.\" Copyright (c) 2016 Pavel Emelyanov <xemul@virtuozzo.com>
.\" Copyright (c) 2016 Dmitry V. Levin <ldv@altlinux.org>
.\"
.\" SPDX-License-Identifier: GPL-2.0-or-later
.TH sock_diag 7 2024-06-15 "Linux man-pages (unreleased)"
.SH NAME
sock_diag \- obtaining information about sockets
.SH SYNOPSIS
.nf
.B #include <sys/socket.h>
.B #include <linux/sock_diag.h>
.BR "#include <linux/unix_diag.h>" " /* for UNIX domain sockets */"
.BR "#include <linux/inet_diag.h>" " /* for IPv4 and IPv6 sockets */"
.P
.BI "diag_socket = socket(AF_NETLINK, " socket_type ", NETLINK_SOCK_DIAG);"
.fi
.SH DESCRIPTION
The sock_diag netlink subsystem provides a mechanism for obtaining
information about sockets of various address families from the kernel.
This subsystem can be used to obtain information about individual
sockets or request a list of sockets.
.P
In the request, the caller can specify additional information it would
like to obtain about the socket, for example, memory information or
information specific to the address family.
.P
When requesting a list of sockets, the caller can specify filters that
would be applied by the kernel to select a subset of sockets to report.
For now, there is only the ability to filter sockets by state (connected,
listening, and so on.)
.P
Note that sock_diag reports only those sockets that have a name;
that is, either sockets bound explicitly with
.BR bind (2)
or sockets that were automatically bound to an address (e.g., by
.BR connect (2)).
This is the same set of sockets that is available via
.IR /proc/net/unix ,
.IR /proc/net/tcp ,
.IR /proc/net/udp ,
and so on.
.\"
.SS Request
The request starts with a
.I "struct nlmsghdr"
header described in
.BR netlink (7)
with
.I nlmsg_type
field set to
.BR SOCK_DIAG_BY_FAMILY .
It is followed by a header specific to the address family that starts with
a common part shared by all address families:
.P
.in +4n
.EX
struct sock_diag_req {
    __u8 sdiag_family;
    __u8 sdiag_protocol;
};
.EE
.in
.P
The fields of this structure are as follows:
.TP
.I sdiag_family
An address family.
It should be set to the appropriate
.B AF_*
constant.
.TP
.I sdiag_protocol
Depends on
.IR sdiag_family .
It should be set to the appropriate
.B IPPROTO_*
constant for
.B AF_INET
and
.BR AF_INET6 ,
and to 0 otherwise.
.P
If the
.I nlmsg_flags
field of the
.I "struct nlmsghdr"
header has the
.B NLM_F_DUMP
flag set, it means that a list of sockets is being requested;
otherwise it is a query about an individual socket.
.\"
.SS Response
The response starts with a
.I "struct nlmsghdr"
header and is followed by an array of objects specific to the address family.
The array is to be accessed with the standard
.B NLMSG_*
macros from the
.BR netlink (3)
API.
.P
Each object is the NLA (netlink attributes) list that is to be accessed
with the
.B RTA_*
macros from
.BR rtnetlink (3)
API.
.\"
.SS UNIX domain sockets
For UNIX domain sockets the request is represented in the following structure:
.P
.in +4n
.EX
struct unix_diag_req {
    __u8    sdiag_family;
    __u8    sdiag_protocol;
    __u16   pad;
    __u32   udiag_states;
    __u32   udiag_ino;
    __u32   udiag_show;
    __u32   udiag_cookie[2];
};
.EE
.in
.P
The fields of this structure are as follows:
.TP
.I sdiag_family
The address family; it should be set to
.BR AF_UNIX .
.P
.I sdiag_protocol
.PD 0
.TP
.PD
.I pad
These fields should be set to 0.
.TP
.I udiag_states
This is a bit mask that defines a filter of sockets states.
Only those sockets whose states are in this mask will be reported.
Ignored when querying for an individual socket.
Supported values are:
.P
.RS 12
1 <<
.B TCP_ESTABLISHED
.P
1 <<
.B TCP_LISTEN
.RE
.TP
.I udiag_ino
This is an inode number when querying for an individual socket.
Ignored when querying for a list of sockets.
.TP
.I udiag_show
This is a set of flags defining what kind of information to report.
