RFC2075 - IP Echo Host Service

Network Working Group C. Partridge

Request for Comments: 2075 BBN

Category: EXPerimental January 1997

IP Echo Host Service

Status of this Memo

This memo defines an Experimental Protocol for the Internet

community. This memo does not specify an Internet standard of any

kind. Discussion and suggestions for improvement are requested.

Distribution of this memo is unlimited.

Abstract

This memo describes how to implement an IP echo host. IP echo hosts

send back IP datagrams after exchanging the source and destination IP

addresses. The effect is that datagrams sent to the echo host are

sent back to the source, as if they originated at the echo host.

IntrodUCtion

An IP echo host returns IP datagrams to their original source host,

with the IP source and destination addresses reversed, so that the

returning datagram appears to be coming from the echo host to the

original source. IP echo hosts are tremendously useful for debugging

applications and protocols. They allow researchers to create looped

back conversations across the Internet, exposing their traffic to all

the vagaries of Internet behavior (congestion, cross traffic,

variable round-trip times and the like) without having to distribute

prototype software to a large number of test machines.

IP echo hosts were heavily used on the Internet in the late 1970s and

early 1980s to debug various Internet transport and application

protocols. But, for reasons unclear, at the current date there are

no echo hosts on the Internet and few people are even aware of the

concept. The goal of this memo is to document the concept in the

hopes it will be revived.

Implementation Details

While the basic idea of a echo host is simple, there are a few

implementation details that require attention. This section

describes those implementation details. The presentation works from

the simplest to most difficult issues.

The most straightforward situation is when an echo host receives an

IP datagram with no options and whose protocol field has a value

other than 1 (ICMP). In this case, the echo host modifies the header

by exchanging the source and destination addresses, decrements the

TTL by one and updates the IP header checksum. The host then

transmits the updated IP datagram back to the original source of the

datagram.

NOTE: If the TTL is zero or less after decrementing, the datagram

MUST not be echoed. In general, an echo host is required to do

all the various sanity checks that a router or host would do to an

IP datagram before accepting the datagram for echoing (see STD 3,

RFC1122, and RFC1812).

The TTL MUST be decremented for security reasons noted below.

Observe, however, that the effect is that hosts using an echo path

through an echo host SHOULD set their TTL to twice the normal

value to be sure of achieving connectivity over the echo path.

If an arriving IP datagram has options, the echo host's

responsibilities are more complex. In general, the IP source and

destination are always exchanged and TTL and checksum updated, but in

certain situations, other special actions may have to take place.

If the datagram contains an incomplete source route option (i.e. the

echo host is not the final destination), the datagram MUST be

discarded. If the datagram contains a complete source route option,

the source route option MUST be reversed, and the datagram (with

source and destination IP addresses exchanged and updated TTL) MUST

be sent back along the reverse source route.

More generally, the goal with any option is to update the option such

that when the echoed packet is received at the original source, the

option fields will contain data which makes sense for a datagram

originating at the echo host.

There is one option for which it is unclear what the correct action.

The timestamp option is sometimes used for round-trip time

estimation. If the option is reset at the echo host, then a history

of roughly half of the trip delay will be lost. But if the option is

not reset, then the timestamp option will appear inconsistent with

the source and destination addresses of the datagram. To try to

balance these two issues, the following rules are suggested:

1. If the first entry in the timestamp option contains the IP

address of the source host, the entry SHOULD be rewritten to

contain the IP address of the echo host, and the timestamp option

pointer SHOULD be truncated so that this timestamp is the only one

in the list. (This rewrite makes the option appear consistent

with the new source and destination IP addresses, and retains the

source timestamp, while losing information about the path to the

echo host).

2. If the first entry in the timestamp option does not contain the

IP address of the source host, the entry SHOULD be echoed back

unchanged. The echo host SHOULD NOT appear in the timestamp

option. (This approach retains the entire history of the path,

though observe that on a symmetric route, it means every router

may appear twice in the path).

Finally, if the IP datagram contains an ICMP packet (i.e. the IP

protocol field value is 1), the datagram SHOULD be discarded. The

reason for this rule is that the most likely reason for receiving an

ICMP datagram is that an echoed datagram has encountered a problem at

some router in the path and the router has sent back an ICMP

datagram. Echoing the ICMP datagram back to the router may confuse

the router and thus SHOULD be avoided. (This rule simply follows the

Internet maxim of being conservative in what we send).

