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1 perry 1 ARP
2 !!!ARP
3 NAME
4 DESCRIPTION
5 IOCTLS
6 SYSCTLS
7 BUGS
8 VERSIONS
9 SEE ALSO
10 ----
11 !!NAME
12
13
14 arp - Linux ARP kernel module.
15 !!DESCRIPTION
16
17
18 This kernel protocol module implements the Address
19 Resolution Protocol defined in RFC 826. It is used to
20 convert between Layer2 hardware addresses and IPv4 protocol
21 addresses on directly connected networks. The user normally
22 doesn't interact directly with this module except to
23 configure it; instead it provides a service for other
24 protocols in the kernel.
25
26
27 A user process can receive ARP packets by using
28 packet(7) sockets. There is also a mechanism for
29 managing the ARP cache in user-space by using
30 netlink(7) sockets. The ARP table can also be
31 controlled via __ioctl (2)__ on any __PF_INET__
32 socket.
33
34
35 The ARP module maintains a cache of mappings between
36 hardware addresses and protocol addresses. The cache has a
37 limited size so old and less frequently used entries are
38 garbage-collected. Entries which are marked as permanent are
39 never deleted by the garbage-collector. The cache can be
40 directly manipulated by the use of ioctls and its behaviour
41 can be tuned by the sysctls defined below.
42
43
44 When there is no positive feedback for an existing mapping
45 after some time (see the sysctls below) a neighbour cache
46 entry is considered stale. Positive feedback can be gotten
47 from a higher layer; for example from a successful TCP ACK.
48 Other protocols can signal forward progress using the
49 __MSG_CONFIRM__ flag to sendmsg(2). When there is
50 no forward progress ARP tries to reprobe. It first tries to
51 ask a local arp daemon __app_solicit__ times for an
52 updated MAC address. If that fails and an old MAC address is
53 known an unicast probe is send __ucast_solicit__ times.
54 If that fails too it will broadcast a new ARP request to the
55 network. Requests are only send when there is data queued
56 for sending.
57
58
59 Linux will automatically add a non-permanent proxy arp entry
60 when it receives a request for an address it forwards to and
61 proxy arp is enabled on the receiving interface. When there
62 is a reject route for the target no proxy arp entry is
63 added.
64 !!IOCTLS
65
66
67 Three ioctls are available on all __PF_INET__ sockets.
68 They take a pointer to a __struct arpreq__ as their
69 parameter.
70
71
72 struct arpreq
73 {
74 struct sockaddr arp_pa; /* protocol address */
75 struct sockaddr arp_ha; /* hardware address */
76 int arp_flags; /* flags */
77 struct sockaddr arp_netmask; /* netmask of protocol address */
78 char arp_dev[[16];
79 };
80 __SIOCSARP__, __SIOCDARP__ and __SIOCGARP__ respectively set, delete and get an ARP mapping. Setting __CAP_NET_ADMIN__ capability or an effective UID of 0.
81
82
83 ''arp_pa'' must be an __AF_INET__ socket and
84 ''arp_ha'' must have the same type as the device which is
85 specified in ''arp_dev''. ''arp_dev'' is a
86 zero-terminated string which names a device.
87
88
89 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.
90 !!SYSCTLS
91
92
93 ARP supports a sysctl interface to configure parameters on a
94 global or per-interface basis. The sysctls can be accessed
95 by reading or writing the
96 __/proc/sys/net/ipv4/neigh/*/*__ files or with the
97 sysctl(2) interface. Each interface in the system has
98 its own directory in /proc/sys/net/ipv4/neigh/. The setting
99 in the `default' directory is used for all newly created
100 devices. Unless otherwise specified time related sysctls are
101 specified in seconds.
102
103
104 __anycast_delay__
105
106
107 The maximum number of jiffies to delay before replying to a
108 IPv6 neighbour solicitation message. Anycast support is not
109 yet implemented. Defaults to 1 second.
110
111
112 __app_solicit__
113
114
115 The maximum number of probes to send to the user space ARP
116 daemon via netlink before dropping back to multicast probes
117 (see ''mcast_solicit''). Defaults to 0.