Each requested kind of information is reported back as a netlink
attribute as described below:
.RS
.TP
.B UDIAG_SHOW_NAME
The attribute reported in answer to this request is
.BR UNIX_DIAG_NAME .
The payload associated with this attribute is the pathname to which
the socket was bound (a sequence of bytes up to
.B UNIX_PATH_MAX
length).
.TP
.B UDIAG_SHOW_VFS
The attribute reported in answer to this request is
.BR UNIX_DIAG_VFS .
The payload associated with this attribute is represented in the following
structure:
.IP
.in +4n
.EX
struct unix_diag_vfs {
    __u32 udiag_vfs_dev;
    __u32 udiag_vfs_ino;
};
.EE
.in
.IP
The fields of this structure are as follows:
.RS
.TP
.I udiag_vfs_dev
The device number of the corresponding on-disk socket inode.
.TP
.I udiag_vfs_ino
The inode number of the corresponding on-disk socket inode.
.RE
.TP
.B UDIAG_SHOW_PEER
The attribute reported in answer to this request is
.BR UNIX_DIAG_PEER .
The payload associated with this attribute is a __u32 value
which is the peer's inode number.
This attribute is reported for connected sockets only.
.TP
.B UDIAG_SHOW_ICONS
The attribute reported in answer to this request is
.BR UNIX_DIAG_ICONS .
The payload associated with this attribute is an array of __u32 values
which are inode numbers of sockets that has passed the
.BR connect (2)
call, but hasn't been processed with
.BR accept (2)
yet.
This attribute is reported for listening sockets only.
.TP
.B UDIAG_SHOW_RQLEN
The attribute reported in answer to this request is
.BR UNIX_DIAG_RQLEN .
The payload associated with this attribute is represented in the following
structure:
.IP
.in +4n
.EX
struct unix_diag_rqlen {
    __u32 udiag_rqueue;
    __u32 udiag_wqueue;
};
.EE
.in
.IP
The fields of this structure are as follows:
.RS
.TP
.I udiag_rqueue
For listening sockets:
the number of pending connections.
The length of the array associated with the
.B UNIX_DIAG_ICONS
response attribute is equal to this value.
.IP
For established sockets:
the amount of data in incoming queue.
.TP
.I udiag_wqueue
For listening sockets:
the backlog length which equals to the value passed as the second argument to
.BR listen (2).
.IP
For established sockets:
the amount of memory available for sending.
.RE
.TP
.B UDIAG_SHOW_MEMINFO
The attribute reported in answer to this request is
.BR UNIX_DIAG_MEMINFO .
The payload associated with this attribute is an array of __u32 values
described below in the subsection "Socket memory information".
.P
The following attributes are reported back without any specific request:
.TP
.B UNIX_DIAG_SHUTDOWN
The payload associated with this attribute is __u8 value which represents
bits of
.BR shutdown (2)
state.
.RE
.TP
.I udiag_cookie
This is an array of opaque identifiers that could be used along with
.I udiag_ino
to specify an individual socket.
It is ignored when querying for a list
of sockets, as well as when all its elements are set to \-1.
.P
The response to a query for UNIX domain sockets is represented as an array of
.P
.in +4n
.EX
struct unix_diag_msg {
    __u8    udiag_family;
    __u8    udiag_type;
    __u8    udiag_state;
    __u8    pad;
    __u32   udiag_ino;
    __u32   udiag_cookie[2];
};
.EE
.in
.P
followed by netlink attributes.
.P
The fields of this structure are as follows:
.TP
.I udiag_family
This field has the same meaning as in
.IR "struct unix_diag_req" .
.TP
.I udiag_type
This is set to one of
.BR SOCK_PACKET ,
.BR SOCK_STREAM ,
or
.BR SOCK_SEQPACKET .
.TP
.I udiag_state
This is set to one of
.B TCP_LISTEN
or
.BR TCP_ESTABLISHED .
.TP
.I pad
This field is set to 0.
.TP
.I udiag_ino
This is the socket inode number.
.TP
.I udiag_cookie
This is an array of opaque identifiers that could be used in subsequent
queries.
