Setting Timing Parameters for IP Masquerade When each new connection is established, the IP masquerade software creates an association in memory between each of the hosts involved in the connection. You can view these associations at any time by looking at the /proc/net/ip_masquerade file. These associations will timeout after a period of inactivity, though. You can set the timeout values using the ipfwadm command. The general syntax for this is: ipfwadm -M -s and for the ipchains command it is: ipchains -M -S The iptables implementation uses much longer default timers and does not allow you to set them. Each of these values represents a timer used by the IP masquerade software and are in units of seconds. The following table summarizes the timers and their meanings: Name Description tcp TCP session timeout. How long a TCP connection may remain idle before the association for it is removed. tcpfin TCP timeout after FIN. How long an association will remain after a TCP connection has been disconnected. udp UDP session timeout. How long a UDP connection may remain idle before the association for it is removed. Handling Name Server Lookups Handling domain name server lookups from the hosts on the LAN with IP masquerading has always presented a problem. There are two ways of accomodating DNS in a masquerade environment. You can tell each of the hosts that they use the same DNS that the Linux router machine does, and let IP masquerade do its magic on their DNS requests. Alternatively, you can run a caching name server on the Linux machine and have each of the hosts on the LAN use the Linux machine as their DNS. Although a more aggressive action, this is probably the better option because it reduces the volume of DNS traffic travelling on the Internet link and will be marginally faster for most requests, since they’ll be served from the cache. The downside to this configuration is that it is more complex. “Caching-only named Configuration” in Chapter 6 describes how to configure a caching name server. More About Network Address Translation The netfilter software is capable of many different types of Network Address Translation. IP Masquerade is one simple application of it. It is possible, for example, to build NAT rules that translate only certain addresses or ranges of addresses and leave all others untouched, or to translate addresses into pools of addresses rather than just a single address, as masquerade does. You can in fact use the iptables command to generate NAT rules that map just about anything, with combinations of matches using any of the standard attributes, such as source address, destination address, protocol type, port number, etc. Translating the Source Address of a datagram is referred to as “Source NAT,” or SNAT, in the netfilter documentation. Translating the Destination Address of a datagram is known as “Destination NAT,” or DNAT. Translating
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