118
119
120 __base_reachable_time__
121
122
123 Once a neighbour has been found, the entry is considered to
124 be valid for at least a random value between
125 ''base_reachable_time''/2 and
126 3*''base_reachable_time''/2. An entry's validity will be
127 extended if it receives positive feedback from higher level
128 protocols. Defaults to 30 seconds.
129
130
131 __delay_first_probe_time__
132
133
134 Delay before first probe after it has been decided that a
135 neighbour is stale. Defaults to 5 seconds.
136
137
138 __gc_interval__
139
140
141 How frequently the garbage collector for neighbour entries
142 should attempt to run. Defaults to 30 seconds.
143
144
145 __gc_stale_time__
146
147
148 Determines how often to check for stale neighbour entries.
149 When a neighbour entry is considered stale it is resolved
150 again before sending data to it. Defaults to 60
151 seconds.
152
153
154 __gc_thresh1__
155
156
157 The minimum number of entries to keep in the ARP cache. The
158 garbage collector will not run if there are fewer than this
159 number of entries in the cache. Defaults to
160 128.
161
162
163 __gc_thresh2__
164
165
166 The soft maximum number of entries to keep in the ARP cache.
167 The garbage collector will allow the number of entries to
168 exceed this for 5 seconds before collection will be
169 performed. Defaults to 512.
170
171
172 __gc_thresh3__
173
174
175 The hard maximum number of entries to keep in the ARP cache.
176 The garbage collector will always run if there are more than
177 this number of entries in the cache. Defaults to
178 1024.
179
180
181 __locktime__
182
183
184 The minimum number of jiffies to keep an ARP entry in the
185 cache. This prevents ARP cache thrashing if there is more
186 than one potential mapping (generally due to network
187 misconfiguration). Defaults to 1 second.
188
189
190 __mcast_solicit__
191
192
193 The maximum number of attempts to resolve an address by
194 multicast/broadcast before marking the entry as unreachable.
195 Defaults to 3.
196
197
198 __proxy_delay__
199
200
201 When an ARP request for a known proxy-ARP address is
202 received, delay up to ''proxy_delay'' jiffies before
203 replying. This is used to prevent network flooding in some
204 cases. Defaults to 0.8 seconds.
205
206
207 __proxy_qlen__
208
209
210 The maximum number of packets which may be queued to
211 proxy-ARP addresses. Defaults to 64.
212
213
214 __retrans_time__
215
216
217 The number of jiffies to delay before retransmitting a
218 request. Defaults to 1 second.
219
220
221 __ucast_solicit__
222
223
224 The maximum number of attempts to send unicast probes before
225 asking the ARP daemon (see ''app_solicit''). Defaults to
226 3.
227
228
229 __unres_qlen__
230
231
232 The maximum number of packets which may be queued for each
233 unresolved address by other network layers. Defaults to
234 3.
235 !!BUGS
236
237
238 Some timer settings are specified in jiffies, which is
239 architecture related. On the Alpha a jiffy is 1/1024 of a
240 second, on most other architectures it is
241 1/100s.
242
243
244 There is no way to signal positive feedback from user space.
245 This means connection oriented protocols implemented in user
246 space will generate excessive ARP traffic, because ndisc
247 will regularly reprobe the MAC address. The same problem
248 applies for some kernel protocols (e.g. NFS over
249 UDP).
250
251
252 This man page mashes IPv4 specific and shared between IPv4
253 and IPv6 functionality together.
254 !!VERSIONS
255
256
257 The __struct arpreq__ changed in Linux 2.0 to include the
258 ''arp_dev'' member and the ioctl numbers changed at the
259 same time. Support for the old ioctls was dropped in Linux
260 2.2.
261
262
263 Support for proxy arp entries for networks (netmask not
264 equal 0xffffffff) was dropped in Linux 2.2. It is replaced
265 by automatic proxy arp setup by the kernel for all reachable
266 hosts on other interfaces (when forwarding and proxy arp is
267 enabled for the interface).
268
269
270 The neigh/* sysctls did not exist before Linux
271 2.2.
272 !!SEE ALSO
273
274
275 ip(7)
276
277
278 RFC826 for a description of ARP.
279 RFC2461 for a description of IPv6 neighbour discovery and
280 the base algorithms used.
281
282
283 Linux 2.2+ IPv4 ARP uses the IPv6 algorithms when
284 applicable.
285 ----
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