In a typical enterprise environment with dual WAN connections, we often need to route specific traffic through secondary gateways. Here's the exact infrastructure I'm working with:

• Primary gateway: Windows Server 2008 (192.168.16.1)
• Secondary gateway: Router at 192.168.1.254
• Linux client IP: 192.168.16.22 (able to ping 192.168.1.254)
• Target server: 123.123.123.123

When attempting to create a static route using the classic route command:

sudo route add -host 123.123.123.123 gw 192.168.1.254 dev eth0
SIOCADDRT: No such process

This error typically indicates one of several potential issues, despite successful ping connectivity.

The traditional route command is deprecated in favor of the ip utility. Try this instead:

sudo ip route add 123.123.123.123/32 via 192.168.1.254 dev eth0

If this fails with "Network is unreachable", we need to verify the gateway's accessibility.

First, confirm ARP resolution works:

arping -I eth0 192.168.1.254

Then test the actual route with traceroute:

traceroute -n 192.168.1.254

Examine your current routing table for conflicts:

ip route show
route -n

Look for existing routes that might conflict with your new route.

Check if firewall rules are blocking route setup:

sudo iptables -L -n -v

ICMP (ping) might be allowed while routing traffic is blocked.

For Debian-based systems, add to /etc/network/interfaces:

post-up ip route add 123.123.123.123/32 via 192.168.1.254 dev eth0

For RHEL-based systems, create /etc/sysconfig/network-scripts/route-eth0:

123.123.123.123/32 via 192.168.1.254 dev eth0

For more complex scenarios, consider policy routing:

sudo ip rule add from 192.168.16.22 table 100
sudo ip route add default via 192.168.1.254 dev eth0 table 100

After adding the route, verify with:

ip route get 123.123.123.123

And test actual connectivity:

curl --interface eth0 http://123.123.123.123

---

While you can ping the gateway (192.168.1.254), the route addition fails with SIOCADDRT: No such process. This typically indicates one of these scenarios:

# First verify the gateway is truly reachable with advanced ping:
ping -c 4 -I eth0 192.168.1.254

# Check if the gateway responds to ARP requests
arping -I eth0 -c 3 192.168.1.254

# Verify route table before modification
ip route show

Modern Linux systems prefer ip route over the deprecated route command. Try these alternatives:

# Method 1: Using ip route command
sudo ip route add 123.123.123.123 via 192.168.1.254 dev eth0

# Method 2: Persistent route (Ubuntu/Debian)
echo "123.123.123.123 via 192.168.1.254 dev eth0" | sudo tee -a /etc/network/interfaces.d/static-routes

# Method 3: NetworkManager CLI (if used)
nmcli connection modify eth0 +ipv4.routes "123.123.123.123/32 192.168.1.254"

On your Windows Server 2008 router, ensure proper routing is configured:

# Check existing routes on Windows
route print

# Verify RRAS NAT configuration
netsh routing ip nat show interface

ICMP (ping) might be allowed while routing protocols are blocked:

# Check Linux firewall rules
sudo iptables -L -n -v

# Temporarily disable firewall for testing
sudo systemctl stop firewalld  # For RHEL/CentOS
sudo ufw disable               # For Ubuntu

Capture network traffic during route addition attempt:

sudo tcpdump -i eth0 host 192.168.1.254 -w route_debug.pcap

# After reproducing the error, analyze with:
tcpdump -r route_debug.pcap -nn -v

For a production environment, consider these robust approaches:

# Create a network configuration script
#!/bin/bash
ip route flush cache
ip route add 123.123.123.123 via 192.168.1.254 dev eth0 || \
echo "Failed to add route; investigating..." >> /var/log/network_routes.log

# Add to cron for persistence
(crontab -l 2>/dev/null; echo "@reboot /path/to/route_script.sh") | crontab -

For more granular control when multiple interfaces exist:

# Create custom routing table
echo "200 custom_route" >> /etc/iproute2/rt_tables

# Add rule for specific traffic
ip rule add from 192.168.16.22 table custom_route
ip route add default via 192.168.1.254 dev eth0 table custom_route

# Verify configuration
ip rule show
ip route show table custom_route