White Papers

Denying and Permitting Access to a Terminal Line
Dell Networking recommends applying only standard access control lists (ACLs) to deny and permit access to VTY lines.
● Layer 3 ACLs deny all traffic that is not explicitly permitted, but in the case of VTY lines, an ACL with no rules does not deny
traffic.
● You cannot use the show ip accounting access-list command to display the contents of an ACL that is applied
only to a VTY line.
● When you use the access-class access-list-name command without specifying the ipv4 or ipv6 attribute, both
IPv4 as well as IPv6 rules that are defined in that ACL are applied to the terminal. This is a generic way of configuring access
restrictions.
● To be able to filter access exclusively using either IPv4 or IPv6 rules, you must use either the ipv4 or ipv6 attribute along
with the access-class access-list-name command. Depending on the attribute that you specify (ipv4 or ipv6),
the ACL processes either IPv4 or IPv6 rules, but not both. Using this configuration, you can set up two different types of
access classes with each class processing either IPv4 or IPv6 rules separately.
To apply an IP ACL to a line, Use the following command.
● Apply an ACL to a VTY line.
LINE mode
ip access-class access-list [ipv4 | ipv6]
NOTE: If you already have configured generic IP ACL on a terminal line, then you cannot further apply IPv4 or IPv6
specific filtering on top of this configuration. Similarly, if you have configured either IPv4 or IPv6 specific filtering on a
terminal line, you cannot apply generic IP ACL on top of this configuration. Before applying any of these configurations,
you must first undo the existing configuration using the no access-class access-list-name [ipv4 | ipv6]
command.
To view the configuration, use the show config command in LINE mode.
Dell(config-std-nacl)#show config
!
ip access-list standard myvtyacl
seq 5 permit host 10.11.0.1
Dell(config-std-nacl)#line vty 0
Dell(config-line-vty)#show config
line vty 0
access-class myvtyacl
Dell OS Behavior: Prior to Dell OS version 7.4.2.0, in order to deny access on a VTY line, apply an ACL and accounting,
authentication, and authorization (AAA) to the line. Then users are denied access only after they enter a username and
password. Beginning in Dell OS version 7.4.2.0, only an ACL is required, and users are denied access before they are prompted
for a username and password.
Configuring Login Authentication for Terminal Lines
You can use any combination of up to six authentication methods to authenticate a user on a terminal line.
A combination of authentication methods is called a method list. If the user fails the first authentication method, the
system prompts the next method until all methods are exhausted, at which point the connection is terminated. The available
authentication methods are:
enable
Prompt for the enable password.
line
Prompt for the password you assigned to the terminal line. Configure a password for the terminal line to
which you assign a method list that contains the line authentication method. Configure a password using
the password command from LINE mode.
local
Prompt for the system username and password.
none
Do not authenticate the user.
radius
Prompt for a username and password and use a RADIUS server to authenticate.
tacacs+
Prompt for a username and password and use a TACACS+ server to authenticate.
76 Management