.\"
.SS IPv4 and IPv6 sockets
For IPv4 and IPv6 sockets,
the request is represented in the following structure:
.P
.in +4n
.EX
struct inet_diag_req_v2 {
    __u8    sdiag_family;
    __u8    sdiag_protocol;
    __u8    idiag_ext;
    __u8    pad;
    __u32   idiag_states;
    struct inet_diag_sockid id;
};
.EE
.in
.P
where
.I "struct inet_diag_sockid"
is defined as follows:
.P
.in +4n
.EX
struct inet_diag_sockid {
    __be16  idiag_sport;
    __be16  idiag_dport;
    __be32  idiag_src[4];
    __be32  idiag_dst[4];
    __u32   idiag_if;
    __u32   idiag_cookie[2];
};
.EE
.in
.P
The fields of
.I "struct inet_diag_req_v2"
are as follows:
.TP
.I sdiag_family
This should be set to either
.B AF_INET
or
.B AF_INET6
for IPv4 or IPv6 sockets respectively.
.TP
.I sdiag_protocol
This should be set to one of
.BR IPPROTO_TCP ,
.BR IPPROTO_UDP ,
or
.BR IPPROTO_UDPLITE .
.TP
.I idiag_ext
This is a set of flags defining what kind of extended information to report.
Each requested kind of information is reported back as a netlink attribute
as described below:
.RS
.TP
.B INET_DIAG_TOS
The payload associated with this attribute is a __u8 value
which is the TOS of the socket.
.TP
.B INET_DIAG_TCLASS
The payload associated with this attribute is a __u8 value
which is the TClass of the socket.
IPv6 sockets only.
For LISTEN and CLOSE sockets, this is followed by
.B INET_DIAG_SKV6ONLY
attribute with associated __u8 payload value meaning whether the socket
is IPv6-only or not.
.TP
.B INET_DIAG_MEMINFO
The payload associated with this attribute is represented in the following
structure:
.IP
.in +4n
.EX
struct inet_diag_meminfo {
    __u32 idiag_rmem;
    __u32 idiag_wmem;
    __u32 idiag_fmem;
    __u32 idiag_tmem;
};
.EE
.in
.IP
The fields of this structure are as follows:
.RS
.TP 12
.I idiag_rmem
The amount of data in the receive queue.
.TP
.I idiag_wmem
The amount of data that is queued by TCP but not yet sent.
.TP
.I idiag_fmem
The amount of memory scheduled for future use (TCP only).
.TP
.I idiag_tmem
The amount of data in send queue.
.RE
.TP
.B INET_DIAG_SKMEMINFO
The payload associated with this attribute is an array of __u32 values
described below in the subsection "Socket memory information".
.TP
.B INET_DIAG_INFO
The payload associated with this attribute is specific to the address family.
For TCP sockets, it is an object of type
.IR "struct tcp_info" .
.TP
.B INET_DIAG_CONG
The payload associated with this attribute is a string that describes the
congestion control algorithm used.
For TCP sockets only.
.RE
.TP
.I pad
This should be set to 0.
.TP
.I idiag_states
This is a bit mask that defines a filter of socket states.
Only those sockets whose states are in this mask will be reported.
Ignored when querying for an individual socket.
.TP
.I id
This is a socket ID object that is used in dump requests, in queries
about individual sockets, and is reported back in each response.
Unlike UNIX domain sockets, IPv4 and IPv6 sockets are identified
using addresses and ports.
All values are in network byte order.
.P
The fields of
.I "struct inet_diag_sockid"
are as follows:
.TP
.I idiag_sport
The source port.
.TP
.I idiag_dport
The destination port.
.TP
.I idiag_src
The source address.
.TP
.I idiag_dst
The destination address.
.TP
.I idiag_if
The interface number the socket is bound to.
.TP
.I idiag_cookie
This is an array of opaque identifiers that could be used along with
other fields of this structure to specify an individual socket.
It is ignored when querying for a list of sockets, as well as
when all its elements are set to \-1.
.P
The response to a query for IPv4 or IPv6 sockets is represented as an array of
.P
.in +4n
.EX
struct inet_diag_msg {
    __u8    idiag_family;
    __u8    idiag_state;
    __u8    idiag_timer;
    __u8    idiag_retrans;
\&
    struct inet_diag_sockid id;
\&
    __u32   idiag_expires;
    __u32   idiag_rqueue;
    __u32   idiag_wqueue;
    __u32   idiag_uid;
    __u32   idiag_inode;
};
.EE
.in
.P
followed by netlink attributes.
.P
The fields of this structure are as follows:
.TP
.I idiag_family
This is the same field as in
.IR "struct inet_diag_req_v2" .
