1RAW(7)                     Linux Programmer's Manual                    RAW(7)
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NAME

6       raw, SOCK_RAW - Linux IPv4 raw sockets
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SYNOPSIS

9       #include <sys/socket.h>
10       #include <netinet/in.h>
11       raw_socket = socket(AF_INET, SOCK_RAW, int protocol);
12

DESCRIPTION

14       Raw  sockets  allow new IPv4 protocols to be implemented in user space.
15       A raw socket receives or sends the  raw  datagram  not  including  link
16       level headers.
17
18       The  IPv4 layer generates an IP header when sending a packet unless the
19       IP_HDRINCL socket option is enabled on the socket.  When it is enabled,
20       the  packet  must contain an IP header.  For receiving the IP header is
21       always included in the packet.
22
23       Only processes with an effective user ID of 0 or the CAP_NET_RAW  capa‐
24       bility are allowed to open raw sockets.
25
26       All  packets  or  errors matching the protocol number specified for the
27       raw socket are passed to this socket.  For a list of the allowed proto‐
28       cols see RFC 1700 assigned numbers and getprotobyname(3).
29
30       A  protocol  of  IPPROTO_RAW  implies enabled IP_HDRINCL and is able to
31       send any IP protocol that is specified in the passed header.  Receiving
32       of all IP protocols via IPPROTO_RAW is not possible using raw sockets.
33
34              ┌───────────────────────────────────────────────────┐
35              │IP Header fields modified on sending by IP_HDRINCL 
36              ├──────────────────────┬────────────────────────────┤
37              │IP Checksum           │Always filled in.           │
38              ├──────────────────────┼────────────────────────────┤
39              │Source Address        │Filled in when zero.        │
40              ├──────────────────────┼────────────────────────────┤
41              │Packet Id             │Filled in when zero.        │
42              ├──────────────────────┼────────────────────────────┤
43              │Total Length          │Always filled in.           │
44              └──────────────────────┴────────────────────────────┘
45
46       If  IP_HDRINCL is specified and the IP header has a nonzero destination
47       address then the destination address of the socket is used to route the
48       packet.   When  MSG_DONTROUTE  is  specified,  the  destination address
49       should refer to a local interface, otherwise a routing table lookup  is
50       done anyway but gatewayed routes are ignored.
51
52       If IP_HDRINCL isn't set, then IP header options can be set on raw sock‐
53       ets with setsockopt(2); see ip(7) for more information.
54
55       In Linux 2.2, all IP header fields and options  can  be  set  using  IP
56       socket options.  This means raw sockets are usually only needed for new
57       protocols or protocols with no user interface (like ICMP).
58
59       When a packet is received, it is passed to any raw sockets  which  have
60       been  bound  to its protocol before it is passed to other protocol han‐
61       dlers (e.g., kernel protocol modules).
62
63   Address Format
64       Raw sockets use the standard sockaddr_in address structure  defined  in
65       ip(7).   The  sin_port  field  could be used to specify the IP protocol
66       number, but it is ignored for sending in Linux 2.2 and should be always
67       set to 0 (see BUGS).  For incoming packets, sin_port is set to the pro‐
68       tocol of the packet.  See the <netinet/in.h> include file for valid  IP
69       protocols.
70
71   Socket Options
72       Raw socket options can be set with setsockopt(2) and read with getsock‐
73       opt(2) by passing the IPPROTO_RAW family flag.
74
75       ICMP_FILTER
76              Enable  a  special  filter  for  raw  sockets   bound   to   the
77              IPPROTO_ICMP  protocol.   The  value has a bit set for each ICMP
78              message type which should be filtered out.  The  default  is  to
79              filter no ICMP messages.
80
81       In  addition,  all  ip(7)  IPPROTO_IP socket options valid for datagram
82       sockets are supported.
83
84   Error Handling
85       Errors originating from the network are only passed to  the  user  when
86       the  socket  is  connected or the IP_RECVERR flag is enabled.  For con‐
87       nected sockets, only EMSGSIZE and EPROTO are passed for  compatibility.
88       With IP_RECVERR, all network errors are saved in the error queue.
89

