Extracted from [de49a77](https://github.com/NVIDIA/nccl/tree/de49a77074e884758e689ff0204f066eac7aae46 )

nccl_socket.cpp

/*************************************************************************
 * Copyright (c) 2016-2019, NVIDIA CORPORATION. All rights reserved.
 *
 * See LICENSE.txt for license information
 ************************************************************************/

#include <arpa/inet.h>
#include <errno.h>
#include <ifaddrs.h>
#include <net/if.h>
#include <netdb.h>
#include <netinet/tcp.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.h>

#define ENABLE_TRACE 1
#define ncclDebugNoWarn 0

typedef int ncclResult_t;

#define ncclSuccess 0
#define ncclSystemError 1
#define ncclInvalidArgument 2

struct netIf {
  char prefix[64];
  int port;
};

#define TRACE(place_holder, pattern, ...) \
  do {                                    \
    printf("TRACE: ");                    \
    printf(pattern, __VA_ARGS__);         \
    printf("\n");                         \
  } while (0)

#define INFO(place_holder, pattern, ...) \
  do {                                   \
    printf("INFO: ");                    \
    printf(pattern, __VA_ARGS__);        \
    printf("\n");                        \
  } while (0)

#define WARN(...)        \
  do {                   \
    printf("WARN: ");    \
    printf(__VA_ARGS__); \
    printf("\n");        \
  } while (0)

#define SYSCHECK(call, name)         \
  do {                               \
    int retval;                      \
    SYSCHECKVAL(call, name, retval); \
  } while (false)

#define SYSCHECKVAL(call, name, retval)                      \
  do {                                                       \
    SYSCHECKSYNC(call, name, retval);                        \
    if (retval == -1) {                                      \
      WARN("Call to " name " failed : %s", strerror(errno)); \
      return ncclSystemError;                                \
    }                                                        \
  } while (false)

#define SYSCHECKSYNC(call, name, retval)                                               \
  do {                                                                                 \
    retval = call;                                                                     \
    if (retval == -1 && (errno == EINTR || errno == EWOULDBLOCK || errno == EAGAIN)) { \
      INFO(NCCL_ALL, "Call to " name " returned %s, retrying", strerror(errno));       \
    } else {                                                                           \
      break;                                                                           \
    }                                                                                  \
  } while (true)

// Propagate errors up
#define NCCLCHECK(call)                                                                 \
  do {                                                                                  \
    ncclResult_t res = call;                                                            \
    if (res != ncclSuccess) {                                                           \
      /* Print the back trace*/                                                         \
      if (ncclDebugNoWarn == 0) INFO(NCCL_ALL, "%s:%d -> %d", __FILE__, __LINE__, res); \
      return res;                                                                       \
    }                                                                                   \
  } while (0);

#define MAX_IFS 16
#define MAX_IF_NAME_SIZE 16
#define SLEEP_INT 1000           // connection retry sleep interval in usec
#define RETRY_REFUSED_TIMES 2e4  // connection refused retry times before reporting a timeout (20 sec)
#define RETRY_TIMEDOUT_TIMES 3   // connection timed out retry times (each one can take 20s)

int parseStringList(const char* string, struct netIf* ifList, int maxList) {
  if (!string) return 0;

  const char* ptr = string;

  int ifNum = 0;
  int ifC = 0;
  char c;
  do {
    c = *ptr;
    if (c == ':') {
      if (ifC > 0) {
        ifList[ifNum].prefix[ifC] = '\0';
        ifList[ifNum].port = atoi(ptr + 1);
        ifNum++;
        ifC = 0;
      }
      while (c != ',' && c != '\0') c = *(++ptr);
    } else if (c == ',' || c == '\0') {
      if (ifC > 0) {
        ifList[ifNum].prefix[ifC] = '\0';
        ifList[ifNum].port = -1;
        ifNum++;
        ifC = 0;
      }
    } else {
      ifList[ifNum].prefix[ifC] = c;
      ifC++;
    }
    ptr++;
  } while (ifNum < maxList && c);
  return ifNum;
}

static bool matchIf(const char* string, const char* ref, bool matchExact) {
  // Make sure to include '\0' in the exact case
  int matchLen = matchExact ? strlen(string) + 1 : strlen(ref);
  return strncmp(string, ref, matchLen) == 0;
}

static bool matchPort(const int port1, const int port2) {
  if (port1 == -1) return true;
  if (port2 == -1) return true;
  if (port1 == port2) return true;
  return false;
}

bool matchIfList(const char* string, int port, struct netIf* ifList, int listSize, bool matchExact) {
  // Make an exception for the case where no user list is defined
  if (listSize == 0) return true;

  for (int i = 0; i < listSize; i++) {
    if (matchIf(string, ifList[i].prefix, matchExact) && matchPort(port, ifList[i].port)) {
      return true;
    }
  }
  return false;
}