.TP
.I idiag_state
This denotes socket state as in
.IR "struct inet_diag_req_v2" .
.TP
.I idiag_timer
For TCP sockets, this field describes the type of timer that is currently
active for the socket.
It is set to one of the following constants:
.IP
.PD 0
.RS 12
.TP
.B 0
no timer is active
.TP
.B 1
a retransmit timer
.TP
.B 2
a keep-alive timer
.TP
.B 3
a TIME_WAIT timer
.TP
.B 4
a zero window probe timer
.RE
.PD
.IP
For non-TCP sockets, this field is set to 0.
.TP
.I idiag_retrans
For
.I idiag_timer
values 1, 2, and 4, this field contains the number of retransmits.
For other
.I idiag_timer
values, this field is set to 0.
.TP
.I idiag_expires
For TCP sockets that have an active timer, this field describes its expiration
time in milliseconds.
For other sockets, this field is set to 0.
.TP
.I idiag_rqueue
For listening sockets:
the number of pending connections.
.IP
For other sockets:
the amount of data in the incoming queue.
.TP
.I idiag_wqueue
For listening sockets:
the backlog length.
.IP
For other sockets:
the amount of memory available for sending.
.TP
.I idiag_uid
This is the socket owner UID.
.TP
.I idiag_inode
This is the socket inode number.
.\"
.SS Socket memory information
The payload associated with
.B UNIX_DIAG_MEMINFO
and
.B INET_DIAG_SKMEMINFO
netlink attributes is an array of the following __u32 values:
.TP
.B SK_MEMINFO_RMEM_ALLOC
The amount of data in receive queue.
.TP
.B SK_MEMINFO_RCVBUF
The receive socket buffer as set by
.BR SO_RCVBUF .
.TP
.B SK_MEMINFO_WMEM_ALLOC
The amount of data in send queue.
.TP
.B SK_MEMINFO_SNDBUF
The send socket buffer as set by
.BR SO_SNDBUF .
.TP
.B SK_MEMINFO_FWD_ALLOC
The amount of memory scheduled for future use (TCP only).
.TP
.B SK_MEMINFO_WMEM_QUEUED
The amount of data queued by TCP, but not yet sent.
.TP
.B SK_MEMINFO_OPTMEM
The amount of memory allocated for the socket's service needs (e.g., socket
filter).
.TP
.B SK_MEMINFO_BACKLOG
The amount of packets in the backlog (not yet processed).
.SH VERSIONS
.B NETLINK_INET_DIAG
was introduced in Linux 2.6.14 and supported
.B AF_INET
and
.B AF_INET6
sockets only.
In Linux 3.3, it was renamed to
.B NETLINK_SOCK_DIAG
and extended to support
.B AF_UNIX
sockets.
.P
.B UNIX_DIAG_MEMINFO
and
.B INET_DIAG_SKMEMINFO
were introduced in Linux 3.6.
.SH STANDARDS
Linux.
.SH EXAMPLES
The following example program prints inode number, peer's inode number,
and name of all UNIX domain sockets in the current namespace.
.P
.EX
#include <errno.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <linux/netlink.h>
#include <linux/rtnetlink.h>
#include <linux/sock_diag.h>
#include <linux/unix_diag.h>
\&
static int
send_query(int fd)
{
    struct sockaddr_nl nladdr = {
        .nl_family = AF_NETLINK
    };
    struct
    {
        struct nlmsghdr nlh;
        struct unix_diag_req udr;
    } req = {
        .nlh = {
            .nlmsg_len = sizeof(req),
            .nlmsg_type = SOCK_DIAG_BY_FAMILY,
            .nlmsg_flags = NLM_F_REQUEST | NLM_F_DUMP
        },
        .udr = {
            .sdiag_family = AF_UNIX,
            .udiag_states = \-1,
            .udiag_show = UDIAG_SHOW_NAME | UDIAG_SHOW_PEER
        }
    };
    struct iovec iov = {
        .iov_base = &req,
        .iov_len = sizeof(req)
    };
    struct msghdr msg = {
        .msg_name = &nladdr,
        .msg_namelen = sizeof(nladdr),
        .msg_iov = &iov,
        .msg_iovlen = 1
    };
\&
    for (;;) {
        if (sendmsg(fd, &msg, 0) < 0) {
            if (errno == EINTR)
                continue;
\&
            perror("sendmsg");
            return \-1;
        }
\&
        return 0;
    }
}
\&
static int
