When diagnosing network issues, one particularly frustrating scenario occurs when packets are visible at the network interface level but never reach the application. This manifests in RHEL6 systems as:

# tcpdump shows packets arriving
$ sudo tcpdump -i eth0 udp port 1234
17:42:35.123456 IP 192.168.1.100.4321 > 192.168.1.200.1234: UDP, length 100

# But netstat shows zero received packets
$ netstat -s -u
Udp:
    0 packets received
    0 packets to unknown port received

Before diving deep, let's confirm the basic network configuration:

# Check interface configuration
$ ifconfig eth0
# Verify routing
$ route -n
# Confirm ARP resolution
$ arp -an

When packets disappear between the interface and application, we need to trace their path through the kernel:

# Check kernel ring buffer
$ dmesg | grep -i udp
# Monitor kernel drops
$ cat /proc/net/snmp | grep -A1 Udp:
# Trace packet flow
$ strace -f -e trace=network -p $(pgrep your_udp_app)

Here's an enhanced test script that includes socket debugging:

#!/usr/bin/perl
use strict;
use Socket;

# Create UDP socket
socket(SOCK, PF_INET, SOCK_DGRAM, getprotobyname('udp')) 
    or die "socket: $!";

# Enable debug
setsockopt(SOCK, SOL_SOCKET, SO_DEBUG, 1);

# Bind to port
bind(SOCK, sockaddr_in(1234, INADDR_ANY)) 
    or die "bind: $!";

print "UDP server ready\n";
while (1) {
    my $data;
    my $client_addr = recv(SOCK, $data, 1024, 0);
    if (defined $client_addr) {
        my ($port, $ip) = sockaddr_in($client_addr);
        print "Received from " . inet_ntoa($ip) . ":$port\n";
    } else {
        warn "recv error: $!\n";
    }
}

In RHEL environments, SELinux often interferes with network operations:

# Check current SELinux status
$ getenforce
# Check audit logs
$ ausearch -m avc -ts recent
# Temporarily test with SELinux disabled
$ setenforce 0
# If it works, create proper policy
$ ausearch -m avc -ts recent | audit2allow -M myudppolicy
$ semodule -i myudppolicy.pp

Several kernel parameters can affect UDP delivery:

# Check current UDP buffer sizes
$ sysctl net.core.rmem_default net.core.rmem_max
# Check UDP early demux setting
$ sysctl net.ipv4.udp_early_demux
# Temporary adjustment for testing
$ sudo sysctl -w net.core.rmem_max=16777216

In some cases, NIC offloading features can cause issues:

# Check current offload settings
$ ethtool -k eth0
# Disable problematic features
$ sudo ethtool -K eth0 rx off tx off sg off tso off

Comparative packet capture helps identify where packets are dropped:

# Interface level capture
$ tcpdump -i eth0 -w interface.pcap udp port 1234
# Raw socket capture (bypasses normal stack)
$ tcpdump -i any -w raw.pcap udp port 1234
# Compare the two captures in Wireshark

When Perl isn't available, consider these alternatives:

# Simple Python UDP server
import socket
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.bind(('0.0.0.0', 1234))
while True:
    data, addr = sock.recvfrom(1024)
    print(f"Received from {addr}: {data.decode()}")

# Using netcat for testing
$ nc -ul 1234

In rare cases, kernel modules might be involved:

# Check loaded network modules
$ lsmod | grep udp
# Monitor kernel module activity
$ strace -f -e trace=open,close,ioctl -p 1

---

You're observing a peculiar network behavior where UDP packets appear in interface-level captures (tcpdump) but never reach the application layer. The key indicators are:

# tcpdump shows packets arriving
$ sudo tcpdump -i eth0 udp port 1234
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
15:32:45.123456 IP 192.168.1.100.4321 > 192.168.1.200.1234: UDP, length 512

# But netstat shows zero received packets
$ netstat -s -u
Udp:
    0 packets received
    0 packets to unknown port received.
    0 packet receive errors

1. Kernel-level Packet Filtering

Even with iptables showing ACCEPT policies, check for these:

# Check for nf_conntrack issues
$ dmesg | grep nf_conntrack
$ sysctl net.netfilter.nf_conntrack_udp_timeout

# Verify rp_filter settings
$ sysctl -a | grep .rp_filter
net.ipv4.conf.all.rp_filter = 0
net.ipv4.conf.default.rp_filter = 0
net.ipv4.conf.eth0.rp_filter = 0

2. NIC Offloading Features

Hardware checksum offloading can sometimes cause issues:

$ ethtool -k eth0 | grep checksum
rx-checksumming: on
tx-checksumming: on
$ sudo ethtool -K eth0 rx off tx off  # Test disabling

3. Application Socket Configuration

Try this enhanced test script with socket debugging:

#!/usr/bin/perl -w
use strict;
use Socket;

my $port = 1234;
socket(SERVER, PF_INET, SOCK_DGRAM, getprotobyname('udp')) or die "socket: $!";
setsockopt(SERVER, SOL_SOCKET, SO_REUSEADDR, 1) or die "setsockopt: $!";
bind(SERVER, sockaddr_in($port, INADDR_ANY)) or die "bind: $!";

# Enable debug
setsockopt(SERVER, SOL_SOCKET, SO_DEBUG, 1) or warn "SO_DEBUG failed: $!";

print "UDP server ready on port $port\n";
while (1) {
    my $msg;
    my $cli_addr = recv(SERVER, $msg, 1024, 0);
    if (defined $cli_addr) {
        my ($cli_port, $cli_ip) = sockaddr_in($cli_addr);
        print "Received from ", inet_ntoa($cli_ip), ":$cli_port - $msg\n";
    } else {
        warn "recv error: $!\n";  # This should show errors if occurring
    }
}

Kernel Trace:

$ sudo strace -f -e trace=network -p $(pgrep your_application)
$ sudo perf trace -e 'net:*' -p $(pgrep your_application)

Firewall Deep Check:

# Check for ebtables/arptables
$ sudo ebtables -L
$ sudo arptables -L

# Verify conntrack
$ sudo conntrack -L 2>/dev/null | grep udp

Socket Buffer Inspection:

$ ss -ulnp
$ cat /proc/net/udp
$ sysctl net.core.rmem_default net.core.rmem_max

• Virtualization layer filtering (if running under KVM/Xen/VMware)
• Network interface bonding/teaming issues
• SELinux restrictions (check audit.log)
• FIB rules affecting packet routing (ip rule list)
• Kernel UDP early demux (sysctl net.ipv4.udp_early_demux)

When testing changes, always monitor both netstat -s -u and application behavior simultaneously to identify which layer the packets are being dropped.