Loose Source Routing - Detailed Trace
═══════════════════════════════════════════════════════════════════
 
INITIAL: Host A creates packet
─────────────────────────────────────────────────────────────────
Source:      192.168.1.10
Destination: 172.16.0.1 (first waypoint)
LSR Option:  Type=131, Length=11, Pointer=4
Route Data:  [10.20.0.1 (WP2)] [10.30.40.50 (final dest)]
                   ↑
                 Pointer
 
AT R1, R2: (Non-waypoint routers)
─────────────────────────────────────────────────────────────────
- Destination 172.16.0.1 ≠ our IP
- Look up 172.16.0.1 in routing table
- Forward toward 172.16.0.1
- LSR option unchanged
 
AT WP1 (172.16.0.1): WAYPOINT PROCESSING!
─────────────────────────────────────────────────────────────────
1. "Destination 172.16.0.1 = my IP, I'm a waypoint!"
2. "Pointer (4) ≤ Length (11), more route to go"
3. "Read next waypoint at offset 4: 10.20.0.1"
4. "Record my IP (172.16.0.1) at offset 4"
5. "Set destination = 10.20.0.1"
6. "Advance pointer: 4 → 8"
7. "Route to 10.20.0.1 via normal routing"
 
After WP1:
Destination: 10.20.0.1 (second waypoint)
Route Data:  [172.16.0.1*] [10.30.40.50]
              ↑recorded       ↑
                           Pointer(8)
 
AT R3, R4: (Non-waypoint routers)
─────────────────────────────────────────────────────────────────
- Forward toward 10.20.0.1 using normal routing
 
AT WP2 (10.20.0.1): WAYPOINT PROCESSING!
─────────────────────────────────────────────────────────────────
1. "Destination = my IP, I'm a waypoint!"
2. "Pointer (8) ≤ Length (11), more route to go"
3. "Read next address at offset 8: 10.30.40.50"
4. "Record my IP (10.20.0.1) at offset 8"
5. "Set destination = 10.30.40.50"
6. "Advance pointer: 8 → 12"
 
After WP2:
Destination: 10.30.40.50 (final destination)
Route Data:  [172.16.0.1] [10.20.0.1*]
                           ↑recorded
Pointer: 12 (now > Length 11)
 
AT R5: (Non-waypoint router)
─────────────────────────────────────────────────────────────────
- Forward toward 10.30.40.50
 
AT Host B (10.30.40.50): FINAL DESTINATION!
─────────────────────────────────────────────────────────────────
1. "Destination = my IP"
2. "Pointer (12) > Length (11) → I'm the final destination!"
3. "Route completed: WP1 → WP2 now recorded in option"
4. "Deliver to upper layer"
 
RESULT: Packet traversed exactly the waypoints specified!
        Route Data contains: [172.16.0.1, 10.20.0.1] - the reverse path

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#include <stdint.h>
#include <stdbool.h>
#include <string.h>
 
#define OPT_LSR 131  // Loose Source Route type
 
typedef struct {
    uint32_t src_ip;
    uint32_t dst_ip;
    uint8_t options[40];
    uint8_t options_len;
} IPv4Packet;
 
/**
 * Process Loose Source Route option
 * 
 * Key difference from Strict: NO direct reachability check
 * We simply route to the next waypoint using normal routing
 */
int process_lsr(IPv4Packet *packet, uint32_t my_ip) {
    
    // Find LSR option in options area
    int offset = 0;
    while (offset < packet->options_len) {
        uint8_t type = packet->options[offset];
        
        if (type == 0) break;  // EOL
        if (type == 1) {       // NOP
            offset++;
            continue;
        }
        
        if (type == OPT_LSR) {
            // Found Loose Source Route option
            uint8_t length = packet->options[offset + 1];
            uint8_t pointer = packet->options[offset + 2];
            
            // Is this packet addressed to us (are we a waypoint)?
            if (packet->dst_ip != my_ip) {
                // Not a waypoint for us - continue normal routing
                // The packet passes through us toward its current destination
                return 0;
            }
            
            // We ARE the destination in the header - we're a waypoint!
            
