写一个c语言的linux服务程序，实现l2tp协议的服务端，要求高性能。

在Linux环境中实现L2TP协议的服务端可以使用C语言编写。一个简单的实现方法是使用Berkeley sockets API来实现网络通信。

下面是一个示例代码，实现了一个简单的L2TP服务器：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

#define PORT 1701
#define BUFSIZE 2048

int main(int argc, char *argv[]) {
    int sockfd, new_sockfd;
    struct sockaddr_in server_addr, client_addr;
    socklen_t client_len;
    char buffer[BUFSIZE];

    // create a socket
    if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
        perror("socket");
        exit(1);
    }

    // set up the server address
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(PORT);
    server_addr.sin_addr.s_addr = htonl(INADDR_ANY);

    // bind the socket to the server address
    if (bind(sockfd, (struct sockaddr *) &server_addr, sizeof(server_addr)) == -1) {
        perror("bind");
        exit(1);
    }

    // listen for incoming connections
    if (listen(sockfd, 5) == -1) {
        perror("listen");
        exit(1);
    }

    printf("L2TP server is running on port %d\n", PORT);

    // accept incoming connections
    while (1) {
        client_len = sizeof(client_addr);
        if ((new_sockfd = accept(sockfd, (struct sockaddr *) &client_addr, &client_len)) == -1) {
            perror("accept");
            continue;
        }

        printf("Connection from %s\n", inet_ntoa(client_addr.sin_addr));

        // receive data
        int n = recv(new_sockfd, buffer, BUFSIZE, 0);
        if (n > 0) {
            // process the data
            // ...

            // send a response
            send(new_sockfd, buffer, n, 0);
        }

        close(new_sockfd);
    }

    close(sockfd);
    return 0;
}

这个代码只是一个示例