Request for Comments: 4146
Simple New Mail Notification
Status of This Memo
This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
Copyright © The Internet Society (2005).
This memo documents a long-standing technique, supported by a large number of mail servers, which allows users to be notified of new mail. In addition to server support, there are a number of clients that support this, ranging from full email clients to specialized clients whose only purpose is to receive new mail notifications and alert a mail client.
In brief, the server sends the string "nm_notifyuser" CRLF to the finger port on the IP address (either configured or last used) of the user who has received new mail.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions Used in this Document . . . . . . . . . . . . . . 2 3. Simple Mail Notification . . . . . . . . . . . . . . . . . . 2 4. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 5. Security Considerations . . . . . . . . . . . . . . . . . . 3 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 3 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 3
There is a long-standing technique supported by a large number of mail servers that allows users to be notified of new mail. In addition to server support, there are a number of clients that support this, ranging from full email clients to specialized clients whose only purpose is to receive new mail notifications and alert a mail client. This technique is sometimes known as "notify mail" (after a shareware client of the same name), "biff", and the "finger hack".
2. Conventions Used in This Document
In examples, "C:" indicates lines sent by the client, and "S:" indicates those sent by the server. Line breaks within a command example are for editorial purposes only. Line breaks in the protocol are indicated by "CRLF".
3. Simple Mail Notification
With this technique, the server sends the string "nm_notifyuser" immediately followed by CRLF to the finger port on the IP address for the user who has received new mail. The finger port is 79. Note that only the port for finger is used; the finger protocol itself is not used.
The IP address to use may be configured, or the server may use the IP address that was last used to check mail by the user. Typically, this is a per-account configuration option.
On the client system, a process must be listening to the finger port to be useful. When it receives the "nm_notifyuser" string, it takes a configured action, typically instructing a mail client to fetch mail.
Normally, a TCP connection to the target computer is opened, the "nm_notifyuser" string is sent, and the connection is closed without waiting for a response.
In some cases, UDP is used instead of TCP, but the default and general practice is TCP. Even though only a single message in one direction is sent (with no reply), TCP is used most often, probably for reliability.
There is an assumption that the client listening on an IP address only has responsibility for one email account. If a client can check multiple accounts and receives the "nm_notifyuser" string, it does not know which account received the mail.
There is a requirement that a finger daemon not be active on the client.
This example assumes that new mail has arrived at the server mail.isp.example.com for account email@example.com. The server has determined an IP address to which notification should be sent.
C: <listens on TCP port 79>
S: <opens TCP connection to IP address port 79> C: <accepts inbound connection on port 79> S: nm_notifyuserCRLF
S: <closes TCP connection>
5. Security Considerations
There is no assurance that the "nm_notifyuser" message is being sent to the correct IP address. Nor does the listening agent on the client system have any assurance that an "nm_notifyuser" string was sent by a mail server that has received new mail for the user.
It is trivial for an attacker to send large numbers of "nm_notifyuser" messages to a targeted system. Client systems that are listening for this message SHOULD implement protections against being flooded with notifications. Many server systems already implement protections against users logging in and checking mail too frequently.
Because use of this protocol requires that a port be open with an agent listening on it, if that agent contains vulnerabilities, it may create a remotely exploitable attack (for example, buffer overflows that permit an attacker to execute arbitrary code on the client system with the permissions of the agent). As usual, with a process listening on a port, the process should execute with the least possible privilege level and access.
6. IANA Considerations
The notify mail hack (and this document) should be included as an additional usage for port 79.
The NotifyMail shareware utility was written by Scott Gruby.
Randall Gellens QUALCOMM Incorporated 6455 Lusk Blvd. San Diego, CA 92121-2779 USA EMail: firstname.lastname@example.org
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