            // Check if there's more route
            if (pointer > length) {
                // Route exhausted - we're the FINAL destination
                // Deliver to upper layer
                return 1;  // 1 = deliver locally
            }
            
            // More route to go - get next waypoint
            int route_offset = offset + pointer - 1;  // 1-indexed to 0-indexed
            
            if (route_offset + 4 > offset + length) {
                // Malformed option
                return -1;
            }
            
            // Extract next waypoint IP
            uint32_t next_waypoint;
            memcpy(&next_waypoint, &packet->options[route_offset], 4);
            next_waypoint = ntohl(next_waypoint);
            
            // ════════════════════════════════════════════════════
            // KEY DIFFERENCE FROM STRICT SOURCE ROUTING:
            // NO direct reachability check here!
            // We simply route to next_waypoint using normal routing
            // ════════════════════════════════════════════════════
            
            // Perform the swap:
            // 1. Record our IP where we read the waypoint
            uint32_t my_ip_network = htonl(my_ip);
            memcpy(&packet->options[route_offset], &my_ip_network, 4);
            
            // 2. Set new destination (next waypoint)
            packet->dst_ip = next_waypoint;
            
            // 3. Advance pointer
            packet->options[offset + 2] = pointer + 4;
            
            // Return 0 = forward the packet (use normal routing)
            return 0;
        }
        
        // Skip this option by its length
        uint8_t length = packet->options[offset + 1];
        offset += length;
    }
    
    return 0;  // No LSR found, normal processing
}
 
/**
 * Simplified main forwarding logic with LSR support
 */
void forward_packet(IPv4Packet *packet, uint32_t my_ip) {
    // Check for LSR option
    int lsr_result = process_lsr(packet, my_ip);
    
    if (lsr_result == 1) {
        // We're the final destination
        deliver_to_upper_layer(packet);
        return;
    }
    
    if (lsr_result == -1) {
        // Malformed LSR option
        send_icmp_parameter_problem(packet);
        return;
    }
    
    // Normal forwarding (destination may have been swapped by LSR)
    uint32_t next_hop = lookup_route(packet->dst_ip);
    send_to_interface(packet, next_hop);
}

LSR Error Scenarios
═══════════════════════════════════════════════════════════════
 
Scenario 1: Waypoint Unreachable
────────────────────────────────
Route specified: A → [WP1] → [WP2] → B
Actual state:    WP2's network has no route from WP1
 
Packet path:
A → (routers) → WP1 → (tries to route to WP2) → FAIL
                       └── No route to 10.20.0.0/24
                       └── ICMP Type 3, Code 0 sent to A
 
Scenario 2: Waypoint Down
────────────────────────────────
Route specified: A → [WP1] → [WP2] → B
Actual state:    WP1 is powered off
 
Packet path:
A → (routers reach subnet, but WP1 doesn't respond)
    └── ARP fails for WP1's IP
    └── Eventually: ICMP Type 3, Code 1 (Host Unreachable)
 
Scenario 3: TTL Exhaustion
────────────────────────────────
Route specified: A → [WP1] → B
Actual state:    30 hops between A and WP1, TTL starts at 64
 
Packet path:
A → R1 (TTL 63) → R2 (62) → ... → R30 (34) → WP1
WP1 → R31 (33) → ... → (TTL reaches 0 before B)
    └── ICMP Type 11, Code 0 (TTL Exceeded)

# Linux: Disable loose source routing
echo 0 > /proc/sys/net/ipv4/conf/all/accept_source_route
echo 0 > /proc/sys/net/ipv4/conf/default/accept_source_route
 
# Permanent setting in /etc/sysctl.conf:
net.ipv4.conf.all.accept_source_route = 0
net.ipv4.conf.default.accept_source_route = 0
 
# iptables: Explicitly drop source-routed packets
iptables -A INPUT -m ipv4options --lsrr -j DROP
iptables -A FORWARD -m ipv4options --lsrr -j DROP
 
# Cisco IOS: Disable source routing
Router(config)# no ip source-route
 
# FreeBSD
sysctl net.inet.ip.sourceroute=0
sysctl net.inet.ip.accept_sourceroute=0
 
# Windows PowerShell (check status)
Get-NetIPv4Protocol | Select-Object SourceRouting
# Should show: Disabled