/* Common socket address storage structure for IPv4/IPv6 */
union socketAddress {
  struct sockaddr sa;
  struct sockaddr_in sin;
  struct sockaddr_in6 sin6;
};

/* Format a string representation of a (struct sockaddr *) socket address using getnameinfo()
 *
 * Output: "IPv4/IPv6 address<port>"
 */
static inline const char* socketToString(struct sockaddr* saddr, char* buf) {
  if (buf == NULL || saddr == NULL) return NULL;
  if (saddr->sa_family != AF_INET && saddr->sa_family != AF_INET6) {
    buf[0] = '\0';
    return buf;
  }
  char host[NI_MAXHOST], service[NI_MAXSERV];
  (void)getnameinfo(saddr, sizeof(union socketAddress), host, NI_MAXHOST, service, NI_MAXSERV, NI_NUMERICHOST | NI_NUMERICSERV);
  sprintf(buf, "%s<%s>", host, service);
  return buf;
}

static inline uint16_t socketToPort(struct sockaddr* saddr) {
  return ntohs(saddr->sa_family == AF_INET ? ((struct sockaddr_in*)saddr)->sin_port : ((struct sockaddr_in6*)saddr)->sin6_port);
}

/* Allow the user to force the IPv4/IPv6 interface selection */
static inline int envSocketFamily(void) {
  int family = -1;  // Family selection is not forced, will use first one found
  char* env = getenv("NCCL_SOCKET_FAMILY");
  if (env == NULL) return family;

  INFO(NCCL_ENV, "NCCL_SOCKET_FAMILY set by environment to %s", env);

  if (strcmp(env, "AF_INET") == 0)
    family = AF_INET;  // IPv4
  else if (strcmp(env, "AF_INET6") == 0)
    family = AF_INET6;  // IPv6
  return family;
}

static int findInterfaces(const char* prefixList, char* names, union socketAddress* addrs, int sock_family, int maxIfNameSize, int maxIfs) {
#ifdef ENABLE_TRACE
  char line[1024];
#endif
  struct netIf userIfs[MAX_IFS];
  bool searchNot = prefixList && prefixList[0] == '^';
  if (searchNot) prefixList++;
  bool searchExact = prefixList && prefixList[0] == '=';
  if (searchExact) prefixList++;
  int nUserIfs = parseStringList(prefixList, userIfs, MAX_IFS);

  int found = 0;
  struct ifaddrs *interfaces, *interface;
  getifaddrs(&interfaces);
  for (interface = interfaces; interface && found < maxIfs; interface = interface->ifa_next) {
    if (interface->ifa_addr == NULL) continue;

    /* We only support IPv4 & IPv6 */
    int family = interface->ifa_addr->sa_family;
    if (family != AF_INET && family != AF_INET6) continue;

    TRACE(NCCL_INIT | NCCL_NET, "Found interface %s:%s", interface->ifa_name, socketToString(interface->ifa_addr, line));

    /* Allow the caller to force the socket family type */
    if (sock_family != -1 && family != sock_family) continue;

    /* We also need to skip IPv6 loopback interfaces */
    if (family == AF_INET6) {
      struct sockaddr_in6* sa = (struct sockaddr_in6*)(interface->ifa_addr);
      if (IN6_IS_ADDR_LOOPBACK(&sa->sin6_addr)) continue;
    }

    // check against user specified interfaces
    if (!(matchIfList(interface->ifa_name, -1, userIfs, nUserIfs, searchExact) ^ searchNot)) {
      continue;
    }

