Linux Security Cookbook

2.9. Blocking Outgoing Access to All Web Servers on a Network

Problem

You want to prevent outgoing access to a network, e.g., all web servers at yahoo.com.

Solution

Figure out how to specify the yahoo.com network, e.g., 64.58.76.0/24, and reject web access:

For iptables :

# iptables -A OUTPUT -p tcp -d 64.58.76.0/24 --dport www -j REJECT

For ipchains :

# ipchains -A output -p tcp -d 64.58.76.0/24 --dport www -j REJECT

Discussion

Here the network is specified using Classless InterDomain Routing (CIDR) mask format, a.b.c.d/N, where N is the number of bits in the netmask. In this case, N=24, so the first 24 bits are the network portion of the address.

See Also

iptables(8), ipchains(8).

Tip

You can supply hostnames instead of IP addresses in your firewall rules. If DNS reports multiple IP addresses for that hostname, a separate rule will be created for each IP address. For example, www.yahoo.com has (at this writing) 11 IP addresses:

$ host www.yahoo.com
www.yahoo.com is an alias for www.yahoo.akadns.net.
www.yahoo.akadns.net has address 216.109.125.68
www.yahoo.akadns.net has address 64.58.76.227
...

So you could block access to Yahoo, for example, and view the results by:

iptables:

# iptables -A OUTPUT -d www.yahoo.com -j REJECT
# iptables -L OUTPUT

ipchains:

# ipchains -A output -d www.yahoo.com -j REJECT
# ipchains -L output

Security experts recommend that you use only IP addresses in your rules, not hostnames, since an attacker could poison your DNS and circumvent rules defined for hostnames. ...