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AUTH_OPTIONS in Sendmail

The authentication methods that Sendmail will employ when connecting to remote mail servers are specified by the auth_options parameter in the Sendmail.mc config file. We can find a set of general tuning parameters affecting authentication in AuthOptions. It is specified as the following:

For configuration file (V8.10 and later): AuthOptions=string

For configuration file (V8.10 and later): -OAuthOptions=string

And for mc configuration (V8.10 and later): define(`confAUTH_OPTIONS’, `string’)

The argument is a list of characters of type string, chosen from the characters listed as below, where each character specifies a unique tuning parameter. If more than one character is listed, we must use a space or comma to separate each character from the next.

AUTH_OPTIONS: Parameters

A: Only once authentication is successful should the MAIL FROM: command’s AUTH= option be used. As a fix, this character can be given for malfunctioning MTAs that improperly carry out RFC2554. (Only the client)

a: Defend the authentication exchange against active attacks. (Only the server)

c: Permit the use of client details only for specific mechanisms (those that are able to pass client details). (Only the server)

d: Allowing the use of technologies vulnerable to passive dictionary attacks should be prohibited. (Only the server)

f: Demand that sessions be kept secret from one another so that breaking one won’t aid in breaking the other. (Only the server)

p: It is advisable to avoid the usage of techniques that are vulnerable to basic passive attacks, such as PLAIN and LOGIN unless a security layer such as STARTTLS is already in place. (Only the server)

T: A’s opposite (client only, pre-V8.12 only)

y: Don’t allow any system to be used that enables invalid login. (Only the server)

Sendmail will skip this option declaration and give the following error if the string is missing:

If a letter is supplied that is not one of the ones in the table, Sendmail will disregard this option declaration and will issue the following warning:

It is not possible to define the character list using macros. Only when sendmail is compiled with SASL specified as true is the AuthOptions option available. It is unsafe to use the AuthOptions option. In the event that it is given on the CLI, sendmail may give up its special rights.

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Conclusion

The article offers a brief note on defining AUTH_OPTIONS in Sendmail.