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ARP(4) Kernel Interfaces Manual ARP(4) NAME arp -- Address Resolution Protocol SYNOPSIS device ether DESCRIPTION The Address Resolution Protocol (ARP) is used to dynamically map be- tween Protocol Addresses (such as IP addresses) and Local Network Ad- dresses (such as Ethernet addresses). This implementation maps IP ad- dresses to Ethernet addresses. It is used by all the Ethernet inter- face drivers. ARP caches Internet-Ethernet address mappings. When an interface re- quests a mapping for an address not in the cache, ARP queues the mes- sage which requires the mapping and broadcasts a message on the associ- ated network requesting the address mapping. If a response is pro- vided, the new mapping is cached and any pending message is transmit- ted. ARP will queue at most net.link.ether.inet.maxhold packets while waiting for a response to a mapping request; only the most recently ``transmitted'' packets are kept. If the target host does not respond after several requests, the host is considered to be down allowing an error to be returned to transmission attempts. Further demand for this mapping causes ARP request retransmissions, that are ratelimited to one packet per second. The error is EHOSTDOWN for a non-responding desti- nation host, and EHOSTUNREACH for a non-responding router. The ARP cache is stored in per-interface link-level table. ARP entries may be added, deleted or changed with the arp(8) utility. Manually-added entries may be temporary or permanent, and may be "published", in which case the system will respond to ARP requests for that host as if it were the target of the request. In the past, ARP was used to negotiate the use of a trailer encapsula- tion. This is no longer supported. ARP watches passively for hosts impersonating the local host (i.e., a host which responds to an ARP mapping request for the local host's ad- dress). Proxy ARP is a feature whereby the local host will respond to requests for addresses other than itself, with its own address. Normally, proxy ARP in FreeBSD is set up on a host-by-host basis using the arp(8) util- ity, by adding an entry for each host inside a given subnet for which proxying of ARP requests is desired. However, the "proxy all" feature causes the local host to act as a proxy for all hosts reachable through some other network interface, different from the one the request came in from. It may be enabled by setting the sysctl(8) MIB variable net.link.ether.inet.proxyall to 1. MIB Variables The ARP protocol implements a number of configurable variables in net.link.ether.inet branch of the sysctl(3) MIB. allow_multicast Install ARP entries with the multicast bit set in the hardware address. Installing such entries is an RFC 1812 violation, but some proprietary load balancing techniques require routers to do so. Turned off by default. garp_rexmit_count Retransmit gratuitous ARP (GARP) packets when an IPv4 address is added to an interface. A GARP is always transmitted when an IPv4 ad- dress is added to an interface. A non-zero value causes the GARP packet to be retrans- mitted the stated number of times. The in- terval between retransmissions is doubled each time, so the retransmission intervals are: {1, 2, 4, 8, 16, ...} (seconds). The default value of zero means only the initial GARP is sent; no additional GARP packets are retransmitted. The maximum value is sixteen. The default behavior of a single GARP packet is usually sufficient. However, a single GARP might be dropped or lost in some circum- stances. This is particularly harmful when a shared address is passed between cluster nodes. Neighbors on the network link might then work with a stale ARP cache and send packets destined for that address to the node that previously owned the address, which might not respond. log_arp_movements Log movements of IP addresses from one hard- ware address to another. See "DIAGNOSTICS" below. Turned on by default. log_arp_permanent_modify Log attempts by a remote host to modify a permanent ARP entry. See "DIAGNOSTICS" be- low. Turned on by default. log_arp_wrong_iface Log attempts to insert an ARP entry on an in- terface when the IP network to which the ad- dress belongs is connected to another inter- face. See "DIAGNOSTICS" below. Turned on by default. max_log_per_second Limit the number of remotely triggered log- ging events to a configured value per second. Default is 1 log message per second. max_age How long an ARP entry is held in the cache until it needs to be refreshed. Default is 1200 seconds. maxhold How many packets to hold in the per-entry output queue while the entry is being re- solved. Default is 16 packets. maxtries Number of retransmits before a host is con- sidered down and an error is returned. De- fault is 5 tries. proxyall Enables ARP proxying. Turned off by default. wait Lifetime of an incomplete ARP entry. Default is 20 seconds. DIAGNOSTICS arp: %x:%x:%x:%x:%x:%x is using my IP address %d.%d.%d.%d on %s! ARP has discovered another host on the local network which responds to map- ping requests for its own Internet address with a different Ethernet address, generally indicating that two hosts are attempting to use the same Internet address. arp: link address is broadcast for IP address %d.%d.%d.%d! ARP re- quested information for a host, and received an answer indicating that the host's ethernet address is the ethernet broadcast address. This indicates a misconfigured or broken device. arp: %d.%d.%d.%d moved from %x:%x:%x:%x:%x:%x to %x:%x:%x:%x:%x:%x on %s ARP had a cached value for the ethernet address of the referenced host, but received a reply indicating that the host is at a new ad- dress. This can happen normally when host hardware addresses change, or when a mobile node arrives or leaves the local subnet. It can also indicate a problem with proxy ARP. This message can only be issued if the sysctl net.link.ether.inet.log_arp_movements is set to 1, which is the system's default behaviour. arpresolve: can't allocate llinfo for %d.%d.%d.%d The route for the referenced host points to a device upon which ARP is required, but ARP was unable to allocate a routing table entry in which to store the host's MAC address. This usually points to a misconfigured routing ta- ble. It can also occur if the kernel cannot allocate memory. arp: %d.%d.%d.%d is on if0 but got reply from %x:%x:%x:%x:%x:%x on if1 Physical connections exist to the same logical IP network on both if0 and if1. It can also occur if an entry already exists in the ARP cache for the IP address above, and the cable has been disconnected from if0, then reconnected to if1. This message can only be issued if the sysctl net.link.ether.inet.log_arp_wrong_iface is set to 1, which is the sys- tem's default behaviour. arp: %x:%x:%x:%x:%x:%x attempts to modify permanent entry for %d.%d.%d.%d on %s ARP has received an ARP reply that attempts to over- write a permanent entry in the local ARP table. This error will only be logged if the sysctl net.link.ether.inet.log_arp_permanent_modify is set to 1, which is the system's default behaviour. arp: %x:%x:%x:%x:%x:%x is multicast Kernel refused to install an entry with multicast hardware address. If you really want such addresses be- ing installed, set the sysctl net.link.ether.inet.allow_multicast to a positive value. SEE ALSO inet(4), route(4), arp(8), ifconfig(8), route(8), sysctl(8) Plummer, D., "RFC826", An Ethernet Address Resolution Protocol. Leffler, S.J. and Karels, M.J., "RFC893", Trailer Encapsulations. FreeBSD 14.3 October 7, 2016 ARP(4)
NAME | SYNOPSIS | DESCRIPTION | MIB Variables | DIAGNOSTICS | SEE ALSO
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