分类： 系统运维
2012-05-14 14:29:08
Introduction
The Traceroute program, written by Van Jacobson, is a handy debugging tool that allows us to further explore the TCP/IP protocols. Although there are no guarantees that two consecutive IP datagrams from the same source to the same destination follow the same route, most of the time they do. Traceroute lets us see the route that IP datagrams follow from one host to another. Traceroute also lets us use the IP source route option.
Traceroute Program Operation
Traceroute uses ICMP and the TTL field in the IP header. When a router gets an IP datagram whose TTL is either 0 or 1 it must not forward the datagram. (A destination host that receives a datagram like this can deliver it to the application, since the datagram does not have to be routed. Normally, however, no system should receive a datagram with a TTL of 0.) Instead the router throws away the datagram and sends back to the originating host an ICMP "time exceeded" message. The key to Traceroute is that the IP datagram containing this ICMP message has the router's IP address as the source address.
We can now guess the operation of Traceroute. It sends an IP datagram with a TTL of 1 to the destination host. The first router to handle the datagram decrements the TTL, discards the datagram, and sends back the ICMP time exceeded. This identifies the first router in the path. Traceroute then sends a datagram with a TTL of 2, and we find the IP address of the second router. This continues until the datagram reaches the destination host. But even though the arriving IP datagram has a TTL of 1, the destination host won't throw it away and generate the ICMP time exceeded, since the datagram has reached its final destination. How can we determine when we've reached the destination?
Traceroute sends UDP datagrams to the destination host, but it chooses the destination UDP port number to be an unlikely value (larger than 30,000), making it improbable that an application at the destination is using that port. This causes the destination host's UDP module to generate an ICMP "port unreachable" error (Section 6.5) when the datagram arrives. All Traceroute needs to do is differentiate between the received ICMP messages-time exceeded versus port unreachable-to know when it's done.
The Traceroute program must be able to set the TTL field in the outgoing datagram. Not all programming interfaces to TCP/IP support this, and not all implementations support the capability, but most current systems do, and are able to run Traceroute. This programming interface normally requires the user to have superuser privilege, meaning it may take special privilege to run it on your host.
IP Source Routing Option
Normally IP routing is dynamic with each router making a decision about which next-hop router to send the datagram to. Applications have no control of this, and are normally not concerned with it. It takes tools such as Traceroute to figure out what the route really is. The idea behind source routing is that the sender specifies the route. Two forms are provided:
1. Strict source routing. The sender specifies the exact path that the IP datagram must follow. If a router encounters a next hop in the source route that isn't on a directly connected network, an ICMP "source route failed" error is returned.
2. Loose source routing. The sender specifies a list of IP address that the datagram must traverse, but the datagram can also pass through other routers between any two addresses in the list.
Traceroute provides a way to look at source routing, as we can specify an option allowing us to force a source route, and see what happens.
The format of the source route option:
| 39bytes | |||||||
| code | len | ptr | IP address #1 | IP address #2 | IP address #3 | ... | IP address #9 |
| 1 | 1 | 1 | 4 | 4 | 4 | 4 | |
The source route options are actually called "source and record route" (LSRR and SSRR, for loose and strict) since the list of IP addresses is updated as the datagram passes along the path. What happens is as follows:
1. The sending host takes the source route list from the application, removes the first entry (it becomes the destination address of the datagram), moves all the remaining entries left by one entry, and places the original destination address as the final entry in the list. The pointer still points to the first entry in the list (e.g., the value of the pointer is 4).
2. Each router that handles the datagram checks whether it is the destination address of the datagram. If not, the datagram is forwarded as normal. (In this case loose source routing must have been specified, or we wouldn't have received the datagram.)
3. If the router is the destination, and the pointer is not greater than the length, then (1) the next address in the list (where ptr points) becomes the destination address of the datagram, (2) the IP address corresponding to the outgoing interface replaces the source address just used, and (3) the pointer is incremented by 4.
The -g option to traceroute lets us specify intermediate routers to be used with loose source routing. This option can be specified up to eight times. (The reason this is eight and not nine is that the programming interface being used requires that the final entry be the destination.)
The -G option in the author's version of traceroute is identical to the -g option described earlier, but the source route is strict instead of loose.