    // Check that this interface has not already been saved
    // getifaddrs() normal order appears to be; IPv4, IPv6 Global, IPv6 Link
    bool duplicate = false;
    for (int i = 0; i < found; i++) {
      if (strcmp(interface->ifa_name, names + i * maxIfNameSize) == 0) {
        duplicate = true;
        break;
      }
    }

    if (!duplicate) {
      // Store the interface name
      strncpy(names + found * maxIfNameSize, interface->ifa_name, maxIfNameSize);
      // Store the IP address
      int salen = (family == AF_INET) ? sizeof(sockaddr_in) : sizeof(sockaddr_in6);
      memcpy(addrs + found, interface->ifa_addr, salen);
      found++;
    }
  }

  freeifaddrs(interfaces);
  return found;
}

static bool matchSubnet(struct ifaddrs local_if, union socketAddress* remote) {
  /* Check family first */
  int family = local_if.ifa_addr->sa_family;
  if (family != remote->sa.sa_family) {
    return false;
  }

  if (family == AF_INET) {
    struct sockaddr_in* local_addr = (struct sockaddr_in*)(local_if.ifa_addr);
    struct sockaddr_in* mask = (struct sockaddr_in*)(local_if.ifa_netmask);
    struct sockaddr_in& remote_addr = remote->sin;
    struct in_addr local_subnet, remote_subnet;
    local_subnet.s_addr = local_addr->sin_addr.s_addr & mask->sin_addr.s_addr;
    remote_subnet.s_addr = remote_addr.sin_addr.s_addr & mask->sin_addr.s_addr;
    return (local_subnet.s_addr ^ remote_subnet.s_addr) ? false : true;
  } else if (family == AF_INET6) {
    struct sockaddr_in6* local_addr = (struct sockaddr_in6*)(local_if.ifa_addr);
    struct sockaddr_in6* mask = (struct sockaddr_in6*)(local_if.ifa_netmask);
    struct sockaddr_in6& remote_addr = remote->sin6;
    struct in6_addr& local_in6 = local_addr->sin6_addr;
    struct in6_addr& mask_in6 = mask->sin6_addr;
    struct in6_addr& remote_in6 = remote_addr.sin6_addr;
    bool same = true;
    int len = 16;                    // IPv6 address is 16 unsigned char
    for (int c = 0; c < len; c++) {  // Network byte order is big-endian
      char c1 = local_in6.s6_addr[c] & mask_in6.s6_addr[c];
      char c2 = remote_in6.s6_addr[c] & mask_in6.s6_addr[c];
      if (c1 ^ c2) {
        same = false;
        break;
      }
    }
    // At last, we need to compare scope id
    // Two Link-type addresses can have the same subnet address even though they are not in the same scope
    // For Global type, this field is 0, so a comparison wouldn't matter
    same &= (local_addr->sin6_scope_id == remote_addr.sin6_scope_id);
    return same;
  } else {
    WARN("Net : Unsupported address family type");
    return false;
  }
}

static int findInterfaceMatchSubnet(char* ifNames, union socketAddress* localAddrs, union socketAddress* remoteAddr, int ifNameMaxSize,
                                    int maxIfs) {
#ifdef ENABLE_TRACE
  char line[1024];
#endif
  char line_a[1024];
  int found = 0;
  struct ifaddrs *interfaces, *interface;
  getifaddrs(&interfaces);
  for (interface = interfaces; interface && !found; interface = interface->ifa_next) {
    if (interface->ifa_addr == NULL) continue;

    /* We only support IPv4 & IPv6 */
    int family = interface->ifa_addr->sa_family;
    if (family != AF_INET && family != AF_INET6) continue;

    // check against user specified interfaces
    if (!matchSubnet(*interface, remoteAddr)) {
      continue;
    }

    // Store the local IP address
    int salen = (family == AF_INET) ? sizeof(sockaddr_in) : sizeof(sockaddr_in6);
    memcpy(localAddrs + found, interface->ifa_addr, salen);

    // Store the interface name
    strncpy(ifNames + found * ifNameMaxSize, interface->ifa_name, ifNameMaxSize);

