IP-Sysctl variables

3 minute read

A list of interesting IP-Sysctl variables.


ICMP configuration

Here is what the documentation says about the ICMP configuration values :

icmp_echo_ignore_all - BOOLEAN

  • If set non-zero, then the kernel will ignore all ICMP ECHO requests sent to it.
  • Default: 0

icmp_echo_ignore_broadcasts - BOOLEAN

  • If set non-zero, then the kernel will ignore all ICMP ECHO and TIMESTAMP requests sent to it via broadcast/multicast.
  • Default: 1

ARP configuration

rp_filter - BOOLEAN

  • 1 - Allows you to have multiple network interfaces on the same subnet, and have the ARPs for each interface be answered based on whether or not the kernel would route a packet from the ARP’d IP out that interface (therefore you must use source based routing for this to work). In other words it allows control of which cards (usually 1) will respond to an arp request.
  • 0 - (default) The kernel can respond to arp requests with addresses from other interfaces. This may seem wrong but it usually makes sense, because it increases the chance of successful communication. IP addresses are owned by the complete host on Linux, not by particular interfaces. Only for more complex setups like load- balancing, does this behaviour cause problems.

arp_filter for the interface will be enabled if at least one of conf/{all,interface}/arp_filter is set to TRUE, it will be disabled otherwise

arp_announce - INTEGER

Define different restriction levels for announcing the local source IP address from IP packets in ARP requests sent on interface:

  • 0 - (default) Use any local address, configured on any interface
  • 1 - Try to avoid local addresses that are not in the target’s subnet for this interface. This mode is useful when target hosts reachable via this interface require the source IP address in ARP requests to be part of their logical network configured on the receiving interface. When we generate the request we will check all our subnets that include the target IP and will preserve the source address if it is from such subnet. If there is no such subnet we select source address according to the rules for level 2.
  • 2 - Always use the best local address for this target. In this mode we ignore the source address in the IP packet and try to select local address that we prefer for talks with the target host. Such local address is selected by looking for primary IP addresses on all our subnets on the outgoing interface that include the target IP address. If no suitable local address is found we select the first local address we have on the outgoing interface or on all other interfaces, with the hope we will receive reply for our request and even sometimes no matter the source IP address we announce.

The max value from conf/{all,interface}/arp_announce is used.

Increasing the restriction level gives more chance for receiving answer from the resolved target while decreasing the level announces more valid sender’s information.

arp_ignore - INTEGER

Define different modes for sending replies in response to received ARP requests that resolve local target IP addresses:

  • 0 - (default): reply for any local target IP address, configured on any interface
  • 1 - reply only if the target IP address is local address configured on the incoming interface
  • 2 - reply only if the target IP address is local address configured on the incoming interface and both with the sender’s IP address are part from same subnet on this interface
  • 3 - do not reply for local addresses configured with scope host, only resolutions for global and link addresses are replied
  • 4-7 - reserved
  • 8 - do not reply for all local addresses

The max value from conf/{all,interface}/arp_ignore is used when ARP request is received on the {interface}

arp_notify - BOOLEAN

Define mode for notification of address and device changes.

  • 0 - (default): do nothing
  • 1 - Generate gratuitous arp requests when device is brought up or hardware address changes.

arp_accept - BOOLEAN

Define behavior for gratuitous ARP frames who’s IP is not already present in the ARP table:

  • 0 - don’t create new entries in the ARP table
  • 1 - create new entries in the ARP table

Both replies and requests type gratuitous arp will trigger the ARP table to be updated, if this setting is on.

If the ARP table already contains the IP address of the gratuitous arp frame, the arp table will be updated regardless if this setting is on or off.