ip addr命令实现源码分析

Posted 2021-03-28 linux进阶

执行命令ip addr  show  dev eth2

执行strace 查看相应的系统调用如下

root@TopSecSystem:~/iproute2-5.6.0/tc>strace ip addr show dev eth2execve("/sbin/ip", ["ip", "addr", "show", "dev", "eth2"], [/* 22 vars */]) = 0brk(0) = 0x848000mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f3c85d3c000access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3fstat(3, {st_mode=S_IFREG|0644, st_size=32941, ...}) = 0mmap(NULL, 32941, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f3c85d33000close(3) = 0open("/lib64/libdl.so.2", O_RDONLY|O_CLOEXEC) = 3read(3, "177ELF2113>120016`U0"..., 832) = 832fstat(3, {st_mode=S_IFREG|0755, st_size=17384, ...}) = 0mmap(0x3055600000, 2109712, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x3055600000mprotect(0x3055603000, 2093056, PROT_NONE) = 0mmap(0x3055802000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x2000) = 0x3055802000close(3) = 0open("/lib64/libc.so.6", O_RDONLY|O_CLOEXEC) = 3read(3, "177ELF2113>135"U0"..., 832) = 832fstat(3, {st_mode=S_IFREG|0755, st_size=1723928, ...}) = 0mmap(0x3055200000, 3832352, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x3055200000mprotect(0x305539f000, 2093056, PROT_NONE) = 0mmap(0x305559e000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x19e000) = 0x305559e000mmap(0x30555a4000, 14880, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x30555a4000close(3) = 0mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f3c85d32000mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f3c85d31000mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f3c85d30000arch_prctl(ARCH_SET_FS, 0x7f3c85d31700) = 0mprotect(0x3055802000, 4096, PROT_READ) = 0mprotect(0x305559e000, 16384, PROT_READ) = 0mprotect(0x305381f000, 4096, PROT_READ) = 0munmap(0x7f3c85d33000, 32941) = 0socket(PF_NETLINK, SOCK_RAW|SOCK_CLOEXEC, 0) = 3setsockopt(3, SOL_SOCKET, SO_SNDBUF, [32768], 4) = 0setsockopt(3, SOL_SOCKET, SO_RCVBUF, [1048576], 4) = 0bind(3, {sa_family=AF_NETLINK, pid=0, groups=00000000}, 12) = 0getsockname(3, {sa_family=AF_NETLINK, pid=27974, groups=00000000}, [12]) = 0access("/proc/net", R_OK) = 0access("/proc/net/unix", R_OK) = 0socket(PF_LOCAL, SOCK_DGRAM|SOCK_CLOEXEC, 0) = 4ioctl(4, SIOCGIFINDEX, {ifr_name="eth2", ifr_index=4}) = 0close(4) = 0sendto(3, "(2213@2505`"..., 40, 0, NULL, 0) = 40recvmsg(3, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000}, msg_iov(1)=[{"04202@2505`Fm431I1"..., 16384}], msg_controllen=0, msg_flags=0}, 0) = 3240brk(0) = 0x848000brk(0x869000) = 0x869000recvmsg(3, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000}, msg_iov(1)=[{"@4202@2505`Fm14C201"..., 16384}], msg_controllen=0, msg_flags=0}, 0) = 1088recvmsg(3, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000}, msg_iov(1)=[{"2432@2505`Fm", 16384}], msg_controllen=0, msg_flags=0}, 0) = 20sendto(3, "(2613A2505`"..., 40, 0, NULL, 0) = 40recvmsg(3, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000}, msg_iov(1)=[{"D242A2505`Fm21020037611011771"..., 16384}], msg_controllen=0, msg_flags=0}, 0) = 148recvmsg(3, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000}, msg_iov(1)=[{"@242A2505`Fm
2002003761241"..., 16384}], msg_controllen=0, msg_flags=0}, 0) = 128recvmsg(3, {msg_name(12)={sa_family=AF_NETLINK, pid=0, groups=00000000}, msg_iov(1)=[{"2432A2505`Fm", 16384}], msg_controllen=0, msg_flags=0}, 0) = 20fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(3, 1), ...}) = 0ioctl(1, SNDCTL_TMR_TIMEBASE or SNDRV_TIMER_IOCTL_NEXT_DEVICE or TCGETS, {B38400 opost isig icanon echo ...}) = 0mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f3c85d3b000write(1, "4: eth2: <BROADCAST,MULTICAST,UP"..., 884: eth2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000) = 88write(1, " link/ether 00:0c:29:43:96:78"..., 55 link/ether 00:0c:29:43:96:78 brd ff:ff:ff:ff:ff:ff) = 55write(1, " inet 192.168.74.55/24 brd 19"..., 63 inet 192.168.74.55/24 brd 192.168.74.255 scope global eth2) = 63write(1, " valid_lft forever preferr"..., 47 valid_lft forever preferred_lft forever) = 47open("/etc/iproute2/rt_scopes", O_RDONLY) = 4fstat(4, {st_mode=S_IFREG|0644, st_size=92, ...}) = 0mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f3c85d3a000read(4, "#
# reserved values
#
0	global
2"..., 4096) = 92read(4, "", 4096) = 0close(4) = 0munmap(0x7f3c85d3a000, 4096) = 0write(1, " inet6 fe80::20c:29ff:fe56:77"..., 70 inet6 fe80::20c:29ff:fe56:7771/64 scope link tentative dadfailed ) = 70write(1, " valid_lft forever preferr"..., 47 valid_lft forever preferred_lft forever) = 47exit_group(0) = ?+++ exited with 0 +++

