This chapter will go through each and one of the IPv4 variables possible to set via sysctl or the proc filesystem. You will be provided with a basic explanation on what behaviour the variable will change and how, as well as default behaviour, if possible, and what values the variable may be set to. We will not go into any deeper discussion about why each variable should be changed unless there are any very normal reasons to change the values. The structure used within this reference chapter will follow the same structure as the structure used within ipsysctl structure, as well as the default ipv4 directory being further structured due to its large size and mix of many different variables.
This list contains all of the variables available in a standard 2.4.x kernel that pertains to the IP settings. As you will see, there is a huge set of them, and some should be properly set from the beginning for you, and others may not be so properly set. Most of them should look quite proper, however, some do require some extra configuration depending on your needs, but most should be decently set for you as is.
The ip_default_ttl variable tells the kernel what Time To Live to set as default on packets that leaves this host. This tells how long the packets may live on the internet before they are dropped. Each time the packet passes a router, firewall, computer, etcetera, the TTL is decremented with one step.
The default value for ip_default_ttl is 64, which is a fairly good TTL which will not cause too much trouble. It is very unlikely to time out in transit to the host in question. This variable takes an unsigned integer, but the actual TTL field is only 8 bit long. The value may in other words be as high as 255 and as low as 0, however 255 could be considered rude and 0 wouldn’t leave your computer at all. 64 is a good value, unless you are trying to connect to computers extremely far away counted in hops or jumps. These would then time out. As it looks today, I have pretty much never seen a host that lives more than 30 hops away on the internet, so I don’t think there is any need to make this value higher than the default value for now.
Setting the TTL to 255 would be considered rude since this would make a packet live an extremely long time on the internet. If there would be a glitch in 2 routers, this packet could bounce back and forth for a huge amount of time, eating away on the bandwidth without any reason at all. Normally, don’t set this value higher than 100 or something alike.
The ip_dynaddr variable is used to allow a few problems with dynamic addressing to be fixed. This allows diald oneshot connections to get established by dynamically changing packet source address, and sockets if local processes. This option was implemented for TCP diald-box connections and Masquerading connections. Masquerading will in other words work 100% with this option, letting Masquerading switch source adress of packets if the boxes own address change.
This option takes an integer, but only makes use of 3 possible states, 0, 1 or 2.
0 means that this option is turned off, which is also the default behaviour.
1 means that the option is enabled and running.
Any non 0 or 1 values means that we have turned on verbose mode, which in turn will add extra debugging messages that you may use to get things to work properly.
If this variable is turned on and forwarding interface changes, this is what may happen
Socket and packet source address is rewritten on retransmissions while in SYN_SENT state. This is the diald-box processes.
Outbound masqueraded source address changes on output, when internal host does retransmission, until a packet from the outside is received by the tunnel.
This is especially helpful for auto-dialup links (diald), where the actual outgoing address is unknown at the moment the link is going up. This enables the same, local and masqueraded, connection requests that brought the link up to actually establish their connections. This means that we will not have to first issue an connection request just to bring the connection up, and then have to issue the “real” connection request when we have actually established the connection.
The ip_forward variable is used to turn IP forwarding on or off. This means that we can turn off the functions for forwarding packets between interfaces, which lets the computer act as a firewall, or router. Note that this is an extremely important variable for Network Address Translation, firewalling, routing, masquerading, and all other things where we actually let packets through the box to another network, as you can understand.
This is an boolean variable. In other words, it will take a 1 or a 0. The default value for this variable is 0, or disabled. As you can understand, 0 means disabled and 1 means enabled.
Note that this is an very special variable since it will reset all configuration parameters to their default states if it is changed. For a complete list of the exact states, look closer at RFC1122 for hosts and RFC1812 for routers.
The ip_local_port_range variable consists of two integers which tells the kernel which ports to use for client connections. This means, all connections going from our box to some other box and where we are the client. The first port is the lower bound and the second one is the upper bound.
