatory parameter is the destination host name or IP number. The default probe datagram length is 40 bytes, but this may be increased by specifying a packet length (in bytes) after the destination host name.
Other options are:
- -f Set the initial time-to-live used in the first outgoing probe packet.
- -F Set the “don’t fragment” bit.
- -d Enable socket level debugging.
- -g Specify a loose source route gateway (8 maximum).
- -i Specify a network interface to obtain the source IP address for outgoing probe packets. This is normally only useful on a multi-homed host. (See the -s flag for another way to do this.)
- -I Use ICMP ECHO instead of UDP datagrams.
- Set the max time-to-live (max number of hops) used in outgoing probe packets. The default is 30 hops (the same default used for TCP connections).
- Print hop addresses numerically rather than symbolically and numerically (saves a nameserver address-to-name lookup for each gateway found on the path).
- Set the base UDP port number used in probes (default is 33434). Traceroute hopes that nothing is listening on UDP ports base to base + nhops – 1 at the destination host (so an ICMP PORT_UNREACHABLE message will be returned to terminate the route tracing). If something is listening on a port in the default range, this option can be used to pick an unused port range.
- 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 interface that has no route through it (e.g., after the interface was dropped by routed(8C)).
- Use the following IP address (which usually is given as an IP number, not a hostname) as the source address in outgoing probe packets. On multi-homed hosts (those with more than one IP address), this option can be used to force the source address to be something other than the IP address of the interface the probe packet is sent on. If the IP address is not one of this machine’s interface addresses, an error is returned and nothing is sent. (See the -i flag for another way to do this.)
- Set the type-of-service in probe packets to the following value (default zero). The value must be a decimal integer in the range 0 to 255. This option can be used to see if different types-of-service result in different paths. (If you are not running 4.4bsd, this may be academic since the normal network services like telnet and ftp don’t let you control the TOS). Not all values of TOS are legal or meaningful – see the IP spec for definitions. Useful values are probably `-t 16‘ (low delay) and `-t 8‘ (high throughput).
- Verbose output. Received ICMP packets other than TIME_EXCEEDED and UNREACHABLEs are listed.
- Set the time (in seconds) to wait for a response to a probe (default 5 sec.).
- Toggle ip checksums. Normally, this prevents traceroute from calculating ip checksums. In some cases, the operating system can overwrite parts of the outgoing packet but not recalculate the checksum (so in some cases the default is to not calculate checksums and using -x causes them to be calcualted). Note that checksums are usually required for the last hop when using ICMP ECHO probes (-I). So they are always calculated when using ICMP.
- Set the time (in milliseconds) to pause between probes (default 0). Some systems such as Solaris and routers such as Ciscos rate limit icmp messages. A good value to use with this this is 500 (e.g. 1/2 second).