源码分析

ip 对应的命令及主要实现接口定义在struct cmd结构中，如下

今天我们主要分析ip addr  show 命令， 所以对应的接口实现为do_ipaddr。

进入main函数，在执行具体的流程分支前，代码会先初始化netlink套接字信息，流程如下：

rtnl_open的接口实现为

rtnl_open_byproto接口的实现为

int rtnl_open_byproto(struct rtnl_handle *rth, unsigned int subscriptions, int protocol){ socklen_t addr_len; int sndbuf = 32768; int one = 1; memset(rth, 0, sizeof(*rth)); rth->proto = protocol; rth->fd = socket(AF_NETLINK, SOCK_RAW | SOCK_CLOEXEC, protocol); if (rth->fd < 0) { perror("Cannot open netlink socket"); return -1; } if (setsockopt(rth->fd, SOL_SOCKET, SO_SNDBUF, &sndbuf, sizeof(sndbuf)) < 0) { perror("SO_SNDBUF"); return -1; }
 if (setsockopt(rth->fd, SOL_SOCKET, SO_RCVBUF, &rcvbuf, sizeof(rcvbuf)) < 0) { perror("SO_RCVBUF"); return -1; }
 /* Older kernels may no support extended ACK reporting */ setsockopt(rth->fd, SOL_NETLINK, NETLINK_EXT_ACK, &one, sizeof(one));
 memset(&rth->local, 0, sizeof(rth->local)); rth->local.nl_family = AF_NETLINK; rth->local.nl_groups = subscriptions;
 if (bind(rth->fd, (struct sockaddr *)&rth->local, sizeof(rth->local)) < 0) { perror("Cannot bind netlink socket"); return -1; } addr_len = sizeof(rth->local); if (getsockname(rth->fd, (struct sockaddr *)&rth->local, &addr_len) < 0) { perror("Cannot getsockname"); return -1; } if (addr_len != sizeof(rth->local)) { fprintf(stderr, "Wrong address length %d
", addr_len); return -1; } if (rth->local.nl_family != AF_NETLINK) { fprintf(stderr, "Wrong address family %d
", rth->local.nl_family); return -1; } rth->seq = time(NULL); return 0;}

通过cmd定义，ip addr命令对应的实现为 do_ipaddr接口中。继续分析do_ipaddr接口实现

根据ip addr 命令后携带参数类型及数量，执行不同的代码分支。根据代码可以知道，在终端执行 ip addr 、ip addr show、 ip addr list或者 ip addr lst效果一致。

我们刚开始执行的命令为ip  addr  show  dev  eth2，就会进入ipaddr_list_flush_or_save(argc-1, argv+1, IPADD_LIST);分支。继续分析ipaddr_list_flush_or_save接口，忽略掉不相关代码，  程序执行

分析代码，最终获取eth2网口信息的实现在 ipaddr_link_get接口中，实现如下：

最后遍历linfo链表打印到终端，代码如下：

应用

正所谓它山之石可以攻玉，学习的目的在于应用，如果我们在实际的项目开发中，需要通过netlink接口获取内核信息， 完全可以参考iproute2的代码实现， 但是我们完全没有必要自己全部分装netlink的各个与内核交互的接口和信息， 通过分析iproute2源码的makefile文件和目录结构如下：

我们完全只需要将include和lib拷贝到自己的工程中，如果需要获取ip相关信息，那只需要调用lib提供的相关接口，再参考ip目录提供的实现例子对获取的信息进行解析即可。

lib目录中提供了两个静态库，名字为libnetlink.a 和 libutil.a。Makefile实现如下，如果读者习惯使用动态库，可以稍加修改Makefile脚本，编译为动态库即可。

include目录提供了相关接口的声明。