The default value in this variable depends on how much memory you have. If you have more than 128 megabytes of physical memory, the lower bound will be 32768 and the upper bound will be 61000. If the computer has less than 128 megabytes of physical memory, the lower bound will be 1024 and the upper bound will be 4999, or even less.
This number defines the possible active connections which this system can issue simultaneously (ie, at the same time) to other systems that does not support the TCP extension timestamps.
If you have tcp_tw_recycle enabled (the default behaviour) range 1024-4999 is enough to issue up to 2000 connections per second to systems supporting timestamps. In other words, this should be more than enough for most of us.
The ip_no_pmtu_disc disables PMTU (Path Maximum Transfer Unit) discovery if enabled. In most cases this is good, so it is per default set to FALSE (ie, Path Maximum Transfer Unit is used). However, in some cases this is bad and may lead to broken connectivity. If you are experiencing problems like this, you should turn this option off and set your MTU to a reasonable value yourself.
Do note that MTU and PMTU are two different things. MTU tells the kernel the maximum transfer unit for our connection, but not over the whole connection to the other end. PMTU discovery tries to discover the maximum transfer unit to specific hosts, including all the intermediate hops on the way there.
The default value is that the ip_no_pmtu_disc is FALSE, as already stated. If this is set to TRUE, PMTU discovery is turned off. The ip_no_pmtu_disc takes a boolean value, in other words either an 1 or a 0, where 1 is on and 0 is off.
The ip_nonlocal_bind variable allows us to set if local processes should be able to bind to non-local IP addresses. This could be quite useful, in such cases where we want specific programs or applications to be able to listen to non-local IP adresses, such as sniffing for traffic to a specific host which may commit bad things, etcetera. The variable may, however, break some applications and they will no longer work.
The ip_nonlocal_bind variable takes a boolean value which can be set to 1 or 0. If the variable is set to 0, this option is turned off and if it is set to 1 it is turned on. The default value is to turn this option off, or 0 in other words.
The ipfrag_high_thresh tells the kernel the maximum amount of memory to use to reassemble IP fragments. When and if the high threshold is reached, the fragment handler will toss all packets until the memory usage reaches ipfrag_low_thresh instead. This means that all fragments that reached us during this time will have to be retransmitted.
Packets are fragmented if they are too large to pass through a certain pipe. If they are to large, the box that is trying to transmit them breaks them down into smaller pieces and send each piece one by one. When these fragments reaches their destination, they need to be defragmented (ie, put together again) to be read properly. Note that IP Fragmentation are in general a good thing, but there are a lot of people that do bad things with them since fragments are inherently a security problem.
The ipfrag_high_thresh variable takes an integer value, which would mean 0 through 2147483647 bytes can be assigned to be the upper limit of this function. The default value is 262144 bytes, or 256 kilobytes, which should work well in even the most extreme cases.
This option has a lot to do with the ipfrag_high_thresh option. The ipfrag_low_thresh is the lower limit at which packets should start being assembled again. What this means, all in all, is that our fragmentation handler has an queue that grows larger the more packets are waiting in the queue to be defragmentized, when this queue grows to ipfrag_high_thresh byte size, the fragmentation handler queue will stop queueing any further fragments until we reach the ipfrag_low_thresh again. This stops our system from being overloaded with fragmentized packets and may stop certain Denial of Service attacks.
This variable takes an integer value between 0 and 2147483647, and refers to the amount of bytes used at which the fragmentation handler should resume the receiving of IP fragments again. Per default it is set to 196608 bytes, or 192 kilobytes which should be a reasonable amount of memory set aside for this task even in the hardest of attacks. This value should be lower than ipfrag_high_thresh, or else it will be invalid.
The ipfrag_time variable tells the IP fragmentation handler how long to keep an IP fragment in memory, counted in seconds. This only refers to fragments that has been impossible to reassemble since fragments that has been assembled most probably has already been sent on to either the next layer, or to the next host.
The ipfrag_time variable takes an integer as its input and the value is counted as seconds. In other words, if you input 5 to this variable, it counts as 5 seconds.