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Recipe 2.13 Prohibiting Outgoing Telnet Connections
2.13.1 Problem
You want to block outgoing Telnet
connections.
2.13.2 Solution
To block all outgoing Telnet connections:
For
iptables:
# iptables -A OUTPUT -p tcp --dport telnet -j REJECT
For ipchains:
# ipchains -A output -p tcp --dport telnet -j REJECT
To block all outgoing Telnet connections except to yourself from
yourself:For iptables:
# iptables -A OUTPUT -p tcp -o lo --dport telnet -j ACCEPT
# iptables -A OUTPUT -p tcp --dport telnet -j REJECT
For ipchains:
# ipchains -A output -p tcp -i lo --dport telnet -j ACCEPT
# ipchains -A output -p tcp --dport telnet -j REJECT
2.13.3 Discussion
Telnet is notoriously insecure in its most
common form, which transmits your login name and password in
plaintext over the network. This recipe is a sneaky way to encourage
your users to find a more secure alternative, such as
ssh. (Unless your users are running Telnet in a
secure fashion with Kerberos authentication. [Recipe 4.15])
2.13.4 See Also
iptables(8), ipchains(8), telnet(1).