ERRORS

91       EACCES User  tried  to  send  to a broadcast address without having the
92              broadcast flag set on the socket.
93
94       EFAULT An invalid memory address was supplied.
95
96       EINVAL Invalid argument.
97
98       EMSGSIZE
99              Packet too big.  Either  Path  MTU  Discovery  is  enabled  (the
100              IP_MTU_DISCOVER socket flag) or the packet size exceeds the max‐
101              imum allowed IPv4 packet size of 64KB.
102
103       EOPNOTSUPP
104              Invalid flag has been passed to a socket call (like MSG_OOB).
105
106       EPERM  The user doesn't have permission to open raw sockets.  Only pro‐
107              cesses  with  an  effective  user  ID  of  0  or the CAP_NET_RAW
108              attribute may do that.
109
110       EPROTO An ICMP error has arrived reporting a parameter problem.
111

VERSIONS

113       IP_RECVERR and ICMP_FILTER are new in Linux 2.2.  They are Linux exten‐
114       sions and should not be used in portable programs.
115
116       Linux  2.0  enabled  some  bug-to-bug compatibility with BSD in the raw
117       socket code when the SO_BSDCOMPAT socket option was set —  since  Linux
118       2.2, this option no longer has that effect.
119

NOTES

121       By default, raw sockets do path MTU (Maximum Transmission Unit) discov‐
122       ery.  This means the kernel will keep track of the MTU  to  a  specific
123       target  IP  address and return EMSGSIZE when a raw packet write exceeds
124       it.  When this happens, the  application  should  decrease  the  packet
125       size.   Path MTU discovery can be also turned off using the IP_MTU_DIS‐
126       COVER socket option or the /proc/sys/net/ipv4/ip_no_pmtu_disc file, see
127       ip(7) for details.  When turned off, raw sockets will fragment outgoing
128       packets that exceed the interface MTU.  However, disabling  it  is  not
129       recommended for performance and reliability reasons.
130
131       A raw socket can be bound to a specific local address using the bind(2)
132       call.  If it isn't bound, all packets with the  specified  IP  protocol
133       are  received.   In  addition,  a RAW socket can be bound to a specific
134       network device using SO_BINDTODEVICE; see socket(7).
135
136       An IPPROTO_RAW socket is send only.  If you really want to receive  all
137       IP  packets,  use  a packet(7) socket with the ETH_P_IP protocol.  Note
138       that packet sockets don't reassemble IP fragments, unlike raw sockets.
139
140       If you want to receive all ICMP packets for a datagram  socket,  it  is
141       often better to use IP_RECVERR on that particular socket; see ip(7).
142
143       Raw sockets may tap all IP protocols in Linux, even protocols like ICMP
144       or TCP which have a protocol module in the kernel.  In this  case,  the
145       packets  are  passed  to  both the kernel module and the raw socket(s).
146       This should not be relied upon in portable  programs,  many  other  BSD
147       socket implementation have limitations here.
148
149       Linux never changes headers passed from the user (except for filling in
150       some zeroed fields as described for  IP_HDRINCL).   This  differs  from
151       many other implementations of raw sockets.
152
153       RAW  sockets  are  generally rather unportable and should be avoided in
154       programs intended to be portable.
155
156       Sending on raw sockets should take the IP protocol from sin_port;  this
157       ability was lost in Linux 2.2.  The workaround is to use IP_HDRINCL.
158

BUGS

160       Transparent proxy extensions are not described.
161
162       When the IP_HDRINCL option is set, datagrams will not be fragmented and
163       are limited to the interface MTU.
164
165       Setting the IP protocol for sending in sin_port got lost in Linux  2.2.
166       The  protocol that the socket was bound to or that was specified in the
167       initial socket(2) call is always used.
168

SEE ALSO

170       recvmsg(2), sendmsg(2), capabilities(7), ip(7), socket(7)
171
172       RFC 1191 for path MTU discovery.
173
174       RFC 791 and the <linux/ip.h> include file for the IP protocol.
175

COLOPHON

177       This page is part of release 3.25 of the Linux  man-pages  project.   A
178       description  of  the project, and information about reporting bugs, can
179       be found at http://www.kernel.org/doc/man-pages/.
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183Linux                             2008-11-20                            RAW(7)
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