UNIX domain sockets provide inter-process communication (IPC) on the same host. Unlike network sockets, they operate entirely within the kernel and use filesystem paths as addresses. When troubleshooting applications like Asterisk that use /var/run/asterisk/asterisk.ctl, we often need to identify communicating processes and inspect traffic.

The lsof approach you tried shows socket ownership but not active connections. For deeper inspection:

sudo ss -xlp | grep asterisk.ctl

This reveals both listening and connected processes with their PIDs. The -x flag specifies UNIX sockets, while -p shows process information.

For sniffing UNIX domain socket traffic, consider these approaches:

Method 1: socat for Traffic Relay

Create a relay to intercept traffic:

sudo mv /var/run/asterisk/asterisk.ctl /var/run/asterisk/asterisk.ctl.original
sudo socat -t100 -x -v UNIX-LISTEN:/var/run/asterisk/asterisk.ctl,mode=777,reuseaddr,fork \
UNIX-CONNECT:/var/run/asterisk/asterisk.ctl.original

The -v flag enables verbose output showing all transmitted data.

Method 2: strace for System Call Monitoring

Attach to the suspected process:

sudo strace -p PID -e trace=write -s 1024 -xx

This captures write operations including socket data (replace PID with actual process ID).

Method 3: bpftrace for Advanced Tracing

Modern Linux systems can use BPF tools:

sudo bpftrace -e 'tracepoint:syscalls:sys_enter_sendto /comm == "asterisk"/ { printf("%s: %s\\n", comm, str(args->buff)); }'

When direct sniffing isn't possible:

• Configure Asterisk for verbose logging
• Use auditd to monitor filesystem access
• Check system logs for relevant entries

Remember that:

• UNIX domain socket traffic isn't encrypted by default
• Monitoring tools may impact system performance
• Some methods require root privileges

When debugging inter-process communication on Linux systems, UNIX domain sockets present unique challenges for monitoring. Unlike network sockets that can be inspected with tools like tcpdump, UNIX sockets require different approaches for traffic analysis and process identification.

The lsof command shows socket ownership but not active connections. For deeper inspection, use these methods:

# Find all processes with open connections to the socket
sudo ss -x -a -p | grep asterisk.ctl

# Alternative using lsof with more detailed output
sudo lsof -U -a +E -c asterisk

For sniffing socket traffic, consider these approaches:

# Method 1: Using strace to monitor specific process
sudo strace -p PID -e trace=write -s 1000 2>&1 | grep asterisk.ctl

# Method 2: Network namespace trick (requires root)
sudo socat -t0 -x UNIX-LISTEN:/tmp/monitor,mode=777,reuseaddr,fork \ 
UNIX-CONNECT:/var/run/asterisk/asterisk.ctl

For comprehensive monitoring, SystemTap provides deep inspection:

# SystemTap script to monitor socket traffic
probe kernel.function("unix_stream_sendmsg").call {
    if (__file_ino($sock->sk->sk_socket->file) == inode) {
        printf("PID %d sending %d bytes\n", pid(), $size)
        print_ubacktrace()
    }
}

The Linux audit system can track socket access:

# Add audit rule for the socket file
sudo auditctl -w /var/run/asterisk/asterisk.ctl -p rwxa -k asterisk_comms

# View audit logs
sudo ausearch -k asterisk_comms | aureport -f -i

For the specific case of asterisk.ctl, combine multiple tools:

# First identify the Asterisk main process
pgrep -f "asterisk.*asterisk.ctl"

# Then trace its socket activity
sudo strace -f -p $(pgrep -f "asterisk.*asterisk.ctl") \
-e trace=network,read,write,recvfrom,sendto 2>&1 | \
grep -A 10 "asterisk.ctl"

Remember that some methods may impact performance on production systems. For long-term monitoring, consider building dedicated monitoring using eBPF programs or the newer Linux kernel observation capabilities.