main.c

#define _GNU_SOURCE

#include <fcntl.h>
#include <sched.h>
#include <stdio.h>
#include <stdlib.h>

int main(int argc, const char *argv[]) {
	if (argc != 3) {
		printf("Usage:\n");
		printf("\t%s NETNS_FILE COMMAND\n", argv[0]);
		return 1;
	}

	int fd = open(argv[1], 0);
	if (fd < 0) {
		printf("Cloudn't open netns file\n");
		return 1;
	}

	int err = setns(fd, CLONE_NEWNET);
	if (err != 0) {
		printf("Failed to setns\n");
		return 1;
	}

	system(argv[2]);
}

proc-1-ns-net

$ sudo ./a.out /proc/1/ns/net "cat /proc/net/dev"
Inter-|   Receive                                                |  Transmit
 face |bytes    packets errs drop fifo frame compressed multicast|bytes    packets errs drop fifo colls carrier compressed
    lo: 19169789308 3292433    0    0    0     0          0         0 19169789308 3292433    0    0    0     0       0          0
enp3s0: 54061868186 93390689    0 1601    0     0          0    193967 52834018320 90656143    0    0    0     0       0          0
wlp2s0: 4710068   25551    0    0    0     0          0         0 28146620   31022    0    0    0     0       0          0
Management: 4339712   25470    0    0    0     0          0         0     4130       7    0    0    0     0       0          0
private: 11043357669 30494619    0    0    0     0          0      1070 38382690789 44373108    0    0    0     0       0          0
external: 39295920892 47042469    0 163965    0     0          0    164002 11910273444 32507959    0    0    0     0       0          0
br-a108010ddf73:       0       0    0    0    0     0          0         0        0       0    0    0    0     0       0          0
docker0:       0       0    0    0    0     0          0         0        0       0    0    0    0     0       0          0
 guest: 116642051  161708    0    0    0     0          0      1434 482205883  357946    0    0    0     0       0          0
   dmz: 325641255 1741815    0    0    0     0          0         0 828399876 1301762    0    0    0     0       0          0

proc-110156-ns-net

$ sudo ./a.out /proc/110156/ns/net "cat /proc/net/dev"
[sudo] password for proelbtn:
Inter-|   Receive                                                |  Transmit
 face |bytes    packets errs drop fifo frame compressed multicast|bytes    packets errs drop fifo colls carrier compressed
    lo:       0       0    0    0    0     0          0         0        0       0    0    0    0     0       0          0

run-netns-test

$ sudo ./a.out "/run/netns/test" "cat /proc/net/dev"
Inter-|   Receive                                                |  Transmit
 face |bytes    packets errs drop fifo frame compressed multicast|bytes    packets errs drop fifo colls carrier compressed
    lo:       0       0    0    0    0     0          0         0        0       0    0    0    0     0       0          0