However, in some cases the ICMP datagram will have useful information

for the source host which it would be desirable to echo. A

sophisticated echo host MAY choose to echo ICMP datagrams according

to the following rules:

1. Any ICMP datagram in which the destination address in the

encapsulated IP header (the header within the ICMP datagram)

matches the source address of the ICMP datagram MAY be safely

echoed.

2. ICMP Source Quench and ICMP Destination Unreachable with a code

of 4 (fragmentation needed and DF set) MAY be sent to the

*destination* of the encapsulated IP datagram if the source IP

address of the encapsulated IP datagram is that of the echo host.

When the ICMP message is sent on, it SHOULD be rewritten as an

ICMP message from the echo host to the source.

3. All other ICMP messages MUST be discarded.

These rules were chosen to try to ensure that end-to-end ICMP

messages are passed through, as are messages from routers which are

fairly safe and useful (or necessary) to the end system, but that

potentially dangerous messages such as Redirects are suppressed.

(The ICMP Destination Unreachable with code 4 is required for MTU

discovery under RFC-1191).

Security Considerations

Echo hosts pose a number of security concerns related to address

spoofing.

First, echo hosts provide obvious ways to extend attacks that make

use of address spoofing. A malevolent host can write an third

party's IP address as the source address of a datagram sent to an

echo host and thus cause the echo host to send a datagram to the

third party. In general, this trick does not create a new security

hole (the malevolent host could just as well have sent the datagram

with a forged source address straight to the third party host). But

there are some new twists to the problem.

One exception is if the echo host is a host inside a firewall that

accepts datagrams from hosts outside the firewall. In that case, a

malevolent host outside the firewall may be able to use the echo host

to make its packets appear to originate from inside the firewall

(from the echo host). In general, a good firewall will catch these

cases (the source address of the datagrams sent to the echo host will

be for a host inside the firewall and testing for interior source

addresses on datagrams arriving at an exterior interface is a

standard firewall filter) but since the primary purpose of echo hosts

is for wide scale Internet testing, there seems no reason to invite

danger. So we recommend that echo hosts SHOULD NOT be placed inside

firewalls.

Second, address spoofing can be used to cause flooding of the

network. In this case, a malevolent host sends a datagram to an echo

host with the source address of another echo host. This trick will

cause datagrams to circulate between the two echo hosts. The

requirement that the echo host decrement the TTL by one ensures that

each datagram will eventually die, but a sufficiently malevolent host

sending a large number of datagrams with high TTLs to an echo host

can cause considerable disruption. There are a number of possible

ways to repair this problem (such as requiring sources to

authenticate themselves before sending datagrams to be echoed). A

simple protection is simply to limit the number of packets echoed

back to any one source per second. For instance, one might limit a

source to a packet rate equal to 10% of the interface bandwidth (for

a 10 Mb/s Ethernet this would be about 75 maximum sized packets per

second).

One variation of this attack is to generate e-mail addressed to the

echo host (e.g., user@echo.xxx.com). This e-mail will loop over the

network a number of times until the SMTP server determines the

message has too many Received-From: lines.

A third variation of the flooding trick is to place a multicast or

broadcast address as the source of the IP datagram sent to an echo

server. Since this results in an illegal arriving IP datagram, the

echo server MUST discard the datagram. (This warning serves as a

reminder that echo servers MUST do the standard checks for an illegal

datagram before echoing).

Implementation Note

Echo hosts are often implemented as virtual interfaces on an existing

host or router. One can think of the echo host's IP address as a

second IP address for the host, with the semantics that all datagrams

sent to that address get echoed. Observe that when an echo host is

supported as a module within a larger host implementation, an easy

implementation mistake to make is to accidentally put the non-echo

address of a host into an echoed packet. For a variety of reasons

(including security and correct operation of echo paths) implementors

MUST ensure this NEVER happens.

Acknowledgements

This memo was stimulated by a conversation with Jon Crowcroft in

which we both lamented the demise of some beloved IP echo hosts

(e.g., goonhilly-echo.arpa). It has been considerably improved by

comments from various members of the End2End-Interest mailing list,

including Bob Braden, Mark Handley, Christian Huitema, Dave Mills,

Tim Salo, Vern Schryver, Lansing Sloan, and Rich Stevens.

The author is emphatically not the inventor of echo hosts. Enquiries

to the usual suspects suggest that echo hosts were created by persons

unknown (probably at BBN) very early in the development of IP. I'd

like to thank those persons who created echo hosts and apologize for

any errors in describing their invention.

Author's Address

Craig Partridge

BBN Corporation

10 Moulton St

Cambridge MA 02138

EMail: craig@bbn.com