Transcript UNIX domain protocols

Unix Domain Protocols
• Unix domain socket address structure
• Unix domain socket client-server
• Descriptor passing with Unix domain socket
• Readings
– UNP Sections 14.4 and 14.5, Ch15
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Unix Domain Protocols
• Used for client-server communication on a single
machine
– Use same API as with regular client-server
– Both stream and datagram sockets are supported
– Local IPC (inter-process communication)
• Why do we need this?
– Efficiency (faster than TCP/UDP socket on same machine)
– Passing (file) descriptors
– More security checks (passing credentials such as pid)
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Address Structure
• Open a socket using UNIX domain socket
socket(AF_LOCAL, SOCK_STREAM, 0)
socket(AF_LOCAL, SOCK_DGRAM, 0)
– AF_LOCAL, AF_UNIX (historical)
• UNIX domain socket address structure
#include <sys/un.h>
struct sockaddr_un {
sa_family_t sun_family; /*AF_LOCAL or AF_UNIX */
char
sun_path[104]; /* null-terminated path name */
}
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A Few Notes
• sun_path is null-terminated file pathname
– Must not exist in file system
– Should be absolute pathname
– connect: access permission of sun_path is verified like open
with write-only access
• Unix domain stream socket similar to TCP
• Unix domain datagram socket similar to UDP
• For Unix domain datagram client
– connect and sendto will not automatically bind socket to
local address
– Server cannot return datagram to client without bind
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Binding a Unix Domain Socket
• example1.c
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–
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Use absolute path for path name
bind will fail if the file exists
Pathname can potentially overrun the sun_path buffer
Otherwise, it is very similar to a TCP socket
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Client-Server Using Unix Domain Protocols
• example2.c and example3.c
– Echo server and client
– connect needs a pathname that is currently bound to an
open UNIX domain socket of the same type
– Permission test for connect is the same as that for open with
write-only access
– connect will fail if the listening queue is full
• UNIX domain datagram sockets are similar to UDP
sockets
– Sender must bind first for the receiver to reply
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Full-Duplex Stream Pipe
• #include <sys/socket.h>
• int socketpair(int family, int type, int protocol, int sockfd[2]);
–
–
–
–
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Create two sockets connected together
Family: AF_LOCAL
Type: SOCK_STREAM or SOCK_DGRAM
Protocol: 0
Full-duplex stream pipe. Both can be used for read and write
– Example example4.c
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Passing Descriptors
• What we have learned
– Parent open files and pass to child processes
• What if child wants to pass back file descriptors to
parent?
• What if two unrelated processes want to pass file
descriptors?
• Unix domain socket can handle all these
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Passing Descriptors
• Involves creating a new descriptor in receiving
process
– referring to the same system open file table entry
– It is normal that we see different descriptor numbers in
sending and receiving processes
– It is OK for the sending process to close the file after calling
sendmsg
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recvmsg and sendmsg Functions
#include <sys/socket.h>
ssize_t recvmsg(int sockfd, struct msghdr *msg, int flags);
ssize_t sendmsg(int sockfd, struct msghdr *msg, int flags);
struct msghdr {
void
socklen_t
struct iovec
int
void
socklen_t
int
*msg_name; //protocol address
msg_namelen;
*msg_iov; // array to hold data
msg_iovlen;
*msg_control; // ancillary data
// link to cmsghdr struct
msg_controllen;
msg_flags;
};
struct iovec {
void
*iov_base; // starting address of buffer
size_t iov_len; // size of buffer
};
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Control Message Header
struct cmsghdr {
socklen_t
cmsg_len; // length in bytes
int
cmsg_level; // socket option level
int
cmsg_type; // socket option type
// followed by unsigned char cmsg_data[]
};
• A number of macros provided to access cmsghdr
from msghdr
– struct cmsghdr *CMSG_FIRSTHDR(struct msghdr *)
– struct cmsghdr *CMSG_NXTHDR(struct msghdr *, struct
cmsghdr*)
– unsigned char *CMSG_DATA(struct cmsghdr *)
– etc
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Passing Descriptor
• Goal: passing open file descriptor in the kernel
(process file descriptor entries are used to reference
to the kernel entry).
– Create a UNIX domain socket
– Send open the descriptor with open, pipe etc (get the
reference)
– Send process build msghdr structure containing the
descriptor to pass (using the reference), and call sendmsg
– Receiving process call recvmsg to receive the descriptor on
the domain socket (the reference can be different, but the
kernel open file entry would be the same).
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Passing Descriptor Example
• Example5.c
– Waiting for incoming request to open a specified file
– After opening the file, send back the file descriptor
• Example6.c
– Send specified file to server
– After receiving file descriptor returned from server, print the
content of the file
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Passing Credentials
• cmsg_level: SOL_SOCKET
• cmsg_type: SCM_CREDS
struct cmsgcred {
pid_t cmcred_pid; // PID of sending process
uid_t cmcred_uid; // real UID
uid_t cmcred_euid; // effective UID
gid_t cmcred_gid; // real GID
short cmcred_ngroups; // num of grps
gid_t cmcred_groups[CMGROUP_MAX];
};
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