print_diag(const struct unix_diag_msg *diag, unsigned int len)
{
    if (len < NLMSG_LENGTH(sizeof(*diag))) {
        fputs("short response\[rs]n", stderr);
        return \-1;
    }
    if (diag\->udiag_family != AF_UNIX) {
        fprintf(stderr, "unexpected family %u\[rs]n", diag\->udiag_family);
        return \-1;
    }
\&
    unsigned int rta_len = len \- NLMSG_LENGTH(sizeof(*diag));
    unsigned int peer = 0;
    size_t path_len = 0;
    char path[sizeof(((struct sockaddr_un *) 0)\->sun_path) + 1];
\&
    for (struct rtattr *attr = (struct rtattr *) (diag + 1);
             RTA_OK(attr, rta_len); attr = RTA_NEXT(attr, rta_len)) {
        switch (attr\->rta_type) {
        case UNIX_DIAG_NAME:
            if (!path_len) {
                path_len = RTA_PAYLOAD(attr);
                if (path_len > sizeof(path) \- 1)
                    path_len = sizeof(path) \- 1;
                memcpy(path, RTA_DATA(attr), path_len);
                path[path_len] = \[aq]\[rs]0\[aq];
            }
            break;
\&
        case UNIX_DIAG_PEER:
            if (RTA_PAYLOAD(attr) >= sizeof(peer))
                peer = *(unsigned int *) RTA_DATA(attr);
            break;
        }
    }
\&
    printf("inode=%u", diag\->udiag_ino);
\&
    if (peer)
        printf(", peer=%u", peer);
\&
    if (path_len)
        printf(", name=%s%s", *path ? "" : "@",
                *path ? path : path + 1);
\&
    putchar(\[aq]\[rs]n\[aq]);
    return 0;
}
\&
static int
receive_responses(int fd)
{
    long buf[8192 / sizeof(long)];
    struct sockaddr_nl nladdr;
    struct iovec iov = {
        .iov_base = buf,
        .iov_len = sizeof(buf)
    };
    int flags = 0;
\&
    for (;;) {
        struct msghdr msg = {
            .msg_name = &nladdr,
            .msg_namelen = sizeof(nladdr),
            .msg_iov = &iov,
            .msg_iovlen = 1
        };
\&
        ssize_t ret = recvmsg(fd, &msg, flags);
\&
        if (ret < 0) {
            if (errno == EINTR)
                continue;
\&
            perror("recvmsg");
            return \-1;
        }
        if (ret == 0)
            return 0;
\&
        if (nladdr.nl_family != AF_NETLINK) {
            fputs("!AF_NETLINK\[rs]n", stderr);
            return \-1;
        }
\&
        const struct nlmsghdr *h = (struct nlmsghdr *) buf;
\&
        if (!NLMSG_OK(h, ret)) {
            fputs("!NLMSG_OK\[rs]n", stderr);
            return \-1;
        }
\&
        for (; NLMSG_OK(h, ret); h = NLMSG_NEXT(h, ret)) {
            if (h\->nlmsg_type == NLMSG_DONE)
                return 0;
\&
            if (h\->nlmsg_type == NLMSG_ERROR) {
                const struct nlmsgerr *err = NLMSG_DATA(h);
\&
                if (h\->nlmsg_len < NLMSG_LENGTH(sizeof(*err))) {
                    fputs("NLMSG_ERROR\[rs]n", stderr);
                } else {
                    errno = \-err\->error;
                    perror("NLMSG_ERROR");
                }
\&
                return \-1;
            }
\&
            if (h\->nlmsg_type != SOCK_DIAG_BY_FAMILY) {
                fprintf(stderr, "unexpected nlmsg_type %u\[rs]n",
                        (unsigned) h\->nlmsg_type);
                return \-1;
            }
\&
            if (print_diag(NLMSG_DATA(h), h\->nlmsg_len))
                return \-1;
        }
    }
}
\&
int
main(void)
{
    int fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_SOCK_DIAG);
\&
    if (fd < 0) {
        perror("socket");
        return 1;
    }
\&
    int ret = send_query(fd) || receive_responses(fd);
\&
    close(fd);
    return ret;
}
.EE
.SH SEE ALSO
.BR netlink (3),
.BR rtnetlink (3),
.BR netlink (7),
.BR tcp (7)