Routing from one IP to another is not working

Hi,
I tried below code for testing the IP routing of data packets.

iplist.py -

```
`from bcc import BPF, table
import socket
import struct
import ctypes as ct
import utils
import json
import os
# from ctypes import *
from datetime import datetime
import time
import ipaddress

device = "lo"
src_file = './filter.c'

ip_list = []
ip_list.append(utils.ip_to_uint32('127.0.0.1'))

#open eBPF c source code
with open(src_file, 'r') as f:
file = ''.join(f.readlines())

b = BPF(text=file)
functions = b.load_func('udpfilter', BPF.XDP)
print("eBPF progam loaded")
b.attach_xdp(device, functions, 0)
print("eBPF program attached")

#load blacklist map
blacklist= b.get_table("iplist")
blacklist.clear()

#put the to block ip addreses in map
for item in ip_list:
blacklist[ct.c_uint32(item)] = ct.c_uint32(utils.ip_to_uint32('127.0.0.2'))

while True:
try:
b.trace_print()
time.sleep(5)
except KeyboardInterrupt:
print("Removing filter from device")
break

b.remove_xdp(device, 0)
print('done')`

```
filter.c -
```

`#define KBUILD_MODNAME "filter"
#include
#include
#include
#include
#include

BPF_TABLE("percpu_hash", uint32_t, uint32_t, iplist, 6524288);

int udpfilter(struct xdp_md *ctx) {
bpf_trace_printk("got a packet\n");
void *data = (void *)(long)ctx->data;
void *data_end = (void *)(long)ctx->data_end;
struct ethhdr *eth = data;
if ((void*)eth + sizeof(*eth) <= data_end) {
struct iphdr *ip = data + sizeof(*eth);
if ((void*)ip + sizeof(*ip) <= data_end) {
if (ip->protocol == IPPROTO_UDP) {
struct udphdr *udp = (void*)ip + sizeof(*ip);
if ((void*)udp + sizeof(*udp) <= data_end) {
uint32_t *value;
value = iplist.lookup(&(ip->daddr));
// bpf_trace_printk("%d value\n", *value);
if(value){
bpf_trace_printk("ip daddr found in the ip blacklist %lu\n",*value);
// bpf_trace_printk("port of the %lu\n",*value);

// *value += 1;
ip->daddr = *value;
bpf_trace_printk("ip daddr changed to %lu\n", ip->daddr);

}
}
}
}
}
return XDP_PASS;
}`

```
But this is not routing the packet coming at 127.0.0.1 to 127.0.0.2
I tested it with nc (netcat) commands. Is there something wrong in the code?

Also, sometimes it prints correct updated address(127.0.0.2) in uint32 format in ip->daddr and sometimes it randomly prints 0 as updated daddr.

Read iplist.py and filter.c first, then reproduce the reported netcat test while inspecting the BPF trace output. Compare the observed destination-address updates and packet behavior with the intended transfer from 127.0.0.1 to 127.0.0.2; done means the behavior is consistent and the zero-address result is explained or eliminated.

Written by the indexing model from the issue text.