Day54 - UNIX Domain Socket Advanced IPC

Objective

Learn advanced AF_UNIX IPC concepts:

• Abstract namespace socket
• SO_PEERCRED
• Permission control
• SOCK_DGRAM
• Datagram request/reply
• Datagram truncation

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Environment

• Linux userspace
• GCC
• AF_UNIX socket APIs

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Part 1 - Abstract Namespace Socket

Server Bind

Example:

addr.sun_family = AF_UNIX;
addr.sun_path[0] = '\0';

memcpy(&addr.sun_path[1],
       SERVER_SOCKET_NAME,
       strlen(SERVER_SOCKET_NAME));

Address length:

addr_len =
    offsetof(struct sockaddr_un, sun_path)
    + 1
    + strlen(SERVER_SOCKET_NAME);

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Verification

Run:

ss -xl

Expected:

@mypoll.sock

Verify filesystem socket does not exist:

ls /tmp/mypoll.sock

Expected:

No such file or directory

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Part 2 - SO_PEERCRED

Retrieve Peer Credential

Example:

struct ucred cred;

getsockopt(fd,
           SOL_SOCKET,
           SO_PEERCRED,
           &cred,
           &len);

Expected log:

pid=1234 uid=1000 gid=1000

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Part 3 - Permission Control

Restrict privileged commands:

if (!client_is_privileged(&client->cred))
    return server_reply_perm_denied(q);

Test:

• Normal user
• Root user

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Part 4 - SOCK_DGRAM Server

Create DGRAM Socket

fd = socket(AF_UNIX, SOCK_DGRAM, 0);

No:

• listen()
• accept()

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Part 5 - recvfrom()

Example:

recvfrom(fd,
         buf,
         sizeof(buf) - 1,
         0,
         (struct sockaddr *)&client_addr,
         &addr_len);

Verify sender address:

client-1
client-2

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Part 6 - Request / Reply

Client

sendto(server)

Server

recvfrom(client)
sendto(client)

Client

recvfrom(server)

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Part 7 - Message Boundary Test

Client sends multiple packets:

msg1
msg2
msg3

Verify server receives separate packets.

Expected:

msg1
msg2
msg3

Not:

msg1msg2msg3

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Part 8 - Datagram Truncation Test

Setup

• Server RX buffer: 128 bytes
• Client packet: >128 bytes

Example:

recvfrom(..., MSG_TRUNC, ...)

Expected behavior:

• Packet truncated
• Remaining bytes discarded
• Next packet unaffected

Expected warning:

Message truncated because it is too long.

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Summary

This lab demonstrates:

• Linux-specific AF_UNIX features
• Local IPC authentication
• Datagram packet semantics
• STREAM vs DGRAM behavior
• Truncation handling

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