    TRACE(NCCL_INIT | NCCL_NET, "NET : Found interface %s:%s in the same subnet as remote address %s", interface->ifa_name,
          socketToString(&(localAddrs[found].sa), line), socketToString(&(remoteAddr->sa), line_a));
    found++;
    if (found == maxIfs) break;
  }

  if (found == 0) {
    WARN("Net : No interface found in the same subnet as remote address %s", socketToString(&(remoteAddr->sa), line_a));
  }
  freeifaddrs(interfaces);
  return found;
}

static ncclResult_t GetSocketAddrFromString(union socketAddress* ua, const char* ip_port_pair) {
  if (!(ip_port_pair && strlen(ip_port_pair) > 1)) {
    WARN("Net : string is null");
    return ncclInvalidArgument;
  }

  bool ipv6 = ip_port_pair[0] == '[';
  /* Construct the sockaddress structure */
  if (!ipv6) {
    struct netIf ni;
    // parse <ip_or_hostname>:<port> string, expect one pair
    if (parseStringList(ip_port_pair, &ni, 1) != 1) {
      WARN("Net : No valid <IPv4_or_hostname>:<port> pair found");
      return ncclInvalidArgument;
    }

    struct addrinfo hints, *p;
    int rv;
    memset(&hints, 0, sizeof(hints));
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;

    if ((rv = getaddrinfo(ni.prefix, NULL, &hints, &p)) != 0) {
      WARN("Net : error encountered when getting address info : %s", gai_strerror(rv));
      return ncclInvalidArgument;
    }

    // use the first
    if (p->ai_family == AF_INET) {
      struct sockaddr_in& sin = ua->sin;
      memcpy(&sin, p->ai_addr, sizeof(struct sockaddr_in));
      sin.sin_family = AF_INET;  // IPv4
      // inet_pton(AF_INET, ni.prefix, &(sin.sin_addr));  // IP address
      sin.sin_port = htons(ni.port);  // port
    } else if (p->ai_family == AF_INET6) {
      struct sockaddr_in6& sin6 = ua->sin6;
      memcpy(&sin6, p->ai_addr, sizeof(struct sockaddr_in6));
      sin6.sin6_family = AF_INET6;      // IPv6
      sin6.sin6_port = htons(ni.port);  // port
      sin6.sin6_flowinfo = 0;           // needed by IPv6, but possibly obsolete
      sin6.sin6_scope_id = 0;           // should be global scope, set to 0
    } else {
      WARN("Net : unsupported IP family");
      return ncclInvalidArgument;
    }

    freeaddrinfo(p);  // all done with this structure

  } else {
    int i, j = -1, len = strlen(ip_port_pair);
    for (i = 1; i < len; i++) {
      if (ip_port_pair[i] == '%') j = i;
      if (ip_port_pair[i] == ']') break;
    }
    if (i == len) {
      WARN("Net : No valid [IPv6]:port pair found");
      return ncclInvalidArgument;
    }
    bool global_scope = (j == -1 ? true : false);  // If no % found, global scope; otherwise, link scope

    char ip_str[NI_MAXHOST], port_str[NI_MAXSERV], if_name[IFNAMSIZ];
    memset(ip_str, '\0', sizeof(ip_str));
    memset(port_str, '\0', sizeof(port_str));
    memset(if_name, '\0', sizeof(if_name));
    strncpy(ip_str, ip_port_pair + 1, global_scope ? i - 1 : j - 1);
    strncpy(port_str, ip_port_pair + i + 2, len - i - 1);
    int port = atoi(port_str);
    if (!global_scope) strncpy(if_name, ip_port_pair + j + 1, i - j - 1);  // If not global scope, we need the intf name

    struct sockaddr_in6& sin6 = ua->sin6;
    sin6.sin6_family = AF_INET6;                                      // IPv6
    inet_pton(AF_INET6, ip_str, &(sin6.sin6_addr));                   // IP address
    sin6.sin6_port = htons(port);                                     // port
    sin6.sin6_flowinfo = 0;                                           // needed by IPv6, but possibly obsolete
    sin6.sin6_scope_id = global_scope ? 0 : if_nametoindex(if_name);  // 0 if global scope; intf index if link scope
  }
  return ncclSuccess;
}

