arp(7)
NAME
arp - Linux ARP kernel module.
DESCRIPTION
This kernel protocol module implements the Address Resolution Protocol defined in RFC 826. It is used to convert between Layer2 hardware addresses and IPv4 protocol addresses on directly connected networks. The user normally doesn't interact directly with this module except to configure it; instead it provides a service for other protocols in the kernel. A user process can receive ARP packets by using packet(7) sockets. There is also a mechanism for managing the ARP cache in user-space by using netlink(7) sockets. The ARP table can also be controlled via ioctl(2) on any AF_INET socket. The ARP module maintains a cache of mappings between hardware addresses and protocol addresses. The cache has a limited size so old and less frequently used entries are garbage-collected. Entries which are marked as permanent are never deleted by the garbage-collector. The cache can be directly manipulated by the use of ioctls and its behavior can be tuned by the /proc interfaces described below. When there is no positive feedback for an existing mapping after some time (see the /proc interfaces below), a neighbor cache entry is considered stale. Positive feedback can be gotten from a higher layer; for example from a successful TCP ACK. Other protocols can signal forward progress using the MSG_CONFIRM flag to sendmsg(2). When there is no forward progress, ARP tries to reprobe. It first tries to ask a local arp daemon app_solicit times for an updated MAC address. If that fails and an old MAC address is known, a unicast probe is sent ucast_solicit times. If that fails too, it will broadcast a new ARP request to the network. Requests are sent only when there is data queued for sending. Linux will automatically add a nonpermanent proxy arp entry when it receives a request for an address it forwards to and proxy arp is enabled on the receiving interface. When there is a reject route for the target, no proxy arp entry is added. Ioctls Three ioctls are available on all AF_INET sockets. They take a pointer to a struct arpreq as their argument. struct arpreq { struct sockaddr arp_pa; /* protocol address */ struct sockaddr arp_ha; /* hardware address */ int arp_flags; /* flags */ struct sockaddr arp_netmask; /* netmask of protocol address */ char arp_dev[16]; }; SIOCSARP, SIOCDARP and SIOCGARP respectively set, delete and get an ARP mapping. Setting and deleting ARP maps are privileged operations and may be performed only by a process with the CAP_NET_ADMIN capability or an effective UID of 0. arp_pa must be an AF_INET address and arp_ha must have the same type as the device which is specified in arp_dev. arp_dev is a zero-terminated string which names a device. arp_flags flag meaning ATF_COM Lookup complete ATF_PERM Permanent entry ATF_PUBL Publish entry ATF_USETRAILERS Trailers requested ATF_NETMASK Use a netmask ATF_DONTPUB Don't answer If the ATF_NETMASK flag is set, then arp_netmask should be valid. Linux 2.2 does not support proxy network ARP entries, so this should be set to 0xffffffff, or 0 to remove an existing proxy arp entry. ATF_USETRAILERS is obsolete and should not be used. /proc interfaces ARP supports a range of /proc interfaces to configure parameters on a global or per-interface basis. The interfaces can be accessed by reading or writing the /proc/sys/net/ipv4/neigh/*/* files. Each interface in the system has its own directory in /proc/sys/net/ipv4/neigh/. The setting in the "default" directory is used for all newly created devices. Unless otherwise specified, time- related interfaces are specified in seconds. anycast_delay (since Linux 2.2) The maximum number of jiffies to delay before replying to a IPv6 neighbor solicitation message. Anycast support is not yet implemented. Defaults to 1 second. app_solicit (since Linux 2.2) The maximum number of probes to send to the user space ARP daemon via netlink before dropping back to multicast probes (see mcast_solicit). Defaults to 0. base_reachable_time (since Linux 2.2) Once a neighbor has been found, the entry is considered to be valid for at least a random value between base_reachable_time/2 and 3*base_reachable_time/2. An entry's validity will be extended if it receives positive feedback from higher level protocols. Defaults to 30 seconds. This file is now obsolete in favor of base_reachable_time_ms. base_reachable_time_ms (since Linux 2.6.12) As for base_reachable_time, but measures time in milliseconds. Defaults to 30000 milliseconds. delay_first_probe_time (since Linux 2.2) Delay before first probe after it has been decided that a neighbor is stale. Defaults to 5 seconds. gc_interval (since Linux 2.2) How frequently the garbage collector for neighbor entries should attempt to run. Defaults to 30 seconds. gc_stale_time (since Linux 2.2) Determines how often to check for stale neighbor entries. When a neighbor entry is considered stale, it is resolved again before sending data to it. Defaults to 60 seconds. gc_thresh1 (since Linux 2.2) The minimum number of entries to keep in the ARP cache. The garbage collector will not run if there are fewer than this number of entries in the cache. Defaults to 128. gc_thresh2 (since Linux 2.2) The soft maximum number of entries to keep in the ARP cache. The garbage collector will allow the number of entries to exceed this for 5 seconds before collection will be performed. Defaults to 512. gc_thresh3 (since Linux 2.2) The hard maximum number of entries to keep in the ARP cache. The garbage collector will always run if there are more than this number of entries in the cache. Defaults to 1024. locktime (since Linux 2.2) The minimum number of jiffies to keep an ARP entry in the cache. This prevents ARP cache thrashing if there is more than one potential mapping (generally due to network misconfiguration). Defaults to 1 second. mcast_solicit (since Linux 2.2) The maximum number of attempts to resolve an address by multicast/broadcast before marking the entry as unreachable. Defaults to 3. proxy_delay (since Linux 2.2) When an ARP request for a known proxy-ARP address is received, delay up to proxy_delay jiffies before replying. This is used to prevent network flooding in some cases. Defaults to 0.8 seconds. proxy_qlen (since Linux 2.2) The maximum number of packets which may be queued to proxy-ARP addresses. Defaults to 64. retrans_time (since Linux 2.2) The number of jiffies to delay before retransmitting a request. Defaults to 1 second. This file is now obsolete in favor of retrans_time_ms. retrans_time_ms (since Linux 2.6.12) The number of milliseconds to delay before retransmitting a request. Defaults to 1000 milliseconds. ucast_solicit (since Linux 2.2) The maximum number of attempts to send unicast probes before asking the ARP daemon (see app_solicit). Defaults to 3. unres_qlen (since Linux 2.2) The maximum number of packets which may be queued for each unresolved address by other network layers. Defaults to 3.
VERSIONS
The struct arpreq changed in Linux 2.0 to include the arp_dev member and the ioctl numbers changed at the same time. Support for the old ioctls was dropped in Linux 2.2. Support for proxy arp entries for networks (netmask not equal 0xffffffff) was dropped in Linux 2.2. It is replaced by automatic proxy arp setup by the kernel for all reachable hosts on other interfaces (when forwarding and proxy arp is enabled for the interface). The neigh/* interfaces did not exist before Linux 2.2.
BUGS
Some timer settings are specified in jiffies, which is architecture- and kernel version-dependent; see time(7). There is no way to signal positive feedback from user space. This means connection-oriented protocols implemented in user space will generate excessive ARP traffic, because ndisc will regularly reprobe the MAC address. The same problem applies for some kernel protocols (e.g., NFS over UDP). This man page mashes together functionality that is IPv4-specific with functionality that is shared between IPv4 and IPv6.
SEE ALSO
capabilities(7), ip(7) RFC 826 for a description of ARP. RFC 2461 for a description of IPv6 neighbor discovery and the base algorithms used. Linux 2.2+ IPv4 ARP uses the IPv6 algorithms when applicable.
COLOPHON
This page is part of release 4.09 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.
Opportunity
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
Free Books
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Education
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.