static int findInterfaces(char* ifNames, union socketAddress* ifAddrs, int ifNameMaxSize, int maxIfs) {
  static int shownIfName = 0;
  int nIfs = 0;
  // Allow user to force the INET socket family selection
  int sock_family = envSocketFamily();
  // User specified interface
  char* env = getenv("NCCL_SOCKET_IFNAME");
  if (env && strlen(env) > 1) {
    INFO(NCCL_ENV, "NCCL_SOCKET_IFNAME set by environment to %s", env);
    // Specified by user : find or fail
    if (shownIfName++ == 0) INFO(NCCL_NET, "NCCL_SOCKET_IFNAME set to %s", env);
    nIfs = findInterfaces(env, ifNames, ifAddrs, sock_family, ifNameMaxSize, maxIfs);
  } else {
    // Try to automatically pick the right one
    // Start with IB
    nIfs = findInterfaces("ib", ifNames, ifAddrs, sock_family, ifNameMaxSize, maxIfs);
    // else see if we can get some hint from COMM ID
    if (nIfs == 0) {
      char* commId = getenv("NCCL_COMM_ID");
      if (commId && strlen(commId) > 1) {
        INFO(NCCL_ENV, "NCCL_COMM_ID set by environment to %s", commId);
        // Try to find interface that is in the same subnet as the IP in comm id
        union socketAddress idAddr;
        GetSocketAddrFromString(&idAddr, commId);
        nIfs = findInterfaceMatchSubnet(ifNames, ifAddrs, &idAddr, ifNameMaxSize, maxIfs);
      }
    }
    // Then look for anything else (but not docker or lo)
    if (nIfs == 0) nIfs = findInterfaces("^docker,lo", ifNames, ifAddrs, sock_family, ifNameMaxSize, maxIfs);
    // Finally look for docker, then lo.
    if (nIfs == 0) nIfs = findInterfaces("docker", ifNames, ifAddrs, sock_family, ifNameMaxSize, maxIfs);
    if (nIfs == 0) nIfs = findInterfaces("lo", ifNames, ifAddrs, sock_family, ifNameMaxSize, maxIfs);
  }
  return nIfs;
}

static ncclResult_t createListenSocket(int* fd, union socketAddress* localAddr) {
  /* IPv4/IPv6 support */
  int family = localAddr->sa.sa_family;
  int salen = (family == AF_INET) ? sizeof(sockaddr_in) : sizeof(sockaddr_in6);

  /* Create socket and bind it to a port */
  int sockfd = socket(family, SOCK_STREAM, 0);
  if (sockfd == -1) {
    WARN("Net : Socket creation failed : %s", strerror(errno));
    return ncclSystemError;
  }

  if (socketToPort(&localAddr->sa)) {
    // Port is forced by env. Make sure we get the port.
    int opt = 1;
#if defined(SO_REUSEPORT)
    SYSCHECK(setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR | SO_REUSEPORT, &opt, sizeof(opt)), "setsockopt");
#else
    SYSCHECK(setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt)), "setsockopt");
#endif
  }

  // localAddr port should be 0 (Any port)
  SYSCHECK(bind(sockfd, &localAddr->sa, salen), "bind");

  /* Get the assigned Port */
  socklen_t size = salen;
  SYSCHECK(getsockname(sockfd, &localAddr->sa, &size), "getsockname");

#ifdef ENABLE_TRACE
  char line[1024];
  TRACE(NCCL_INIT | NCCL_NET, "Listening on socket %s", socketToString(&localAddr->sa, line));
#endif

  /* Put the socket in listen mode
   * NB: The backlog will be silently truncated to the value in /proc/sys/net/core/somaxconn
   */
  SYSCHECK(listen(sockfd, 16384), "listen");
  *fd = sockfd;
  return ncclSuccess;
}

static ncclResult_t connectAddress(int* fd, union socketAddress* remoteAddr) {
  /* IPv4/IPv6 support */
  int family = remoteAddr->sa.sa_family;
  int salen = (family == AF_INET) ? sizeof(sockaddr_in) : sizeof(sockaddr_in6);

  /* Connect to a hostname / port */
  *fd = socket(family, SOCK_STREAM, 0);
  if (*fd == -1) {
    WARN("Net : Socket creation failed : %s", strerror(errno));
    return ncclSystemError;
  }

  const int one = 1;
  SYSCHECK(setsockopt(*fd, IPPROTO_TCP, TCP_NODELAY, (char*)&one, sizeof(int)), "setsockopt");

  /*  const int bufsize = 128*1024;
    SYSCHECK(setsockopt(*fd, SOL_SOCKET, SO_SNDBUF, (char*)&bufsize, sizeof(int)), "setsockopt");
    SYSCHECK(setsockopt(*fd, SOL_SOCKET, SO_RCVBUF, (char*)&bufsize, sizeof(int)), "setsockopt");*/

  char line[1024];
#ifdef ENABLE_TRACE
  TRACE(NCCL_INIT | NCCL_NET, "Connecting to socket %s", socketToString(&remoteAddr->sa, line));
#endif

  int ret;
  int timedout_retries = 0;
  int refused_retries = 0;
retry:
  SYSCHECKSYNC(connect(*fd, &remoteAddr->sa, salen), "connect", ret);
  if (ret == 0) return ncclSuccess;
  if ((errno == ECONNREFUSED || errno == ETIMEDOUT)) {
    if ((errno == ECONNREFUSED && ++refused_retries < RETRY_REFUSED_TIMES) ||
        (errno == ETIMEDOUT && ++timedout_retries < RETRY_TIMEDOUT_TIMES)) {
      if (refused_retries % 1000 == 0) INFO(NCCL_ALL, "Call to connect returned %s, retrying", strerror(errno));
      usleep(SLEEP_INT);
      goto retry;
    }
  }
  WARN("Connect to %s failed : %s", socketToString(&remoteAddr->sa, line), strerror(errno));
  return ncclSystemError;
}

#define NCCL_SOCKET_SEND 0
#define NCCL_SOCKET_RECV 1
static ncclResult_t socketProgressOpt(int op, int fd, void* ptr, int size, int* offset, int block) {
  int bytes = 0;
  char* data = (char*)ptr;
  do {
    if (op == NCCL_SOCKET_RECV) bytes = recv(fd, data + (*offset), size - (*offset), block ? 0 : MSG_DONTWAIT);
    if (op == NCCL_SOCKET_SEND) bytes = send(fd, data + (*offset), size - (*offset), block ? 0 : MSG_DONTWAIT);
    if (op == NCCL_SOCKET_RECV && bytes == 0) {
      WARN("Net : Connection closed by remote peer");
      return ncclSystemError;
    }
    if (bytes == -1) {
      if (errno != EINTR && errno != EWOULDBLOCK && errno != EAGAIN) {
        WARN("Call to recv failed : %s", strerror(errno));
        return ncclSystemError;
      } else {
        bytes = 0;
      }
    }
    (*offset) += bytes;
  } while (bytes > 0 && (*offset) < size);
  return ncclSuccess;
}

static ncclResult_t socketProgress(int op, int fd, void* ptr, int size, int* offset) {
  return socketProgressOpt(op, fd, ptr, size, offset, 0);
}

static ncclResult_t socketWait(int op, int fd, void* ptr, int size, int* offset) {
  while (*offset < size) NCCLCHECK(socketProgressOpt(op, fd, ptr, size, offset, 1));
  return ncclSuccess;
}

static ncclResult_t socketSend(int fd, void* ptr, int size) {
  int offset = 0;
  NCCLCHECK(socketWait(NCCL_SOCKET_SEND, fd, ptr, size, &offset));
  return ncclSuccess;
}

static ncclResult_t socketReceive(int fd, void* ptr, int size) {
  int offset = 0;
  NCCLCHECK(socketWait(NCCL_SOCKET_RECV, fd, ptr, size, &offset));
  return ncclSuccess;
}

int main() {
  char names[MAX_IF_NAME_SIZE*MAX_IFS];
  union socketAddress addrs[MAX_IFS];
  int ncclNetIfs = findInterfaces(names, addrs, MAX_IF_NAME_SIZE, MAX_IFS);

  printf("\n======== Result ========\n");
  printf("Found %d interface.\n", ncclNetIfs);
  for(int i=0; i<ncclNetIfs; i++) {
    printf(" - %d: %s\n", i, &names[MAX_IF_NAME_SIZE * i]);
  }
}

If you want to know how the f*** NCCL pick network interface.

g++ nccl_socket.cpp -o nccl_socket

# run with with some additional envs, and see if the result match your expectation.
NCCL_COMM_ID=10.1.23.45 ./nccl_socket

available env:

NCCL_SOCKET_FAMILY
NCCL_SOCKET_IFNAME
NCCL_COMM_ID

NCCL_COMM_ID is missing from documentation